The 21st century water quality challenges for managed aquifer recharge: towards a risk-based regulatory approach

Abstract

Sustained environmental and human health protection is threatened by ~350,000 chemicals available in global markets, plus new biological entities including coronaviruses. These water-quality hazards challenge the proponents of managed aquifer recharge (MAR) who seek to ensure the integrity of groundwater. A risk-based regulatory framework accounting for groundwater quality changes, adoption in subsurface attenuation zones, and use of advanced monitoring methods is required to support confidence in the sustainability of MAR.

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Hydrogeology Journal
https://doi.org/10.1007/s10040-022-02543-z
ESSAY
The 21st century water quality challenges formanaged aquifer
recharge: towardsarisk‑based regulatory approach
YanZheng1 · JoanneVanderzalm2· NielsHartog3· EnriqueFernándezEscalante4· CatalinStefan5
Received: 27 February 2022 / Accepted: 11 September 2022
© The Author(s), under exclusive licence to International Association of Hydrogeologists 2022
Abstract
Sustained environmental and human health protection is threatened by ~350,000 chemicals available in global markets, plus new bio-
logical entities including coronaviruses. These water-quality hazards challenge the proponents of managed aquifer recharge (MAR) who
seek to ensure the integrity of groundwater. A risk-based regulatory framework accounting for groundwater quality changes, adoption in
subsurface attenuation zones, and use of advanced monitoring methods is required to support confidence in the sustainability of MAR.
Keywords Water-resources management· Managed aquifer recharge (MAR)· Groundwater sustainability· Waterquality risks·
Attenuation zone
Introduction
Managed aquifer recharge (MAR) is the purposeful recharge of
water to aquifers for subsequent recovery or environmental ben-
efit (IAH 2022). A rigorous environmental and social sustaina-
bility assessment of 28 schemes from 21 countries demonstrates
that MAR is a sustainable technology (Zheng etal. 2021). This
nature-based engineering approach is poised to play an increas-
ingly significant role in climate change adaptation through aug-
menting water supply and environmental flows, and recycling
treated wastewater. This essay calls upon hydrogeologists world-
wide to rise to the 21st-century water-quality challenges using
MAR, to maintain the integrity of groundwater resources and to
meet humanity’s demand for good quality freshwater.
Here it is argued that “novel entities” (NE), defined as “new
substances, new forms of existing substances and modified life
forms” (Steffen etal. 2015), need to be considered. These NEs
include “chemicals and other new types of engineered materials
or organisms not previously known to the Earth system as well
as naturally occurring elements (for example, heavy metals)
mobilized by anthropogenic activities” (Steffen etal. 2015).
Considering NEs means that proponents of MAR must go
beyond managing risks associated with known legacy pollutants,
such as hydrocarbons, pesticides, and disinfection by-products,
which can amount to several hundred regulated water quality
parameters (Escalante etal. 2020). It also requires addressing not
(yet) regulated, and sometimes novel (unknown) water quality
threats. Clearly, the capacity to manage current, emerging, and
unforeseen water quality risks is critical and relies upon chemical
and biological reactions to “purify” purposefully recharged water
within a subsurface attenuation zone, a concept originating in
Australia (Fig.1). To gain regulatory approval for this attenuation
zone, MAR practitioners have had to demonstrate, using
laboratory and field monitoring, the aquifer’s treatment capacity
and protection of the aquifer’s groundwater environmental values
beyond the attenuation zone. However, regulators may still be
inclined to regard the subsurface environment as “pristine”
and which should not be “disturbed” by any means. In reality,
the interaction of “unmanaged” recharge with a wide range of
anthropogenic activities has led to widespread groundwater
quality decline. This call to action begins with a historical
perspective on water quality issues frequently encountered
in MAR. Then, a resolution to tackle this challenge to ensure
sustained MAR implementation globally is discussed.
This article is part of the topical collection “International Year of
Groundwater”
* Yan Zheng
yan.zheng@sustech.edu.cn
1 School ofEnvironmental Science andEngineering, Southern
University ofScience andTechnology, Shenzhen518055, China
2 CSIRO Land andWater, Waite Road, Urrbrae,
SouthAustralia5064, Australia
3 KWR Water Research Institute, Groningenhaven 7, 3433, PE,
Nieuwegein, theNetherlands
4 Grupo Tragsa, Maldonado 58, 28006Madrid, Spain
5 Research Group INOWAS, Technische Universität Dresden,
01062Dresden, Germany
Historical perspective
An account of 60 years of global progress of MAR estimated
that purposeful recharge has reached 10 km3/year, ~2.4% of

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groundwater extraction in countries reporting MAR, or ~1.0%
of global groundwater extraction (Dillon etal. 2019). A global
inventory of MAR, including 1,136 pilot and full-scale MAR
schemes from 60 countries (Fig.2, Stefan and Ansems 2018)
found that the influent water source, the main objective of
the scheme, and the final use of recovered water were well
reported (96, 82 and 73% of the total number of cases, respec-
tively). Although a detailed assessment of water quality (over
100 considered) was available in less than 5% of cases, water
quality changes were mentioned in many papers, especially in
conference papers and specific technical reports.
The important role of water quality investigations in MAR
is illustrated by a search of the Science Citation Index (SCI)
Expanded database (period from year 1900 to 12 February
2022) using combined topics of MAR and artificial recharge
(AR, as it was widely used in the past), with and without ‘water
quality’ as a topic. Just above one-third of publications, or 118
out of 391 papers, included water quality, and this proportion
remained fairly constant through the years. This is consistent
with presentations made at recent International Symposiums
on Managed Aquifer Recharge (ISMAR, in 2016 and 2019)
where ~34% mentioned water quality (125 out of 371 papers).
Presenters at ISMAR conferences (many MAR practitioners
do not publish SCI papers) acknowledge that water quality is
one aspect to be considered during MAR planning, construc-
tion, and operation. Studies aimed at improving understanding
of processes regulating water quality and clogging, managing
potential degradation or enhancing treatment, are pursued with
vigor. Comprehensive laboratory and field-scale experiments
which provide sufficient data for reactive transport modelling
(e.g., (Prommer and Stuyfzand 2005)) have been invaluable in
elucidating the controlling processes and developing manage-
ment strategies as required. This advancement has led to a recent
focus on strategies to optimize water quality treatment, such as
through advanced pre-treatment or by a combination of MAR
types (Hellauer etal. 2018), incorporation of reactive barriers
(Valhondo etal. 2020), or manipulation of the subsurface redox
zones (Bartak etal. 2017).
Water quality investigations during MAR projects serve mul-
tiple aims. While monitoring for regulatory compliance is a basic
starting point, it is not sufficient to adequately manage water qual-
ity. Here inorganic arsenic is used to illustrate the importance
of having a good understanding of hydrogeochemical processes
and their potential impact on MAR operations, with the ability to
make prediction a plus. Arsenic is such a highly toxic chemical
that even the regulatory limit of 10 μg/L adopted by most coun-
tries is not entirely protective of public health. In Florida (USA),
injecting oxygenated Tampa City supply water into the Suwannee
Limestone of the Upper Floridan aquifer containing pyrite (Price
and Pichler 2006) resulted in pyrite-oxidation-driven arsenic
release, with recovered water arsenic concentrations frequently
exceeding 10 μg/L and rising to as high as 130 μg/L (Jones and
Pichler 2007). Recharging a reduced, As-rich coastal aquifer in
Khulna, Bangladesh, with pond water resulted in arsenic concen-
trations in recovered water of >100 μg/L (Sultana etal. 2015) and
was attributed to reductive dissolution of As-bearing Fe-oxyhy-
droxide. A recent critical review on mobilization of arsenic and
other naturally occurring contaminants during MAR (Fakhred-
dine etal. 2021) concludes that arsenic poses the most wide-
spread challenge at MAR sites due to its ubiquity in subsurface
sediments and toxicity at trace concentrations; other geogenic
contaminants of concern include iron, molybdenum, manganese,
chromium, and fluoride. Fortunately, the same review points out
many approaches to mitigate MAR-induced arsenic problems
in recovered water, but these need process understanding and
predictive capability to ensure such risks are managed appropri-
ately. A key step in prediction is an early stage hydrogeochemical
investigation to characterize the aquifer and source of water for
recharge for conceptual understanding of geochemical reactions
and their potential impact (or risk).
Furthermore, risk-based management is essential for the
future of MAR (Imig etal. 2022), to ensure the protection of
public and environmental health, while also fully utilizing the
potential of MAR to provide natural treatment and facilitate recy-
cling and reuse (Fig.1). Increasing reliance on multiple source
Fig. 1 A schematic diagram illustrating how, with the example of an
infiltration pond, MAR has been used to purify purposefully recharged
water through a series of natural treatment processes occurring in the
unsaturated and saturated zones of an aquifer to facilitate the removal
of organic pollutants and pathogenic microbes. Here, it is recom-
mended that an attenuation zone (after NRMMC, EPHC, NHMRC
2009) is defined as an independent regulatory unit so that groundwater
quality beyond this zone is sustainably protected. Note that the diagram
is not to scale because the attenuation zone is usually confined beneath
the land owned by the MAR operator and normally <50–300 m
Fig. 2 Global inventory of MAR schemes presented as an online por-
tal with the database being continuously updated (IGRAC 2022)

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waters (e.g., agricultural return flow, urban storm water, and
reclaimed water) also expands complexity in water-quality-risk
management and regulations for MAR, making the designation
of an attenuation zone in MAR regulation ever more relevant
(Fig.1). Such complex and uncertain risks can be dealt with
through decades of experience in water quality improvement and
management in MAR, furthered by targeted research—for exam-
ple, a study that evaluates the die-off of plant pathogenic bacteria
when stormwater is used to recharge a brackish anoxic aquifer in
the Netherlands can enhance confidence in the recovered water’s
intended use for irrigation (Eisfeld etal. 2021). Knowledge of
biodegradation of trace organic chemicals or contaminants of
concern has been advanced through the application of genomic
markers to infer the prevailing trophic state of microbial com-
munities in a MAR scheme, and subsequently, predict favorable
conditions for removal (Filter etal. 2021). While it is understood
that microbially mediated processes are an important control
on water quality, and in particular, water quality improvement,
approaches are required to assess aquifer microbial communi-
ties, their potential to augment treatment, response to changing
geochemical conditions, and ultimately the sustainability of treat-
ment. Leveraging the natural treatment capacity, where available,
allows for the design of a sustainable treatment train and avoids
overuse of energy-intensive engineered pretreatment without
overtreating water prior to MAR—for example, a current envi-
ronmental challenge is the widespread use and environmental
impact of perfluoroalkyl and polyfluoroalkyl substances (PFAS).
The fate of PFAS in MAR is uncertain and thus pretreatment or
posttreatment technologies may be required to manage this risk
(Page etal. 2019). Considering MAR as a step in a treatment train
enables one to manage the complex topic of water quality when
MAR alone cannot provide sufficient treatment, fate cannot be
predicted, or where water quality degradation may occur.
The natural treatment processes endowed by storage in the
aquifer are credited for helping the public to overcome the
“yuck factor” associated with recycling treated wastewater for
drinking water supply (Alley and Alley 2022). Faced with the
unknowns and uncertainties of regulated and unregulated water
quality threats, the assumption that storage time mitigates risk,
especially of pathogens, biodegradable organic matter, and trace
organic chemicals, is likely to hold (NRMMC, EPHC, NHMRC
2009), although more research is warranted to determine the
time scale and environmental conditions (Hübner etal. 2022)
for complete mineralization, including mostly unknown bio-
transformation byproducts (Ma etal. 2021). This should be of
interest to many water banking authorities such as those in the
western US states. The Kern Water Bank in the USA, initiated
in the early 1970s, recharged 1.13 billion m3 through 44 km2 of
recharge/spreading basins between 1995 and 2000 to alluvial
fan deposits of the Kern River. Meillier etal. (2008) used dis-
solved chlorofluorocarbons (CFC-11 and CFC-12) to estimate
groundwater ages of the target alluvial aquifer, finding that the
youngest apparent ages (younger than 1985) were found in the
shallow wells in the northern and central sections of the study
area where MAR is usually performed. The recovered water
here is suitable for irrigation but needs further treatment if used
for drinking. With the new analytical capability regarding con-
taminants of emerging concerns and microbial genomes, water
banking authorities, out of their fiduciary duty, can expand their
monitoring programs to track the recharged water as it “ages”
in the aquifer. For instance, it would be desirable to understand
the storage time required under particular redox conditions to
completely mineralize the myriad of trace organic contaminants.
The way forward
To enhance climate resilience and other social, economic, and
environmental benefits of groundwater through MAR, water
quality threats from novel entities need to be addressed to
maintain resource integrity. The aforementioned water qual-
ity challenges can be approached from a risk-based perspec-
tive grounded by precautionary principles, developed over time
through practice to solve clogging issues, and overcome eco-
nomic and policy barriers (Megdal etal. 2015). Strengthening
institutional capacity for regulatory frameworks for water alloca-
tion, permit granting and water quality protection are especially
relevant. It is important to balance the need to protect ground-
water quality integrity (ecocentrism ethic) and the desire to use
the natural treatment ability of the aquifer to improve water
quality (anthropocentrism ethic). It is worth noting that when
it comes to groundwater recharge laws in the United States, a
communitarian ethic has been suggested to underpin regulatory
processes (Owen 2021). Debate is encouraged on how to arrive
at a sensible regulatory framework for MAR to manage water
quality risks. Here, the perspective grounded in a communitarian
ethic and the precautionary principle provides a starting point.
The Australian risk-based approach to MAR (NRMMC,
EPHC, NHMRC 2009) is a model that sustainably protects
groundwater quality, accounting for water quality changes, both
improvements and deteriorations in the subsurface, and can be
expanded geographically because many countries use the highly
prescriptive approach of measuring compliance against uniform
water quality parameters. In Europe, both the development and
application of a legislative framework for MAR have varied
among different countries, with current legislation ranging from
strict and uniform water quality requirements to site-specific
risk-based evaluation in the Netherlands (similar to Australia). A
soon-to-be-effective European Union Directive 2020/741 has set
minimum requirements for water quality, as well as monitoring
and provisions on risk management applications for agricultural
use of reclaimed water. A risk-based directive specific for MAR
to further expand water reuse and recycling is a logical next step
for the EU and any designated regulatory entities to consider.
The way forward clearly depends on regulations that value
and enable the sustained use of natural treatment capacity pro-
vided by MAR, seamlessly integrated into a treatment train with

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pretreatment or posttreatment technologies as required. Advanced
tools, including but not limited to real-time monitoring, data
assimilation, and reactive-transport modeling, are required to
predict the fate of chemicals and pathogens and to assess risks to
human health and aquifer integrity. Currently, the natural attenu-
ation or assimilatory capacity of aquifers has been relied upon for
the degradation of many organic pollutants. As such, one could
view this “attenuation zone” simultaneously as a subsurface natu-
ral treatment zone with a finite hydraulic retention time (Fig.1).
In addition to the determination of the hydraulic retention time,
the understanding of the fate of pathogenic organisms, includ-
ing attachment and inactivation and the variables that influence
these, is necessary. Surrogates that can be used for laboratory
and field verification, and genomic approaches for characterizing
the health of subsurface microbial communities, also provide a
broader perspective on the sustainability of microbial and trace
organic removal processes. The IAH-MAR Commission strives
to develop the body of scientific knowledge needed to have confi-
dence in enhancing the sustainable and beneficial use of aquifers
for humanity within the Earth’s safe operating space.
Acknowledgement We are grateful for the insightful comments pro-
vided by Dr. Peter Dillon and for discussions with Drs. John Cherry,
Bill Alley, Beth Parker, Junjiang Wang and Xiuyu Liang. Xiangshuan
Meng helped with drafting Fig.1.
Funding A DANIDA Fellowship 17-M08-GEU is acknowledged for
partial support.
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