Coastal urban reliance on groundwater during drought cycles: Opportunities, threats and state of knowledge

Abstract

Urbanisation and population growth are concentrated on the coast with approximately 40% of the human population living within 100 km of the ocean. The freshwater systems on which coastal urban areas rely are vulnerable to bidirectional pressures including coastal processes such as sea-level rise and coastal erosion coupled with land use changes and pollution occurring in inland catchment areas. These threats are likely to be amplified in the future under climate change conditions and more frequent and severe drought periods are expected to jeopardise already constrained water supply systems. Groundwater is used as a freshwater resource globally and is especially important as a conjunctive supply during drought periods due to perceived buffer capabilities. However, several threats impact coastal aquifers due to over-abstraction, such as salinisation, land subsidence and groundwater flooding and often these subterranean resources are “out of sight and out of mind” when it comes to management strategies. Here, we present an assessment of current issues and management options relevant to coastal aquifers using recent literature. These insights provide knowledge on global issues relevant to groundwater resources, especially regarding water use during droughts. This is exemplified using a South African case study of two metropolitan municipalities that have experienced or are experiencing severe multi-year droughts. Both municipalities have grappled with the depletion of surface water resources, which constitutes the bulk of the local water supply systems. Consequently, groundwater resources have been explored as an augmentation strategy. Although groundwater resources may be useful in alleviating drought effects, it is crucial that a local understanding of the aquifers is developed through baseline hydrological studies and long-term monitoring. Furthermore, unregistered groundwater use needs to be quantified. Finally, a holistic groundwater management view, and the communication thereof, is required to ensure the sustainable management of coastal aquifers.

Full text

Coastal urban reliance on groundwater during
drought cycles: Opportunities, threats and state
of knowledge
Carla Dodd
1,2
and Gavin M. Rishworth
2,3
1
Department of Geosciences, Nelson Mandela University, Gqeberha, South Africa;
2
SARChI: Shallow Water Ecosystems,
Institute for Coastal and Marine Research (CMR), Nelson Mandela University, Gqeberha, South Africa and
3
Department
of Zoology, Nelson Mandela University, Gqeberha, South Africa
Abstract
Urbanisation and population growth are concentrated on the coast with approximately 40% of
the human population living within 100 km of the ocean. The freshwater systems on which
coastal urban areas rely are vulnerable to bidirectional pressures including coastal processes such
as sea-level rise and coastal erosion coupled with land use changes and pollution occurring in
inland catchment areas. These threats are likely to be amplified in the future under climate
change conditions and more frequent and severe drought periods are expected to jeopardise
already constrained water supply systems. Groundwater is used as a freshwater resource globally
and is especially important as a conjunctive supply during drought periods due to perceived
buffer capabilities. However, several threats impact coastal aquifers due to over-abstraction, such
as salinisation, land subsidence and groundwater flooding and often these subterranean
resources are “out of sight and out of mind”when it comes to management strategies. Here,
we present an assessment of current issues and management options relevant to coastal aquifers
using recent literature. These insights provide knowledge on global issues relevant to ground-
water resources, especially regarding water use during droughts. This is exemplified using a
South African case study of two metropolitan municipalities that have experienced or are
experiencing severe multi-year droughts. Both municipalities have grappled with the depletion
of surface water resources, which constitutes the bulk of the local water supply systems.
Consequently, groundwater resources have been explored as an augmentation strategy.
Although groundwater resources may be useful in alleviating drought effects, it is crucial that
a local understanding of the aquifers is developed through baseline hydrological studies and
long-term monitoring. Furthermore, unregistered groundwater use needs to be quantified.
Finally, a holistic groundwater management view, and the communication thereof, is required
to ensure the sustainable management of coastal aquifers.
Impact statement
This study provides an overview of recent literature related to groundwater use in coastal urban
areas, especially as an emergency resource during drought periods. The primary threats to
coastal aquifers in the past, present and future are highlighted, as well as potential sustainable
management solutions. A global comparison of risks and possible solutions regarding coastal
aquifer management is imperative given increasing pressures on these systems related to socio-
economic development, population growth and climate change. This is especially important in
the context of droughts as these events affect both the quantity and quality of water available to
densely populated coastal urban areas, which are vulnerable to resource degradation from both
catchment and marine processes. Droughts coupled with groundwater abstraction also increase
saltwater intrusion in aquifers with marine connectivity. As an area susceptible to severe
droughts, a South African case study is used to illustrate the historical reliance of two coastal
metropolitan municipalities on surface water resources and how groundwater abstraction has
been used to supplement diminishing water supply. To our knowledge, this is the first study to
review the literature available on the water crises experienced by both coastal municipalities.
This assessment reveals priority management principles linked to sustainable water resource use
during drought disasters in the coastal zone.
Introduction
Sustainable and reliable potable freshwater supply has been a limiting feature for human
occupation of landscapes historically, especially at the coast (Rishworth et al., 2020a; Rosinger,
2021), and will restrict emergent economic development in the future in all regions (Garrick et al.,
2017). Behavioural changes surrounding how water use is minimised across all sectors, how its
Cambridge Prisms: Coastal
Futures
www.cambridge.org/cft
Review
Cite this article: Dodd C and Rishworth GM
(2023). Coastal urban reliance on groundwater
during drought cycles: Opportunities, threats
and state of knowledge. Cambridge Prisms:
Coastal Futures,1, e11, 1–13
https://doi.org/10.1017/cft.2022.11
Received: 01 August 2022
Revised: 16 December 2022
Accepted: 16 December 2022
Keywords:
coastal aquifers; hydrological drought; urban
water supply; water crisis; integrated
groundwater management
Author for correspondence:
Carla Dodd,
Email: cdodd49@gmail.com
© The Author(s), 2023. Published by Cambridge
University Press. This is an Open Access article,
distributed under the terms of the Creative
Commons Attribution licence (http://
creativecommons.org/licenses/by/4.0), which
permits unrestricted re-use, distribution and
reproduction, provided the original article is
properly cited.

lifecycle is optimised to restrict wastage and how alternative water
supplies are utilised present solutions to managing water scarcity.
The latter of which includes sources such as groundwater which
have been used for millennia as dependable freshwater sources but
are not always well-understood locally in terms of hydrological
cycling.
Many groundwater reserves have been unsustainably managed
and are at risk of depletion globally (Famiglietti and Ferguson, 2021;
Jasechko and Perrone, 2021), although in other areas, such as Sub-
Saharan Africa, groundwater resources are generally underdeveloped
(Cobbing, 2020). Coastal aquifers (groundwater resources situated
along the coast and linked to or influenced by coastal or marine
ecosystems) face additional threats driven by climate change and
urbanisation compared to their inland counterparts (e.g., Boretti and
Rosa, 2019). These include sea-level rise (SLR), reduced recharge,
storm surges (including hurricanes and cyclones), coastal flooding
and erosion (Ferguson and Gleeson, 2012;Barbier,2015;Erostate
et al., 2020; Chesnaux et al., 2021). Coastal urbanisation is more
concentrated compared to inland, and this is exacerbated by the
increasing trend of coastal tourism, migration and population
growth to coastal areas (Neumann et al., 2015; Parisi et al., 2018).
As a result, increased anthropogenic pressures at the coast may
accelerate pollution, over-abstraction and compaction of coastal
aquifers causing water quality degradation, seawater intrusion and
land subsidence (e.g., Ferguson and Gleeson, 2012; Chang et al., 2019;
Han and Currell, 2022). Thus, the urbanisation of continental coast-
lines and islands coupled with increasing threats of climate change,
coastal vulnerability and associated extreme events such as droughts
adds pressure to an already constrained system of freshwater supply
(Boretti and Rosa, 2019;seeFigure 1). Therefore management and
supply of alternative resources such as groundwater are becoming
more crucial under these circumstances along coastlines.
Droughts may be classified according to their origin (e.g.,
meteorological drought) or effect (e.g., agricultural, hydrological).
A prolonged meteorological drought, which is caused by below-
average precipitation for a given time, may develop into an
agricultural or hydrological drought. An agricultural drought is
characterised by deficits in soil moisture and often has negative
effects on crop yields. On the other hand, a hydrological drought
can be sub-categorised (e.g., groundwater drought) and refers to
reductions in water levels and reservoir storage (e.g., Schreiner-
McGraw and Ajami, 2021). In this assessment, “drought”generally
refers to a persistent meteorological drought, which has manifested
as a hydrological drought.
This study evaluates and reviews recent literature discussing the
drivers, pressures, states and impacts influencing coastal aquifer
functioning and potential management responses, especially in a
drought context.
Perspective
Groundwater use and risks in the coastal zone
As surface water resources become more limited and progressively
degraded, coastal groundwater abstraction has increased markedly
and is expected to increase even further under predicted drier
conditions in the future with more frequent, long-lasting and intense
droughts anticipated (e.g., Parisi et al., 2018; Erostate et al., 2020).
Figure 1. Schematic portraying the issues, opportunities and dynamics related to ground water and its management at the coast in light of future climate change, sustainable
development and drought threats. This demonstrates key issues relevant to coastal urban reliance on groundwater. Based on and adapted from graphical abstract of Han and
Currell (2022).
2 Carla Dodd and Gavin M. Rishworth

These climatic changes (higher temperatures, evapotranspiration
and decreased precipitation) will further affect groundwater
resources through decreased recharge (e.g., Stigter et al., 2014; Unsal
et al., 2014). The consequent impacts of declines in precipitation and
groundwater recharge are expected to be especially pronounced in
arid and semi-arid regionsand coastal regions, such as small islands,
that rely exclusively on rainwater harvesting (RWH) and ground-
water resources for water supply (e.g., Stigter et al., 2014; Unsal et al.,
2014; Kopsiaftis et al., 2017). Urbanisation may further decrease
groundwater recharge by the replacement of natural permeable
landscapes with hard, impermeable surfaces such as roads. These
urban surfaces linearly direct surface runoff to lenticwaterways more
effectively and therefore reduce infiltration opportunities (Han et al.,
2017; Lorenzo-Lacruz et al., 2017;Minnigetal.,2018;Frommen
et al., 2021). As such, this will contribute to the coastal squeeze of
coastal urban aquifers since these systems are wedged between
developed urban areas and rising seas (Vitousek et al., 2017).
Coastal threats and changes (e.g., SLR, storm surges, coastal
erosion and land subsidence) may occur simultaneously and/or
have knock-on effects leading to the occurrence or acceleration of
other pressures, usually intensified through urbanisation and devel-
opment. The onset of coastal pressures may, however, also be
triggered inland such as in the case of coastal erosion caused by
dam construction and the resultant changes in sediment source and
coastal sediment dynamics (e.g., Huang and Jin, 2018 and refer-
ences therein; Hzami et al., 2021). Coastal erosion may accelerate
land losses beyond those caused by SLR, or erosion may amplify the
effects of SLR, causing groundwater salinisation and reductions in
groundwater recharge (Baharuddin et al., 2018; Chesnaux et al.,
2021). This is of further concern since the decrease in groundwater
recharge may be a more important issue than SLR in flat low-lying
coastal areas (Parisi et al., 2018). Diminished groundwater dis-
charge levels due to declines in groundwater levels, whether
induced by abstraction or reductions in recharge, affect the rest
of the hydrologic system through decreased river baseflows, lake
and spring levels and ultimately result in landscape desiccation
(de Graaf et al., 2019). Furthermore, seaward erosion during storm
surges of coastal barriers (e.g., dunes), could result in breaching
events that may subsequently affect coastal aquifers kilometres
inland through vertical saltwater infiltration, with the delayed
recovery of the aquifers to a freshwater state lasting longer than
the storm event (Giambastiani et al., 2017; Elsayed et al., 2018).
Many coastal hazards, whether caused by anthropogenic or
climatic drivers, ultimately lead to coastal inundation (permanent)
or flooding (temporary) (sensu Flick et al., 2012) (hereafter collect-
ively referred to as submergence). Three types may be recognised,
namely: 1) marine submergence (surface submergence with marine
water through, e.g., tidal flooding, SLR, storm drain backflow); 2)
groundwater submergence (rise in groundwater levels as ground-
water is displaced in response to, e.g., SLR or wastewater network
leaks) and 3) precipitation-driven submergence (through large
rainfall events) (Han et al., 2017; Minnig et al., 2018; Habel et al.,
2020; Rahimi et al., 2020; Su et al., 2020; Frommen et al., 2021).
Furthermore, these submergence types may compound each other,
which can result in “snowballing”impacts compared to isolated
occurrences (Habel et al., 2020; Rahimi et al., 2020). For example,
modelled scenarios of the Oakland Flatlands, a low-lying coastal
area of California, USA indicate that SLR alone will not pose
significant threats to urban infrastructure, but coupled with
groundwater submergence more than 310 ha will be negatively
affected (Rahimi et al., 2020). Excluding the damage to coastal
infrastructure and threats to human life, submergence is usually
accompanied by groundwater salinisation through seawater intru-
sion and pollution of aquifers (Habel et al., 2020; Rakib et al., 2020),
which exacerbates the degradation of aquifers already under pres-
sure from over-abstraction and droughts by decreasing potability
(e.g., in California, USA –Rahimi et al., 2020).
Despite being identified as a hazard to drinking water supply
systems more than a century ago (Michael et al., 2017), seawater
intrusion and resultant groundwater salinisation are still the pri-
mary threat related to coastal groundwater abstraction (Chang
et al., 2019). The source of salinity, however, may be natural
(seawater trapped in pores from earlier sea level transgressions
and dissolution of evaporite minerals, e.g., halite and gypsum) or
anthropogenic (related to over-extraction and agricultural return
flow) (Nogueira et al., 2019).
Seawater intrusion triggered by over-abstraction from coastal
aquifers can result in both landward (lateral) and vertical (upconing)
intrusion (Parisi et al., 2018). Coastal aquifers are increasingly at risk
from bidirectional contamination as shorelines migrate landward
with SLR and seawater intrusion and with pollution pressure from
land sources –especially agricultural and industrial (Michael et al.,
2017; summarised in Figures 1 and 2). Not only does this leave
groundwater resource unsuitable for human use but may also have
dire effects on coastal groundwater-dependent ecosystems (GDEs)
such as estuaries, lagoons, microbialite ecosystems and coral reefs
(Michael et al., 2017; Erostate et al., 2020; Rishworth et al., 2020b).
These GDEs are also at risk of state changes or even localised
extinction as groundwater levels are reduced or depleted and
surface-groundwater connections are disturbed. This is especially
relevant in arid or semi-arid coastal regions with a Mediterranean
climate, such as those of the Mediterranean basin and the south-
western coasts of Australia, Chile, California (USA) and
South Africa (Erostate et al., 2020). For example, salinity increases
in the microbialite-bearing Lake Clifton,Western Australia has been
observed since the 1990s. This is attributed to hydrological changes
driven by increased evaporation and decreased freshwater recharge
to Lake Clifton due to lower rainfall, groundwater abstraction in the
catchment areas and the construction of the Dawesville Channel
(Forbes and Vogwill, 2016; Warden et al., 2019). The higher salinity,
in addition to higher nutrient loads, is thought to have contributed
to a change in the microbial communities of the Lake, although this
does not necessarily translate to unfavourable conditions for micro-
bialite formation (Gleeson et al., 2016; Warden et al., 2019). The
effects of coastal water crises associated with groundwater salinisa-
tion globally may be summarised as conflict of uses between differ-
ent water users, wetland degradation, loss of crops, soil salinisation,
aridity, desertification and health issues (Parisi et al., 2018).
Furthermore, an emerging issue related to coastal groundwater
use is that of land subsidence and increased rates of relative SLR.
Natural landsubsidence of unconsolidated coastal sediments may be
accelerated by groundwater abstraction and urban development (see
below) and when combined with SLR and storm surges increase the
risk of seawaterintrusion into aquifers(Huang and Jin, 2018; Shirzaei
et al., 2021). Thiswill likely be exacerbated by drought conditions and
climate change. For example, model predictions underestimated land
subsidence in Yuanchang, Taiwan for the 2011–2012 period due to
increased groundwater pumping and reduced recharge during a dry
spell (Shirzaei et al., 2021). Relative SLR may pose a further risk to
coastal communities through groundwater flooding as unconfined
coastal aquifer levels will increase in response to sea level changes. It
is anticipated that water tables will either rise the same amount as sea
levels, where the additional groundwater can be accommodated, or
that the seawater intrusion will displace fresh groundwater, which
Cambridge Prisms: Coastal Futures 3

will discharge through original or new drainage systems (Befus et al.,
2020). For instance, coastal cities in Indonesia such as Semarang and
the Indonesian capital, Jakarta, are already experiencing the impacts
of land subsidence, coastal flooding during high tides and seawater
intrusion into aquifers (Abidin et al., 2015). This is partly due to the
demand for clean water outstripping surface water supply and
consequently the over-abstraction of groundwater to make up the
water supply deficit. Furthermore, urban development results in the
consolidation of sediments, increased surface runoff and decreased
groundwater recharge during rain events (Abidin et al., 2015;Pra-
mono, 2021; Taftazani et al., 2022). As a result, more than 80% of
groundwater samples from monitoring wells in northern Jakarta’s
groundwater basins exceed the recommended limits of salinity for
human consumption (Taftazani et al., 2022) and aquifers of entire
small islands are brackish or saline during dry seasons (Cahyadi,
2018).
Water quality degradation through anthropogenic and geogenic
pollution linked to increased water demand will further decrease
both surface and groundwater resources available for use at the
coast (e.g., Han and Currell, 2022). Urban aquifers are often asso-
ciated with pollution by emerging contaminants (e.g., pharmaceut-
icals and personal care products) and nutrient loading through
aqua- and agricultural activities, septic tanks, cemeteries and land-
fills (e.g., Lapworth et al., 2017; Boretti and Rosa, 2019; Burri et al.,
2019; Preziosi et al., 2019; Han and Currell, 2022). Slow-moving
aquifers and groundwater-fed rivers can release persistent organic
pollutants, such as anthropogenic per- and polyfluoroalkyl sub-
stances (PFAS), into marine environments long after its initial
release into the upstream environment (Sunderland et al., 2019;
Zhang et al., 2019a; Ruyle et al., 2021). Coastal point sources include
airports and ports (Li et al., 2022), while wastewater treatment
works, landfills and urban runoff also contribute to the transpor-
tation of PFAS (e.g., Hepburn et al., 2019; Cui et al., 2020). The
residence time of certain PFAS species may be further lengthened
by adsorption driven by tidal and salinity effects in the coastal zone
resulting in bioaccumulation in benthic organisms. In fact, seafood
is considered the primary human dietary exposure pathway to
PFAS compounds (Sunderland et al., 2019; Zhang et al., 2019a).
However, PFAS adsorption in coastal sandy aquifers may act as a
natural attenuation process (Li et al., 2022). These pathways are
therefore not fully understood but present an emerging research
priority and likely threat to coastal aquifers.
Management of water crises and groundwater use in the
coastal zone
When facing a water scarcity crisis, there are two management strat-
egies that can be employed, namely, supply management (i.e., increase
the available resources) and demand management (i.e., decrease water
use) (Lam et al., 2016). Although droughts occur naturally as a part of
the hydrological cycle, their frequency and intensity are increasing
under climate change scenarios. Emergency procedures during
droughts often mean that more wells are drilled, which may accelerate
aquifer depletion (Petersen-Perlman et al., 2022) and are therefore
likely not a sustainable solution (Figure 2). Water supply management
in urbanised coastal areas during droughts cannot rely on a “wait ‘til
it rains’” approach (Parisi et al., 2018). Rather, socio-ecological and
collaborative strategic management of water supply is required to
effectively manage water resources and prevent aquifer salinisation
(Parisi et al., 2018).
Some problems related to drought and groundwater manage-
ment are caused by governance misalignments (Petersen-Perlman
Figure 2. Driver-Pressure-State-Impact-Response (DPSIR) framework regarding groundwater use in coastal urban areas. Responses with a cross indicate there are large negative
implications reported with the use thereof.
4 Carla Dodd and Gavin M. Rishworth

et al., 2022). For example, the Salento aquifer in Italy is experiencing
groundwater salinisation and despite droughts in the area, salinity
effects are not considered in groundwater management practices
and no systematic monitoring takes place (Parisi et al., 2018).
Furthermore, although the national legislation of selected water-
scarce and drought-prone countries protects GDEs (e.g., Australia)
or advocates for environmental requirements (e.g., South Africa)
(Rohde et al., 2017; Erostate et al., 2020), monitoring of actual
regional groundwater abstraction is often lacking or management
regulations are not met (Bekesi et al., 2009; Erostate et al., 2020;
Kent et al., 2020; Robertson, 2020). That being said, in Australia
progress in GDE management has been achieved through compre-
hensive adaptive management frameworks that are informed by
legislation as well as scientific and technological advancements
coupled with regional datasets (Rohde et al., 2017).
Other accounts of successful groundwater management strat-
egies and implementation of policy during droughts in coastal
urban areas include the employment of managed aquifer recharge
(MAR).ThemainapplicationsofMARaretoincreasewater
storage and provide a water resource more resilient to climate
change, while water quality is also often improved by pollutant
attenuation during infiltration (e.g., Kazakis, 2018; Dillon et al.,
2020). This strategy of MAR is often implemented in regions with
seasonal precipitation cycles to mitigate surface-water shortages
during the dry months, such as in India (Glendenning and
Vervoort, 2011). A small-scale example, stemming from ancestral
knowledge, is that of coastal Ecuador using constructed “tapes”or
artisanal dikes to increase groundwater storage during periods of
water scarcity (Carrión et al., 2018). Furthermore, MAR has been
widely applied to prevent seawater intrusion into coastal aquifers
by maintaining groundwater levels and driving the saltwater
wedge seaward or decreasing aquifer salinisation through blend-
ing with lower salinity surface water, such as treated wastewater
(e.g., Masciopinto, 2013; Bachtouli and Comte, 2019; Alam et al.,
2021). For example, in Jakarta, Indonesia the effects of land
subsidence and seawater intrusion are combatted by increasing
groundwater recharge through the construction of water traps and
drainage reservoirs and increasing vegetation cover (Pramono,
2021). However, if not managed properly MAR can introduce
pollutants and pathogens into groundwater resources (e.g., Raicy
et al., 2012; Casanova et al., 2016; Alam et al., 2021). For example, a
review of studies using stormwater for MAR revealed that dis-
solved organic carbon, selected metals and E. coli are effectively
removed from recharge water during the infiltration process, but
that trace organics and Enterococcus bacteria were still present
(Alam et al., 2021).
Policy-driven groundwater demand management has been suc-
cessful in the drought-prone Central Coast region of California
(USA) where groundwater supplies 90% of the drinking water
(Langridge and Van Schmidt, 2020). In response to the severe
drought of 2012–2016 experienced in the area, a Sustainable
Groundwater Management Act (SGMA) was passed to mitigate
groundwater storage loss (Langridge and Van Schmidt, 2020).
Although the SGMA does not take into account groundwater losses
prior to the passing of the Act, mitigation strategies related to
groundwater include using flood flows rather than groundwater
for MAR and irrigation purposes and developing local groundwater
drought reserves to avoid irreversible groundwater losses during
drought periods. The latter is achieved through, for example, using
agricultural return flow for recharge, of which a portion is then
available to farmers for irrigation during droughts (Langridge and
Van Schmidt, 2020).
Another example of a coastal urban area that is prone to severe
droughts and is predicted to be impacted by increased temperatures
and decreases in surface water resources is that of Perth, Australia
(population of 1.6 million) (Serrao-Neumann et al., 2017). During
the Millennium Drought (1996–2010) in southern Australia, it was
proposed that the water supply in Perth be further augmented from
the West Yarragadee aquifer, however, desalination of seawater was
implemented instead (Lam et al., 2016). During 2013–2014 about
42% of Perth’s water supply was from groundwater, while desalin-
ation provided 39% of supply. More recently, MAR is also being
considered to supplement water supply. Interestingly, the urban
water use in Perth remained lower post-drought than that of pre-
drought use, despite increased urban population, suggesting the
value of social awareness and behavioural shifts in managing water
scarcity. However, the energy use of the water supply system has
doubled (Lam et al., 2016).
Several other interventions have been successful in decreasing/
managing pollution and salinisation risks such as alternating/sea-
sonal pumping regimes, pumping limits and hard-engineering
approaches (e.g., subsurface dams, cut-off walls, semi-pervious
subsurface barriers; Chang et al., 2019), rehabilitation of dune
systems and the removal of alien trees (Giambastiani et al., 2017),
and identifying sources of groundwater-borne pollutants (Michael
et al., 2017). Some of these interventions are not without adverse
effects. For example, sub-surface dams can result in the accumula-
tion of landward pollutants behind the barrier, increase inland soil
salinisation and prevent groundwater discharge to the coastal zone
(Chang et al., 2019).
Depending on the scale of a drought event, groundwater demand
may be effectively managed by a range of options in the short to
medium term (Parisi et al., 2018), including alternative sources such
as RWH, the recycling of water and desalinisation. RWH is perhaps
the most universally applied method (Wurthmann, 2019) and can be
used at a household (e.g., tank) or catchment (impoundment or for
groundwater recharge) scale. For example, in Bangladesh, rainfall in
coastal areas is higher than inland and RWH in urban areas is
considered a suitable resource for drinking and residential purposes
(Islam et al., 2015). Conversely in Florida USA, RWH used for
residential outdoor irrigation could meet more than half the future
water demand under a high population growth scenario and may
perform better than other alternative sources such as reclaimed
water (Wurthmann, 2019). The long-term reliability of RWH is,
however, a concern since harvesting units are usually designed for
current precipitation patterns and future rainfall variability is immi-
nent due to climate change (Islam et al., 2015). In addition, at a
catchment scale RWH through impoundments could reduce
streamflow (Glendenning and Vervoort, 2011).
The recycling of storm- and/or wastewater for irrigation pur-
poses (Lavrnićet al., 2017), MAR (Alam et al., 2021 and references
therein) and even potable use (Lee and Tan, 2016) is a climate-
resilient water source. However, integration into water supply
systems is not as readily accepted due to public perceptions as well
as contamination and health risks (Gu et al., 2015; Garcia-Cuerva
et al., 2016). The use of reclaimed water for irrigation is considered a
suitable option since the water already contains nutrients and
therefore reduced fertiliser application is required (Lavrnićet al.,
2017). Other innovations such as altered agricultural water supply
systems (e.g., drip irrigation) or drought-resistant crops will be
essential for future agricultural practices (Oude Essink, 2001;
Lavrnićet al., 2017). Finally, desalination of brackish and seawater
is a viable option for coastal urban water supply but is often
expensive, energy-intensive and can have negative impacts on
Cambridge Prisms: Coastal Futures 5

coastal biodiversity through brine by-products (Singh et al., 2021
and see below case study).
Other management solutions include aquifer protection,
through for example financial incentives to reduce groundwater
abstraction and associated negative impacts. For instance, ground-
water abstraction tax has been implemented in Jakarta and it was
suggested that groundwater abstraction is spatially controlled (e.g.,
limiting abstraction to volcanic lithologies rather than subsidence-
prone sedimentary lithologies) (Taftazani et al., 2022).
Drought management successes have also been achieved through
adjusted water tariffs, integrated management of water resources,
conservative water-use practices (residents and tourists), as well as
the re-definition of management and policy systems (Parisi et al.,
2018; Singh et al., 2021 and the references therein). Furthermore, it
is crucial that groundwater modelling of water supply systems is
included into drought policies and long-term planning. These models
must take into account the present status of the aquifer(s) and be
informed by baseline monitoring assessments and extraction rate
information. However, a lack of monitoring and unmetered ground-
water use poses a major challenge for groundwater management
modelling (Hunter et al., 2016; Keir et al., 2019; Rochford et al.,
2022). Recent innovations in groundwater modelling have meant that
hydrological deficits can be distinguished from the effects of over-
abstraction and that aquifer responses can be linked to decision-
making (Petersen-Perlman et al., 2022). In addition, advances have
been made in the understanding of aquifer vulnerability in terms of
both volume and quality during droughts related to pumping and
pollution. However, the effects of augmentation strategies such as
MAR need to be incorporated more sufficiently (Petersen-Perlman
et al., 2022). Finally, it is essential that issues related to groundwater
use during droughts are effectively communicated and that local
stakeholders participate in managing groundwater resources
(Petersen-Perlman et al., 2022).
Both land and water management need to be proactive rather
than reactive to ensure that coastal groundwater resources are
managed adequately for the economy, health and environment
(Michael et al., 2017). Integrated coastal groundwater management
needs to take into account geological, hydrological and biogeo-
chemical complexity whilst also accounting for human complexity
in terms of economic, cultural and decision-making factors
(Michael et al., 2017). Thus, environmental and hydrological con-
nections between cities and the surrounding areas, future uncer-
tainties related to water availability, and a holistic landscape view is
required in planning (Serrao-Neumann et al., 2017; see Figure 2).
Case study: A comparison of two South African metropolitan
municipalities
Several South African urban areas are struggling to meet water
supply requirements due to growing demand, decreased rainfall
and poor governance (e.g., Olivier and Xu, 2019; Mahlalela et al.,
2020; Pamla et al., 2021). Two large, coastal metropolitan munici-
palities, the City of Cape Town (CoCT) in the Western Cape
province and the Nelson Mandela Bay Metropolitan Municipality
(NMBM) in the Eastern Cape province (see Figure 3), have faced or
are facing “Day Zero”where the water supply systems are expected
to fail (e.g., Pamla et al., 2021).
Although all droughts do not necessarily evolve into water crises
(Wolski, 2018), both municipalities rely primarily on rain-fed
surface water resources (e.g., Luker and Harris, 2019; NMBM,
2022), which makes them vulnerable to reduction in precipitation
in the short to medium term (see Table 1). This is especially
Figure 3. Locations of City of Cape Town and Nelson Mandela Bay Metropolitan Municipalities and their major supply dams. Urban population size, unconstrained and constrained
surface water supplies available to the municipalities and water use during the drought are also indicated.
6 Carla Dodd and Gavin M. Rishworth

pertinent given the coastal setting of these municipalities and the
additional climate change challenges that they face (e.g., SLR,
coastal flooding and erosion) (e.g., Bornman et al., 2016; Williams
and Lück-Vogel, 2020; Dube et al., 2022). Consequently, ground-
water reserves are viable alternative freshwater supplies for the
metros (e.g., Miller et al., 2017; NMBM, 2022) to mitigate the
unsustainable water supply scenario presented by anticipated cli-
mate change effects and population growth.
Large-scale, systematic development of groundwater resources
for water supply in the CoCT and NMBM was generally limited to
times of drought (e.g., 1982–1994 drought in NMBM; Lomberg
et al., 1996) or where no other water supply options were available
(e.g., rural/small town settings) as groundwater was/is predomin-
antly considered an emergency resource (Cobbing et al., 2015;
Luker and Harris, 2019). Furthermore, groundwater resource
development for private use was likely driven by socio-economic
factors rather than hydrogeological features (Lomberg et al., 1996).
This is despite both municipalities having access to suitable aquifers
for potential development or water supply diversification (e.g.,
Pietersen and Parsons, 2002). The recent interest in large-scale
abstraction from regional and local aquifers, therefore, provides
the municipalities with a comparatively “clean slate”and an oppor-
tunity to learn from management successes and failures of other
coastal urban areas reliant on groundwater (as discussed above).
This case study highlights the groundwater resources available for
development in the CoCT and NMBM, municipal groundwater
schemes already in place, anticipated/experienced climate change
threats and management priorities for sustainable groundwater
development in these coastal cities.
For both the CoCT and NMBM an important regional aquifer is
that of the Palaeozoic Table Mountain Group aquifer (TMGA),
which predominantly consists of quartzitic sandstones that gener-
ally yield good quality water (e.g., Rosewarne, 2002;Figure 4).
Water is stored in pore space formed by secondary deformation
associated with faults, bedding planes and joints (e.g., Pietersen and
Parsons, 2002). Shallow, unconfined Cenozoic intergranular aqui-
fers, where water is stored in the primary pore space between
interstices of sand grains of fluvial, marine and aeolian deposits,
also occur in both municipalities (e.g., Saayman and Adams, 2002;
Goedhart et al., 2004; Jovanovic et al., 2017; DWS, 2022). The most
important for development being the Atlantis and Cape Flats
Aquifers (CFA) for the CoCT (Sandveld Group) (e.g., Enqvist
and Ziervogel, 2019; Ziervogel, 2019) and the Algoa Group aquifer
for the NMBM (e.g., Goedhart et al., 2004; DWA, 2010;Figure 4).
In NMBM, about 10–15% of supply for the town of Kariega
(formerly Uitenhage) is derived from the Uitenhage Springs
groundwater (Baron, 2000; DWA, 2010), which equates to about
1.6% of the NMBM water demand (NMBM, 2022). According to
the municipal water outlook reports, it is expected that a further
35 Ml of groundwater can be sustainably abstracted per day from
Table 1. Direct comparison of the case-study scenarios which highlight the relevant regional context and management responses to local drought-related water
scarcity
City of Cape Town Nelson Mandela Bay Metropolitan Municipality
Meteorology 400–2,000 mm MAP; Mediterranean (Ziervogel, 2019) 400–1,500 mm MAP; Temperate (NMBM, 2022)
Urban population size 4 million (Ziervogel, 2019) 1.5 million (NMBM, 2022)
Supply system Western Cape Water Supply System Algoa Water Supply System
Surface water reservoir infrastructure 14 dams; 6 main: Theewaterskloof, Voëlvlei, Berg
River, Wemmershoek, Upper and Lower Steenbras
(Wolski, 2018; Ziervogel, 2019)
9 dams; 5 main: Churchill, Impofu, Kouga, Loerie,
Groendal (NMBM, 2022)
Total surface water resource capacity ~900,000 Ml (Ziervogel, 2019) ~286,000 Ml (bulk water storage); 210 Ml/day
(Nooitgedagt) (DWS, 2018; NMBM, 2022)
Unconstrained available capacity
to municipality
~888 Ml/day (Van Zyl and Jooste, 2022) 400 Ml/day (190 Ml/day bulk water storage; 210 Ml/day
Nooitgedagt) (NMBM, 2022)
Constrained available capacity
to municipality
~488 Ml/day (Van Zyl and Jooste, 2022) 221.5 Ml/day (NMBM, 2022)
Local aquifers TMGA, CFA, Atlantis Aquifer (Ziervogel, 2019) TMGA; Algoa Group Aquifer (Goedhart et al., 2004)
Pre-drought water demand
(Feburary–March 2015)
1,200 Ml/day (Enqvist and Ziervogel, 2019) ~315 Ml/day (NMBM, 2022)
Drought water demand 500 Ml/day (Enqvist and Ziervogel, 2019) 276 Ml/day (NMBM, 2022)
Pre-drought groundwater use 1.5% (Luker and Harris, 2019); that is, ~18 Ml/day 5.92 Ml/day (NMBM, 2022)
Groundwater development during drought 20 Ml/day (Atlantis); 10 Ml/day (TMGA); 19 Ml/day
(CFA) (Ziervogel, 2019)
Anticipated 41.31 Ml/day (NMBM, 2022)
Desalination 16 Ml/day (CoCT, 2018; Ziervogel, 2019) Anticipated minimum 15 Ml/day (NMBM, 2022)
Management response to drought Water restrictions, increased water tariffs, punitive
charges, consumer behaviour and public awareness
campaign, pressure reduction, flow limiting metres,
groundwater development, desalination, water
re-use and alien invasive clearing (CoCT, 2018)
Water restrictions, increased water tariffs, punitive
charges, consumer behaviour and public awareness
campaign, pressure reduction, flow limiting metres,
groundwater development, desalination, water re-use,
alien invasive clearing, stand pipes and water tanks –
informal settlements, leak repairs, upgrade Impofu dam
barge, upgrade Linton Grange WTW, Nooitgedagt/Coega
Low Level Scheme (NMBM, 2022)
Note: Historical usage of groundwater and potential mitigation and alleviating opportunities provided by groundwater are also given.
Cambridge Prisms: Coastal Futures 7

five production well-fields that are being developed in response to
the current drought (NMBM, 2022). However, the local NMBM
aquifers are poorly understood in terms of inflow sources, recharge,
and residence time with scant available hydrogeological literature
(e.g., Goedhart et al., 2004; Murray et al., 2008). There is, therefore,
a risk that groundwater abstraction during droughts may be a
short-term solution should abstraction exceed recharge and aquifer
storage. Furthermore, the ecological implications of reduced
groundwater discharge (e.g., related to GDEs) are either uncertain
or have not been well-considered (e.g., Rishworth et al., 2020b).
Pre-drought groundwater use accounted for merely 1.5% of the
CoCT water supply and was largely limited to the town of Atlantis
(Luker and Harris, 2019). However, groundwater development
became an important augmentation strategy during the recent
drought for the CoCT, as well as the surrounding agricultural
regions, due to the lack of upstream surface water releases and
agricultural releases (Watson et al., 2022). Large-scale groundwater
projects prompted by the 2015–2018 drought included the prop-
osition to abstract 100 Ml/day from the TMGA and CFA and
further MAR schemes in the West Coast area (Luker and Harris,
2019; Zhang et al., 2019b). However, the former was considered
unfeasible by experts given the timeline (De Villiers, 2017; Luker
and Harris, 2019) with production well-fields still under develop-
ment years later (Blake et al., 2021; McGibbon et al., 2021) and
operational issues due to over-recharge and groundwater flooding
were previously encountered with the latter (Zhang et al., 2019b).
South Africa boasts the most reported cases of MAR in Africa of
which one of the most notable is the Atlantis scheme (e.g., Dillon
et al., 2020; Ebrahim et al., 2020). The scheme has operated since the
1970s (Bugan et al., 2016; LaVanchy et al., 2021) and uses storm-
water, domestic and industrial effluent for recharge to meet domes-
tic demand and simultaneously prevent seawater intrusion.
Stormwater and treated domestic wastewater are used in the main
recharge basins for the former, while industrial wastewater is
released into coastal recharge basins for the latter before discharging
into the Atlantic Ocean (Bugan et al., 2016). It is estimated that up to
30% of the Atlantis Town water is supplied from the recharge
scheme and future expansion has been proposed to increase water
resilience (Ebrahim et al., 2020). Although a largely successful
example of MAR, groundwater abstraction from the scheme
decreased following access to surface water resources and issues
related to borehole clogging and turbidity (Bugan et al., 2016).
Decreased abstraction and continued MAR resulted in groundwater
level increases, which pose threats in terms of groundwater pollu-
tion, flooding risks and ecosystem functioning (Bugan et al., 2016;
LaVanchy et al., 2021). The management of the scheme has since
been improved including actions to prevent iron clogging through
ozone injection, monitoring of groundwater levels and quality, and
addressing illegal discharges (Bugan et al., 2016). More recently, it
has been proposed that MAR is also implemented in the CFA (e.g.,
Hay et al., 2018; LaVanchy et al., 2021). This would entail that
stormwater and domestic wastewater were intercepted before dis-
charging to the ocean. Treated water would be directly injected into
the aquifer or recharge could occur through infiltration at wetland
areas (Hay et al., 2018). Similarly to the Atlantis scheme, this would
increase groundwater storage and a total sustainable yield of
18,000 Ml/a is suggested as feasible (Mauck and Winter, 2021).
Threats of groundwater integration into the water supplysystems
of the CoCT and NMBM include overexploitation and drawdown.
Furthermore, contamination of groundwater resources is a concern
(Rosewarne, 2002;LukerandHarris,2019). For example, seawater
intrusion in the coastal suburb of Summerstrand in the NMBM was
already reported in the 1990s (Lomberg et al., 1996) and continues to
be a concern associated with groundwater use in coastal regions
Figure 4. Location and regional extent of the lithological groups of interest for groundwater development in the City of Cape Town (CoCT) and Nelson Mandela Bay Metropolitan
(NMBM) municipal areas.
8 Carla Dodd and Gavin M. Rishworth

(Rosewarne, 2002). Developed coastal areas and estuaries within the
CoCT and NMBM are also amongst the most threatened areas in
South Africa in terms of SLR and the anticipated increase in storm
surges (e.g., Theron and Rossouw, 2008;Bornmanetal.,2016;Raw
et al., 2021; Allison et al., 2022). Resultant saltwater intrusion into
estuaries is expected to be amplified by decreased freshwater inflows
and altered sediment sources (e.g., Bornman et al., 2016) and aquifer
salinisation and higher groundwater levels are expected (Williams
and Lück-Vogel, 2020). Coastal flooding has also increased in the
CoCT region and is driven by high-intensity rainfall events and
larger tidal amplitudes (Dube et al., 2022). Already certain suburbs
(e.g., Strand) are subject to frequent coastal flooding and erosion
(Williams and Lück-Vogel, 2020) and SLR has caused significant
economic losses and damage to infrastructure, threatening tourism
activities and World Heritage sites (e.g., Robben Island) (Dube et al.,
2021), while dune erosion leaves coastal aquifers vulnerable to
seawater intrusion (LaVanchy et al., 2021).
In addition, biofouling and iron-related clogging of boreholes
may occur (e.g., Maclear, 2001; Fortuin et al., 2004; Luker and
Harris, 2019). Geogenic contamination of the TMGA by high
concentrations of radionuclides and metals is a further concern
(Mohuba et al., 2022). Other issues include a lack of institutional
knowledge, the human capacity for maintenance and monitoring,
and the absence of regulatory requirements and implementation of
restrictions for domestic groundwater use in South Africa (e.g.,
Wright and Jacobs, 2016; Luker and Harris, 2019).
Given the coastal setting of the CoCT and NMBM, it is worth
noting that desalination of seawater has been proposed as a poten-
tial long-term water supply augmentation strategy for both muni-
cipalities that is resilient in terms of climate change (Blersch and Du
Plessis, 2017; NMBM, 2022). For example, this has been employed
as a drought management option at another coastal South African
town, Mossel Bay (Sorensen, 2017). However, desalination is an
expensive, energy-intensive option with a high carbon footprint
compared to other more conventional sources (Gobin et al., 2019).
The high electricity costs and frequent electricity outages
(“loadshedding”) in South Africa is problematic for both desalin-
ation as well as groundwater abstraction and pumping, although
this may be mitigated through renewable energy supply in the
future (Sorensen, 2017; Hattingh, 2022).
From a demand management perspective, both the CoCT and
NMBM effectively used several measures to curb water use/loss
from the surface water supply systems. These measures included
water restrictions, stepped water tariffs, water metres and fixing of
water leaks, to mention a few (e.g., Enqvist and Ziervogel, 2019;
Ziervogel, 2019; NMBM, 2022). However, mismanagement of
water supply systems, ageing infrastructure and maintenance issues
contributed to the water crises. For instance, alien invasive vegeta-
tion significantly reduces surface runoff and although clearing
programmes (viz. Working for Water) exist and is listed as a
demand management strategy (NMBM, 2022), it is essential that
routine follow-up clearing is conducted in catchments (Enqvist and
Ziervogel, 2019; Ziervogel, 2019; Holden et al., 2022). Clearing
invasives from dune systems may also have a positive impact on
shallow unconfined aquifers, as indicated elsewhere along the coast
(Görgens and van Wilgen, 2004). Of concern is that in contrast to
surface water resources, systematic management of groundwater
resources, restriction of use at a municipal level and detailed
information on private groundwater use is lacking (Ziervogel,
2019). For example, the number of registered private boreholes in
the CoCT increased from 1,500 to 23,000 within the calendar year
of 2017 (Visser, 2018).
Therefore, sustainable groundwater abstraction in the CoCT
and NMBM requires comprehensive groundwater monitoring,
especially related to baseline datasets for future groundwater
scheme locations, and offers the opportunity for knowledge
building and the advancement of groundwater management
institutions (Luker and Harris, 2019). Routine long-term moni-
toring of groundwater resources and GDEs is a crucial manage-
ment priority. The removal of alien vegetation from catchments
and dune systems is likely to be beneficial for both surface- and
groundwater resources. Furthermore, previous economic valu-
ations of water remediation programs in the case study areas (e.g.,
Hosking and Du Preez, 2002)shouldbeupdatedandincludedin
a comprehensive cost–benefit analysis of the development of
groundwater monitoring programs for the CoCT and NMBM
water supply systems. Finally, sustainable groundwater use may
be achieved by the conjunctive use of groundwater resources.
This is especially needed during supply deficiencies, surface water
infrastructure failures (e.g., blocked/damaged pipes) and con-
tamination issues (Luker and Harris, 2019), such as experienced
in the Voëlvlei Dam, CoCT (Luker and Harris, 2019) and in the
NMBM (Capa, 2022).
Strategies and solutions linked to managing hydrological vari-
ability relevant to metropolitan cities on dynamic coastlines could
include inland MAR schemes that effectively would then allow for
higher dependency on accessible groundwater aquifers at the coast
while minimising risks of saltwater intrusion and groundwater
submergence or land subsidence (Dillon et al., 2020; Alam et al.,
2021; Pramono, 2021). An urgent research priority should be
directed towards quantifying the inland point sources of ground-
water recharge, both spatially and volumetrically (e.g., Hall et al.,
2020), as well as through legislative policies regulating and moni-
toring the quantities of residential groundwater abstraction (e.g.,
Langridge and Van Schmidt, 2020)–one cannot manage what one
does not measure. Such research is now underway regionally in
South Africa. Knowledge of source-to-sink hydrological connect-
ivity of coastal groundwater cycles would allow interventions to be
more effectively directed, such as the removal of water-intensive
alien invasive plants in the areas that most directly recharge coastal
groundwater-fed aquifers to both improve coastal supply and
quality.
Concluding remarks
Coastal aquifers are under increasing pressure due to a myriad of
threats affecting the quality and quantity of fresh groundwater
resources, often associated with anthropogenic drivers and magni-
fied by climate change and climate extremes. In addition to the
“standard”climate change driven impacts, hazards specific to
coastal aquifers include SLR, storm surges, seawater intrusion,
coastal flooding and erosion. Furthermore, although not unique
to coastal aquifers, pressures may be aggravated in coastal areas
such as land subsidence due to the unconsolidated nature of sedi-
ment and pollution risks since coasts are a nexus for inland and
marine contaminants.
Remediation of over-abstracted and contaminated groundwater
resources is difficult, costly and in some cases irreversible. Proactive
protection of aquifers is therefore crucial for sustainable manage-
ment of urban water supply. These issues are arguably compounded
in coastal cities because of increased water demand due to rapid
population growth and urbanisation coupled with decreased
recharge. On the other hand, several management solutions and
Cambridge Prisms: Coastal Futures 9

strategies, such as MAR and desalination are practical and poten-
tially the most cost-effective in coastal settings.
To emphasise the issues and management imperatives related to
coastal urban water supply and the mitigating role of groundwater
during drought-related crises, the investigation of the water supply
systems of two South African metropolitan municipalities provides
examples of large cities (population > 1 million) experiencing water
crises due to anthropogenic (e.g., urbanisation, socio-economic
development) and environmental (e.g., decreased precipitation
and runoff, increased evapotranspiration) pressures. We specific-
ally chose to highlight this scenario given the regional climate-
related future predictions related to increased drought severity.
Furthermore, these coastal municipalities rely primarily on surface
water resources and therefore have the opportunity to develop
integrated water resource management through incorporating
alternative water sources, including easily accessible groundwater,
into water supply systems. However, the unique coastal pressures
facing these groundwater aquifers (e.g., saltwater intrusion and
reduced recharge to GDEs) necessitate unique solutions and strat-
egies. Both municipalities implemented several interventions to
decrease water demand and increase water supply and although
several of these were effective to postpone “Day Zero,”it may not be
enough to prevent water supply failure in the future.
Future research priorities related to groundwater at the coast
need to account for the following most especially: (1) updating and
implementing existing policies to develop integrated groundwater
management; (2) long-term groundwater monitoring in terms of
quantity and quality for both urban and natural systems, especially
related to salinity effects and (3) communication of issues, such as
land subsidence, seawater intrusion and coastal ecosystem degrad-
ation through, for example, decreased discharge related to large-
scale groundwater use during droughts, to seek a balance between
human-driven water needs and the sustainability of coastal aquifers
both in terms of supply quantity and quality.
Open peer review. To view the open peer review materials for this article,
please visit http://doi.org/10.1017/cft.2022.11.
Acknowledgements. Ms Larika van Vuu ren is thanked for provi ding admin-
istrative support during the writing of this manuscript. The Editors, Dr
Matthew Forbes and an anonymous reviewer are thanked for their valuable
input and fee dback, which especial ly helped strengthen the coastal focus of t he
manuscript.
Author contributions. Conceptualisation: C.D., G.M.R.; Funding acquisition:
G.M.R.; Investigation: C.D.; Visualisation: C.D., G.M.R.; Writing –original
draft: C.D., G.M.R.
Financial support. This research is funded by the Water Research Commis-
sion (WRC) through grant number 2022/2023-00833, “Coastal seeps and
groundwater connectivity”awarded to G.M.R. C.D. is funded by the National
Research Foundation of South Africa and the Deutscher Akademischer Aus-
tausch Diens (UID: 131592). Opinions expressed herein are those of the authors
and not necessarily also those of the funders.
Competing interest. The authors declare none.
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