Urban aquifers are a valuable resource of freshwater for
cities, however, their quality is degraded due to the presence of organic
contaminants of emerging concern (CECs). The effects of organic CECs are
largely unknown, but there is evidence that they pose a risk for human
health, soil, plants and animals. Organic CECs are naturally degraded in
aquifers and their degradation rates depend on the physico-chemical
properties, i.e., redox conditions and groundwater temperature. Some
anthropogenic activities, like low-enthalpy geothermal energy (LEGE), may
modify subsurface physico-chemical conditions altering the behaviour of
organic CECs. LEGE is a renewable and carbon-free energy that allows
obtaining cooling and heating energy. The utilization of LEGE is currently
growing and it is expected that in a near future the density of LEGE systems
will increase. LEGE modifies the groundwater temperature and in some
situations the redox state (i.e., if the dissolved oxygen increases when
groundwater is returned to the aquifer as a result of a poorly design),
thus, it is of paramount importance to determine the impact of LEGE related
activities on the behaviour of organic CECs. The behaviour of organic CECs
under the influence of LEGE is investigated by means of
thermo-hydro-chemical numerical modelling. Simulation output shows that LEGE
activities have the potential to modify the degradation rates of organic
CECs, and thus, their concentrations in aquifers. In the simulated scenario,
the concentration of the chosen CEC decreases by the 77 % at the
downgradient boundary of the model. The results of this study have
significant implications for predicting the behaviour of organic CECs in
urban aquifers and suggest specific changes in the design of LEGE facilities
aiming to improve the quality of urban groundwater by boosting in-situ
attenuation mechanisms.
Introduction
In the current context of climate change and urban sprawl, it is necessary
to take advantage of urban groundwater bodies to cover the increasing water
demand. However, urban groundwater bodies are usually highly polluted by a
wide range of anthropogenic contaminants, such as organic contaminants of
emerging concern (CECs) and their transformation products (TPs)
(Lapworth et al., 2019). Organic CECs comprise natural and
anthropogenic substances of organic nature such as pharmaceuticals and
personal care products. These pollutants, which are not removed in
wastewater treatment plants, reach groundwater bodies through different
recharge sources such as water leakage from sewer and septic systems,
seepage from rivers or artificial recharge activities (Jurado et al., 2020). Organic CECs
pose human health and ecological risks (Arnold et al., 2014;
Tran et al., 2013) and for this reason they deserve being investigated to
assure the safe usage of urban groundwater resources.
Organic CECs are degraded or transformed in aquifers by microbial activity (Greskowiak et al., 2017), which is reflected in
lower concentrations found in aquifers in comparison to rivers (Jurado et al., 2021).
Physico-chemical conditions of aquifers play an important role in the
degradation of organic CECs since it is a redox-dependent process (Burke et al., 2014) that varies with
temperature (Burke et al., 2017).
Groundwater temperature is not a commonly considered parameter when
investigating the degradation of organic CECs since it is almost constant
during the whole year, except close of surface water bodies that may
influence the groundwater temperature. However, it can be altered due to
anthropogenic activities, like the use of the subsurface for energetic
purposes by means of low-enthalpy geothermal energy (LEGE). LEGE is a
carbon-free and renewable energy that is used for cooling and heating
buildings and has a great potential to supply a large portion of the heating
and cooling demand. In fact, its use is growing continuously in recent years
(García-Gil et al., 2020). LEGE
provides cooling and heating energy by exchanging the temperature of
groundwater with the temperature of buildings through heat exchangers
(Cui et al., 2014). Cooling energy is
obtained by transferring the building heat to groundwater, resulting in an
increase in groundwater temperature. In contrast, for heating, the
groundwater heat is transferred to the building, decreasing the temperature
of groundwater. Consequently, as a result of LEGE activities, the
groundwater temperature varies.
The hypothesis of this study is that groundwater temperature variations,
produced by LEGE facilities, impact the behaviour of organic CECs by
increasing or decreasing their degradation/transformation rates. The
investigation of this hypothesis is not only important to better understand
the behaviour of organic CECs, but also to evaluate the possibility of
designing LEGE facilities to improve the quality of groundwater by enhancing
the degradation capacity of aquifers against organic CECs. This hypothesis
is deduced from previous investigations in the context of river bank
filtration and polluted aquifers by chlorinated organic compounds (COCs). On
the one hand, investigations developed in the context of river bank
filtration show that the seasonal variations of the river temperature modify
the evolution of organic CECs in the aquifer (Barkow et
al., 2021). On the other hand, groundwater temperature variations, induced
by LEGE facilities, seem to improve the dichlorination capacity of aquifers
polluted by industrial compounds (Hoekstra
et al., 2020; Pellegrini et al., 2019). Despite of these relevant
investigations, the influence of LEGE on the behaviour of organic CECs at
the aquifer scale remains to be investigated.
Thus, our main objective is to assess the potential impact of LEGE on the
behaviour of organic CECs present in groundwater bodies. The problem is
addressed numerically with a synthetic thermo-hydro-chemical numerical model
that simulates an aquifer polluted by phenazone and in which a LEGE facility
used for cooling purposes is implemented.
MethodologyProblem statement
The problem consists in a 20 m thick fully-saturated and homogeneous sandy
aquifer made of unconsolidated materials. The aquifer has a length of 2000 m
and a width of 1000 m (Fig. 1a).
(a) General view of the problem. (b) Detailed view of the modelled
area including the boundary conditions (BC).
The LEGE facility, which is located in the middle of the considered aquifer,
is of the groundwater heat pump (GWHP) type (Lee et al., 2006) and consists of two wells,
the production and the injection well. The production well is located
upgradient while the injection one is located downgradient. It is assumed a
negligible conductive heat loss to overlying or underlying strata (i.e.,
groundwater temperature is not dissipated thought surrounding strata).
Pumping and injection rates (Q) are assumed to be constant and equal to 432 m3 d-1 (i.e., 5 L s-1). The injection rate has been chosen according with
the transmissivity to slightly modify the hydraulic gradient and minimize
the risk of thermal breakthrough. The wells are separated 290 m to avoid
thermal breakthrough during the simulated time. The minimum distance between
wells to avoid thermal breakthrough has been computed according to Banks (2009) considering hydraulic parameters shown in Table 1 and a hydraulic
gradient of 0.005.
Aquifer parameters. Values are chosen according to those typical
for homogenous sandy aquifers (K: Domenico and Schwartz, 1998;
θ: Woessner and Poeter, 2020; DL: Schulze-Makuch, 2005). DT is chosen
ten times lower than DL, which is common practice
(Zech et al., 2019) and Dm is chosen according
to the reference values from the German Engineer Association guidelines for
thermal use of the underground (VDI, 2019).
It is assumed that the simulated LEGE facility is only used for cooling and
the obtained cooling potential is constant. Though LEGE facilities are
commonly used for cooling and heating purposes depending on the season, they
can be also used only for cooling. Continuous cooling energy can be used by
neighbourhood factories or other infrastructures that need to be
continuously refrigerated, such as data centres containing information
technology equipment or high-performance computing systems (Zurmuhl et al., 2019). We
consider that the temperature of the pumped groundwater is increased by
15 ∘C to obtain cooling energy. Then, considering a
groundwater temperature under unperturbed conditions of 20 ∘C and that water is injected at 35 ∘C, the thermal potential
(PGW) of the facility is 313 800 W, i.e., 225 936 kWh per month.
PGW is computed as follows:
PGW=SVCwatQΔT,
where SVCwat is the is the volumetric heat capacity of water at
20 ∘C (4180 Jkg-1 K-1) and ΔT is the
temperature difference between the wells.
It is considered that the modelled aquifer is polluted by phenazone, which
is homogenously distributed at a concentration of 1×10-9 mol L-1. Phenazone is an analgesic drug that is commonly reported in urban
aquifers (Reddersen
et al., 2002; Jurado et al., 2021). Its degradation is oxygen and
temperature dependent (Greskowiak et al., 2006) and its
behaviour is similar to other redox and temperature dependent organic CECs.
The aquifer is characterized with aerobic conditions and has a moderate
value of dissolved oxygen (2 mg L-1). This value of dissolved oxygen is chosen
according with reported data from Barcelona's aquifers
(Jurado et al., 2013).
Parameters used to simulate the dynamics of phenazone.
The numerical model is developed with the code PHT3D (Prommer et al., 2003). Theoretically,
if the distance between the production and injection wells is correctly
chosen, the hydraulic head at the middle of the domain should be constant
and equal to that under initial conditions. Thus, taking advantage of the
symmetry of the problem, only 1/4 of the problem is modelled
(Fig. 1b). The simulated domain covers, in the flow direction, from the
centre of the aquifer to the downgradient boundary, and, in the
perpendicular direction to the flow, from the centre until the southern
boundary. Twenty years is the total simulated time and the time steps are of
30 and 1 d for the flow and reactive transport problems, respectively. The
difference between flow and transport time steps are due to convergence
issues. Smaller time steps for the flow are not required since the pumping
and injection rates are constant, however, short time steps are used for the
transport to avoid convergence problems. A regular mesh consisting of one
layer divided into 5000 elements is used.
Two types of flow boundary conditions (BCs) are implemented. Specified head
BCs are implemented at the upgradient and downgradient boundaries to reach a
hydraulic gradient under unperturbed conditions of 0.005, while specified
flow BCs are adopted in the injection well. No-flow BCs are assumed in the
other boundaries. Concerning the transport BCs, concentrations of
1×10-9 mol L-1 of phenazone and 2 mg L-1 of oxygen (O2) are
prescribed to the upgradient boundary and in the injection well. The
concentration of phenazone is in the same range of magnitude than reported
data in previous works (Barkow et al., 2021). In
addition, the required concentration of phenazone in the whole domain is
reached by implementing a constant input mass of phenazone of 8.4×10-7 mg d-1 per square meter. This BC represents the mechanisms driving
the recharge of phenazone into groundwater since the recharge of organic
CECs is a diffuse process (Drew and
Hotzl, 1999; Wolf et al., 2012).
Distribution of normalized concentration of phenazone (a) and
groundwater temperature (b) at the end of the simulated period (i.e., 20
years). (c) Evolution of normalized concentration of phenazone (blue dashed
line) and groundwater temperature (red continuous line) at the observation
point located 500 m away from the injection well in the downgradient
direction.
The degradation rate of phenazone (rPhena) is modelled considering
Monod kinetics (Lu et al.,
1999; Greskowiak et al., 2006) as follows:
rPhena=-λPhenaMXCPhenaCO2KPhenaO2+CO2fT,
where CPhena is the concentration of phenazone, λPhenaMX is the maximum degradation rate constant of phenazone,
KPhenaO2 is the Monod half-saturation constant of phenazone,
CO2 is O2 concentration, and fT is a temperature-dependent
function. Equation (2) approximates the degradation/transformation of phenazone
as a 1st order degradation. In addition, it includes a term accounting
for the concentration of O2, which is needed because the degradation
rate of phenazone decreases with low concentration of O2. Finally, the
impact of groundwater temperature is considering by implementing the
additional factor fT. A normalized form of the Arrhenius equation is
used to define fT as:
fT=βAe-EART,
where A is a pre-exponential factor, EA is the activation energy, R is the
gas constant, T the temperature in Kelvin and β is a factor to
normalize the equation to 1 when the water temperature ranges between 35 and
40 ∘C. The required parameters to apply Eqs. (2) and (3)
have been derived from bibliography and are presented in Table 2. A and
EA are derived from Barkow et al. (2021), and
λMAX and KPhenaO2 from Greskowiak
et al. (2006).
Results and Discussion
Figure 2 shows the concentration of phenazone normalized by its initial
concentration (A) after 20 years of simulation, the distribution of
groundwater temperature (B) after 20 years of simulation and the evolution
of phenazone (dashed blue line) and groundwater temperature (red continuous
line) in an observation point located 500 m away from the injection well (C). Groundwater temperature increases up to 35 ∘C around
the injection well and on the downgradient side. As a result, the
degradation rate of phenazone in the area with high groundwater temperature
increases according to Eqs. (2) and (3). The concentration of phenazone in
the area affected by the LEGE facility decreases by 77 % in comparison to
the initial concentration that does not vary far from the injection point
where the aquifer is not affected by the injected water. Unlike the
distribution of groundwater temperature, an area with high concentration of
phenazone is observed around the injection well. This high concentration is
the result of injecting the groundwater pumped in the production well, where
the concentration of phenazone is high. In the top left corner of the model,
it is possible to observe that the concentration of phenazone decreases.
This occurs because in this corner, groundwater is not flowing as a result
of the head distribution, and thus, groundwater with high concentration of
phenazone does not inflow the model through this area. In addition, despite
it is not observed in the figure because of colour bar limitations, the
temperature is a little bit higher than that under unperturbed conditions
that increases the degradation rate of phenazone. The measurements at the
observation point (Fig. 2c) show how the concentration of phenazone
decreases and the temperature increases during the simulated period. The
concentration of phenazone starts to decrease before that the hot
groundwater reaches the observation point. This behaviour is the consequence
of the higher retardation factor for the heat transport than for phenazone
transport. Note that no retardation factor was applied to phenazone because
its sorption can be considered negligible (Greskowiak et
al., 2006).
Our results are complementary to the conclusions of Hartog (2011), who assessed the influence of groundwater temperature induced by
geothermal energy on groundwater quality. Hartog (2011) concluded
that the impact of the temperature is relatively low if groundwater
temperature does not exceed 25 ∘C. Here we probe that,
certainly, the groundwater quality is substantially improved by reaching
35 ∘C around the injection well. Despite this is the first
work analysing the impact of geothermal energy on organic CECs, other
authors have assessed the behaviour of chlorinated compounds under the
influence of geothermal facilities obtaining similar results. Among others,
Ni et al. (2016) demonstrated through
laboratory experiments that the degradation velocity increases (up to 13
times in their experiments) in comparison with natural conditions by the
high groundwater temperature occurred around geothermal facilities.
Beyer et al. (2016) also reached the same conclusions
but in base of a numerical model. However, unlike us,
Beyer et al. (2016) considered a geothermal facility of
the closed-loop type where heat exchangers are introduced into the ground
and groundwater is not pumped. This suggests that the concentration of
organic CECs would also decrease under the influence of a geothermal
facility of the closed-loop type.
Conclusions
This paper aims at investigating whether the thermal impact generated by
LEGE facilities in aquifers can modify the behaviour of organic CECs. The
synthetic numerical model considers the presence of phenazone, which is
commonly reported in urban aquifers, and a LEGE system of the GWHP type that
is used only for cooling purposes. A synthetic numerical model is used to
allow the generalisation of the results. Obviously, site-specific numerical
models will be needed when quantifying the influence of LEGE on CECs at real
cases. The main conclusions that can be drawn from the results are:
The thermal impact produced by LEGE facilities on aquifers can considerably
modify the behaviour of organic CECs. Thus, it is needed to consider the
influence of these facilities when investigating the fate and evolution of
organic CECs in groundwater bodies, especially in urban areas, where LEGE
are expected to grow significantly in the coming years.
This investigation only considers a LEGE facility used for cooling. Thus,
additional studies to investigate the behaviour of organic CECs under the
influence of a LEGE facility used for heating and cooling will be needed.
Theoretically, the degradation rate should decrease when using LEGE for
heating. However, the exponential relation between degradation rate and
temperature suggests that, overall, LEGE facilities used for heating and
cooling will also enhance the degradation capacity of aquifers against
organic CECs.
Simulation results suggest the possibility of designing LEGE facilities
specifically to increase the degradation capacity of aquifers against
organic contaminants which, consequently, improve the groundwater quality.
This possibility worths to be deeply investigated in future research.
Data availability
Data relative to the results and input files for the numerical model are available at 10.17605/OSF.IO/JXEAW (Pujades, 2022).
Author contributions
EP wrote the original draft of the paper and obtained the needed funding. EP, VV and AJ contributed to the conceptualization of the investigation. EP, MT, VV and AJ defined the methodology to reach the objectives. LS, MT, RC, ON, EVS, VV, AJ contributed to writing – review and editing the paper.
Competing interests
At least one of the (co-)authors is a guest member of the editorial board of Advances in Geosciences for the special issue “Quality and quantity issues in urban hydrogeology (EGU2022 HS8.2.8 session)”. The peer-review process was guided by an independent editor, and the authors also have no other competing interests to declare.
Disclaimer
Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
Special issue statement
This article is part of the special issue “Quality and quantity issues in urban hydrogeology (EGU2022 HS8.2.8 session)”. It is a result of the EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022.
Acknowledgements
Estanislao Pujades, Laura Scheiber and Rotman Criollo would like to thank the Ibero-American Programme of Science and technology for development (CYTED-Programa de ciencia y tecnologia para el desarrollo) under project 719RT0585. Rotman Criollo also thanks the support from the Balearic Island Government through the Margalida Comas postdoctoral fellowship programme (grant no. PD/036/2020). Finally, Victor Vilarrasa acknowledges support from the Agencia Estatal de Investigación (10.13039/501100011033), Spanish Ministry of Science and Innovation through the grant HydroPore (grant no. PID2019-106887GB-C32).
Financial support
This research has been supported by the Barcelona city council through the Award for Scientific Research into Urban Challenges in the City of Barcelona 2020 (20S08708) and by the grants PID2021-128995OA-I00 funded by MCIN/AEI/10.13039/501100011033T and FEDER “one way to make Europe”, CEX2018-000794-S funded by MCIN/AEI/10.13039/501100011033 and RYC2020-029225-I funded by MCIN/AEI/10.13039/501100011033 and “ESF Investing in your future”.
We acknowledge support of the publication fee by the CSIC Open Access Publication Support Initiative through its Unit of Information Resources for Research (URICI).
Review statement
This paper was edited by Miao Jing and reviewed by Oluwaseun Olabode, Laura Balzani, and Jannis Epting.
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