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Diffuse agricultural pollution is one of the greatest challenges to achieving good chemical and ecological status of Scotland’s water bodies. The River Ythan in Aberdeenshire was designated a Nitrate Vulnerable Zone (NVZ) in the year 2000, due to the eutrophication of the Ythan Estuary and rising nitrate trends in Private Water Supply (PWS) groundwater abstractions. The third River Basin Management Plan (RBMP) for Scotland reported the Ellon groundwater body of the River Ythan catchment to be of poor chemical status as of 2021 with respect to nitrate, and forecasted groundwater recovery beyond 2027. Following two decades of NVZ designation, we investigated the drivers of groundwater nitrate across the River Ythan catchment through an analysis of long-term (2009–2018) groundwater quality monitoring data collected by the Scottish Environmental Protection Agency (SEPA) and a recent synoptic groundwater nitrate sampling survey of PWSs. Groundwater nitrate was found to remain elevated across the catchment area, and appeared to be highly sensitive to agricultural practices and meteorological forcing, indicating a high sensitivity of groundwater quality to environmental change. Further hydrogeological characterisation is recommended to better understand the effects of agricultural practices on groundwater quality, and to facilitate achievement of future RBMP goals under a changing climate.
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Citation: Johnson, H.; Simpson, E.M.;
Troldborg, M.; Ofterdinger, U.;
Cassidy, R.; Soulsby, C.; Comte, J.-C.
Evaluating Groundwater Nitrate
Status across the River Ythan
Catchment (Scotland) following Two
Decades of Nitrate Vulnerable Zone
Designation. Environments 2023,10,
67. https://doi.org/10.3390/
environments10040067
Academic Editor: Marianne Stuart
Received: 21 March 2023
Revised: 14 April 2023
Accepted: 14 April 2023
Published: 18 April 2023
Copyright: © 2023 by the authors.
Licensee MDPI, Basel, Switzerland.
This article is an open access article
distributed under the terms and
conditions of the Creative Commons
Attribution (CC BY) license (https://
creativecommons.org/licenses/by/
4.0/).
environments
Article
Evaluating Groundwater Nitrate Status across the River Ythan
Catchment (Scotland) following Two Decades of Nitrate
Vulnerable Zone Designation
Hamish Johnson 1, * , Emma May Simpson 1, Mads Troldborg 2, Ulrich Ofterdinger 3, Rachel Cassidy 4,
Chris Soulsby 1and Jean-Christophe Comte 1
1School of Geosciences, University of Aberdeen, Aberdeen AB24 3FX, UK;
emma.may.simpson@outlook.de (E.M.S.); c.soulsby@abdn.ac.uk (C.S.); jc.comte@abdn.ac.uk (J.-C.C.)
2Information and Computational Sciences, The James Hutton Institute, Aberdeen AB15 8QH, UK;
mads.troldborg@hutton.ac.uk
3School of Natural and Built Environment, Queen’s University Belfast, Belfast BT9 5AG, UK;
u.ofterdinger@qub.ac.uk
4Agri-Environment Branch, Agri-Food and Biosciences Institute, Belfast BT9 5PX, UK;
rachel.cassidy@afbini.gov.uk
*Correspondence: h.johnson.19@abdn.ac.uk
Abstract:
Diffuse agricultural pollution is one of the greatest challenges to achieving good chemical
and ecological status of Scotland’s water bodies. The River Ythan in Aberdeenshire was designated
a Nitrate Vulnerable Zone (NVZ) in the year 2000, due to the eutrophication of the Ythan Estuary
and rising nitrate trends in Private Water Supply (PWS) groundwater abstractions. The third River
Basin Management Plan (RBMP) for Scotland reported the Ellon groundwater body of the River
Ythan catchment to be of poor chemical status as of 2021 with respect to nitrate, and forecasted
groundwater recovery beyond 2027. Following two decades of NVZ designation, we investigated the
drivers of groundwater nitrate across the River Ythan catchment through an analysis of long-term
(2009–2018) groundwater quality monitoring data collected by the Scottish Environmental Protection
Agency (SEPA) and a recent synoptic groundwater nitrate sampling survey of PWSs. Groundwater
nitrate was found to remain elevated across the catchment area, and appeared to be highly sensitive
to agricultural practices and meteorological forcing, indicating a high sensitivity of groundwater
quality to environmental change. Further hydrogeological characterisation is recommended to better
understand the effects of agricultural practices on groundwater quality, and to facilitate achievement
of future RBMP goals under a changing climate.
Keywords:
diffuse nutrient pollution; nitrate; trends; NVZ; groundwater vulnerability; groundwater
quality; groundwater monitoring
1. Introduction
Anthropogenic nitrogen (N) loadings to the environment have contaminated aquifers
worldwide [
1
,
2
], resulting in the exposure of human and ecological receptors through
drinking water abstractions from aquifers and groundwater discharge to surface water
bodies, respectively [
3
5
]. Nitrate consumption in drinking water poses a variety of
human health risks [
6
], and surface water eutrophication is a key driver for biodiversity
loss [
7
]. Furthermore, water treatment for nitrate removal and clearance of algal mats
from eutrophicated reservoirs, recreational water bodies and protected habitats present
undesirable burdens to water management [8].
In European Union (EU) member states, the European Commission Drinking Water
Directive (80/778/EEC) designates the maximum permissible limit for nitrate in water
intended for human consumption as 50 mg/L of NO
3
(11.3 mg/L of NO
3
-N), due to the
perceived risks of nitrate consumption to human health. This threshold value was adopted
Environments 2023,10, 67. https://doi.org/10.3390/environments10040067 https://www.mdpi.com/journal/environments
Environments 2023,10, 67 2 of 22
in the Nitrates Directive (91/676/EEC) as an environmental water quality threshold, con-
tributing to the chemical status classification of water bodies under the Water Framework
Directive (WFD) (2000/60/EC). Under the WFD, the catchment areas supplying water
bodies exceeding the regulatory environmental nitrate limit or exhibiting poor ecological
status due to the impacts of eutrophication, are designated as Nitrate Vulnerable Zones
(NVZs), typically justified by the accompaniment of groundwater vulnerability mapping [
9
].
Agricultural land managers operating within NVZs are legally obligated to follow Nitrate
Action Plans (NAPs) regulating agricultural activities to reduce N loading to water bodies.
Although the original WFD goal for achieving good chemical and ecological status for
all European water bodies by 2015 was never realised, the pressure to achieve these environ-
mental targets has ultimately reduced N loadings to water bodies, where NVZ designation
has provided a powerful legislative tool [
10
]. Disregarding socioeconomic and policy im-
plementation challenges, the delayed recovery of water bodies to nitrate pollution has been
largely due to the practicality of reducing diffuse agricultural pollution losses from modern
agriculture, and the lag times of nitrate removal from catchments due to the groundwater
residence times of aquifers, that are often poorly constrained in rural regions [
11
,
12
]. In
addition, there is a growing risk of current diffuse nutrient management efforts being
undermined by climate change impacts to hydrological and biogeochemical cycles that
govern water quality [
13
]. Increased precipitation volume and intensity over the winter is
expected to increase the proportion of N lost from agroecosystems to groundwater, coupled
with increasing water scarcity over the summer due to droughts are expected to increase
the greywater footprint of agriculture [
14
], a metric representing the volume of freshwater
required to dilute contaminants below environmental limits. Increasing water scarcity and
temperatures over summer low flow periods will also increase the dependence of aquatic
ecosystems on the chemical quality of groundwater-derived baseflows, and increase the
severity of the impacts to biodiversity associated with eutrophication events [15].
In Scotland, diffuse nitrate pollution by agriculture represents the most significant
water quality pressure, affecting >80% groundwaters bodies by area [
16
]. NVZ designation
and revision is determined by water body chemical and ecological quality monitoring by the
Scottish Environmental Protection Agency (SEPA) and groundwater vulnerability mapping
supported by the British Geological Survey (BGS) [
17
20
]. EU water quality legislation
was transposed into Scots law through the Water Environment and Services (Scotland)
Act 2003 (WEWS), and has since been retained following withdrawal from the EU [
10
].
Under the WEWS, SEPA is responsible for the coordination of River Basin Management
Plans (RBMPs), which define targets for achieving good chemical and ecological status
of water bodies and their implementation, including groundwater quality monitoring on
behalf of the Scottish government. The current Scottish RBMP cycle (2021–2027) aims to
increase the number of water bodies achieving good water quality status from 87% to 98%,
in consideration of a climate emergency and biodiversity crisis [21].
Groundwater bodies with delayed water quality recovery to diffuse nutrient pollution
following long-term nutrient management improvements warrant further investigation
to validate the effectiveness of current agri-environmental policies [
22
24
]. This study
aimed to evaluate the factors limiting the recovery of the Ellon groundwater body of
the River Ythan catchment following two decades of NVZ designation. The specific
objectives were the following: (1) to relate the spatial distribution of groundwater nitrate to
the physiographic factors governing groundwater vulnerability; (2) to assess the relative
recovery rates of groundwaters from nitrate pollution observed at different long-term
groundwater quality Monitoring Stations (MSs); (3) to identify climate and land use drivers
determining groundwater nitrate dynamics; and (4) to use the factors associated with
delayed groundwater recovery to define recommendations for nutrient management to
ensure achievement of future water quality targets.
Environments 2023,10, 67 3 of 22
2. Materials and Methods
2.1. Case Study Area
The River Ythan catchment is located in the Aberdeenshire, northeast Scotland (
Figure 1
),
discharging into the Ythan Estuary, home to the Newburgh and Sands of Forvie RAMSAR
site and Site of Special Scientific Interest (SSSI). Agriculture is the dominant land use (87%),
where physiographic factors determine 61% of land capable of supporting mixed agriculture
and 38% of land capable of supporting arable agriculture [
25
]. Post-war agricultural
intensification of the catchment was associated with the widespread installation of artificial
drainage and conversion of grassland to arable land uses, with the introduction of the
Common Agricultural Policy motivating the cultivation and fertilisation of wheat and
barley at the expense of less demanding crops [
26
]. Deteriorating surface water quality and
eutrophication of the Ythan Estuary over the 1960s–1990s was concluded by SEPA to be
largely associated with agriculture, particularly changes to winter cropping [
27
]. The River
Ythan was finally designated as an NVZ in 2000 on the basis of estuarine eutrophication
and rising nitrate trends in surface waters and Private Water Supply (PWS) groundwater
abstractions. Subsequent groundwater vulnerability mapping by the British Geological
Survey (BGS) and the Macaulay Land Research Institute (now the James Hutton Institute)
motivated the expansion of the original NVZ in 2002 to cover the entirety of the coastal
lowlands of northeast Scotland [17].
Environments 2023, 10, x FOR PEER REVIEW 3 of 24
groundwater recovery to dene recommendations for nutrient management to ensure
achievement of future water quality targets.
2. Materials and Methods
2.1. Case Study Area
The River Ythan catchment is located in the Aberdeenshire, northeast Scotland (Fig-
ure 1), discharging into the Ythan Estuary, home to the Newburgh and Sands of Forvie
RAMSAR site and Site of Special Scientic Interest (SSSI). Agriculture is the dominant
land use (87%), where physiographic factors determine 61% of land capable of supporting
mixed agriculture and 38% of land capable of supporting arable agriculture [25]. Post-war
agricultural intensication of the catchment was associated with the widespread installa-
tion of articial drainage and conversion of grassland to arable land uses, with the intro-
duction of the Common Agricultural Policy motivating the cultivation and fertilisation of
wheat and barley at the expense of less demanding crops [26]. Deteriorating surface water
quality and eutrophication of the Ythan Estuary over the 1960s1990s was concluded by
SEPA to be largely associated with agriculture, particularly changes to winter cropping
[27]. The River Ythan was nally designated as an NVZ in 2000 on the basis of estuarine
eutrophication and rising nitrate trends in surface waters and Private Water Supply (PWS)
groundwater abstractions. Subsequent groundwater vulnerability mapping by the British
Geological Survey (BGS) and the Macaulay Land Research Institute (now the James Hut-
ton Institute) motivated the expansion of the original NVZ in 2002 to cover the entirety of
the coastal lowlands of northeast Scotland [17].
Figure 1. Map of the River Ythan catchment area and Ellon groundwater body in relation to the
wider Moray, Aberdeenshire/Ban and Buchan Nitrate Vulnerable Zone (NVZ). SEPA groundwater
quality monitoring stations are labelled 110. Sources: Scoish Government 2015; SEPA 2023; Con-
tains Ordnance Survey data © Crown copyright and database right, 2023.
A catchment-based environmental management project, The Ythan Project(2001
2005), was sponsored by the European LIFE Environmental fund, resulting in the
Figure 1.
Map of the River Ythan catchment area and Ellon groundwater body in relation to the wider
Moray, Aberdeenshire/Banff and Buchan Nitrate Vulnerable Zone (NVZ). SEPA groundwater quality
monitoring stations are labelled 1–10. Sources: Scottish Government 2015; SEPA 2023; Contains
Ordnance Survey data © Crown copyright and database right, 2023.
A catchment-based environmental management project, “The Ythan Project” (2001–
2005), was sponsored by the European LIFE Environmental fund, resulting in the restoration
of a significant tract of riparian zone along the main thalweg, provision of nutrient bud-
geting tools and training for farmers, in addition to supporting rural stewardship scheme
applications resulting in 70 km of buffer strip installation [
28
]. Despite improvements
in environmental management and surface water quality [
29
,
30
], the Ellon groundwater
Environments 2023,10, 67 4 of 22
body maintains a poor chemical quality status that is not expected to reach good environ-
mental status within the current RBMP (2021–2027) due to persistent groundwater nitrate
exceedances above the environmental limit [2130].
The basement rocks of the Ellon groundwater body include Precambrian (Dalradian)
metasediments and Ordovician igneous intrusions, providing low and very low produc-
tivity aquifers, respectively (Figure 2). In the metasedimentary aquifers, the majority of
groundwater flow occurs along the upper weathered zone at rock head, formed from
Tertiary weathering and/or (peri)glacial processes [
17
,
31
,
32
], with some deeper ground-
water flow controlled by complex fracture networks [
33
]. The igneous intrusions range
from granitic to ultrabasic compositions, with the latter typically associated with a deeper
degree of preglacial weathering [
32
]. The Turriff groundwater body is defined as a basin of
Devonian sedimentary rocks providing moderate productivity aquifers that supplement a
public water supply reservoir in the summer months.
Quaternary geology is mapped at the 1:50,000 scale in the west and at the 1:10,000 scale in
the east, resulting in a N–S division in diamicton coverage across the centre of the catchment,
creating artefacts within subsequent BGS thematic maps (Figure 2). Overall, diamicton rarely
exceeds 3 m in thickness, and is not considered to be a significantly exploitable aquifer, but
within an environmental context contributes to baseflow discharges, as well as feeds numerous
shallow PWS wells and springs [
17
]. Moderate productivity heterogeneous alluvial aquifers
are distributed throughout the river network, whereas highly permeable glaciofluvial sand
and gravel aquifers are restricted to the valley sides of the central thalweg.
The drainage properties of soils are related to their parent materials and position along
hillslopes, with well-drained podzols and brown earths occupying hilltops, and poorly
drained gleys occupying lower slopes [
34
]. The high spatial heterogeneity in geology and
landscape has favoured the persistence of mixed farming enterprises despite agricultural
intensification, where inclusion of grassland into arable rotations is required to maintain
soil structure [25,35].
Environments 2023, 10, x FOR PEER REVIEW 4 of 24
restoration of a signicant tract of riparian zone along the main thalweg, provision of nu-
trient budgeting tools and training for farmers, in addition to supporting rural steward-
ship scheme applications resulting in 70 km of buer strip installation [28]. Despite im-
provements in environmental management and surface water quality [29,30], the Ellon
groundwater body maintains a poor chemical quality status that is not expected to reach
good environmental status within the current RBMP (20212027) due to persistent
groundwater nitrate exceedances above the environmental limit [2130].
The basement rocks of the Ellon groundwater body include Precambrian (Dalradian)
metasediments and Ordovician igneous intrusions, providing low and very low produc-
tivity aquifers, respectively (Figure 2). In the metasedimentary aquifers, the majority of
groundwater ow occurs along the upper weathered zone at rock head, formed from Ter-
tiary weathering and/or (peri)glacial processes [17,31,32], with some deeper groundwater
ow controlled by complex fracture networks [33]. The igneous intrusions range from
granitic to ultrabasic compositions, with the laer typically associated with a deeper de-
gree of preglacial weathering [32]. The Turrigroundwater body is dened as a basin of
Devonian sedimentary rocks providing moderate productivity aquifers that supplement
a public water supply reservoir in the summer months.
Quaternary geology is mapped at the 1:50,000 scale in the west and at the 1:10,000
scale in the east, resulting in a NS division in diamicton coverage across the centre of the
catchment, creating artefacts within subsequent BGS thematic maps (Figure 2). Overall,
diamicton rarely exceeds 3 m in thickness, and is not considered to be a signicantly ex-
ploitable aquifer, but within an environmental context contributes to baseow discharges,
as well as feeds numerous shallow PWS wells and springs [17]. Moderate productivity
heterogeneous alluvial aquifers are distributed throughout the river network, whereas
highly permeable glaciouvial sand and gravel aquifers are restricted to the valley sides
of the central thalweg.
The drainage properties of soils are related to their parent materials and position
along hillslopes, with well-drained podzols and brown earths occupying hilltops, and
poorly drained gleys occupying lower slopes [34]. The high spatial heterogeneity in geol-
ogy and landscape has favoured the persistence of mixed farming enterprises despite ag-
ricultural intensication, where inclusion of grassland into arable rotations is required to
maintain soil structure [25,35].
Figure 2.
Spatial distribution of SEPA groundwater quality monitoring stations 1–10 and Private Water
Supply (PWS) groundwater sampling points by construction type (well, spring, borehole) overlayed
onto BGS aquifer productivity (Scotland) datasets derived from 1:100,000 scale BGS Digital Data under
License (2023/007) British Geological Survey. © and Database Right UKRI. All rights reserved [36,37].
Environments 2023,10, 67 5 of 22
2.2. Description of Groundwater Quality Datasets
SEPA established 10 groundwater quality MSs across the River Ythan catchment in
2008 [
30
]. Their selection of groundwater monitoring points was based on typical land use
classes to assess their relative effects on diffuse nitrate loadings to groundwater (Table 1).
These monitoring points include five purpose-drilled groundwater environmental monitor-
ing boreholes (n = 4), and farm PWS abstraction sites (n = 6) that include traditional shallow,
wide diameter dug wells, and spring collection chambers (the most common PWS construc-
tion type in Northeast Scotland) [
38
]. Following an initial high frequency monitoring during
a characterisation stage, groundwater quality monitoring became quarterly post-2013.
Table 1. Summary of the SEPA Ythan groundwater Monitoring Stations (MSs).
MS SEPA Site Code Construction Aquifer Type Geomorphology Current Land Use
1 365224 Monitoring borehole
Weathered bedrock
Hilltop Improved grassland
2 340586 PWS spring Superficial Slope break Improved grassland
3 345069 PWS spring Superficial Meltwater channel Mixed agriculture
4 338000 PWS well Superficial Meltwater channel Arable (cereals)
5 337736 PWS well Superficial Meltwater channel Arable (cereals)
6 338297 PWS well Superficial Hillslope Arable (cereals)
7 326656 PWS spring Superficial Fracture valley Mixed agriculture
8365223 1Monitoring borehole Fractured bedrock Hillslope Forestry
9365228 1Monitoring borehole Fractured bedrock Valley bottom Mixed agriculture
10 365226 1Monitoring borehole
Weathered bedrock
Valley bottom Bog peatland
1Nitrate concentrations consistently below analytical detection limit.
In addition to the SEPA groundwater quality monitoring data, groundwater sam-
ples were collected from 27 PWS groundwater abstraction points within the River Ythan
catchment between July 2021 and March 2022, representing a mixture of land uses and
construction styles (well, spring, borehole). Samples were bottled from the outflow pipes
of spring collection chambers, bailed from wells, and from purged, unfiltered borehole
taps. These groundwater samples were refrigerated prior to nitrate determination with
flow injection analysis (FIA) by the University of Aberdeen School of Biological Sciences
Analytical Services Unit. A further 38 groundwater samples from different PWS abstrac-
tion sites (2013–2020) were obtained from PWS water quality certificates obtained from
the Aberdeenshire Council public access and building register database. These samples
were collected and analysed by Aberdeenshire Council Environmental Health in accor-
dance with the Private Water Supplies (Scotland) Regulations 2006, representing unfiltered
groundwater samples to establish filtration requirements for safe human consumption.
Figure 2provides the spatial distribution of all groundwater samples plotted overlaying
BGS bedrock and superficial aquifer productivity maps for Scotland [36,37].
2.3. Groundwater Vulnerability Assessment
The current BGS groundwater vulnerability map for Scotland uses a decision tree-
based approach to evaluate the travel time and attenuation of contaminants reaching the
water table [
18
20
]. The conceptual model invokes the concept of recharge acceptance [
17
],
where low recharge rates associated with the low permeability fractured bedrock aquifers
that typify Scotland may result in low total contaminant loading rates, but with dispro-
portionate impacts to groundwater quality and loadings to receptors, due to negligible
opportunities for contaminant attenuation within fractured matrices [
16
]. Here, lower
groundwater vulnerability ratings may be provided by thick and/or low permeability
superficial cover. Where superficial deposits are absent, vulnerability is exclusively deter-
mined by soil permeability [1820].
The DRASTIC method [
39
] precedes the development of the BGS methodology tai-
lored for Scotland, yet remains one of the most widely applied groundwater vulnerability
mapping approaches [
39
,
40
]. Groundwater vulnerability is determined by the relative
Environments 2023,10, 67 6 of 22
weightings of several thematic layers representing the spatial distribution of hydrogeo-
logical parameters rated according to their influence on contaminant transport (Depth
to groundwater table, Recharge, Aquifer Media, Soil media, Topography (slope), Impact
of vadose zone, and hydraulic Conductivity). The conceptual model considers higher
permeability media such as glacial sand and gravels to be of higher risk of contaminant
transport relative to poorly permeable materials such as fractured crystalline bedrock, due
to higher advective transport rates. Likewise, higher recharge rates are associated with
greater contaminant fluxes to the saturated zone [
39
]. A DRASTIC groundwater vulnerabil-
ity map using the default methodology was developed for the River Ythan catchment to
assess the risk of N leaching to groundwater [
41
]. A comparison of the parameters used for
the BGS and DRASTIC groundwater vulnerability maps is provided in Table 2.
Table 2.
Comparison of data sources for BGS and DRASTIC groundwater vulnerability map-
ping approaches [20,41].
Hydrogeological Property BGS DRASTIC
Depth to groundwater table River head space [18], corrected by soil
indicators of groundwater depth [42]PWS dip well data collection [41]
Recharge - 2 km2gridded average groundwater recharge
1950–2009 [43]
Aquifer media
Dominant flow mechanism (fracture vs.
intergranular) inferred from BGS bedrock
aquifer productivity [36]
DRASTIC lithology ratings [
39
,
40
] applied to UK
bedrock and superficial geology maps [44,45]
Soil media
Soil hydrology classifications [
42
] used to
define permeability indices
DRASTIC soil texture ratings [39] applied to
Scottish soil map [46]
Topography (slope) - Ordnance Survey 10 m DEM [47]
Impact of vadose zone
Superficial deposit thickness model [48].
Maximum superficial geology
permeability map [49]
DRASTIC superficial geology texture ratings [
39
]
applied to BGS superficial geology map [45]
Impact of vadose zone
Superficial deposit thickness model [48].
Maximum superficial geology
permeability map [49]
DRASTIC superficial geology texture ratings [
39
]
applied to BGS superficial geology map [45]
Bedrock aquifer hydraulic
conductivity - Literature review [41]
2.4. Groundwater Nitrate Time Series Analyses
All statistical analyses of SEPA groundwater quality monitoring data were completed
within the open access trend analysis and equivalence testing of the environmental data
software package Time Trends v9.0 [
50
]. The software provides several statistical anal-
ysis tools that are widely applied to the assessment of groundwater quality time series
data [5154]
. Within the Time Trends package, seasonality and trend analyses were applied
to the entire time series for each SEPA groundwater quality MS to derive long-term trend
and seasonality statistics. The analyses were then repeated for discrete sections of the
monitoring period that demonstrated contrasting temporal dynamics, i.e., sub-trends. The
latter were screened for using a combination of piecewise linear regression and cumulative
sum charts, before iteratively applying trend tests to optimise fits to the data.
The strength of seasonality in groundwater nitrate time series data was evaluated by
applying Mann–Whitney Utests (
α
= 0.05) to observations grouped into four seasons that
were defined by the quarterly monitoring frequency towards the end of the monitoring
period (January–March, April–June, July–September, October–December). In addition
to the seasonality test, effective rainfall (precipitation (P)—potential evapotranspiration
(PET)) was plotted alongside time series of SEPA groundwater quality monitoring data
to identify seasonal climate drivers to nitrate dynamics. Daily total P and PET at a 1 km
2
spatial resolution were obtained from the HadUK-Grid meteorological and the Hydro-PE
HadUK-Grid datasets, respectively [54,55].
Environments 2023,10, 67 7 of 22
The significance of potential upward and downward trends was determined using
Mann Kendall correlation and Seasonal Kendall tests at the 95% significance level, for time
series with non-significant and significant seasonality components, respectively. Sen slope
estimation was then applied to all time series data, to provide a magnitude of the trend. The
relative slopes of groundwater nitrate trends for comparisons between MSs were derived
by expressing the calculated Sen slope as a percentage of median nitrate concentration over
the observed trend (% yr1).
2.5. Determination of Land Use
The local and/or dominant land use upgradient to SEPA groundwater MSs were
catalogued over the monitoring period, in order to provide probable explanations to drivers
of shallow groundwater nitrate dynamics. Google earth aerial imagery [
56
] provided
the primary determinant of land use due to the high-resolution imagery, whereas lower
resolution, satellite-derived Land Cover Maps (LCMs) and multispectral satellite imagery
provided secondary sources of land use classification to fill in time gaps between aerial
photographs. In aerial photographs, the presence of livestock was indicated by grazed land
(pasture), and grass fodder by the homogeneity of vegetative cover. Arable systems were
defined by the presence of tramlines and other linear heterogeneities in vegetative cover, as
well as seasonal occurrences of bare earth associated with harvest and tillage.
LCMs developed by the UK Centre for Ecology and Hydrology (CEH) from classifica-
tion of satellite imagery 2007, 2015 and 2017 were obtained from the EDINA Environment
Digimap Service [
57
]. CEH LCMs categorise agricultural land covers into improved grass-
land (grazing and grass fodder) and arable land uses, with some years providing rough
grassland as a separate category (grazing land).
Landsat imagery was viewed on the ArcGIS online Landsat Explorer web applica-
tion [
58
], using the agricultural band combination (6,5,2), to distinguish differences in
vegetative cover between fields. Under this band combination, vegetation appeared bright
green during early growth, becoming dark green at maturity, whereas stressed vegetation
associated with heavy grazing or dry grasses prior to cutting appeared a dull or yellowish
green. Despite improved grassland and arable systems both appearing green, different
shades were apparent between adjacent fields of contrasting land use. In addition, recently
tilled or harvested arable land with bare earth appeared brown or magenta.
3. Results
3.1. Spatial Distribution of Nutrients in Groundwater and Groundwater Vulnerability Assignment
Figure 3provides all groundwater samples plotted as graduated symbols overlaying
the BGS and DRASTIC groundwater vulnerability maps for the River Ythan catchment area.
The majority of groundwater nitrate concentrations in the River Ythan catchment greatly
exceeded natural background levels (>1 mg/L of NO
3
-N) [
59
], but ranged from below
the analytical detection limit (<0.3 mg/L of NO3-N) towards a maximum of 17.4 mg/L of
NO
3
-N. Figure 4provides boxplots illustrating the distribution of groundwater nitrate for
groundwater monitoring points grouped by construction details, physiographic factors,
land use and groundwater vulnerability classifications.
There was no significant difference in groundwater nitrate according to groundwa-
ter monitoring point construction details (well, spring, borehole) (Figure 4b). However,
groundwater samples collected from three SEPA boreholes screened exclusively in bedrock
aquifers (MSs 8–10) were consistently below the analytical detection limit for nitrate. MSs 8
and 10 were located in areas of semi-natural land uses, which are shown by Figure 4c to
be associated with lower nitrate concentrations for PWS groundwater abstraction points.
MS 9 was located at the foot of a hillslope associated with an improved grassland land use
category, contrasting with the remaining SEPA borehole MS 1 where consistently detectable
nitrate was located on a hilltop with improved grassland land use. A cluster of PWS
groundwater samples featuring low nitrate concentrations in the centre of the catchment
Environments 2023,10, 67 8 of 22
was associated with a predominantly residential area with a low proportion of agricultural
land uses.
Environments 2023, 10, x FOR PEER REVIEW 9 of 24
Figure 3. Spatial distribution of groundwater nitrate concentrations taken from SEPA Ythan moni-
toring stations and private water supplies, overlayed onto (a) BGS groundwater vulnerability map
(Scotland) derived from 1:100,000 scale BGS Digital Data under License (2023/007) British Geological
Survey. © and Database Right UKRI. All rights reserved; and (b) DRASTIC groundwater vulnera-
bility map for the River Ythan catchment [41].
Figure 3.
Spatial distribution of groundwater nitrate concentrations taken from SEPA Ythan moni-
toring stations and private water supplies, overlayed onto (
a
) BGS groundwater vulnerability map
(Scotland) derived from 1:100,000 scale BGS Digital Data under License (2023/007) British Geological
Survey.
©
and Database Right UKRI. All rights reserved; and (
b
) DRASTIC groundwater vulnerability
map for the River Ythan catchment [41].
PWS groundwater abstractions from aquifers overlain by poorly drained gley soils were
significantly lower than freely drained and imperfectly drained podzols and brown earths
(Figure 4d). There was no significant difference in groundwater nitrate for sampling points
that were spatially associated with different bedrock aquifer productivity classes (Figure 4e).
A small sample size (n = 3) of groundwater samples from intergranular flow, high productivity
glaciofluvial aquifers featured higher nitrate concentrations relative to samples collected from
areas with diamicton deposits or an absence of superficial cover (Figure 4f).
Environments 2023,10, 67 9 of 22
Environments 2023, 10, x FOR PEER REVIEW 10 of 24
Figure 4. Box plots of groundwater sample nitrate concentrations in the River Ythan catchment,
classied by (a) annotated box plot example; (b) groundwater monitoring point construction details;
(c) land use classication; (d) soil type; (e) BGS bedrock aquifer productivity categories (FL) fracture
ow low productivity, (FVL) fracture ow very low productivity, (IFM) intergranular and fracture
ow moderate productivity; (f) BGS supercial aquifer productivity (Absent) no supercial aquifer
present, (IH) intergranular ow high productivity, (NSA) not a signicant aquifer; (g) BGS ground-
water vulnerability rating; and (h) DRASTIC groundwater vulnerability rating (1) very low risk, (2)
low risk, (3) medium risk, (4) high risk, (5) very high risk.
3.2. Land Use Change of SEPA Groundwater Monitoring Stations
The hilltop featuring Monitoring Station (MS) 1 appeared to have consistently re-
mained a grassland throughout the monitoring period. An aerial photograph taken on 1
January 2007 shows overgrown vegetation and patches of bare earth, contrasting with im-
agery from 22 March 2012 onwards, where vegetation appears to be regularly cut and the
patches of bare earth have been reseeded and possibly tilled. Landsat imagery taken
throughout 2012 demonstrates a contrast between a yellowish green towards a seasonal
change between brown and vibrant green, suggesting grass fodder production, but poorly
constrains the timing of land use change due to periods of cloud cover. Subsequent aerial
imagery continues to show characteristics of agricultural management, including
Figure 4.
Box plots of groundwater sample nitrate concentrations in the River Ythan catchment,
classified by (
a
) annotated box plot example; (
b
) groundwater monitoring point construction details;
(
c
) land use classification; (
d
) soil type; (
e
) BGS bedrock aquifer productivity categories (FL) fracture
flow low productivity, (FVL) fracture flow very low productivity, (IFM) intergranular and fracture flow
moderate productivity; (
f
) BGS superficial aquifer productivity (Absent) no superficial aquifer present,
(IH) intergranular flow high productivity, (NSA) not a significant aquifer; (
g
) BGS groundwater
vulnerability rating; and (
h
) DRASTIC groundwater vulnerability rating (1) very low risk, (2) low
risk, (3) medium risk, (4) high risk, (5) very high risk.
The BGS groundwater vulnerability map implies that almost the entire River Ythan
catchment area is of high risk or very high risk of contaminants leaching to groundwater
(Figure 3a), whereas the DRASTIC groundwater vulnerability map provides a larger range
of risk ratings (Figure 3b). For the BGS map, all groundwater samples exceeding the
regulatory environmental limit (>11.3 mg/L of NO
3
-N) were directly located on, or <100 m
downgradient of very high-risk areas, with the exception of two shallow wells associated
with a pig farm. Nevertheless, groundwater nitrate was broadly equivalent between the
high and very high-risk areas (Figure 4g) and did not account for the high variability
in nitrate distributed across categories, with some samples returning nitrate concentra-
tions <10 mg/L of NO
3
-N despite being located in areas of agricultural land use, implying
Environments 2023,10, 67 10 of 22
lower vulnerability to nitrate pollution. We did not obtain groundwater samples for areas
located in the moderate risk class for the BGS map, and so cannot determine whether
groundwaters here featured lower nitrate concentrations. For the DRASTIC map, the sam-
ples with the highest groundwater nitrate concentrations occurred across risk weightings,
with no significant difference in groundwater nitrate between risk categories (Figure 4h).
3.2. Land Use Change of SEPA Groundwater Monitoring Stations
The hilltop featuring Monitoring Station (MS) 1 appeared to have consistently re-
mained a grassland throughout the monitoring period. An aerial photograph taken on
1 January 2007 shows overgrown vegetation and patches of bare earth, contrasting with
imagery from 22 March 2012 onwards, where vegetation appears to be regularly cut and
the patches of bare earth have been reseeded and possibly tilled. Landsat imagery taken
throughout 2012 demonstrates a contrast between a yellowish green towards a seasonal
change between brown and vibrant green, suggesting grass fodder production, but poorly
constrains the timing of land use change due to periods of cloud cover. Subsequent aerial
imagery continues to show characteristics of agricultural management, including homo-
geneous grass texture and forage harvester tracts. Cattle were present on 28 June 2018,
contributing to overgrazing and soil erosion.
Aerial imagery shows most of the fields upgradient of MS 2 were under arable land use
at the beginning of the monitoring period; by 22 March 2012, these fields were converted
to grass fodder and sheep grazing. The field immediately upgradient to the spring was
converted to arable land use by 9 May 2016. LCMs and Landsat data did not provide
further constraint on the timing of these land use changes.
The proportion of improved grassland and arable land uses surrounding MS 3 re-
mained largely consistent throughout the monitoring period. Aerial imagery shows that
by 27 April 2011, a 0.17 km
2
area dedicated to free range chicken runs 56 m upgradient
of the spring was replaced by arable land use. In addition, the MS 3 spring was located
downgradient of a winter housing shed for chickens, indicating that local (point) source
contamination pressures may be evident. Due to the relatively remote area, Landsat data
appeared to be of relatively lower resolution, and did not provide further constraint.
The field hosting MS 4 was under an improved grassland for LCM2007, and aerial
imagery shows conversion to arable land use by 22 March 2012, which it appears to remain
as thereafter. Landsat imagery was unavailable for MS 4 between 2007 and 2012.
Aerial imagery from 1 January 2006 shows the field at the head of the glacial meltwater
channel feeding MS 5 was occupied by arable land use, which was then converted to sheep
grazing by 22 March 2012. The field containing the well was also converted from cattle
grazing to grass fodder production. LCM 2018 shows that the field to the east of the well
was converted from grass fodder to arable land use, with Landsat imagery suggesting
vegetative contrasts with surrounding fields of grasses by 4 July 2018.
All methods of land use determination consistently showed MS 6 to be surrounded by
arable land uses throughout the entire monitoring period.
Aerial imagery and LCM products show that the farmland surrounding the spring
MS 7 underwent multiple rotations of improved grassland and arable land use throughout
the monitoring period, although one 1.25 km
2
field 500 m west of the spring appeared to
have consistently remained under cattle grazing.
3.3. Groundwater Nitrate Trends and Seasonality
The results of the time series analyses are presented in Table 3. The seasonality and
trend p-values report the statistical significance of seasonality and trend identified by the
Mann–Whitney Utests and Mann Kendall/seasonal Kendall tests, respectively. The null
hypothesis may be rejected where these statistical tests return p-values < 0.05, but should be
interpreted with caution where n < 9. The direction of the trend is followed by ‘?’ where
there is low confidence in the significance of the trend. The annual Sen slope provides the
magnitude of the identified trend, which is divided by the median groundwater nitrate over the
Environments 2023,10, 67 11 of 22
monitoring period trend segment to yield the relative rate of change for comparison between
Monitoring Stations (MSs). In previous studies, annual Sen slopes > 0.1 mg/L of N-NO
3
yr
1
and relative slopes > 5% yr
1
have been adopted as thresholds representing environmentally
relevant changes to groundwater quality corresponding to anthropogenic perturbations [
53
].
The slope direction likelihood provides a confidence level for the Sen slope [
49
]. Hyphens in
Table 3indicate there were insufficient data available to perform the Mann–Whitney U-test for
seasonality, or where the magnitudes of Sen slope estimates were negligible.
Table 3.
Nitrate trend analysis results for the SEPA Ythan groundwater quality monitoring stations
1–7 (2009–2018).
Monitoring Station No.
Sub-Trend Duration n
Median
NO3-N
(mg L1)
Seasonality
p-Value
Trend
p-Value
Trend
Direction
Annual Sen
Slope
(mg L1
NO3-N yr1)
Relative
Slope
(% yr1)
Slope
Direction
Likelihood
1 63 12.4 0.025 * 0.000 ** Decreasing 0.369 2.975 1.000
23/02/09–03/12/12 38 13 0.009 * 0.000 ** Decreasing 0.926 7.119 1.000
07/01/13–14/11/18 25 11.6 0.007 * 0.021 * Decreasing 0.158 1.366 0.987
2 55 17.4 0.102 0.000 ** Decreasing 0.361 2.073 1.000
13/05/09–07/11/12 31 18.1 0.277 0.020 * Decreasing 0.300 1.660 0.991
05/12/12–17/11/16 16 16.4 0.419 0.342
Decreasing?
0.190 1.158 0.851
15/02/17–19/11/18 8 16.55 - 0.360 Increasing? 0.471 2.844 0.641
3 54 10.75 0.599 0.748 No trend - - 0.632
13/05/09–21/05/13 35 10.6 0.034 * 0.000 ** Decreasing 0.354 3.337 1.000
26/11/13–12/11/18 19 11 0.069 0.045 *
Decreasing?
0.200 1.817 0.970
4 59 6.37 0.743 0.00 ** Increasing 0.355 5.574 1.000
13/05/09–02/11/11 23 5.3 0.014 ** 0.001 ** Decreasing 0.638 12.039 0.999
08/12/11–07/11/12 12 5.72 0.248 0.000 ** Increasing 1.417 24.780 1.000
05/12/12–12/11/18 23 7.79 0.740 0.665 No trend 0.019 0.238 0.690
5 56 11.6 0.299 0.521
Decreasing?
0.521 4.487 1.000
13/05/09–21/05/13 35 12.3 0.105 0.000 ** Decreasing 0.556 4.522 1.000
26/11/13–06/11/17 17 9.7 0.907 0.021 * Decreasing 0.101 1.043 0.992
06/02/18–12/11/18 4 10.23 - 0.167 Increasing? 1.524 14.895 0.890
6 32 13.65 0.231 0.025 * Increasing 0.135 0.984 0.988
No sub-trends identified
7 56 10.15 0.215 0.00 ** Decreasing 0.156 1.533 1.000
No sub-trends identified
* Significance at the 95% confidence level. ** Significance at the 99% confidence level.
MSs 1, 2, 5 and 7 featured decreasing long-term trends with reductions in ni-
trate >0.15 mg/L of NO
3
-N yr
1
(>1.5% yr
1
) whereas MSs 4 and 6 featured increasing
long-term trends >1.3 mg/L of NO
3
-N yr
1
(>0.9% yr
1
). MS 3 not exhibit a statistically
significant long-term trend. The decomposition of long-term trends into sub-trends typi-
cally yielded more statistically significant trend (Mann Kendall/seasonal Kendall) and
seasonality (Mann–Whitney) statistics and contrasting Sen slope directions and magni-
tudes relative to the overall long-term trends, albeit at a reduced statistical power owing
to the smaller sample size and reduced Sen slope direction likelihoods. Seasonality was
determined to be significant (
α
< 0.05) for MS 1 for the entire monitoring period and the
first sub-trend segments of MSs 3 and 4. For the remaining MSs, seasonality defined
by the quarterly sampling frequency was not determined to be a significant component
of groundwater nitrate time series. However, trend segments at the beginning of the
monitoring period consistently featured smaller Mann–Whitney Utest p-values relative
to trend segments later into the monitoring period for all MSs.
The SEPA groundwater nitrate monitoring data are plotted as time series in Figure 5,
accompanied by Sen slopes derived for the sub-trends and dominant land use, and temporal
occurrence of severe flood and drought events. Here, the identified trends are classified
as increasing, decreasing or nonsignificant on the basis of the Mann Kendall and Seasonal
Kendall test results. Peaks in groundwater nitrate preceded the flood events of 2013 and
2016, and consecutive months of high effective rainfall. Likewise, the severe droughts of
2010 and 2018 were associated with groundwater nitrate peaks.
Environments 2023,10, 67 12 of 22
Environments 2023, 10, x FOR PEER REVIEW 13 of 24
Figure 5. Time series of monthly total eective rainfall with annotated extreme weather events (top)
and groundwater nitrate observations within the SEPA Ythan groundwater quality Monitoring Sta-
tions (MS 17) (SEPA) annotated with identied sub-trends and interpretations of land use (boom).
Major vertical gridlines indicate the rst day of the year.
Figure 5.
Time series of monthly total effective rainfall with annotated extreme weather events
(top) and groundwater nitrate observations within the SEPA Ythan groundwater quality Monitoring
Stations (MS 1–7) (SEPA) annotated with identified sub-trends and interpretations of land use
(bottom). Major vertical gridlines indicate the first day of the year.
Environments 2023,10, 67 13 of 22
4. Discussion
4.1. Spatial Distribution of Groundwater Nitrate and Hydrogeological Interpretation
The overall ubiquity of elevated groundwater nitrate was interpreted to reflect the
dominance of agricultural land uses combined with widespread high groundwater vulner-
ability that was most consistent with the BGS groundwater vulnerability map (Figure 3a).
The majority of dug well and spring PWS sampling points and SEPA MSs 1–7 were consid-
ered to represent groundwater abstractions from shallow aquifers, compromising coarse-
textured glacial deposits and/or shallow weathered zones developed above rockhead [
38
]
acting as a composite aquifer [
32
]. Some proportion of PWS boreholes were known to be
screened to these shallow aquifers, as opposed to exclusively representing deeper fractured
bedrock [
17
,
38
]. The shallow aquifers are reported to be unconfined and dominated by
intergranular flow mechanisms [
30
32
], resulting in a high degree of connection with the
surface and vulnerability to N loading, also supporting the maintenance of oxic conditions
preventative of denitrification [
60
63
]. Groundwater flow pathways associated with these
shallow aquifers were expected to be short, and discharged locally to streams, ditches and
drains adjacent to the recharge areas.
The generally lower, and occasionally negligible groundwater nitrate concentrations
associated with fractured bedrock aquifers (MSs 8–10) and PWS sampling points with low
nitrate indicated the operation of groundwater flow pathways with higher capacities for
denitrification relative to the superficial aquifers represented by dug wells and springs.
The length of these flow pathways and role in diffuse nutrient pollution transport are
poorly constrained, where the extent of fractured bedrock aquifer groundwater–surface
water interaction was, and their hydraulic connection with superficial aquifers could be
important in providing some moderation of water quality. Furthermore, the considerable
flanks of relatively high-permeability glaciofluvial deposits along the River Ythan thalweg
(Figure 2b) were poorly represented by available groundwater quality monitoring data
(Figure 4f), which are expected to provide significant nitrate storage and contributions
to river baseflow, and possible attenuation [
60
,
64
]. The high baseflow index (0.74) of
the River Ythan [
29
,
30
] combined with the extremely damped streamflow stable isotope
composition (
2
H,
18
O) indicate a dominance of well-mixed catchment storage contributions
to streamflow [
65
]. The significant volume of catchment storage is likely to result from a
combination of various aquifer types that are not fully represented by current groundwater
quality monitoring.
4.2. Groundwater Vulnerability
Both the BGS and DRASTIC groundwater vulnerability assessment methodologies
consider the shallow depths to groundwater, and generally thin, permeable glacial deposits
with an absence of clay minerals that typify the River Ythan region [
32
] to be associated with
high groundwater vulnerability [
17
20
,
41
]. However, the DRASTIC map underestimates
groundwater vulnerability in the River Ythan catchment area due to competing weightings
of other physiographic factors in the overall risk calculation that were not considered in the
BGS methodology (i.e., recharge, slope). This was most pronounced for the 2 km
2
pixels
of lower groundwater vulnerability present in Figure 3b corresponding to the resolution
of the recharge layer (Table 2). The spatially distributed groundwater potential recharge
map [
43
] used for the DRASTIC recharge layer was derived from the same digital eleva-
tion model and BGS products used in the other DRASTIC layers [
41
], suggesting some
degree of parameter interaction may have caused disproportionate influences on the final
vulnerability classification. Here, DRASTIC would have benefitted from a recharge layer
derived from groundwater level time series or tracers, or substitution of recharge with
effective rainfall.
The two maps produced contrasting vulnerability realisations across the N–S divide
in diamicton coverage, due to the contrasting conceptual models underpinning their
methodologies. The BGS map indicates higher groundwater vulnerability in the east
of the catchment due to the absence of protective superficial cover overlying fractured
Environments 2023,10, 67 14 of 22
bedrock aquifers with poor contaminant dispersion and dilution characteristics [
18
20
].
Meanwhile, the DRASTIC map indicates a lower groundwater vulnerability relative to
the western region, due the assumption of low advective transport rates of contaminants
within porosity/permeability crystalline bedrock relative to diamicton [39,40].
The DRASTIC groundwater vulnerability map could be improved by calibrating the
layer weightings to the final vulnerability classifications fitted with observed groundwater
nitrate concentrations to inform groundwater management at a higher spatial resolution
provided by the BGS groundwater vulnerability map. This could assist in the design of PWS
risk assessments, assignment of NVZ derogation permits, or spatially targeted nutrient
management. Alternatively, the layer property ratings could be adjusted to represent
current conceptual models of groundwater vulnerability applicable for Scotland [
18
20
,
31
].
4.3. Groundwater Nitrate Trends and Relation to Climate and Land Use Changes
The decreasing long-term trends observed at most SEPA groundwater quality MSs
were interpreted as the receding limbs of groundwater nitrate peaks associated with pre-
NVZ N applications. Likewise, the increasing long-term trend observed at MS 6 could
represent the ascending limb of a groundwater nitrate peak, but in the absence of earlier
monitoring data and groundwater age determination, it is uncertain as to whether the long-
term trend corresponded to pre- or post-NVZ designation nutrient management practices.
Nevertheless, a considerable proportion of groundwater nitrate observed at all monitoring
points likely originated from post-NVZ N loadings, due to the high dynamicity observed
to coincide with land use change and meteorological forcing.
Several of the groundwater quality MSs were typified by strong, decreasing sub-trends
at the beginning of the monitoring period (2009–2013). This was interpreted to reflect the de-
pletion of legacy pre-NVZ N in soils and aquifers associated with land manager compliance
with NVZ rules, including restrictions in fertiliser applications and, for the River Ythan in
particular, reductions in livestock numbers to meet manure storage requirements [
66
]. The
early decreasing sub-trends were interrupted by abrupt trend reversals, step change, or
transient increases in groundwater nitrate followed by a new state of dynamics (post 2013)
characterised by changes in trend magnitude and seasonality behaviour.
Trend reversals and transient increases in groundwater nitrate were associated with the
conversion of grassland systems to arable land uses (MSs 2, 4 and 5). This was interpreted
as reflecting higher N fertiliser crop requirements and the ploughing of grassland soils
to prepare seedbeds and facilitate drainage [
67
]. Organic matter may accumulate in soils
under long-term grassland land uses, and proceed to be oxidized during ploughing/tilling,
resulting in the conversion of immobilised/organic N to higher-mobility N species [
68
]. The
subsequent regime of fertiliser applications, tillage and periods of bare soil cover following
harvest may be responsible for the higher median nitrate concentrations for MSs 2, 4 and 6
under arable land uses [
69
]. A similar, albeit longer term pattern in increased groundwater
nitrate loadings from conversion of grassland to cereal crop production was reported for
the English Chalk aquifer, associated with a longer delay time due to a relatively thicker
unsaturated zone and subsequently greater volume and residence time of N storage [68].
Grassland systems were systematically associated with decreasing groundwater nitrate
trends (MSs 1 and 4) and lower median nitrate concentrations (MSs 4 and 5) relative to
arable land uses [
70
]. Conversion of arable land uses to grassland at MSs 2 and 5 resulted
in a step change decrease in groundwater nitrate [
71
], followed by a decreasing, albeit
weak sub-trend. Nevertheless, it is important to note that to support livestock diets, cereals
need to be grown elsewhere under arable land uses, and manure produced during the
winter housing of livestock may be applied to arable crops. In addition, MS 1 demonstrates
that groundwater recovery rates are inversely proportional to grassland management
intensity [
72
,
73
]. MSs 3 and 7 were both springs with source areas dominated by mixed
land uses, resulting in more complex N fate and transport behaviour that could not be
constrained by the current level of investigation.
Environments 2023,10, 67 15 of 22
Cyclical variation in groundwater nitrate time series is characteristic for shallow
groundwaters in temperate agricultural landscapes [
68
]. Typically, seasonal winter peaks
correspond to seasonally high N leaching rates, due to the accumulation of residual/
mineralised N in soils in excess of summer crop requirements coupled with low N im-
mobilisation and biological uptake rates [
27
,
68
]. Seasonally high water tables, antecedent
unsaturated zone moisture and effective rainfall facilitate the transport of residual N in soils
and the unsaturated zone to groundwater [
74
]. Peaks in groundwater nitrate proceeding
extreme flood events and periods of consecutive high effective rainfall are likely to be
driven by the acceleration of winter N leaching mechanisms [
74
,
75
]. On these bases, the
data gap following the extreme flood event of 2009 was also be expected to feature a similar
peak in groundwater nitrate. The United Kingdom Climate Impacts Program (UKCIP)
forecasts increased winter precipitation volume and intensity in northeast Scotland [
76
],
which is expected to increase residual N leaching from soils over the winter. Warmer
winter temperatures have been observed to reduce snowfall volume and favor snowmelt in
the winter as opposed to spring, additionally contributing to winter recharge [
77
] further
facilitating winter N leaching.
Summer groundwater nitrate peaks were typically associated with periods of extreme
droughts, and were interpreted to result from contemporaneous fertiliser applications and
N excretion by livestock grazing throughout the summer. Although effective rainfall rates
were typically calculated to be negligible in the summer months, SEPA groundwater level
monitoring for shallow aquifers demonstrates that groundwater recharge can occur in
the summer in response to individual precipitation events [
30
]. Alternatively, summer
groundwater nitrate peaks could also arise from upgradient groundwater flow pathway
contributions featuring higher nitrate concentrations. Although groundwater recharge
rates are expected to be significantly lower relative to the winter, small N contributions
to groundwater may produce disproportionately larger impacts to groundwater quality,
resulting from the limited dilutive capacity of aquifers in the summer due to seasonally
low recharge rates and poor groundwater storage of diamicton and fractured bedrock
aquifers [
18
,
77
]. The UKCIP forecasts increased summer temperatures and decreased
summer precipitation volumes in northeast Scotland [
76
], increasing evapotranspiration
rates and soil moisture deficits that will likely motivate increased groundwater abstractions
for crop irrigation [
21
,
77
]. Enhanced N leaching to groundwater associated with irriga-
tion return flows is well-documented [
2
], and irrigation presents additional stresses to
groundwater-dependent ecosystems through reductions in groundwater discharge. Dry
summers in Scotland are also associated with lower groundwater tables in subsequent
winters [
77
], which could be interpreted to reduce the dilutive capacity of aquifers or
increase opportunities for attenuation in the unsaturated zone, depending on the local hy-
drogeological setting and appropriate conceptual model [
18
20
,
39
]. Overall, the impacts of
climate change are expected to increase the greywater footprint of agriculture by increasing
water scarcity in the summer and the proportion of agricultural N lost from agroecosystems
and leached to groundwater [13].
Despite the patterns observed between groundwater nitrate peaks and meteorological
forcing, the majority of the Mann–Whitney Utests failed to return significant indications
of seasonality defined by the quarterly sampling frequency. Here, dynamic factors such
as the timing of nutrient applications and N storage in the unsaturated zone are likely
candidates for driving seasonal dynamics that were not accounted for. However, the
decreased significance of Mann–Whitney Utests reported for sub-trends later into the
monitoring period may correspond to a decrease in seasonality associated with the flushing
of pre-NVZ N from soils and the unsaturated zone over time, resulting in less N avail-
able for winter leaching [
78
]. Although some consistent patterns between meteorological
events and groundwater nitrate were observed, groundwater nitrate dynamics contrast
between monitoring sites, reflecting a combination of site-specific agricultural practices
and intrinsic hydrogeological setting and corresponding groundwater vulnerability to
contamination. The groundwater quality MSs with the most dynamic groundwater nitrate
Environments 2023,10, 67 16 of 22
concentrations are expected to represent relatively younger groundwaters (weeks-months)
with thin unsaturated zones, smaller contributory recharge areas and/or shorter ground-
water flow pathways (<100 m) [
31
]; therefore, these MSs are more sensitive to land use
and meteorological forcings. Meanwhile, the groundwater quality MSs with more damped
nitrate variations may constitute older groundwater and/or feature groundwater flow
pathway contributions from multiple recharge areas. It is expected that the most dynamic
groundwaters feature higher vulnerability to nitrate contamination, but also demonstrate
the greatest recovery rates, and vice versa.
4.4. Implications to Groundwater Management and Limitations of Study
The absence of groundwater quality monitoring data prior to NVZ designation limits
our assessment of aquifer recovery to diffuse nitrate pollution, where the timing and mag-
nitude of the groundwater nitrate peak remains unconstrained. Furthermore, the unknown
hydrogeological setting and construction details for the individual SEPA PWS groundwater
MSs limit interpretations of observed groundwater nitrate dynamics and the ability to draw
comparisons between the different monitoring points (Figure 5). Likewise, the limited
spatiotemporal resolution of aerial and satellite imagery and the limited accuracy of LCMs
can only provide a general timing of land use changes. Regarding the trend analyses, a
level of subjectivity was inherent when dividing the time series into discrete segments of
contrasting trend and seasonality using semi-quantitative change point detection methods.
Despite these shortcomings, the evaluation of groundwater nitrate status of the River
Ythan catchment demonstrates a prevalence of groundwater nitrate in exceedance of the
regulatory environmental limit, posing risks to human and ecological receptors, validating
the current NVZ designation status.
NVZ designation is considered to be responsible for driving the long-term decreasing
groundwater nitrate trends in the River Ythan catchment, demonstrated by several of the
groundwater quality MSs (Table 3). However, some delay to groundwater recovery is
indicated by the persistence of elevated groundwater nitrate throughout the catchment
area (Figure 3) and increasing sub-trends at multiple groundwater quality MSs (Figure 5).
This delay in groundwater recovery was considered to be associated with intensification of
land management upgradient of MSs and the generally high vulnerability of groundwaters
within the shallow aquifers (Figure 3).
Variable groundwater recovery rates reported for European NVZs are largely due to in-
consistencies in NVZ designation criteria, and the design and enforcement of Nitrate Action
Plans (NAPs) [
79
]. Here, the United Kingdom is relatively unique, due to the groundwater
vulnerability mapping approach taken to designate NVZs due to a past scarcity and relia-
bility of groundwater quality MSs that could not effectively represent the chemical status
of all groundwater bodies present [
17
,
38
]. In contrast, Denmark was designated a national
NVZ due to high groundwater nitrate that was evident throughout a pre-existing, high
density groundwater quality monitoring network, and the importance of groundwater
quality for public water supplies [
9
,
80
]. Following the successful implementation of NAPs,
Denmark invested significant resources into spatially targeted nutrient management based
on advanced groundwater vulnerability mapping approaches relying on conceptual models
based on the results of groundwater nutrient management field studies and national, high
resolution geophysical characterisation of groundwater systems [9,8184].
The projected impacts of climate change on biodiversity and food security will require
optimised approaches to nutrient management to achieve a sustainable balance between
water quality and food production [
80
,
85
87
]. Spatially targeted nutrient management
strategies may be appropriate for the River Ythan catchment, due to the strong hetero-
geneity in land use and hydrogeological setting. However, the resolution and accuracy
of available environmental mapping products and the level of conceptual understanding
for hydrogeological settings in northeast Scotland are unable to guide the required policy
development and implementation at present.
Environments 2023,10, 67 17 of 22
In the absence of groundwater nutrient management studies in northeast Scotland,
a series of recommendations are provided to support groundwater quality management,
although their relative effectiveness will have to be verified in the field. Shallow ground-
waters in the River Ythan catchment appear to be susceptible to high rates of N leaching
following extreme flooding events and high effective rainfall during the NVZ closed period,
regardless of land use (Figure 5). General strategies for reducing winter N leaching from
arable land uses include earlier sowing of winter cereals, good post-harvest management
practices, and strategic use of catch crops [
88
90
]. Oilseed rape and brassica crops are not
represented by the SEPA Ythan groundwater quality monitoring network, thus the impacts
of their exclusion from NVZ closed period fertiliser restrictions on groundwater quality
are unknown [91].
The consistency of lower groundwater nitrate levels and stronger decreasing trends
under grassland systems relative to arable land uses suggests that these are more sustain-
able land uses within a water quality perspective. The conversion of arable land uses to
grassland has been demonstrated to reduce N leaching rates within two years [
72
], which
was observed in MSs 2 and 5 (Figure 5). As grassland N leaching rates are proportional
to management intensity [72,73] as indicated by MS 1, environmental subsidies to finance
low-intensity grassland should be targeted in areas with the highest vulnerability to ground-
water pollution. Otherwise, reseeding and fertiliser applications to grassland should be
concentrated earlier into the growing year to minimise winter N losses [
73
,
88
,
92
]. However,
this would need to be optimised due to the relatively wet springs in northeast Scotland, due
to the seasonally high effective rainfall and higher associated risk of N leaching (Figure 5).
Conversion of grassland to arable agriculture is a likely driver for delayed groundwa-
ter recovery in the River Ythan catchment (Figure 5). Direct seed drilling of arable crops
into grassland reduces soil organic N mineralisation and subsequent winter N leaching [
90
],
and the ploughing of grassland in spring as opposed to autumn allows for greater uptake
of mineralised N over the growing season [
69
]. The Nitrogen Risk Assessment Model for
Scotland (NIRAMS) provides estimates of agricultural N exports to surface waters and
groundwaters to support the Scottish government’s review of NVZ designations in addition
to SEPA’s water quality monitoring [
93
]. NIRAMS assumes all organic or immobilised N
is mineralised at the end of the growing season, and becomes available for leaching as
residual N at a rate that is proportional to hydrological fluxes. Long-term organic N storage
and carryover to successive years is unaccounted for [
94
], and so NIRAMS may overes-
timate grassland N losses on annual timescales and underestimate N leaching following
conversion to arable land use [
95
]. Therefore, NIRAMS could be improved by incorporating
a dynamic organic N compartment into the model structure.
Increased drought frequency and severity in northeast Scotland will increase the
greywater footprint of agriculture, due to water scarcity and disproportionate impacts of
contaminants on aquatic ecology. Therefore, the observed summer peaks in groundwater
nitrate (Figure 5) warrant further investigation due to the heightened risks to receptors,
and their discrepancies with traditional conceptual models applied in temperate climates
that emphasise peak N leaching over the winter period. Careful consideration of further
restrictions to fertiliser applications and irrigation are required to minimise impacts to
agricultural enterprises that will be subject to drought-induced crop failure. Limiting tillage
and N fertiliser applications to spring to ensure the biological uptake of N prior to drought
risk periods is an uncertain strategy for the River Ythan, due to the seasonally high effective
rainfall (Figure 5). In the summer of 2022, the SEPA issued emergency suspensions of
surface water abstraction licenses to safeguard river flows, and disseminated advice on
water-conserving irrigation practices and encouraged water-dependent industries to install
boreholes for groundwater abstraction. N leaching and groundwater transport associated
with irrigation return flows [
2
] and migration of shallow groundwater nitrate to deeper
fractured bedrock aquifers due to increasing abstraction rates [
96
] may present future risks
to Scottish PWS users and groundwater-dependent ecosystems.
Environments 2023,10, 67 18 of 22
5. Conclusions
Groundwater nitrate concentrations in the River Ythan catchment remain elevated fol-
lowing two decades of Nitrate Vulnerable Zone designation (NVZ). The majority of ground-
water quality MSs demonstrated long-term decreasing trends over a ten-year monitoring
period (2009–2018), and appear to be approaching equilibrium with the environmental reg-
ulatory limit. The strength of seasonality in groundwater nitrate concentrations decreased
with median groundwater nitrate concentration associated with reduced N storage in soils
and unsaturated zones. Increases in groundwater nitrate concentrations under NVZ rules
were associated with intensification of management for grassland systems, and conversion
to arable land uses. Seasonal peaks in groundwater nitrate preceded extreme flood events
and consecutive months of high effective rainfall, along with prolonged summer droughts.
As a result, climate change is expected to increase the greywater footprint of agriculture in
northeast Scotland. The highly dynamic response of shallow groundwater chemical quality
to environmental change implies high groundwater vulnerability. Further hydrogeological
investigation and higher-resolution groundwater vulnerability mapping is recommended
to inform agri-environmental management and facilitate the future achievement of water
quality targets.
Author Contributions:
Conceptualisation, H.J. and J.-C.C.; methodology, H.J., J.-C.C. and E.M.S.;
investigation, H.J. and E.M.S.; data curation, H.J.; writing—original draft preparation, H.J.;
writing—review and editing, J.-C.C., M.T., U.O., R.C. and C.S.; visualisation, H.J.; supervision,
J.-C.C., U.O., R.C., M.T. and C.S.; project administration, H.J. and J.-C.C.; funding acquisition,
J.-C.C., U.O. and R.C. All authors have read and agreed to the published version of the manuscript.
Funding:
This research was supported by the Natural Environment Research Council and the
QUADRAT Doctoral Training Partnership [NE/S007377/1].
Data Availability Statement:
Groundwater quality monitoring data for the River Ythan catchment
were provided by SEPA. Private Water Supply (PWS) chemical quality compliance certificates were
obtained from the Aberdeenshire Council website. PWS groundwater nitrate sample results are not
presented here to respect the privacy of PWS owners.
Acknowledgments:
We would like to thank SEPA for groundwater quality monitoring and providing
these data upon request, and the BGS for developing and providing various mapping products. We
would also like to thank the residents of the River Ythan catchment for providing access to their
private water supplies, and Michael McGibbon and David Galloway for analytical determination of
nitrate for groundwater samples collected during this investigation.
Conflicts of Interest:
The authors declare no conflict of interest. The funders had no role in the design
of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript; or
in the decision to publish the results.
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... Evidence is provided of some reductions in N runoff in the Eden catchment in eastern Scotland resulting from policy interventions but a worsening of phosphate pollution [44]. Further evidence that 20 years of nitrate reduction policies on the Ythan catchment in north-east Scotland produced a small reduction in nitrate runoff, but that there are still frequent exceedances at monitoring stations [45]. They [45] conclude that "groundwater nitrate was found to remain elevated across the catchment area and appeared to be highly sensitive to agricultural practices and meteorological forcing". ...
... Further evidence that 20 years of nitrate reduction policies on the Ythan catchment in north-east Scotlan