![Conceptual figure of an open ditch showing all potential nitrogen and phosphorus sources (point and diffuse), pathways, and discharge connections [modified from Teagasc (2022) and Simpson et al. (2011)].](https://www.researchgate.net/publication/380095043/figure/fig1/AS:11431281238795031@1714051107691/Conceptual-figure-of-an-open-ditch-showing-all-potential-nitrogen-and-phosphorus-sources_Q320.jpg)
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An integrated connectivity risk
ranking for phosphorus and
nitrogen along agricultural open
ditches to inform targeted and
specific mitigation management
D. G. Opoku
1
,
2
, M. G. Healy
2
, O. Fenton
3
, K. Daly
3
, T. Condon
1
and
P. Tuohy
1
*
1
Teagasc, Animal & Grassland Research and Innovation Centre, Moorepark Fermoy, Cork, Ireland,
2
College of Science and Engineering, Civil Engineering and Ryan Institute, University of Galway, Galway,
Ireland,
3
Teagasc, Crops, Environment and Land Use Centre, Wexford, Ireland
Introduction: On dairy farms with poorly drained soils and high rainfall, open
ditches receive nutrients from different sources along different pathways which
are delivered to surface water. Recently, open ditches were ranked in terms of
their hydrologic connectivity risk for phosphorus (P) along the open ditch
network. However, the connectivity risk for nitrogen (N) was not considered in
that analysis, and there remains a knowledge gap. In addition, the P connectivity
classification system assumes all source–pathway interactions within open
ditches are active, but this may not be the case for N. The objective of the
current study, conducted across seven dairy farms, was to create an integrated
connectivity risk ranking for P and N simultaneously to better inform where and
which potential mitigation management strategies could be considered.
Methods: First, a conceptual figure of known N open ditch source–pathway
connections, developed using both the literature and observations in the field,
was used to identify water grab sampling locations on the farms. During fieldwork,
all open ditch networks were digitally mapped, divided into ditch sections, and
classified in terms of the existing P connectivity classification system.
Results and Discussion: The results showed that not all source–pathway
connections were present across ditch categories for all species of N. This
information was used to develop an improved open ditch connectivity
classification system. Farmyard-connected ditches were the riskiest for
potential point source losses, and outlet ditches had the highest connectivity
risk among the other ditch categories associated with diffuse sources. Tailored
mitigation options for P and N speciation were identified for these locations to
intercept nutrients before reaching receiving waters. In ditches associated with
diffuse sources, nitrate was introduced by subsurface sources (i.e., in-field drains
and groundwater interactions from springs, seepage, and upwelling) and
ammonium was introduced through surface connectivity pathways (i.e., runoff
from internal roadways). On similar dairy farms where open ditches are prevalent,
the integrated classification system and mapping procedure presented herein will
OPEN ACCESS
EDITED BY
Alan Steinman,
Annis Water Resources Institute, United States
REVIEWED BY
Adam Canning,
James Cook University, Australia
Katelyn Lawson,
Auburn University, United States
*CORRESPONDENCE
P. Tuohy,
patrick.Tuohy@teagasc.ie
RECEIVED 13 November 2023
ACCEPTED 23 January 2024
PUBLISHED 19 February 2024
CITATION
Opoku DG, Healy MG, Fenton O, Daly K,
Condon T and Tuohy P (2024), An integrated
connectivity risk ranking for phosphorus and
nitrogen along agricultural open ditches to
inform targeted and specific
mitigation management.
Front. Environ. Sci. 12:1337857.
doi: 10.3389/fenvs.2024.1337857
COPYRIGHT
© 2024 Opoku, Healy, Fenton, Daly, Condon
and Tuohy. This is an open-access article
distributed under the terms of the Creative
Commons Attribution License (CC BY). The use,
distribution or reproduction in other forums is
permitted, provided the original author(s) and
the copyright owner(s) are credited and that the
original publication in this journal is cited, in
accordance with accepted academic practice.
No use, distribution or reproduction is
permitted which does not comply with these
terms.
Frontiers in Environmental Science frontiersin.org01
TYPE Original Research
PUBLISHED 19 February 2024
DOI 10.3389/fenvs.2024.1337857
enable a targeted and nutrient-specific mitigation plan to be developed. The same
methodology may be applied to develop a bespoke integrated connectivity risk
ranking for P and N along agricultural open ditches in other areas.
KEYWORDS
water quality, nutrient loss, grassland, drainage management, connectivity pathways,
North Atlantic Europe, agricultural ditches
1 Introduction
Open ditch networks, also referred to as “surface ditch
network,”are installed in poorly drained soils to remove
excess water, control the water table, and aid with grass
production and utilization (Tuohy et al., 2016;Hertzberger
et al., 2019). These networks comprise a series of connected
and unconnected sections that receive nutrients from a variety of
surface and subsurface pathways, all of which can then be
transported to other sections or associated waterbodies
(Kröger et al., 2007;Herzon and Helenius, 2008;Moloney
et al., 2020). Connectivity is defined as the transfer of energy
and matter across two landscape zones, whereas disconnectivity
is the isolation of these zones (Chorley and Kennedy, 1971).
Identifying the connectivity of these systems enables mitigation
strategies to be implemented at optimal locations where nutrients
can be reduced or restrained (e.g., intercepting the pathway,
slowing the flow, or removing some of the nutrients in the
water) to minimize the impact on the receiving waterbody
(Fenton et al., 2021). Research continues to help farmers
optimize farm management practices (baseline) and
engineering solutions (above baseline) (Moore et al., 2010;
Schoumans et al., 2014;Carstensen et al., 2020). Many studies
on open ditches have focused on nutrient dynamics (Sukias et al.,
2003), sediment attenuation capacity (Ezzati et al., 2020;Mattila
and Ezzati, 2022), nutrient loss attenuation potential by
vegetation (Soana et al., 2017;Zhang et al., 2020), dissolved
organic carbon dynamics (Tiemeyer and Kahle, 2014), organic
matter composition (Hunting et al., 2016), ditch management
(Dollinger et al., 2015;Hertzberger et al., 2019), and indirect
greenhouse gas emissions (Hyvönen et al., 2013;Clagnan et al.,
2019). However, few studies have investigatedtheroleplayedby
open ditch connectivity in the transfer of nutrients from the
source to the receptor. Such studies may provide vital
information to ascertain the positioning of ditch mitigation
option and the dominant nutrient species it is required to
target. Moreover, there is a poor understanding of processes
leading to the immobilization and transformation of nutrients
within soil and drainage systems along the hydrological pathways
into ditches (Deelstra et al., 2014). For efficient mitigation of
nutrient loss from open ditch networks, a conceptual
understanding of how nutrient sources and their pathways
connect to the open ditch system must be established.
The general trend and pathways of agricultural pollutants have
been well-documented and are summarized in Figure 1.In
summary, nutrient entry into ditches is predominantly from
diffuse sources and often through the complex surface and
subsurface pathways determined by soil type, climate, landscape
position, farm management, and nutrient input sources (manure or
fertilizer type) (Granger et al., 2010;Monaghan et al., 2016;
Gramlich et al., 2018). These factors regulate the hydrology, the
primary driver of nutrient transfer, and the terrestrial and aquatic
FIGURE 1
Conceptual figure of an open ditch showing all potential nitrogen and phosphorus sources (point and diffuse), pathways, and discharge connections
[modified from Teagasc (2022) and Simpson et al. (2011)].
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Opoku et al. 10.3389/fenvs.2024.1337857
biogeochemistry that defines the type and form/species of nutrients
entering open ditches and subsequently discharging to associated
waterbodies (Sukias et al., 2003). Conceptually, phosphorus (P),
either as particulate P (PP) or dissolved reactive phosphorus (DRP),
and nitrogen (N), as ammonium (NH
4
+
) or nitrate (NO
3
−
), are
transported from fields or hard surfaces like roadways through
surface flow pathways into open ditches (Figure 1).
As shown in Figure 1, any groundwater-to-open ditch water
connection represents a subsurface interaction distinct from in-field
drain connections. In this scenario, typically, P is in the form of
DRP, and NO
3
−
represents mineralized N that has become
mobilized due to infiltrating water. This N is primarily lost from
diffuse sources in fields due to fertilization and grazing of animals.
Clagnan et al. (2018) have shown the conversion of N to NH
4
+
in
poorly drained soils, which can be discharged in waters from in-field
drains within the groundwater-to-open ditch water connections
(Needelman et al., 2007;Valbuena-Parralejo et al., 2019). The
presence of NO
3
−
in open ditch networks suggests more
permeable connectivity pathways that eventually seep into open
ditches along seepage faces or upwell as the water table rises, whereas
the presence of NH
4
+
suggests less permeable routes before
discharge occurs. Groundwater springs represent a distinct
groundwater storage component that protrudes onto fields, which
are often drained by the installation of an intersecting pipe into an
open ditch below the spring. This creates a direct discharge point
within the open ditch (Figure 1). The presence of this discharge may
change during dry periods as the water level decreases below the base
of the open ditch.
Moloney et al. (2020) used this concept to rank the
connectivity risk (from highest to lowest) for P along
agricultural open ditches. The riskiest open ditches were those
directly connected to farmyards (farmyard connection ditches)
and watercourses (outlet ditches), while the least risky open
ditches included secondary and outflow ditches (disconnected
ditches did not pose any risk of connectivity). The system devised
by Moloney et al. (2020) conceptualized P sources and pathways
with the aim of disconnecting P losses before discharge to
associated waterbodies. The current study takes the same
approach but creates an integrated connectivity risk ranking
that considers both N, which discharges into the open ditch
network via surface and subsurface pathways (Figure 1), and P.
Such integration necessitates a thorough understanding of N and
P biogeochemical cycles, how sources are connected along
different surface and subsurface pathways to the open ditch
network, and how this network is connected and delivered to
the adjoining aquatic system (e.g., river). Accounting for
attenuation along the pathway and within the open ditch
network is a constraint within the current conceptual
framework. Therefore, there is a need to integrate N into the
connectivity risk ranking so that a more holistic mitigation
management strategy may be designed (i.e., source protection
on the farm and “right measure, right place”in the open ditch).
The objective of this study was to derive a farm-scale integrated
open ditch risk ranking for both P and N loss risk based on
connectivity to inform future mitigation management on heavy
textured, grassland dairy farms. To fulfil this objective, seven farms
were selected with open ditch networks on heavy textured soils. A
conceptual figure illustrating the trends and pathways of agricultural
pollutants for an open ditch is presented. The open ditch networks
were mapped during a ground survey, and a qualitative water
sampling campaign was conducted (based on the conceptual
figure) to validate the presence or absence of pathways for N and
P. This enabled an integrated classification of an open ditch network
ranking to be developed. Mitigation options for each ditch class
are presented.
2 Materials and methods
2.1 Site selection and characteristics
Seven grassland dairy farms on poorly drained soils
geographically located across the SW and NE of Ireland were
selected to represent a variety of agronomic dairy production
systems and biophysical settings (Table 1). As per the Ireland
EPA soil and subsoil maps (Fealy et al., 2009), the soil types on
these farms varied from organic to mineral soils. The majority of
these farm fields were imperfectly or poorly drained, necessitating an
ad hoc network of artificial drainage installations on the farms. The
grazing area of each farm ranged from 28 to 45 ha. Intensive dairy
farm management practices were observed on all farms. Morgan’s
extractable soil P test (Wall and Plunkett, 2020) was used to
determine the agronomic excesses and deficiencies in plant
available P for fields of each farm. The farms in this study were
located in high-rainfall areas with an average rainfall of 1092.5 mm.
The average farm slope was measured on all seven farms, as it could
influence open ditch connectivity.
2.2 Ground survey and mapping
connectivity pathways for N into P
connectivity risk ditch categories
A ground survey was carried out on all the farms during winter
(November 2021 to March 2022) to characterize the field
boundaries and surface and subsurface networks on each farm.
This period was selected following multiple field visits carried out
across all seasons in the previous year. This period was identified as
the best hydrological period when connectivity pathways were
active for grab sampling. Drainage network features such as open
ditches connected to the farmyard and the proximity of the open
ditch to waterbodies were noted on each farm during the ground
survey. In addition, the connectivity pathways for N into open
ditches from in-field drains, farm roadways, groundwater springs,
seepage, and upwelling as per the conceptual figure (Figure 1)
throughout the drainage network were noted during this time.
During the ground survey, all drainage network data, such as of
drain locations, flows and connections, and sampling locations,
were recorded by using an electronic device with ESRI ArcGIS
Field Maps mobile software (ESRI, 2024).
Open ditches were identified as manmade open drains usually
sited along the field edges to carry excess water from the field and
farm. Surface waterbodies (1
st
- and 2
nd
-order streams) in and around
each farm, defined as those appearing on the national ordnance
survey maps (6-inch maps) (osi.ie), were mapped onto each farm
map before each ground survey.
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TABLE 1 Summary of agronomic and soil data and associated in-field drainage percentages across case study farms.
Farm
#
Farm
size
NUE
a
% of the
number of
fields with
a high P
index
b
Soil
OM
c
(%)
Annual
rainfall
(mm)
Farm
topography
slope angle
range (°)
Dominant
soil type
Drainage classes
d
(%) Major soil type
d
(%) %of
fields
with in-
field
drains
e
(ha) (kg
N/
ha)
Poor Imperfect Moderate Well Mineral Humic Organic
1 43 27 16.3 16.2 1,086.3 2–3 Humic surface
water gley
30.9 52.9 16.2 0 69.1 30.9 0 48.4
2 40 23 40.0 16.7 1,283.7 3–11 Humic surface
water gley
8.8 39.7 35.1 16.4 68.4 31.6 0 34.1
3 45 24 19.6 30.6 1,002.4 0 Groundwater
gley
50.1 38.5 11.4 0 46.2 31.0 22.8 72.5
4 37 32 10.3 18.0 1,320.2 4–8 Humic brown
podzolic
45.1 0.9 54 0 58.4 41.6 0 13.6
5 41 35 59.4 8.4 900.0 0.6–0.9 Surface water
gley
57.5 17.2 2.1 23.1 88.2 11.8 0 78.4
6 39 45 21.5 14.8 1,035.6 1–8 Typical surface
water gley
42.1 3.5 25.1 29.3 84.3 10.9 4.9 25.2
7 28 42 41.7 12.1 1,019.6 5–7 Typical surface
water gley
50.2 5.1 42.5 2.2 97.1 1.7 1.2 69.6
a
Nitrogen use efficiency.
b
High P index (index 4) fields have soils with excess P concentration (above 8 mg L-1, measured as Morgan’s P, on grassland).
c
OM, organic matter (Corbett et al., 2022a;Corbett et al., 2022b).
d
Data from Tuohy et al. (2018);Tuohy et al. (2021).
e
%field with in-field drain = (size of drained field/total farm size) × 100%.
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Information from the ground survey observations and
qualitative interviews with farmers on drainage networks were
used to digitize and map farm and field boundaries and the open
ditch network (open ditches, sub-surface in-field drains, and
drainage outlets) and associated connectivity pathways for N
(Figure 2). For the open ditch network within each farm, each
ditch was assigned a ditch category using their connection to a
farmyard, watercourse, neighboring farm, other ditches on the same
farm, and also their non-connection to any other part of the open
ditch network after Moloney et al. (2020) (Table 2). These categories
are as follows: (1) farmyard connection ditch, (2) outlet ditch, (3)
outflow ditch, (4) secondary ditch, and (5) disconnected ditch
(Figure 2) using ArcMap GIS software (version 10.5).
On each assigned ditch category, the connectivity pathways for
N(Table 3), where present, were mapped within this open ditch
network using the conceptual figure (Figure 1) as a guide during
fieldwork to integrate the N connectivity pathway risk into the P
connectivity risk open ditch categories. To identify the connectivity
pathways, landscape position was taken into account, especially for
assessing the interaction of groundwater with an open ditch section.
Groundwater seeping through open ditch bank sides and
groundwater upwelling through the base of the open ditch were
identified as groundwater seepage and upwelling, respectively
(Table 3), and were classified together as one connectivity
pathway. Roadways were identified as a connectivity pathway
when there were site observations of water flow and eroded/gully
surface (due to continuous past water flows) from the farm roads
into a nearby open ditch. Groundwater springs were identified as
high-flow groundwater purging out into open ditches either over the
surface or through pipes. Subsurface in-field drains were all piped
drains directed into ditches but were differentiated from piped
springs with their low and intermittent flows into the open ditches.
The length of the open ditches and farm and field boundaries
were measured in ArcGIS and compared for each farm, as shown in
Table 4. In addition, the occurrence of a particular N connectivity
pathway was calculated as a percentage of the total number of N
connectivity pathways observed for each farm and for each open
ditch category.
2.3 Grab water sampling campaign to assess
integrated nutrient connectivity pathways
Water quality parameters change over time, depending on the
local climatic conditions and farming practices (Huebsch et al.,
2013). In the present study, the objective was to establish a link or
connection (see Figure 1) between the source and pathway to the
open ditch network. Therefore, “snapshot”sampling in spring
(March) presented a good opportunity to collect qualitative data.
In spring (March) 2022, a total of 210 water samples were
collected directly from 105 sampling sites in open ditches
throughout the drainage network across all farms during a one-
time sampling event following the procedure of Moloney et al.
(2020). These sampling sites reflected connectivity pathways
presented in Figure 1. March was selected for sampling because
FIGURE 2
Example of a farm output map (for farm 5) showing the ranked
classification risk along the open ditch network for P (color-coded into
categories of connectivity risk) and all conceptualized N open ditch
connectivity pathways to individual open ditch sections. For in-
field drains, arrows indicate fall and flow direction toward open ditch
sections, with a particular P risk indicated by the existing color-coding
scheme of Moloney et al. (2020).
TABLE 2 Definition and description of open ditch categories for the P classification system of Moloney et al. (2020).
Ditch category Description
1. Farmyard A ditch/pipe that connects a farmyard to the drainage connection network or directly to a surface waterbody
2. Outlet A ditch that connects the drainage network to a surface waterbody
3. Outflow/transfer A ditch that carries drainage water across the farm boundary onto the neighboring land
4. Secondary A ditch that typically flows perpendicular to the slope of the land connecting two larger open ditches or running through a field for excess
water removal
5. Disconnected A ditch that is not connected to the overall drainage network but may have groundwater connectivity potential
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this month is hydrologically active in Ireland and all pathways
interact with the open ditch network (e.g., groundwater upwelling,
seepage, and springs), as observed from the previous year’sfield
visits. As this study aimed to validate established connectivity risk
(water and the presence or absence of N and P) between open ditch
types and adjoining surface waterbodies and did not aim to elucidate
the load or impact of this connection, a temporal water sampling
survey was not required. It is acknowledged that the connectivity
level at the time of sampling water is influenced by the precipitation
level (both antecedent and current). Therefore, sampling was
undertaken when both surface and subsurface pathways were
most active, and such data were used to validate the source and
hydrologic connectivity with the open ditch network.
The number of samples collected was dictated mainly by the
observations of connectivity pathways on open ditches during the
initial fieldwork campaign. As such, open ditches that had surface or
subsurface connectivity pathways (Table 3) noted in the earlier
survey were prioritized for sampling. These observations were used
to validate surface, subsurface, and groundwater flows that entered
open ditches on the case study farms. However, some sampling
points had no N connectivity pathways. Only four ditch categories
from Table 2 (farmyard connection, outlet, outflow, and secondary
ditches) were sampled for water across the seven case study farms.
Shallow disconnected ditches (category 5 in Table 2) were dry, which
indicated no N connectivity with perched or true water tables at the
time of sampling. These acted as storage and recharge areas for
groundwater during rainfall periods. At each water sampling
location, two 50-ml samples (filtered on-site using 0.45-μmfilter
paper and unfiltered) were collected for dissolved and total P
analyses, respectively. Grab water sampling was carried out in the
mapped ditch categories on each farm, provided water was present
in the open ditch. The grab water sampling taken directly from an
open ditch was conducted within 1 m downstream of in-field drain
outlets, farm roadways, groundwater springs, and groundwater
seepage/upwelling, where present, in the open ditch categories.
All water samples were kept in an ice box during sampling and
transportation and then tested within 1 day of sample collection.
Filtered water samples were analyzed for DRP and total dissolved
phosphorus (TDP) using a Gallery discrete analyzer (Gallery reference
manual, 2016) and a Hach Ganimede P analyzer, respectively. The total
dissolved phosphorus (TDP) was measured by acid persulfate oxidation
under high temperature and pressure. The unfiltered water samples
were analyzed for nitrite (NO
2
-N), NH
4
-N, total oxidized nitrogen
(TON), and total reactive phosphorus (TRP) using the Gallery analyzer.
Total phosphorus (TP) was analyzed using the Ganimede P analyzer.
Phosphorus was measured colorimetrically by the ascorbic acid
reduction method (Askew and Smith, 2005), where the 12-
molybdophosphoric acid complex is formed by the reaction of
orthophosphate ions with ammonium molybdate and antimony
potassium tartrate (catalyst) and reduced ascorbic acid. All samples,
reagent blanks, and check standards were analyzed at the Teagasc
Johnstown laboratory following the Standard Methods (APHA, 2005).
TABLE 3 Criteria for identifying N connectivity pathways on open ditch categories and associated source of connection.
N connectivity
pathway
Source of
connection
Criteria description
a
In-field drains Subsurface Evidence of in-field pipe drains connecting into ditches, usually with less water flow
Farm roadway Surface Evidence of farm roadway and hard surface runoff connectivity with the open ditch network (directly
during rainfall or indirect signs such as established rills and breakthrough points)
Groundwater springs Subsurface Evidence of natural springs or pipe springs (with high water flow) connecting into ditches
Groundwater upwelling or
seepage
Subsurface Evidence of groundwater seeping from either the base or side of a ditch into the ditch
a
Criteria description (Teagasc, 2022).
TABLE 4 Summary of open ditch data including the proportion of the open ditch network accounted for by different P open ditch categories for each case-
study farm.
Farm
number
Field
perimeter
(m)
% Perimeter
as ditch
Total
ditch
length
(m)
Proportion of total ditch length (%)
1. Farmyard
connection
2.
Outlet
3.
Outflow
4.
Secondary
5.
Disconnected
1 16,471.5 44.3 7,290.4 10.7 0 18.4 70.2 0.7
2 21,524.1 9.0 1,935.1 6.8 59.4 33.8 0 0
3 19,737.9 35.4 6,990.7 5.7 22.6 9.4 62.4 0
4 16,572.3 17.2 2,847.4 28.4 23.3 4.6 10.5 33.2
5 13,085.9 43.5 5,692.4 25.5 39.5 0 34.3 0.7
6 16,966.5 52.6 8,916.3 8.5 22.4 7.2 60.9 0.9
7 9,607.5 28.9 2,773.3 34.2 11.7 15.8 38.3 0
Average 16280.8 33.0 5206.5 17.1 25.6 12.7 39.5 5.1
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All quality control (QC) samples/check standards are prepared from
certified stock standards from a different source than calibration
standards. Quality control sampleswereanalyzedatthebeginning
and end of every batch, for every 10 samples within a batch, and if the
QC fell outside limits, samples were repeated back to the last correct
QC. Blanks were included in every batch, and approximately 10% of
samples were repeated. Tolerances range up to a maximum of ±7.5% of
the nominal value. All instruments used were calibrated in line with the
manufacturers’recommendations. Nitrate-N was calculated by
subtracting NO
2
-N from TON, particulate phosphorus (PP) was
calculated by the difference between TP and TDP, and dissolved
unreactive phosphorus (DUP) was calculated by the difference
between TDP and DRP.
2.4 Data analysis
To validate the link between the conceptualized connectivity
source–pathways and their introduction of N and P into the open
ditch system, data from the spring season synoptic survey were analyzed
statistically to differentiate the nutrient concentrations for the various
open ditch categories and also for the various connectivities to ascertain
if they varied from each other. As the data for each water quality
parameter were not normally distributed, Kruskal–Wallis analysis was
undertaken to find out the significant differences between farmyard
connection, outlet, outflow, and secondary ditch categories and also
between the conceptualized N connectivity pathways (in-field drains,
internal roadways, springs, and seepage/upwelling) within and across
the outlet, outflow, and secondary ditch categories for all the water
quality parameters (NH
4
-N, NO
3
-N, TN, DRP, DUP, TP, and PP).
Data were analyzed using R studio software version 4.0.2 (2020). Where
significant differences were observed using an alpha level of 0.05 (95%
confidencelevel),thepairwiseWilcoxonrank-sumtestwasfurtherused
to find the differences between the means of the pairs. Microsoft Excel
software version 16.0 (2016) was used to find a correlation between the
number of occurrences of in-field drains and the percentage of drained
fields on poorly draining soil farms.
3 Results
3.1 Analysis of the open ditch networks
All five ditch categories, classified by Moloney et al. (2020), were
identified using the criteria outlined in that work. The average
percentage of the total ditch network in all farms was 17.1%,
25.6%, 12.7%, 39.5%, and 5.1% for farmyard connection, outlet,
outflow, secondary, and disconnected ditches, respectively (Table 4).
Farm 2 contained the fewest drainage categories (3 out of 5).
3.2 Observations relating to conceptualized
N connections within the open
ditch networks
Based on the criteria for identifying N connectivity pathways
(Table 3), 52% of all the open ditch network sampling points were
observed to have N connectivity pathways interacting with them.
The N connectivity pathways to open ditches considered in this
study were mainly connected to secondary ditches, followed by
farmyard connection, outflow, and outlet ditches, with no N
connectivity pathway to disconnected ditches (Supplementary
Table S1). For each ditch category (Table 2) sampled in this
study, the percentages of the occurrence of different N
connectivity pathways are shown in Figure 3. Among these N
connectivity pathways across all ditch categories, in-field drains
were the most common (representing 64%), followed by
groundwater springs, internal roadways, and groundwater
upwelling/seepage, respectively, representing 20%, 11%, and 5%
of the sampling points (Supplementary Table S1). The
occurrence of observed in-field drains was positively correlated to
the percentage of drained fields on case study farms (R
2
= 0.35).
Farms 2 and 4, which had the lowest percentage of in-field
drained fields (Table 1), had relatively high connectivity of
groundwater springs to open ditches (Supplementary Table S1).
Aside from farm roadway connectivity pathways to open ditches on
Farm 2, roadway connectivity pathways to open ditches were found
to be highest on farms with a flat topography, particularly farms
3 and 5. Groundwater upwelling/seepage connectivity to ditches was
uncommon. There was an absence of groundwater upwelling and
seepage connectivity pathways on outflow and farmyard connection
ditches and roadway connectivity pathways on outlet ditches across
all farms. In addition, there was evidence of multiple N connectivity
pathways to individual ditches on some farms.
3.3 Validation of N connectivity pathway
using the synoptic survey
The average TN and TP concentrations were significantly higher
in farmyard connection ditches (Figure 4) than in outlet, outflow,
and secondary ditches (p<0.01). Across the outlet, outflow, and
secondary ditch categories, NO
3
-N was the dominant N species,
contributing on average to 44.7% of TN at sampling points near N
connectivity. Only 10.6% of TN comprised NH
4
-N within these
ditch categories. The highest average NO
3
-N across these ditch
categories was observed in groundwater springs (1.90 mg L
-1
),
followed by in-field drains (0.75 mg L
-1
), groundwater upwelling
(0.65 mg L
-1
), and roadways (0.17 mg L
-1
)(Supplementary Table
S1). In addition, NO
3
-N at groundwater springs was dissimilar
(p<0.05) to NO
3
-N at roadways and in-field drains (Figure 5A).
High concentrations of NO
3
-N were also measured on roadways,
where NH
4
-N is conceptualized as being dominant (Figure 1), on
secondary ditches. However, NH
4
-N dominated TN across these
ditches at sample points near roadways, with 25.3% composition as
opposed to 6.9% of NO
3
-N. Ammonium-N concentrations across
these ditch categories were not statistically significant (p>0.05).
No consistent trends in species of TP were observed across the
outlet, outflow, and secondary ditch categories. Among these ditch
categories, TP concentrations were relatively high in secondary
ditches, in which PP was predominant (Figure 5B). Across the
outlet, outflow, and secondary ditch categories, PP was statistically
significant (p<0.05), particularly between in-field drains and
roadway connectivity pathways, and DRP was statistically
significant (p<0.01), particularly between roadways and
groundwater springs. Comparing P species for each N
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Opoku et al. 10.3389/fenvs.2024.1337857
connectivity pathway, average PP concentrations were found to be
the highest in groundwater upwelling/seepage (0.24 mg L
-1
),
followed by roadways (0.12 mg L
-1
), groundwater springs
(0.04 mg L
-1
), and in-field drain (0.02 mg L
-1
) connectivity
pathways, whereas average DRP concentrations were the highest
in roadways (0.19 mg L
-1
), followed by groundwater upwelling/
seepage (0.04 mg L
-1
), in-field drains (0.03 mg L
-1
), and
groundwater springs (0.01 mg L
-1
).
4 Discussion
4.1 Observations on ditch categories and
associated N connectivity pathways
Of the seven farms surveyed, disconnected and secondary
ditches comprised the lowest and highest average percentages of
the total ditch length, respectively. This result is consistent with that
of Moloney et al. (2020), who recorded similarly low and high
average percentages for total ditch length on varying soil grasslands
in Ireland. Disconnected ditches are ineffective for excess field water
removal within the drainage system and exist either as blocked
normal ditches or as created disconnecting ditches that remove field
runoff or precipitation water by infiltration or evaporation.
Disconnected ditches, when wet, may hold water with vegetation
and potentially provide denitrification or create pollution swapping
by the release of greenhouse gases such as nitrous oxide (N
2
O) or
nitric oxide (NO).
Secondary ditches, as the most prevalent ditch category, had the
most N connectivity pathways, of which in-field drains were the
most prevalent (Figure 3). Secondary ditches connect to other ditch
categories from the central farm fields, and due to the farm slopes in
that areas, frequent shallow water table (Clagnan et al., 2018) for
potential for connectivity pathways may occur. As the majority of
the farms in this study contained poorly drained soils (Table 1), a
positive, albeit weak, correlation (R
2
= 0.35) between the number of
occurrences of in-field drains (Supplementary Table S1) and the
percentage of drained fields (Table 1) on poorly draining soil farms
was observed. Both the number of occurrences of in-field drains and
the percentage of drained fields help in regulating water table levels
FIGURE 3
Percentages of the occurred N connectivity pathways for the ditch categories.
FIGURE 4
(A) Nitrogen (N) and (B) phosphorus (P) mean + standard error (SE)
concentrations within the open ditch categories across case study farms.
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Opoku et al. 10.3389/fenvs.2024.1337857
and supporting grass growth functionality, so they were positively
correlated.
4.2 Hydrochemistry across P ditch
categories and consideration of N
connectivity pathways
Higher TN and TP average concentrations were measured in
farmyard connection ditches relative to the other ditch categories,
which were similar to the findings of Moloney et al. (2020),Harrison
et al. (2019), and Ezzati et al. (2020). In the farmyard connection
ditches, the TN and TP concentrations were nearly 3 times higher
than the TN standard limits of 2.5 mg L
-1
set by the European Union
for estuarine waters (Wuijts et al., 2022) and 15 times higher than TP
standards of 0.1 mg L
-1
, as proposed by Wetzel (2001). While both
Edwards et al. (2008) and Mockler et al. (2017) identified farmyards
as point sources for high nutrient loss, the former argued that runoff
from farmyards has been overlooked and not duly considered a
major nutrient loss hotspot. Such runoff may lead to high nutrient
concentration fields near the farmyard relative to fields further away
(Fu et al., 2010), and these potentially may enter open ditches near
the farmyard to create major downstream water quality problems.
Unlike ditches (associated with point sources), the lower TP and TN
concentrations in outlet, outflow, and secondary ditch categories
may be associated with diffuse nutrient sources. Studies have shown
that diffuse sources, relative to point sources, have lower TN and TP
concentrations (Pieterse et al., 2003;Edwards and Withers, 2008).
Management of some of these diffuse sources is problematic as they
are difficult to locate in a landscape (Harrison et al., 2019). However,
their impact on the deterioration of receiving waterbodies is
substantial, and therefore needs to be managed (Andersen et al.,
2014;Bradley et al., 2015). Diffuse sources depend on landscape and
other management factors, which influence diffuse N and P
mobilization, transformation, and delivery into the ditches
(Granger et al., 2010;Schoumans et al., 2014). However, notable
among these factors are the hydrological conditions on which diffuse
nutrient release strongly depends (Edwards and Withers, 2008;
Chen et al., 2013). This, coupled with biogeochemical factors,
which may vary within a landscape, influences the spatial and
temporal distribution patterns of diffuse N and P, including the
pathways by which they enter and leave farms (Clagnan et al., 2019;
Grenon et al., 2021). Nutrient losses from the diffuse sources are
delivered into open ditches along surface and subsurface pathways,
creating hotspots of nutrient loss in certain open ditch categories,
which need to be characterized and potentially mitigated. Climatic,
landscape, and management factors all have a role to play in when
and where impacts occur. These could have contributed to the
higher TN concentrations in water samples that were measured near
N connectivity pathways than at locations with no N connectivity
pathways within the outlet, outflow, and secondary ditch categories,
and also for TP in the outflow ditch category. This observation aligns
with those of the reported works of Ibrahim et al. (2013) and
Valbuena-Parralejo et al. (2019) on in-field drains, Fenton et al.
(2021) and Rice et al. (2022) on roadways, Soana et al. (2017) on
groundwater springs, and O’Callaghan et al. (2018) on groundwater
upwelling/seepage.
Nitrate was the dominating N species in in-field drains,
groundwater springs, and upwelling connectivity pathways in
outlet, outflow, and secondary ditch categories (Figure 5A). This
may be attributed to their connection to a subsurface N source,
which comprises leached N from animal excreta and fertilizer that
may have been nitrified to NO
3
-N (Necpalova et al., 2012). In poorly
drained grasslands, nitrification may have been increased by the
high in-field drainage density (Table 1), which enhanced N
preferential flow (Van Der Grift et al., 2016) and limited
potential N attenuation (Clagnan et al., 2019;Valbuena-Parralejo
et al., 2019). The average NO
3
-N concentration was highest in
groundwater springs and in-field drains. Factors such as the
presence of these N connectivity pathways within the shallow
subsurface region, nearness to the soil surface (where farm
management mostly occurs), and exposure to N sources at the
groundwater–ground surface intersection spots (particularly for
groundwater springs; Infusino et al., 2022) could have
contributed to the high NO
3
-N concentrations in these locations.
In contrast, NH
4
-N was the most dominating N species measured
for roadway connectivity pathways across the outlet, outflow, and
secondary ditch categories, especially where animal excreta were
observed. This observation aligns with that of Fenton et al. (2021),
who observed that roadways draw surface nutrient sources, high in
FIGURE 5
(A) Nitrogen (N) and (B) phosphorus (P) mean + standard error
(SE) concentrations within associated connectivity pathways in
sampled open ditch categories across case study farms.
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Opoku et al. 10.3389/fenvs.2024.1337857
NH
4
-N, as runoff from soil and animal excreta into nearby ditches
and streams. Although important, redox reactions were not
considered in the present study.
For TP concentrations across outlet, outflow, and secondary
ditch categories, P concentrations were relatively low compared to
those in the farmyard connection ditch category. However, such
TP concentrations in the outlet, outflow, and secondary ditch
categories were still high enough to cause eutrophication
downstream, if undiluted. High TP concentrations measured in
secondary ditches may be related to the impacts of farm
management activities including grazing and farm machinery
movement, which is intense within the central fields of most
farms where secondary ditches lie as connecting ditch links.
Thesecontributetotheerosion of ditch sides and associated
deposition of soils in the secondary ditches, as reflected in the
higher PP concentrations observed. High TP concentrations
measured near roadways on outflow ditches may be due to
animal excreta, run-on deposits from farmyards, fields, and
poached surfaces as a result of animal and machinery
movement (Fenton et al., 2021). Both PP and DRP can trigger
eutrophication in waterbodies and may pose a risk to downstream
waterbodies. However, this depends on their closeness,
connection, and mitigation along the pathway to water sources
within agricultural landscapes.
Such information from the study provides an additional insight
into the source, connection, and presence (and transformation
process) of N in ditch categories from a previous study by
Moloney et al. (2020), who observed high NH
4
+
and NO
3
−
concentrations in all ditch categories, except for the outlet ditch,
where high NO
3
−
and low NH
4
+
were measured, and disconnected
ditches, where NO
3
−
concentration was found to be dominant. The
risk ranking of connectivity along the open ditch for N and P does
not determine the impact of the nutrients being lost to the associated
waterbody; it simply establishes the N connectivity pathway if it
is present.
4.3 Deriving a connectivity risk for N into P
agricultural open ditch categories
The evidence of N concentrations in the ditch water chemistry
from Moloney et al. (2020) and the current study informs an
improved ditch connectivity risk category system (Table 5). This
is a valuable information tool for environmental sustainability
officers to enhance water quality management and mitigation
options for N and P losses on dairy grassland farms with heavy
textured soils in high-rainfall areas. It considers both the
connectivity pathways, through which N can be introduced to a
ditch network, and their associated N species.
In the current study, all of the conceptualized N connectivity
pathways (Figure 1) established from the literature were present, but
not in all of the sampled P risk ditch categories developed by
Moloney et al. (2020) (Supplementary Table S1). For instance,
the established general trends and connectivity pathways of
groundwater seepage and upwelling were not present on
farmyard connection and outflow ditches. Moreover, the grab
water sampling data results validated all the conceptualized N
connectivity pathways present in ditches (Figure 5A), except
groundwater seepage and upwelling. The dominance of high
NO
3
-N concentrations at in-field drains and springs and high
NH
4
-N concentrations at roadways within farmyard connection
ditches indicated a point source of pollution arising from their
connection to the farmyard aside from the hydrology-induced N
concentrations. Farmyards pose the greatest nutrient loss risk on
farms due to high nutrient concentration in discharges (Vedder,
2020), and like other point sources, they are independent of
hydrology (Edwards and Withers, 2008). As such, primarily
managing the farmyard wastewater before discharge into
connecting ditches for mitigating nutrient connectivity to water
sources is essential (NFGWS, 2020) before deployment along/within
ditch interventions.
For the other sampled outlet, outflow, and secondary ditch
categories, all N conceptualized pathways were observed, except
for internal farm roadway on outlet ditches and groundwater
seepage and upwelling on outflow ditches (Supplementary Table
S1). In outlet, outflow, and secondary ditch categories, the ditch
water synoptic data validated the conceptualized NO
3
-N and NH
4
-
N for all the observed N connectivity pathways, except farm roadway
connection on secondary ditches (which was invalid with NO
3
-N
dominance over conceptualized NH
4
-N from hard field surface flow
pathways). Nitrate dominated in-field drains, groundwater springs,
upwelling, and seepage connectivity pathways, and NH
4
-N
dominated farm roadways across the outlet, outflow, and
secondary ditch categories, as conceptualized in Figure 1.
Assessment of the N connectivity pathway within ditch category
5 could not be included in the study due to the unavailability of water
samples in this ditch for validating conceptualized N connectivity
pathways. Moloney et al. (2020) showed that disconnected ditches
pose relatively less risk for nutrient loss among the ditch categories,
and therefore, merit less focus during nutrient loss mitigation for
surface water. However, such low nutrient concentrations could be
leached into groundwater, and therefore may require mitigation
interventions to prevent leaching.
To apply this research in practice, once open ditches are
investigated and mapped, a category should be assigned for an
individual open ditch, after which the available N connections for
that ditch are noted. All of these connections, in combination, will
aid in the future mitigation management strategy. It is unlikely, for
example, that more than one mitigation option will be installed in a
single open ditch. Therefore, the information gathered from
Table 5 can be used to ensure that correct nutrients and their
speciation are targeted for mitigation in the open ditch. Mitigation
options may be a combination of those that limit diffuse and point
sources. For example, with respect to diffuse sources, strict
adherence to action programs to reduce losses is important
(e.g., Good Agricultural Practice Regulations, in line with the
Nitrates Directive (91/676/EEC)). With respect to roadway
runoff, NH
4
+
mitigation options are available and have been
outlined in Fenton et al. (2021) and Rice et al. (2022) (e.g.,
diversion bars to move runoff to a buffer area of at least 1.5 m,
cambering farm roadways, and directing flow onto adjacent fields).
Adopting a two-stage ditch design may reduce high PP
concentrations (King et al., 2015;Hodaj et al., 2017;Faust
et al., 2018). With respect to the subsurface N connectivity
pathways (in-field drains, groundwater springs, upwelling, and
seepage), in-ditch management practices may control the flow and
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TABLE 5 An updated integrated ditch connectivity ranking that considers both phosphorus and nitrogen coupled with suggested strategies to reduce
nutrients from ditches on dairy farms.
P Ditch
category
Description Validated N
connection with
category
Associated source Future mitigation
management
1. Farmyard
Connection
A ditch/pipe that connects a farmyard
to the drainage network or directly to a
surface waterbody. These connections
pose the highest risk and should be
prioritized in terms of future
management
Subsurface interaction In-field drains (pipes, moles, or gravel
moles; older variation) bring P and N
from fields to the open ditch
Management practices that disconnect
sub-surface drainage system discharges
into the open ditch
•These may include adherence to
correct land drainage design,
installation guidelines, and
maintenance
•Use of end-of-pipe land drainage
mitigation options including low-
grade weirs Baker et al., (2016),filter
cells, cartridges, and structures
(King et al., (2015);Goeller et al.,
(2020);Liu et al., (2020) (see
discussion for details)
Strict adherence to good farming
practices to minimize diffuse losses
and leaching of nutrients to sub-
surface drainage system that are
connected to the open ditch
•These may include in-ditch measures
such as sediment traps, bioreactors, and
filters to slow the flow and control
nutrient loads (Fenton et al., (2021)
All forms of P and N are potentially lost
through this pathway to the ditch, with
NO
3
−
and DRP dominating
Surface runoff Farmyards and hard surfaces including
farm internal roadways bring P and N
forms, dominated by NH
4
+
and PP from
raw organic waste, loss to the ditch
Management practices that disconnect
the farmyard from the open drainage
ditch and internal farm roadway
network are needed specifically within
100 m of the farmyard in this category
•These may include measures that
prevent roadway runoff from
entering the open ditch using low-
cost diversion bars or surface
modifications Fenton et al., (2021).
There must be a buffer of at least
3mEPA Ireland, (2020) to reduce
the runoff impacts of surface waters
Groundwater interaction Natural springs bring shallow
groundwater P and N, dominated by
NO
3
−
, into open ditches through piped
drains
Strict adherence to good farming
practices to minimize diffuse losses
•These may include end-of-pipe
mitigation measure where the
spring has been piped e.g., vegetated
buffer spots Faust et al., (2018) and
filter cells, cartridges, and structures
using various materials Ibrahim
et al., (2015);King et al., 2015;Penn
et al., (2020) (see discussion for
details). The full list of materials is
reviewed in Ezzati et al. (2020)
2. Outlet A ditch that connects the drainage
network to a surface waterbody
Subsurface interaction In-field drains (pipes, moles, or gravel
moles; older variations) bring P and N
forms, dominated by NO
3
−
, from fields to
the open ditch
Management practices that disconnect
sub-surface drainage system discharges
into the open ditch
•These may include adherence to
correct land drainage design,
installation guidelines, and
maintenance
•Use of end-of-pipe land drainage
mitigation options such as
constructed wetlands (Tanner et al.,
(2005);King et al., (2015) (see
discussion for details)
Strict adherence to good farming
practices to minimize diffuse losses
and leaching of nutrients to sub-
surface drainage system that are
connected to the open ditch
(Continued on following page)
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TABLE 5 (Continued) An updated integrated ditch connectivity ranking that considers both phosp horus and nitrogen coupled with suggested strategies to
reduce nutrients from ditches on dairy farms.
P Ditch
category
Description Validated N
connection with
category
Associated source Future mitigation
management
•These may include in-ditch measures
such as sediment traps, bioreactors,
and filters to slow the flow and
control of nutrient loads (Fenton
et al. 2021)
Groundwater interaction Natural springs bring shallow
groundwater, dominated by NO
3
−
concentration, into ditches through
piped drains
Strict adherence to good farming
practices to minimize diffuse losses
•These may include end-of-pipe
mitigation measures where the
spring has been piped, e.g.,
vegetated buffers (Faust et al. 2018),
filter cells, cartridges, and
structures, using various materials
(Ibrahim et al., 2015;King et al.,
2015;Penn et al., 2020) beneath
piped springs located on the ditch.
The full list of materials is reviewed
in Ezzati et al. (2020)
Groundwater interaction Seeping and upwelling deep
groundwater, dominated by NO
3
−
,
enters
into ditches
Strict adherence to good farming
practices to minimize diffuse losses
•In terms of groundwater upwelling
or spring connectivity, in-ditch
intervention that slows the flow and
mitigates nutrients using
bioreactors, two-stage ditches,
filters, and vegetated ditches (King
et al., (2015);Faust et al., (2018) may
be introduced after spring
connectivity and before the outlet to
reduce dissolved and particulate
nutrients entering waters
3. Outflow/
transfer
A ditch that carries drainage water
across the farm boundary through
neighboring land
Subsurface interaction In-field drains (pipes, moles, or gravel
moles; older variations) bring P and N,
dominated by NO
3
−
,
from fields to the
open ditch
This drainage water will pass to an
adjoining farm and will be mitigated as
another landowner’s arm management
plan. Some mitigation can occur in
outflow ditches using mitigation
management practices provided for
farmyard connection and outlet ditches
as appropriate, which may increase the
efficacy of mitigation across the farm
landscape
Surface runoff Farm internal roadways introduce NH
4
+
-
and DRP-dominated hard surface water
to the ditch
This drainage water will pass to an
adjoining farm and will be mitigated as
another landowner’s farm management
plan. Some mitigation can occur in
outflow ditches using mitigation
management practices provided for
farmyard connection and outlet ditches
as appropriate, which may increase the
efficacy of mitigation across the farm
landscape
Groundwater interaction Natural springs connect shallow
groundwater, dominated by
NO
3
−
concentration, to ditches
This drainage water will pass to an
adjoining farm and will be mitigated as
another landowner’s farm management
plan. Some mitigation can occur in
outflow ditches using mitigation
management practices provided for
farmyard connection and outlet ditches
as appropriate, which may increase the
efficacy of mitigation across the farm
landscape
(Continued on following page)
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the nutrient content leaving the open ditch. These may include
sediment traps (Wilkinson et al., 2014), vegetated ditches (Kröger
et al., 2008;Soana et al., 2017;Faustetal.,2018), or in-ditch filters
or bioreactors (King et al., 2015;Goeller et al., 2020;Liu et al.,
2020). Nutrient filtering through vegetation (Moeder et al., 2017)
or use of a medium (Ezzati et al., 2020) can only aim to mitigate a
small amount of overall nutrients leaving the ditch due to
hydraulic retention times needed and bypass flow during high
storm events. Furthermore, mitigation practices including the
construction of wetlands (Tanner et al., 2005), vegetated buffer
zones (Faustetal.,2018), and low-grade weirs (Kröger et al., 2012;
Littlejohn et al., 2014;Baker et al., 2016) that may be placed at the
end of ditches after the connectivity pathways, especially for
farmyard connection and outlet ditch categories, would help
limit nutrient loss from these farms. Therefore, all measures
need to be considered a package and not in isolation when
trying to minimize nutrient and sediment loads leaving an open
ditch system. It is worth noting that cooperation at the local level is
needed to prevent other mitigation-related problems (such as the
polluter pays principle regarding outflow ditches between
neighboring farmers) to ensure mitigation occurs before waters
are impacted.
TABLE 5 (Continued) An updated integrated ditch connectivity ranking that considers both phosp horus and nitrogen coupled with suggested strategies to
reduce nutrients from ditches on dairy farms.
P Ditch
category
Description Validated N
connection with
category
Associated source Future mitigation
management
4. Secondary A ditch that typically flows
perpendicular to the slope of the land
connecting two larger ditches. It can
also occur as an open ditch running
through a field to collect and remove
large excesses of surface water
Subsurface interaction In-field drains (pipes, moles, or gravel
moles; older variations) bring P and N,
dominated by NO
3
−
from fields to the
open ditch
Mitigation is unlikely to occur in these
open ditches as they do not discharge
directly to waters but act as conduits.
Some mitigation can occur in secondary
ditches using in-ditch mitigation
management practices provided for
farmyard connection and outlet ditches
as appropriate, which may increase the
efficacy of mitigation across an
individual farm
Surface runoff Farm internal roadways introduce PP,
DRP- and NO
3
−
-dominated, within the
water from a hard surface to the ditch
Mitigation is unlikely to occur in these
open ditches as they do not discharge
directly to waters but act as conduits.
Some mitigation can occur in secondary
ditches using in-ditch mitigation
management practices provided for
farmyard connection and outlet ditches
as appropriate, which may increase the
efficacy of mitigation across an
individual farm
Groundwater interaction Natural springs bring shallow
groundwater, dominated by
NO
3
−
concentration, through piped
drains over ditch sides to introduce both
PP and NO
3
−
into the ditch
Mitigation is unlikely to occur in these
open ditches as they do not discharge
directly to waters but act as conduits.
Some mitigation can occur in secondary
ditches using in-ditch mitigation
management practices provided for
farmyard connection and outlet ditches
as appropriate, which may increase the
efficacy of mitigation across an
individual farm
Groundwater interaction Deep groundwater, dominated by NO
3
−
,
seeps through ditch side surfaces and/or
upwells through the ditch base to
introduce PP and NO
3
−
into ditches
Mitigation is unlikely to occur in these
open ditches as they do not discharge
directly to waters but act as conduits.
Some mitigation can occur in secondary
ditches using in-ditch mitigation
management practices provided for
farmyard connection and outlet ditches
as appropriate, which may increase the
efficacy of mitigation across an
individual farm
5. Disconnected A ditch that is not connected to the
overall ditch network and may be
connected with groundwater
Surface and groundwater
interaction
Diffuse source of NO
3
−
interacts with the
open ditch. Runoff may interact with the
open ditch
Connectivity is not present to surface
water within the open network, but
there may be a groundwater connection
which subsequently discharges to
surface water. Precautionary practices
should be taken at these locations to
minimize recharge to groundwater by
provision of a soil buffer
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5 Conclusion
Distinctly different from a P-only classification system, the
integrated connectivity risk classification system for N and P
showed that not all source–pathway interactions within open
ditches are active. This is a valuable information tool that enables
a much more specific and targeted nutrient-specific mitigation
approach to be implemented on open ditches in heavy textured
grassland dairy farms in high-rainfall areas. The new system avoids
the pitfalls of a P-only classification system (i.e., mitigating for P but
allowing N to affect water quality unabated). The findings of this
study are limited to these field sites and may (or may not) differ in
other geographic areas with different soils, climates, agricultural
practices, etc. However, the same methodology may be applied to
other areas to develop a bespoke integrated connectivity risk ranking
for P and N along agricultural open ditches to inform targeted and
specific mitigation strategies on those farms. Further assessment of
the temporal and spatial variability of soil, weather, drainage system,
and general hydrogeochemistry, which influences nutrient
connectivity, may be needed to rank the N and P risk in each
ditch category.
Data availability statement
The raw data supporting the conclusion of this article will be
made available by the authors, without undue reservation.
Author contribution
DO: conceptualization, data curation, formal analysis,
investigation, methodology, validation, visualization,
writing–original draft, and writing–review and editing. MH:
conceptualization, funding acquisition, investigation,
methodology, software, supervision, validation, visualization, and
writing–review and editing. OF: conceptualization, funding
acquisition, investigation, methodology, software, supervision,
validation, visualization, and writing–review and editing. KD:
conceptualization, investigation, methodology, validation,
visualization, and writing–review and editing. TC: funding
acquisition, methodology, and writing–review and editing. PT:
conceptualization, funding acquisition, investigation,
methodology, project administration, resources, software,
supervision, validation, visualization, and writing–review
and editing.
Funding
The author(s) declare that financial support was received for the
research, authorship, and/or publication of this article. The authors
are grateful to Teagasc for the award of a Walsh Scholarship to the
first author (grant number: RMIS-1381) to conduct this research.
Acknowledgments
The authors are grateful to Simon Leach, Asaf Shnel, and Denis
Brennan for GIS, fieldwork, and laboratory assistance provided.
Conflict of interest
The authors declare that the research was conducted in the
absence of any commercial or financial relationships that could be
construed as a potential conflict of interest.
Publisher’s note
All claims expressed in this article are solely those of the authors
and do not necessarily represent those of their affiliated
organizations, or those of the publisher, the editors, and the
reviewers. Any product that may be evaluated in this article, or
claim that may be made by its manufacturer, is not guaranteed or
endorsed by the publisher.
Supplementary material
The Supplementary Material for this article can be found online
at: https://www.frontiersin.org/articles/10.3389/fenvs.2024.1337857/
full#supplementary-material
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