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Driven by the need for integrated management of groundwater (GW) and surface water (SW), quantification of GW–SW interactions and associated contaminant transport has become increasingly important. This is due to their substantial impact on water quantity and quality. In this review, we provide an overview of the methods developed over the past several decades to investigate GW–SW interactions. These methods include geophysical, hydrometric, and tracer techniques, as well as various modeling approaches. Different methods reveal valuable information on GW–SW interactions at different scales with their respective advantages and limitations. Interpreting data from these techniques can be challenging due to factors like scale effects, heterogeneous hydrogeological conditions, sediment variability, and complex spatiotemporal connections between GW and SW. To facilitate the selection of appropriate methods for specific sites, we discuss the strengths, weaknesses, and challenges of each technique, and we offer perspectives on knowledge gaps in the current science. [ABSTRACT FROM AUTHOR]

Current integrated modeling frameworks for simulating nutrient sources and dynamics are inadequate for large regional watersheds dominated by groundwater‐surface water interactions due to their simplistic representations of groundwater. In this study, we develop a coupled model that integrates comprehensive surface water, 3‐D groundwater, soil erosion, and nutrient processes. The model is intended to enhance the understanding of nutrient dynamics and sources in the Pearl River Basin (PRB). The model exhibits satisfactory performance in simulating streamflow and sediment transport patterns, capturing essential seasonal variations in water quality indicators. Hydrological budget assessments from 2002 to 2020 in the PRB reveal that 54% of precipitation drains into the South China Sea as surface water, while groundwater discharge as baseflow accounts for 18% of the streamflow. The nutrient budget for the PRB indicates that non‐point sources are the dominant contributors to both nitrogen (N) and phosphorus (P), ranging between 64% and 90%. Improved sewage collection and treatment have reduced point source nutrient contributions over the evaluation period. Groundwater remains a significant and consistent source of N, contributing between 11% and 19%. Natural disturbances and fertilization have led to an upward trend in river N inputs, while afforestation and sewage reduction efforts have resulted in a downward trend in river P inputs. Increased fertilization emerges as a central concern for the PRB, suggesting cost‐effective mitigation of fertilizer usage a pragmatic solution. The coupled simulation model developed in this study offers a novel systems approach for basin‐wide nutrient analysis and pollution control strategies, considering both natural and human‐induced disturbances. Plain Language Summary: Water nutrient pollution is a global problem caused by both natural and human factors. To better understand nutrient pollution, a new model has been developed that combines water flow, soil erosion, and nutrient transport to simulate water and nutrient cycling on a regional scale. This model can assist in the management of water pollution by providing a better understanding of nutrient sources and quantifying the contribution of climate change and different human activities. Tested in the Pearl River Basin from 2002 to 2020, the model accurately simulated water flow and nutrient patterns. A key finding is that groundwater supplies 18% of water and 14% of nitrogen inputs of the Pearl River. The main sources of pollutants are non‐point agricultural sources, primarily from fertilization. Climate variability leads to more runoff and nutrient input, while improved sewage treatment and afforestation have reduced nutrient pollution. The model allows the exploration of different influencing factors to better control nutrient pollution. Understanding how human activities impact different regions is crucial for effective nutrient pollution management. Key Points: A novel system model integrates water, soil, and nutrient dynamics for effective regional watershed managementNon‐point source nutrients are the major contributors of nitrogen and phosphorus levels in the Pearl River Basin, accounting for 64%–90%The rise in fertilization poses a growing concern for the Pearl River Basin, underscoring the need for cost‐effective mitigation strategies [ABSTRACT FROM AUTHOR]

River deltas typically have high population density and support a wide range of intensive and prosperous socioeconomic activities. The hydrological processes in these regions are complex, primarily due to the interactions among the river, aquifer, and sea. However, a systematic and quantitative elaboration of the river‐aquifer‐sea interactions is still lacking. Here, we developed an integrated hydrological flow model for the Pearl River Delta (PRD), which contains the world's largest urban area in both size and population, to gain a deeper understanding of the complexities in the river‐aquifer‐sea interactions. The model performance was validated and cross‐checked via observations at gauging stations and independent remote‐sensing products (e.g., soil moisture, evapotranspiration, and total water storage anomalies). Based on the 10‐year simulation results (2004–2013), the major findings of this study are as follows: (a) accurate representation of the tidal effect is important not only for simulating short‐term flow dynamics but also for capturing the characteristics of long‐term hydrological fluxes and states; (b) the flow‐model‐computed average groundwater discharge rate per unit length of the coastline for the PRD is 3.01 m3/d/m, which is comparable with those derived from water budget approaches but 1–2 orders of magnitude lower than the total submarine groundwater discharge (SGD) estimated by using isotope tracer‐based methods; (c) the temporal variation of SGD is controlled by tidal forcing on an hourly time scale, but by terrestrial hydrological processes on monthly and annual time scales; and (d) an integrated hydrological flow model can be used to identify distinct and large subsurface zones sensitive to tidal fluctuations, quantifying the pivotal role of ocean tides in shaping the coastal groundwater system. This study represents a first step in using an integrated hydrological model to explore river‐aquifer‐sea interactions and their effects on the regional groundwater system simultaneously driven by meteorological and tidal forcings. Key Points: High‐resolution integrated modeling of river‐aquifer‐sea interactions is accomplished with full consideration of tidal effectsTerrestrial fresh submarine groundwater discharge (SGD) computed by an integrated flow model is 1–2 orders of magnitude less than the total SGD by isotope based methodsA novel method is developed for delineating distinct subsurface zones in an estuarine aquifer that are sensitive to tidal fluctuations [ABSTRACT FROM AUTHOR]

A study describes a rigorous solution developed from first principles to guide the preliminary design of cutoff walls installed to contain the migration of contaminants from source zones and the full analytic solution used to develop a criterion for determining the configuration and hydraulics of optimal wall designs. The results of numerical simulations confirm that for a wall with known transport properties, a specified Darcy flux is associated with a unique outside contaminant concentration.

A large chromium plume that evolved from chromium releases in a valley near the Mojave River was studied to understand the processes controlling fate and migration of chromium in ground water and used as a tracer to study the dynamics of a basin and range ground water system. The valley that was studied is naturally arid with high evapotranspiration such that essentially no precipitation infiltrates to the water table. The dominant natural hydrogeologic processes are recharge to the ground water system from the Mojave River during the infrequent episodes when there is flow in the river, and ground water flow toward a playa lake where the ground water evaporates. Agricultural pumping in the valley from the mid-1930s to the 1970s significantly altered ground water flow conditions by decreasing water levels in the valley by more than 20 m. This pumping declined significantly as a result of dewatering of the aquifer, and water levels have since recovered modestly. The ground water system was modeled using MODFLOW, and chromium transport was simulated using MT3D. Several innovative modifications were made to these modeling programs to simulate important processes in this ground water system. Modifications to MODFLOW include developing a new well package that estimates pumping rates from irrigation wells at each time step based on available drawdown. MT3D was modified to account for mass trapped above the water table when the water table declines beneath nonirrigated areas and to redistribute mass to the system when water levels rise., Introduction Cooling water from an industrial facility containing chromium as a corrosion inhibitor was discharged to unlined ponds for a 14-year period between 1952 and 1965 (Ecology and Environment 1988). [...]

Regional nitrate contamination in groundwater is a management challenge involving multisector benefits. There is always conflict between restricting anthropogenic activities to protect groundwater quality and prioritizing economic development, especially in productive agriculture dominated areas. To mitigate the nitrate contamination in groundwater, it is necessary to develop management alternatives that simultaneously support environmental protection and sustainable economic development. A regional transport modeling framework is applied to evaluate nitrate fate and transport in the Dagu Aquifer, a shallow sandy aquifer that supplies drinking water and irrigation water for a thriving agricultural economy in Shandong Province in east coastal China. The aquifer supports intensive high‐value vegetable farms and nitrate contamination is extensive. Detailed land‐use information and fertilizer use data were compiled and statistical approaches were employed to analyze nitrogen source loadings and the spatiotemporal distribution of nitrate in groundwater to support model construction and calibration. The evaluations reveal that the spatial distribution and temporal trends of nitrate contamination in the Dagu Aquifer are driven by intensive fertilization and vertical water exchange, the dominant flow pattern derived from intensive agricultural pumping and irrigation. The modeling framework is employed to assess the effectiveness of potentially applicable management alternatives. The predictive results provide quantitative comparisons for the trend and extent of groundwater quality mitigation under each scenario. Recommendations are made for measures that can both improve groundwater quality and sustain productive agricultural development. Article impact statement: Assessed nitrogen management measures to balance groundwater protection and agricultural sustainability. [ABSTRACT FROM AUTHOR]

Cutoff walls and liners are used frequently as barriers to isolate contaminants at both controlled and uncontrolled hazardous waste sites. Neville and Andrews (2006) presented a containment criterion for contaminant isolation by a barrier. The analysis of Neville and Andrews (2006) yields the inward Darcy flux that balances the diffusive mass flux from the source so that the net mass flux is zero. A requirement of zero net mass flux may not be achievable in all situations. The analysis developed by Neville and Andrews (2006) is extended to develop straightforward expressions for the long‐term mass fluxes across a barrier for any conditions. In cases where it may not be possible to satisfy a criterion of zero net mass flux, the results from an exact solution for transient solute transport are used to show how the mass fluxes evolve to their long‐term values. Article impact statement: The Neville and Andrews (2006) containment analysis is extended to develop straightforward expressions for the long‐term mass fluxes across a barrier for any conditions. [ABSTRACT FROM AUTHOR]

A modified Water-Table Fluctuation (WTF) method is developed to quantitatively characterize the regional groundwater discharge patterns in stressed aquifers caused by intensive agricultural pumping. Two new parameters are defined to express the secondary information in the observed data. One is infiltration efficiency and the other is discharge modulus (recurring head loss due to aquifer discharge). An optimization procedure is involved to estimate these parameters, based on continuous groundwater head measurements and precipitation records. Using the defined parameters and precipitation time series, water level changes are calculated for individual wells with fidelity. The estimated parameters are then used to further address the characterization of infiltration and to better quantify the discharge at the regional scale. The advantage of this method is that it considers recharge and discharge simultaneously, whereas the general WTF methods mostly focus on recharge. In the case study, the infiltration efficiency reveals that the infiltration is regionally controlled by the intrinsic characteristics of the aquifer, and locally distorted by engineered hydraulic structures that alter surface water-groundwater interactions. The seasonality of groundwater discharge is characterized by the monthly discharge modulus. These results from individual wells are clustered into groups that are consistent with the local land use pattern and cropping structures. [ABSTRACT FROM AUTHOR]