Quantifying climate change impacts on future water resources and salinity transport in a high semi-arid watershed

High salinity mobilization and movement from salt-laden deposits in semi-arid landscapes impair soils and water resources worldwide. Semi-arid regions worldwide are expected to experience rising temperatures and lower precipitation, impacting water supply and spatio-temporal patterns of salinity loads and affecting downstream water quality. This study quantifies the impact of future climate on hydrologic fluxes and salt loads in the Gunnison River Watershed (GRW) (14,608 km2), Colorado, using the APEX-MODFLOW-Salt hydro-chemical watershed model and three different CMIP5 climate models projection downscaled by Multivariate Adaptive Constructed Analogs (MACA) for the period 2020-2099. The APEX-MODFLOW-Salt model accounts for the reactive transport of major salt ions (SO42-, Cl-, CO32-, HCO3-, Ca2+, Na+, Mg2+, and K+) to streams via surface runoff, rainfall erosional runoff, soil lateral flow, quick return flow and groundwater-stream exchange. Model results are analyzed for spatial and temporal trends in water yield and salt loading pathways. Although streamflow is primarily derived from surface runoff (65%), the predominant source of salt loads is the aquifer (73%) due to elevated concentrations of groundwater salt. Annual salt loading from the watershed is 582 Mkg, approximately 10% of the salt load in the Colorado River measured at Lee's Ferry, AZ. For future climate scenarios, annual salt loads from the watershed increased between 4.1% and 9.6% from the historical period due to increased salt loading from groundwater and quick return flow. From the results, applying the APEX-MODFLOW-Salt model with downscaled future climate forcings can be a helpful modeling framework for investigating hydrology and salt mobilization, transport, and export in historical and predictive settings for salt-affected watersheds.

Numerical analysis of surface hydrogeological water budget to estimate unconfined aquifers recharge

Under changing climate, groundwater resources are the main drivers of socioeconomic development and ecosystem sustainability. This study assessed the contribution of two adjacent watersheds, Muse Street (MS) and West Wood (WW), with low and high urban development, to the Memphis aquifer recharge process in central Jackson, Tennessee, USA. The numerical MODFLOW model was created using data from 2017 to 2019 and calibrated using reported water budget components derived from in-situ data. The calibrated MODFLOW model was then used to investigate the impact of high and low urban developments on the recharge rate. The hydraulic parameters and recharge rates were optimized by adjusting the groundwater level based on the observed water level using PEST. The stochastic modeling was also carried out using the Latin Hypercube approach to reduce the uncertainty. The calibration results were satisfactory, with RMSE of 0.124 and 0.63 obtained in the WW and MS watersheds, respectively, indicating accurate estimation of the input parameters, precisely the hydrodynamic coefficients. The study results indicate that, per unit area, the MS watershed contributes 119% more to recharge and 186% more to riverbed leakage compared to the WW watershed. However, regarding total recharge and riverbed leakage, the WW watershed contributed more than the MS watershed. The results of this study have enhanced the knowledge of the impact of urbanization on hydrology and the recharge process in watersheds with diverse land uses.

A preliminary approach based on numerical simulation modelling and evaluation of permeable reactive barrier for aquifer remediation susceptible to selenium contaminant

In this study, numerical groundwater modelling software (GMS) was applied for a 2D transient state predictive (flow and contaminant fate and transport) conceptual model for heavy metal (Selenium in this research) contaminated groundwater, Imamzadeh-Jafar Aquifer, Kohgiluyeh and Boyer-Ahmad Province, Iran. The performances of permeable reactive barrier (PRB) in pollutant removal in the contaminated aquifers were studied by helping the MODFLOW-MT3DMS model. The spatiotemporal distribution of Selenium (Se) contaminant over the aquifer was illustrated using the calibrated flow and contaminant model. According to the findings, the downward movement of Se has resulted in an unsafe and undesirable water quality status in the Imamzadeh-Jafar aquifer, which is supported by field data. The sensitivity analysis of PRB layouts, geometric features, and reactant material characteristics was conducted in groundwater remediation. The numerical model results illustrated that the PRB thickness, ranging from 10 to 500 m, manifested the drop in Se concentration approximately from 40 to 46%. The results shed light on the hydraulic conductivity variations of reactant materials have effects less than 0.5% in Se removals. Furthermore, the decay rate variations in the ranges from 0.0001 to 0.01 d^-1 could result in Se removal from 5 to 100%. According to studies, if the contaminant sources are prevented, in a) installation of PRB and b) not installation of PRB scenarios, the Imamzadeh-Jafar aquifer remediation will take 6 months and 84 months, respectively.

Evaluating the contribution of satellite-derived evapotranspiration in the calibration of numerical groundwater models in remote zones using the EEFlux tool

Assessment of groundwater resources is crucial for developing water management practices for its sustainable exploitation, both for current and future needs. Numerical models are useful tools for such purpose. However, the lack of continuous monitoring networks, mainly due to difficult access to some remote locations, poses a challenge in developing and calibrating groundwater models. Remote sensing offers an alternative for acquiring information on hydrological and climatic variables at multiple spatiotemporal scales that has the potential to strengthen groundwater modeling. The aim of this study is to develop a methodology that uses remote sensing products to support model calibration. With this aim, we used the Parameter Estimation software (PEST) to calibrate a hydrogeological model of an unexploited basin located in the arid Chilean Altiplano using observed groundwater levels and evapotranspiration (ET) derived from the Earth Engine Evapotranspiration Flux (EEFlux) tool as observations. Our results show that the best model calibration is achieved using both EEFlux-ET and heads as observations to calibrate the hydraulic properties (normalized root mean square error = 4.1 %). We analyzed the effect of EEFlux-ET on the calibration of these properties and found a direct effect on specific yield parameters, which regulate the fluctuations of the water table over time. Incorporating EEFlux-ET estimates in the calibration resulted in lower values of specific yield across the aquifer. Therefore, incorporating remotely sensed ET as observations in the calibration of the groundwater model contributes to a better simulation of the spatiotemporal head variations in the basin.

Numerical modeling of groundwater flow and nitrate transport using MODFLOW and MT3DMS in the Karaj alluvial aquifer, Iran

Nitrate is one of the most dangerous pollutants in groundwater, being regarded as a severe environmental hazard and a global-scale problem. The present study aims to analyze the groundwater flow and nitrate contamination in the Karaj unconfined aquifer, Central Iran. Simulation of the quantitative and qualitative status was performed using MODFLOW and MT3DMS. Sampling was done to model groundwater flow for eight seasonal time periods and nitrate pollution transport for four time periods (a total of 420 days), and calibration was carried out by trial-and-error method. Due to the predominance of advection term in the transport of nitrate pollution, the total variation diminishing method was used in transport modeling. The groundwater flow modeling showed a reservoir deficit of - 33 MCUM and 67 cm decrease in the water year 2016-2017 and 173 cm in the entire modeling period (2016-2018) in groundwater level. Also, the RMSE in the calibration and validation stages was obtained from 24.9 to 0.72 and 0.84, respectively. The nitrate contamination transport modeling indicated that there are three nitrate contamination concentration parts with more than safe concentration of nitrate (50 mg/l). The most pollution is in the urban areas in the east of the aquifer. The nitrate pollution is primarily anthropogenic due to industrial and especially domestic wastewater and then fertilizers used in agricultural activities. It can be predicted that with the full implementation of a municipal wastewater collection network, the nitrate pollution concentration in urban areas would reduce significantly to the range of 20 to 25 mg/l.

Modeling groundwater and surface water interaction: An overview of current status and future challenges

The interaction between surface water and groundwater constitutes a critical process to understand the quantitative and qualitative regime of dependent hydrosystems. A multi-scale approach combining cross-disciplinary techniques can considerably reduce uncertainties and provide an optimal understanding of groundwater and surface water exchanges. The simulation process constitutes the most effective tool for such analysis; however, its implementation requires a variety of data, a detailed analysis of the hydrosystem, and time to finalize a reliable solution. The results of the simulation process contribute to the raising of awareness for water protection and the application of better management strategies. Knowledge of models' parameters has great importance to ensure reliable results in the modeling process. In this study, a literature overview of modeling applications in groundwater - surface water interaction is provided. In this context, a comprehensive and holistic approach to groundwater and surface water simulation codes is here presented; results, case studies, and future challenges are also discussed. The main finding of the analysis highlights uncertainties and gaps in the modeling process due to the lack of high frequency and depth dependent field measurements. In many studies, authors underestimate the importance of the hydrogeological regime, and the discretization of hydraulic parameters is often lumped in a simplified manner. The modeling ethics in terms of data transparency and openness should be widely considered to improve the modeling results. The current study contributes to overcome common weaknesses of model applications, fulfils gaps in the existing literature, and highlights the importance of the modeling process in planning sustainable management of water resources.

An integrated approach for an impact assessment of the tank water and groundwater quality in Coimbatore region of South India: implication from anthropogenic activities

Groundwater in the Coimbatore region is very much essential for irrigation and human beings' day-to-day life activities. This research focuses on the impact of anthropogenic activities on the surface water quality of four tanks in Coimbatore city and its influences on disturbing the groundwater quality as well. Water quality is degrading as a result of encroachment and other anthropogenic activities. The disposal of municipal garbage and waste from other enterprises into the nearby tank degrades the quality of the water in the tanks. This pollutant-concentrated surface water, together with the leech organism, may percolate via the pore spaces between soil particles, where it will interact with groundwater. The quality of groundwater will be impacted as a result of this interaction. As a result, water quality metrics for both surface and groundwater have been assessed in this study, and spatial interpolation was performed using ArcGIS. The map of the spatial distribution of water quality was produced using ArcGIS. This spatially interpolated water quality map helped researchers understand how the quality of surface and groundwater changed over time. Visual MODFLOW and MT3D were used to simulate the groundwater flow. The MODFLOW program is used to simulate the direction of groundwater flow based on groundwater level and rainfall data. The output of MT3D allowed for accurate calculation of the pollutant movement's amount and direction. In this study, chloride was used as the pollutant transport parameter. A semi-structured interview has been done in the study region to find out more about the availability of drinking water, how waste is disposed of, and how diseases are spread among residents who live close to the tanks.

Simulation of ground water quality for noyyal river basin of Coimbatore city, Tamilnadu using MODFLOW

In recent years groundwater modelling has become a major part of many projects dealing with groundwater exploitation, protection and remediation. Coimbatore city is located along the Noyyal River which is grown in size and population on either side of the river and the dumping of wastes very close to the river banks, leads to pollution of both surface and groundwater. Study on the quality of groundwater along the Noyyal River in Coimbatore city of Tamilnadu, is necessary to safeguard the interest of the people with respect to quality and quantity of water. The objective of the research is to develop Groundwater quality models to suggest ways and means to contain and remediate the polluted groundwater under various conditions. Data related to the groundwater quality, rainfall and well log were collected from the reputed government departments and fifteen sample well locations are identified near the noyyal river basin. The quality parameters such as pH, TDS, EC, TH, chlorides, alkalinity, sodium, calcium, magnesium, sulphates and nitrates are considered for the study. Groundwater quality modelling is attempted by using MODFLOW/MT3DMS with different scenarios. This study revealed that in all the scenarios the center portion of study area is more affected i.e. from P·N.Palayam to Kalangal. The groundwater flow is moving towards and along the river flow. Hence the pollutants are moving easily from upstream to the downstream side. necessary measures has to be taken to control the groundwater contamination.

Sustainable groundwater management through an optimal water supply system using a coupled simulation-optimization approach

Meeting people's water supply needs in cities is an urgent challenge. Especially in coastal cities, surface water sources are usually inadequate and so groundwater is frequently over-exploited, resulting in its rapid decrease and intrusion of saltwater. Conjunctive use of surface water and groundwater is the best alternative approach to mitigate the overuse of groundwater through effective distribution of surface water sources. It requires numerous runs of the simulation-optimization model to select an optimal pattern of water distribution by keeping the groundwater levels under control. In this study, a new simulation-optimization model is developed using wavelet support vector regression (WSVR) and genetic algorithm (GA) to propose an optimal water distribution system to Visakhapatnam city on the East Coast of India, fulfilling the constraints of surface water quantity, aquifer pumping, and drawdown. Estimation of groundwater pumping and groundwater level variation in spatial context is challenging in urban environment. To overcome this, the calibrated modular finite-difference flow (MODFLOW) model for this study area has been used to prepare the spatial and temporal variation of model inputs such as groundwater pumping and groundwater levels. The WSVR-GA model's performance to reduce the groundwater pumping is evaluated in three distinct cases. The surface water resources from three sources are distributed to different wards in the city source-wise in case I and centralized in cases II and III, while the source-wise surface water constraints are limited to monthly in cases I and II and annual in case III. The WSVR-GA management model suggested ward-wise groundwater pumping restrictions, resulting in 9.57 MCM, 11.64 MCM, and 12.54 MCM increase in total groundwater storage capacity in cases I, II, and III respectively. Cases II and III offer 21% and 25%, respectively, more storage than case I. Thus, centralized distribution systems have increased the sustainability of groundwater supplies by preventing overdrafts caused by a lack of surface water resources. Validation of results using MODFLOW indicates a substantial rise in groundwater levels in the study area.

Assessing the future climate change, land use change, and abstraction impacts on groundwater resources in the Tak Special Economic Zone, Thailand

Groundwater is an important source of water supply in the Tak Special Economic Zone of Thailand. However, groundwater is under stress from climate change, land use change, and an increase in abstraction, affecting the groundwater level and its sustainability. Therefore, this study analyses the impact of these combined stresses on groundwater resources in the near, mid, and far future. Three Global Climate Models are used to project the future climate under SSP2-4.5 and SSP5-8.5 scenarios. According to the results, both maximum and minimum temperatures are likely to show similar increasing trends for both scenarios, with a rise of approximately 1 (1.5), 2 (3), and 3 (5) °C expected for SSP2-4.5 (SSP5-8.5) in each consecutive period. Annual rainfall is expected to continually increase in the future, with around 1500-1600 mm in rainfall (11ꟷ5.43% higher). Land use change is predicted for two scenarios: business as usual (BU) and rapid urbanisation (RU). The forest area is expected to increase to 30% (35%) coverage in 2090 for BU (RU) while agriculture is likely to reduce to 60% (50%) with the urban area increasing to 2.4% (7%). Water demand is predicted to increase in all future scenarios. The SWAT model is used to project recharge, which is likely to increase by 10-20% over time. The highest increase is predicted in the far future under SSP2 and RU scenarios. MODFLOW was used to project future groundwater resources, but due to the lack of consistent data, the time scale is reduced to yearly simulation. The results reveal that the groundwater level is expected to increase in the central part (urban area) of the study area and decrease along the boundary (agricultural area) of the aquifer. This research can aid policymakers and decision-makers in understanding the impact of multiple stressors and formulating adaptation strategies to manage groundwater resources in special economic zones.

The impact of a low-permeability upper layer on transient seawater intrusion in coastal aquifers

This paper provides a thorough investigation of the effect of a top low-permeability (TLK) layer on transient saltwater intrusion dynamics prompted by water table fluctuations and sea level rise. Laboratory experiments were conducted on a 2D-sandbox and numerical simulations were performed using the SEAWAT code. Four cases were investigated, including a homogeneous case and three cases, where the top layer thickness (W_top) was equal to 0.2H, 0.33H and 0.5H, respectively, where H was the aquifer thickness. The experimental and numerical results show that the toe length decreases linearly with increasing the thickness of the TLK layer. The results also suggest that lowering the permeability of the upper part of the aquifer causes faster saltwater removal process. The sensitivity analysis shows that decreasing the top layer permeability causes further reduction of the intrusion length. Nonetheless, the results evidence that this method yields relatively little reduction of the saline water intrusion length if the upper layer thickness is inferior or equal to a fifth of the total aquifer thickness, regardless of the permeability value of the top layer. The field-scale modelling results demonstrate that the performance of the TLK layer weakens noticeably as the hydraulic gradient decreases. The results show that the TLK layer achieved a maximum saltwater wedge reduction of 31% in the case where W_top = 0.75H, which means that lowering the permeability of three fourths of the aquifer thickness only induced a toe length reduction by nearly a third of its original length. In addition to providing a quantitative analysis of SWI dynamics in bi-layered coastal aquifers, this study questions the performance and practicality of the artificial reduction of the upper aquifer permeability as a countermeasure for seawater intrusion control.

Using MODFLOW and RT3D to simulate diffusion and reaction without discretizing low permeability zones

Low permeability zones (LPZs) are major sources of groundwater contamination after active remediation to remove pollutants in adjacent high permeability zones (HPZs). Slow back diffusion from LPZs to HPZs can extend management of polluted sites by decades. Numerical models are often used to simulate back diffusion, estimate cleanup times, and develop site management strategies. Sharp concentration gradients of pollutants are present at the interface between HPZs and LPZs, and hence accurate simulation requires fine grid sizes resulting in high computational burden. Since the MODFLOW family of codes is widely used in practice, we develop a new approach for modeling pollutant back diffusion using MODFLOW/RT3D that eliminates the need for fine discretization of the LPZ. Instead, the LPZ is treated as an impermeable region in MODFLOW, while in RT3D the LPZ is conceptualized as a series of immobile zones coupled with a mobile zone at the HPZ/LPZ interface. Finite volume discretization of diffusion and reaction within the LPZ is then modeled as mass transfer and reaction among several immobile species. This results in a simulation domain with significantly fewer grid cells compared to that required if all LPZs are discretized, providing potential for improved computational efficiency. Cases, including a layer of HPZ over an LPZ, a thin/thick lens of LPZ embedded in HPZ, and multiple lens of LPZs embedded in HPZ are tested by the new approach for tracer and reactive scenarios.

Assessment of the management scenarios for groundwater quality remediation of a nitrate-contaminated aquifer

Nitrate contaminant degrades groundwater quality and threatens the health of the humans, livestock, and the environment. Damaneh-Daran aquifer is located at upstream of the Zayandehrood reservoir in west-central Iran. This aquifer has been highly contaminated by nitrate and is still rapidly being contaminated. Thus, its quality needs to be remediated. This paper is focused on the quantity-quality modeling to predict the average nitrate concentration of the aquifer. Several remediation scenarios are presented in a period beginning from fall 2019, ending in spring 2024. These scenarios address several ways to mitigate the injection of the major sources of contamination in the region, such as equipping the urban regions with wastewater collection and treatment plants and reducing the fertilizers' use. The decreased use of the fertilizers may be achieved through two strategies: directly reducing the amount of the fertilizers by several specific and predefined rates of reduction and indirectly decreasing the amount of the fertilizers used by crop pattern modification. The latter strategy is evaluated to replace all or a part of the areas allocated to the more fertilizer-demanding crops with those of the less fertilizer-demanding crops. Furthermore, some of these scenarios are hybridized to more mitigate groundwater quality degradation. The results of performing the proposed scenarios are once compared together and then compared with the trend scenario letting current case study conditions and facilities be held in the future. The results suggest that the scenario hybridizing the effects of the wastewater treatment plants-equipping scenario with those of the quality-enhancing crop pattern modification scenario is evaluated as the most effective and best-performing scenario, implementation of which offers 20% and 30% reduction of the nitrate concentration for the agricultural and urban areas, respectively.

A proposed modelling towards the potential impacts of climate change on a semi-arid, small-scaled aquifer: a case study of Iran

Several studies have evaluated the impact of climate change on the alluvial aquifers; however, no research has been carried out on a small-scale aquifer without any human influences and pumping wells. The object of this study is to assess the response of such an aquifer to the climate change to observe if it can preserve its storage or not. Pali aquifer, southwest Iran, is solely discharged by Taraz-Harkesh stream and geological formations. On the other hand, it is recharged by precipitation and geological formations. The Taraz-Harkesh stream's discharge rates and the Pali aquifer's groundwater level were simulated by IHACRES and MODFLOW, respectively, in the base (1961-1990) and future (2021-2050) time periods under two Representative Concentration Pathways, i.e., RCP4.5 and RCP8.5. The outputs of IHACRES were regarded as the input of MODFLOW. The groundwater model was calibrated in the steady-state for the hydrological year 2007 and in the unsteady-state for the time period 2008-2014 with annual time steps. Further, the groundwater model was verified for the time period 2015-2018. The statistical criteria maintained the groundwater model's ability, consequently measuring the root mean square error to be 0.69, 0.85, and 1.18 m for the steady calibration, unsteady calibration and verification of the groundwater model, respectively. Results indicate that the stream's discharge rates would decrease in the future time period, especially under RCP8.5. Nevertheless, the groundwater level would not fluctuate considerably. Indeed, the groundwater resources, even a semi-arid, small-scaled aquifer, may be considered as the water supplying systems under the future climate change.

The legacy and drivers of groundwater nutrients and pesticides in an agriculturally impacted Quaternary aquifer system

Nutrient and pesticide pollution are among the major threats to groundwater quality in agriculturally impacted aquifers. Understanding their legacy effects and drivers are important to protect aquifers from exposures to contamination. However, the complexities of groundwater flowpaths make it difficult to predict the time-scales of groundwater flow and contaminant transport. To determine these controls of groundwater nutrient and pesticides in an aquifer system underlying an intensive agricultural area in the Great Barrier Reef catchment, Australia, we sampled tritium (³H) to estimate groundwater-age, nutrient and pesticide concentrations to investigate groundwater contamination, and nitrogen (ẟ¹⁵N-NO₃^-) and oxygen (ẟ¹⁸O-NO₃^-) isotopes to determine groundwater nitrate dynamics. We, then, constructed high-resolution 3D geological and groundwater flow models of the aquifer system to determine the role of the geologic heterogeneity on the observed nutrient and pesticide concentrations. Groundwater ³H derived ages, and nutrient and pesticide concentrations did not follow distinct spatial trends. ẟ¹⁵N-NO₃^- and ẟ¹⁸O-NO₃^- values indicated that nitrification and denitrification processes influenced nitrate dynamics in the aquifer system; however, they were not solely able to explain the entire 3D variability. The 3D geologic modelling identified possible preferential flowpaths and perched systems, which helped to explain the observed groundwater-age, nutrient and pesticide variabilities. Old-groundwater (~100-years) was found in shallow depths (<15 m) where perched systems were identified. In areas with preferential flowpaths, young-groundwater (⁓1-year) with significant nitrate (~12 mg-N/L) and pesticides (up to 315 ng/L) concentrations were detected at deeper depths (>25 m), below perched and locally confined systems. Downward increasing groundwater-age, and decreasing nutrient and pesticide concentrations were detected in the unconfined aquifer, while old-groundwater (~160-years) and lower nitrate (<3 mg-N/L) and pesticides (<2 ng/L) concentrations were detected in the confined systems. This study demonstrates the importance of understanding both the geology and the hydrogeology of an area before deploying monitoring studies and/or making conclusions from tritium, nutrient and pesticide data alone.

Assessing groundwater pollution and potential remediation processes in a multi-layer aquifer system

Pumping-induced leakage across aquitards may induce a deterioration of water quality in multi-layer aquifer systems. It is critical to understand long-term trends of water quality parameters when assessing the sustainability of groundwater abstraction. Daily drinking water needs of 2.2 million people in Yinchuan region of northwest China are solely met by groundwater resources, but long-term groundwater withdrawal has created an extensive cone of depression (294 km² in area) in confined aquifer causing increased vertical recharge. In this study, a model was established and calibrated with head data, then was incorporated with field tracer tests to provide key information on the hydro-dispersive characteristics of the contaminant for assessing both the current and future state of the aquifer system. The results confirmed a close association between water quality deterioration and high downward fluxes of high chloride groundwater, most notably near the center of the cone of depression. On a temporal scale, water quality degradation remains slow, largely due to the high, pre-existing storage of good quality water. Modeling suggests that the water quality in the upper confined aquifer will lose its potability over a 25 km² and 50 km² area within 200 years under the current and intensified pumping conditions, respectively. Elevated chloride values were also detected toward the east of the cone, highlighting the impact of hydrological settings on the vertical groundwater flow. Modeling of potential aquifer remediation shows an even slower response with a further 250 years or more required for potability to be restored in affected areas. The findings can provide valuable guidance to for decision makers and support the sustainable management of aquifer exploitation.

Groundwater flow and arsenic contamination transport modeling for a multi aquifer terrain: Assessment and mitigation strategies

Arsenic contaminated shallow aquifers evaluation, mitigation, and management strategies are the challenging task to all the hydrologist and provide a safe drinking water demand in the Holocene age, alluvial aquifers. To manage and mitigate such problems, we used numerical groundwater modeling software (GMS 10.2), for the development of 3D transient state predictive (groundwater flow and contaminant transport) conceptual model for two topographically different arsenic contaminated regions. The models were built by using the measured hydro-geological data, empirical values, and equations. Groundwater flow calibration, sensitivity analyses, and validation were performed for each soil parameters, varying boundary conditions and for alternate meteorological scenarios. The MODFLOW results suggested that, the distribution of As contaminant was directly controlled by the complex hydrostratigraphy, surface water bodies and indirectly controlled by the change in meteorological conditions. The MT3DMS model, for As contaminant transport, used for the assessment of shallow and deeper aquifers. The results showed that the downward movement of As has made the deeper aquifer unsafe for drinking water and irrigation purposes. However, the aquifers and regions with high flushing capability, negligible vertical hydraulic conductivity can be delineated as As safe groundwater source, irrespective of their sediment color. Therefore, for the geogenic source of As, both the simulation results inferred that to estimate and mitigate As contaminant groundwater aquifers or regions, the numerical modeling solution is a technically viable means an effective decision-making tool.

Use of isotope hydrology in groundwater conceptualization for modeling flow and contaminant transport at northwestern Sinai, Egypt

The study area extends along northwestern Sinai coastal plain, which is considered an integral part of the Mediterranean Sea. It depends mainly on the groundwater resource for different type of human activities such as agricultural and drinking. Many programs and policies should be implemented in this area to concurrently improve the sustainability of groundwater use and manage the risks of its degradation. Leakage from some factories in Bir El-Abd might be a contamination source that would threaten groundwater. In this paper, an attempt was made using an integrated approach of the hydrogeological setting and the conjugation of the hydrogeochemical data with the stable isotope hydrology for representation of the conceptual model of the study area. Those tools give more insights on the characterization of the groundwater system with all relevant boundaries and main recharge sources of the aquifer; which is considered to be the key components of a groundwater modeling. A particular focus is placed on modeling a hypothetical accident for contaminant transport in the groundwater system, using both lead and chromium as a typical contaminant component. Further predication of the concentration of those elements has been estimated, and the safety distances of their plume have been determined. This study would be helpful in dealing with water management issues related to contaminant hydrogeology. As well, it introduces some finding for reducing the environmental risk form the industrial development at the study area.

Investigating effects of climate change, urbanization, and sea level changes on groundwater resources in a coastal aquifer: an integrated assessment

Urbanization and climate change are causing numerous side effects on groundwater resources. In this study, an integrated modeling approach by linking soil and water application tool (SWAT), modular finite difference groundwater flow (MODFLOW), and three-dimensional variable-density groundwater flow coupled with multi-species solute and heat transport (SEAWAT) models were used to exhibit responses of groundwater systems, in terms of flow and salt concentrations to current and future climatic and anthropogenic changes. Future climate scenarios for periods of 2010-2040 were generated from the Canadian Global Coupled Model (CGCM) for scenarios A1B, B1, and A2 which was downscaled by the Long Ashton Research Station weather generator (LARS-WG) providing precipitation and temperature patterns for the period 2018-2040. The GCM's outputs were applied to SWAT model to estimate recharge rate for the ten scenarios designed to assess the sensitivity of the aquifer to urbanization and climate change. The estimated recharge rate from SWAT was utilized as an input in numerical groundwater model to evaluate saltwater intrusion (SWI), changes in freshwater storage within the aquifer system, and changes in groundwater level. Based on the results of each scenario's simulation, increase of pumping rate yield by future population growth will have more adverse effects on the unconfined aquifer. The derived information from this study can be used to improve future works by developing a better understanding of the managed and unmanaged response of freshwater storage and unconfined groundwater systems to climate change and anthropogenic activities.

Assessment of vulnerability and control measures to protect the Salbarua ecosystem from hypothetical spill sites

Population pressure, urbanization, and industrial developments, among other factors, have resulted in severe degradation of environmental resources such as wetlands. Thus, a groundwater model (MODFLOW) was integrated with a particle tracking MODPATH model to simulate the hydrodynamic flow head field and to analyze the vulnerability of the Salburua ecosystem and propose control measures to protect the riparian area. The simulations show that pathways of particle tracking originating at potential contaminant sources will tend to migrate downwards towards the sensitive ecosystem, which suggests that the quality of the hydrological ecosystem is likely to deteriorate in the future. Variation in exit points of particles indicates that the time-related capture areas are affected by changes of the hydraulic gradients. Two control measures of potential sources of pollutants in the vicinity of the Salbarua ecosystem were analyzed. The study results suggest that the travel time-related capture zone with a funnel-and-gate system is much smaller than without the control alternative, which indicates that the gate configuration has an effect on capture zone size and shape and on the residence time with a better attenuation performance. It is also shown that a leakage-proof barrier is less effective for point-source containment, assuming that hydraulic control performance and cost-efficiency are the criteria for pollution control effectiveness. Instead, a program of monitoring wells would effectively characterize water quality in the aquifer and provide a decision support system. This approach may be used in helping water managers to develop more physically based and quantitative protection strategies.

Null-space Monte Carlo particle tracking to assess groundwater PCE (Tetrachloroethene) diffuse pollution in north-eastern Milan functional urban area

The Lombardy Region in Italy is one of the most urbanized and industrialized areas in Europe. The presence of countless sources of groundwater pollution is therefore a matter of environmental concern. The sources of groundwater contamination can be classified into two different categories: 1) Point Sources (PS), which correspond to areas releasing plumes of high concentrations (i.e. hot-spots) and 2) Multiple-Point Sources (MPS) consisting in a series of unidentifiable small sources clustered within large areas, generating an anthropogenic diffuse contamination. The latter category frequently predominates in European Functional Urban Areas (FUA) and cannot be managed through standard remediation techniques, mainly because detecting the many different source areas releasing small contaminant mass in groundwater is unfeasible. A specific legislative action has been recently enacted at Regional level (DGR IX/3510-2012), in order to identify areas prone to anthropogenic diffuse pollution and their level of contamination. With a view to defining a management plan, it is necessary to find where MPS are most likely positioned. This paper describes a methodology devised to identify the areas with the highest likelihood to host potential MPS. A groundwater flow model was implemented for a pilot area located in the Milan FUA and through the PEST code, a Null-Space Monte Carlo method was applied in order to generate a suite of several hundred hydraulic conductivity field realizations, each maintaining the model in a calibrated state and each consistent with the modelers' expert-knowledge. Thereafter, the MODPATH code was applied to generate back-traced advective flowpaths for each of the models built using the conductivity field realizations. Maps were then created displaying the number of backtracked particles that crossed each model cell in each stochastic calibrated model. The result is considered to be representative of the FUAs areas with the highest likelihood to host MPS responsible for diffuse contamination.

Modified MODFLOW-based model for simulating the agglomeration and transport of polymer-modified Fe0 nanoparticles in saturated porous media

The solute transport model MODFLOW has become a standard tool in risk assessment and remediation design. However, particle transport models that take into account both particle agglomeration and deposition phenomena are far less developed. The main objective of the present study was to evaluate the feasibility of adapting the standard code MODFLOW/MT3D to simulate the agglomeration and transport of three different types of polymer-modified nanoscale zerovalent iron (NZVI) in one-dimensional (1-D) and two-dimensional (2-D) saturated porous media. A first-order decay of the particle population was used to account for the agglomeration of particles. An iterative technique was used to optimize the model parameters. The model provided good matches to 1-D NZVI-breakthrough data sets, with R ² values ranging from 0.96 to 0.99, and mass recovery differences between the experimental results and simulations ranged from 0.1 to 1.8 %. Similarly, simulations of NZVI transport in the heterogeneous 2-D model demonstrated that the model can be applied to more complicated heterogeneous domains. However, the fits were less good, with the R ² values in the 2-D modeling cases ranging from 0.75 to 0.95, while the mass recovery differences ranged from 0.7 to 6.5 %. Nevertheless, the predicted NZVI concentration contours during transport were in good agreement with the 2-D experimental observations. The model provides insights into NZVI transport in porous media by mathematically decoupling agglomeration, attachment, and detachment, and it illustrates the importance of each phenomenon in various situations. Graphical Abstract ᅟ.

Floodplain restoration increases hyporheic flow in the Yakima River Watershed, Washington

Hyporheic exchange between a river channel and its floodplain region assists in mediating processes such as nutrient removal and temperature regulation. Floodplain restoration in the form of levee setbacks are often carried out to improve the hyporheic exchange. In this study Light Detection and Ranging (LiDAR) data were used along with the head data from observation wells and stage data from rivers to setup and calibrate a groundwater model for 458 km² of area within Gap to Gap reach of the Yakima River, WA. This area has witnessed several efforts of floodplain restoration in the form of levee setbacks. The groundwater model was used to quantify hyporheic flow emerging from the Yakima River in steady and transient states during pre-restoration (using LiDAR data of 2008) and post-restoration period (after levee setback using LiDAR data of 2013). The comparison of results from the model runs during pre and post-restoration periods showed that the length of the pathlines increased after levee setback for both steady and transient state model simulations. The largest increase of about 62 m was noticed in the month of September 2014 (pre: 398 m and post: 460 m). The study also showed that the direction of the flow changed following levee setback, expanding the area for hyporheic flux exchange between surface and groundwater. The model run during transient state also suggested that pathlines were longer during drier months compared to wet months. Overall, the study showed that levee setbacks improved the hyporheic connection between surface and groundwater in the Yakima floodplain which demonstrates that levee setback can provide a valuable hydrologic tool to restore ecosystem processes in previously leveed rivers.

The land-use legacy effect: Towards a mechanistic understanding of time-lagged water quality responses to land use/cover

Numerous studies have linked land use/land cover (LULC) to aquatic ecosystem responses, however only a few have included the dynamics of changing LULC in their analysis. In this study, we explicitly recognize changing LULC by linking mechanistic groundwater flow and travel time models to a historical time series of LULC, creating a land-use legacy map. We then illustrate the utility of legacy maps to explore relationships between dynamic LULC and lake water chemistry. We tested two main concepts about mechanisms linking LULC and lake water chemistry: groundwater pathways are an important mechanism driving legacy effects; and, LULC over multiple spatial scales is more closely related to lake chemistry than LULC over a single spatial scale. We applied statistical models to twelve water chemistry variables, ranging from nutrients to relatively conservative ions, to better understand the roles of biogeochemical reactivity and solubility on connections between LULC and aquatic ecosystem response. Our study illustrates how different areas can have long groundwater pathways that represent different LULC than what can be seen on the landscape today. These groundwater pathways delay the arrival of nutrients and other water quality constituents, thus creating a legacy of historic land uses that eventually reaches surface water. We find that: 1) several water chemistry variables are best fit by legacy LULC while others have a stronger link to current LULC, and 2) single spatial scales of LULC analysis performed worse for most variables. Our novel combination of temporal and spatial scales was the best overall model fit for most variables, including SRP where this model explained 54% of the variation. We show that it is important to explicitly account for temporal and spatial context when linking LULC to ecosystem response.

Simulation of Regional Karst Aquifer System and Assessment of Groundwater Resources in Manatí-Vega Baja, Puerto Rico

The North Coast karst aquifer system of Puerto Rico, the most productive aquifer of the island, is a vital water source for drinking water and local ecosystems. High freshwater demands alter the coastal groundwater system that impacts both human populations and coastal ecosystems of the island. To predict how this system might respond to rainfall events and high pumping demands, we used the equivalent porous medium (EPM) technique to develop a three-dimensional ground-water flow model to estimate hydrogeological parameters and assess groundwater resources in the Manatí-Vega Baja karst aquifer. The approach is based on the hypothesis that the simplified EPM approach will reproduce groundwater hydrodynamics in this complex karst environment. The steady-state model was calibrated with trial and error and parameter estimation methods using an observed groundwater table of 1995 (r = 0.86, p < 0.0001, n = 39). The large-scale simulation suggested that groundwater flow roughly follows the elevation slope [i.e. south to north). Calibrated hydraulic conductivities range from 0.5 to 86 m/d, whereas the hydro-geologic data strongly suggest higher permeability in the middle karst section of the study area. The transient model adequately estimates the observed groundwater fluctuations in response to rainfall events from 1980 until 2014. The transient results indicate that the conceptual model accuracy is more acceptable with a mean error (ME) of −0.132 m, mean absolute error (MAE) of 0.542 m and root mean square (RMSE) error of 0.365 m. The results of water budget simulation show that the total recharge satisfies the total groundwater withdrawal rate in the past, but continuous closure of more contaminated wells causes groundwater levels to increase in the future. The results indicate that the assumption of applicability of EPM approach is sustained and supported by measured data in the study area. Taking future water demands into account, this model could be applied further to predict the changes of groundwater levels and mass balance under different exploitation scenarios.

Single event time series analysis in a binary karst catchment evaluated using a groundwater model (Lurbach system, Austria)

The Lurbach karst system (Styria, Austria) is drained by two major springs and replenished by both autogenic recharge from the karst massif itself and a sinking stream that originates in low permeable schists (allogenic recharge). Detailed data from two events recorded during a tracer experiment in 2008 demonstrate that an overflow from one of the sub-catchments to the other is activated if the discharge of the main spring exceeds a certain threshold. Time series analysis (autocorrelation and cross-correlation) was applied to examine to what extent the various available methods support the identification of the transient inter-catchment flow observed in this binary karst system. As inter-catchment flow is found to be intermittent, the evaluation was focused on single events. In order to support the interpretation of the results from the time series analysis a simplified groundwater flow model was built using MODFLOW. The groundwater model is based on the current conceptual understanding of the karst system and represents a synthetic karst aquifer for which the same methods were applied. Using the wetting capability package of MODFLOW, the model simulated an overflow similar to what has been observed during the tracer experiment. Various intensities of allogenic recharge were employed to generate synthetic discharge data for the time series analysis. In addition, geometric and hydraulic properties of the karst system were varied in several model scenarios. This approach helps to identify effects of allogenic recharge and aquifer properties in the results from the time series analysis. Comparing the results from the time series analysis of the observed data with those of the synthetic data a good agreement was found. For instance, the cross-correlograms show similar patterns with respect to time lags and maximum cross-correlation coefficients if appropriate hydraulic parameters are assigned to the groundwater model. The comparable behaviors of the real and the synthetic system allow to deduce that similar aquifer properties are relevant in both systems. In particular, the heterogeneity of aquifer parameters appears to be a controlling factor. Moreover, the location of the overflow connecting the sub-catchments of the two springs is found to be of primary importance, regarding the occurrence of inter-catchment flow. This further supports our current understanding of an overflow zone located in the upper part of the Lurbach karst aquifer. Thus, time series analysis of single events can potentially be used to characterize transient inter-catchment flow behavior of karst systems.

Modeling hydrology, groundwater recharge and non-point nitrate loadings in the Himalayan Upper Yamuna basin

The mountainous Himalayan watersheds are important hydrologic systems responsible for much of the water supply in the Indian sub-continent. These watersheds are increasingly facing anthropogenic and climate-related pressures that impact spatial and temporal distribution of water availability. This study evaluates temporal and spatial distribution of water availability including groundwater recharge and quality (non-point nitrate loadings) for a Himalayan watershed, namely, the Upper Yamuna watershed (part of the Ganga River basin). The watershed has an area of 11,600 km(2) with elevation ranging from 6300 to 600 m above mean sea level. Soil and Water Assessment Tool (SWAT), a physically-based, time-continuous model, has been used to simulate the land phase of the hydrological cycle, to obtain streamflows, groundwater recharge, and nitrate (NO3) load distributions in various components of runoff. The hydrological SWAT model is integrated with the MODular finite difference groundwater FLOW model (MODFLOW), and Modular 3-Dimensional Multi-Species Transport model (MT3DMS), to obtain groundwater flow and NO3 transport. Validation of various modules of this integrated model has been done for sub-basins of the Upper Yamuna watershed. Results on surface runoff and groundwater levels obtained as outputs from simulation show a good comparison with the observed streamflows and groundwater levels (Nash-Sutcliffe and R(2) correlations greater than +0.7). Nitrate loading obtained after nitrification, denitrification, and NO3 removal from unsaturated and shallow aquifer zones is combined with groundwater recharge. Results for nitrate modeling in groundwater aquifers are compared with observed NO3 concentration and are found to be in good agreement. The study further evaluates the sensitivity of water availability to climate change. Simulations have been made with the weather inputs of climate change scenarios of A2, B2, and A1B for end of the century. Water yield estimates under climate change scenarios have been made and implications on groundwater and groundwater quality have been assessed. The delicate groundwater resource balance that connects livelihoods of millions of people seems to be under tremendously increasing pressure due to the dynamic conditions of the natural environment of the region and the future climate changes.

Understanding changes in the hydrological behaviour within a karst aquifer (Lurbach system, Austria)

A thorough data analysis combined with groundwater modelling was conducted in an Austrian binary karst aquifer to better understand changes in the hydrological behaviour observed at a karst spring. During a period of 4 years after a major flood event the spring hydrograph appears to be more damped with lower peak flow and higher baseflow than in the years before. The analysis of the hydrograph recession suggests that the observed hydrological change is caused by changes within the karst system rather than by varying hydro-meteorological conditions. The functioning of the aquifer and potential causes of the observed changes are further examined using the groundwater flow model MODFLOW. The simulation results suggest that a modification of hydraulic conductivity and storage within the conduit network, e.g. due to the plugging of the drainage conduits with sediments, may be the cause of the different behaviour. MODFLOW was able to reproduce the observed dynamics of spring flow, although it does not account for turbulent flow within karst conduits. Using a simplified model scenario it is demonstrated that the damping of the hydrograph is much stronger if turbulent conduit flow is taken into account. Thus, a turbulent flow model is needed to assess potential changes in the storage properties quantitatively.