Hydrologic Control on Arsenic Cycling at the Groundwater-Surface Water Interface of a Tidal Channel.

Historical industrial activities have resulted in soil contamination at sites globally. Many of these sites are located along coastlines, making them vulnerable to hydrologic and biogeochemical alterations due to climate change and sea-level rise. However, the impact of hydrologic dynamics on contaminant mobility in tidal environments has not been well studied. Here, we collected data from pressure transducers in wells, multi-level redox sensors, and porewater samplers at an As-contaminated site adjacent to a freshwater tidal channel. Results indicate that sharp redox gradients exist and that redox conditions vary on tidal to seasonal timescales due to sub-daily water level fluctuations in the channel and seasonal groundwater-surface water interactions. The As and Fe2+ concentrations decreased during seasonal periods of net discharge to the channel. The seasonal changes were greater than tidal variations in both Eh and As concentrations, indicating that impacts of the seasonal mechanism are stronger than those of sub-daily water table fluctuations. A conceptual model describing tidal and seasonal hydro-biogeochemical coupling is presented. These findings have broad implications for understanding the impacts of sea-level rise on the mobility of natural and anthropogenic coastal solutes.

Drivers of barrier island water-table fluctuations and groundwater salinization.

Barrier islands are threatened by climate change as sea-level rise and higher frequency storm surge lead to more flooding and saltwater intrusion. Vegetation plays a vital role in preventing erosion of barrier islands due to aeolian and hydrological forces. However, vegetation on barrier islands is threatened by rising water tables causing hypoxic conditions and storm-surge overwash introducing saline water to the root zone. To better protect barrier island ecosystems, it is critical to identify the relative influence of different hydrological drivers on water table elevation and salinity, and understand how this influence varies spatially and temporally. In this study, three barrier island sites were instrumented with groundwater wells monitoring water level and specific conductance. Using these data, a set of transfer function noise models were calibrated and used to determine the relative influence of hydrologic drivers including precipitation, evapotranspiration, bay and ocean water levels, and wave height on groundwater levels and specific conductance. We found that drivers of water-level change and specific conductance vary strongly among sites, depending primarily on the surface water connectivity and the geology of the island. Sites with close connection to inlets showed more salinization and responded to a larger number of drivers, while sites that were poorly connected to the ocean responded to fewer drivers.

Integrated effects of bioturbation, warming and sea-level rise on mobility of sulfide and metalloids in sediment porewater of mangrove wetlands.

Global warming and sea-level rise exert profound impacts on coastal mangrove ecosystems, where widespread benthic crabs change sediment properties and regulate material cycles. How crab bioturbation perturbs the mobilities of bioavailable arsenic (As), antimony (Sb) and sulfide in sediment-water systems and their variability in response to temperature and sea-level rise is still unknown. By combining field monitoring and laboratory experiments, we found that As was mobilized under sulfidic conditions while Sb was mobilized under oxic conditions in mangrove sediments. Crab burrowing greatly enhanced oxidizing conditions, resulting in enhanced Sb mobilization and release but As sequestration by iron/manganese oxides. In control experiments with non-bioturbation, the more sulfidic conditions triggered the contrasting situation of As remobilization and release but Sb precipitation and burial. Moreover, the bioturbated sediments were highly heterogeneous for spatial distributions of labile sulfide, As and Sb as presented by 2-D high-resolution imaging and Moran's Index (patchy at the <1 cm scale). Warming stimulated stronger burrowing activities, which led to more oxic conditions and further Sb mobilization and As sequestration, whilst sea-level rise did the opposite via suppressing crab burrowing activity. This work highlights that global climate changes have the potential to significantly alter element cycles in coastal mangrove wetlands by regulating benthic bioturbation and redox chemistry.

Surface Water-Groundwater Connections as Pathways for Inland Salinization of Coastal Aquifers.

Coastal agricultural zones are experiencing salinization due to accelerating rates of sea-level rise, causing reduction in crop yields and abandonment of farmland. Understanding mechanisms and drivers of this seawater intrusion (SWI) is key to mitigating its effects and predicting future vulnerability of groundwater resources to salinization. We implemented a monitoring network of pressure and specific conductivity (SC) sensors in wells and surface waters to target marsh-adjacent agricultural areas in greater Dover, Delaware. Recorded water levels and SC over a period of three years show that the mechanisms and timescales of SWI are controlled by local hydrology, geomorphology, and geology. Monitored wells did not indicate widespread salinization of deep groundwater in the surficial aquifer. However, monitored surface water bodies and shallow (<4 m deep) wells did show SC fluctuations due to tides and storm events, in one case leading to salinization of deeper (18 m deep) groundwater. Seasonal peaks in SC occurred during late summer months. Seasonal and interannual variation of SC was also influenced by relative sea level. The data collected in this study data highlight the mechanisms by which surface water-groundwater connections lead to salinization of aquifers inland, before SWI is detected in deeper groundwater nearer the coastline. Sharing of our data with stakeholders has led to the implementation of SWI mitigation efforts, illustrating the importance of strategic monitoring and stakeholder engagement to support coastal resilience.

Seasonal oscillations in water exchange between aquifers and the coastal ocean.

Ground water of both terrestrial and marine origin flows into coastal surface waters as submarine groundwater discharge, and constitutes an important source of nutrients, contaminants and trace elements to the coastal ocean. Large saline discharges have been observed by direct measurements and inferred from geochemical tracers, but sufficient seawater inflow has not been observed to balance this outflow. Geochemical tracers also suggest a time lag between changes in submarine groundwater discharge rates and the seasonal oscillations of inland recharge that drive groundwater flow towards the coast. Here we use measurements of hydraulic gradients and offshore fluxes taken at Waquoit Bay, Massachusetts, together with a modelling study of a generalized coastal groundwater system to show that a shift in the freshwater-saltwater interface-controlled by seasonal changes in water table elevation-can explain large saline discharges that lag inland recharge cycles. We find that sea water is drawn into aquifers as the freshwater-saltwater interface moves landward during winter, and discharges back into coastal waters as the interface moves seaward in summer. Our results demonstrate the connection between the seasonal hydrologic cycle inland and the saline groundwater system in coastal aquifers, and suggest a potentially important seasonality in the chemical loading of coastal waters.

Evaluation of the sustainability of deep groundwater as an arsenic-safe resource in the Bengal Basin.

Tens of millions of people in the Bengal Basin region of Bangladesh and India drink groundwater containing unsafe concentrations of arsenic. This high-arsenic groundwater is produced from shallow (<100 m) depths by domestic and irrigation wells in the Bengal Basin aquifer system. The government of Bangladesh has begun to install wells to depths of >150 m where groundwater arsenic concentrations are nearly uniformly low, and many more wells are needed, however, the sustainability of deep, arsenic-safe groundwater has not been previously assessed. Deeper pumping could induce downward migration of dissolved arsenic, permanently destroying the deep resource. Here, it is shown, through quantitative, large-scale hydrogeologic analysis and simulation of the entire basin, that the deeper part of the aquifer system may provide a sustainable source of arsenic-safe water if its utilization is limited to domestic supply. Simulations provide two explanations for this result: deep domestic pumping only slightly perturbs the deep groundwater flow system, and substantial shallow pumping for irrigation forms a hydraulic barrier that protects deeper resources from shallow arsenic sources. Additional analysis indicates that this simple management approach could provide arsenic-safe drinking water to >90% of the arsenic-impacted region over a 1,000-year timescale. This insight may assist water-resources managers in alleviating one of the world's largest groundwater contamination problems.

Injection of desalination brine into the saline part of the coastal aquifer; environmental and hydrological implications.

Seawater desalination, specifically reverse osmosis (RO), has become an important water resource, especially in arid and semi-arid regions. The desalination process generates a brine solution that is usually discharged to the nearshore environment, negatively impacting the marine ecosystem. A different method of disposing of the brine solution is needed to restore and maintain the marine environment. One such method is injecting the desalination brine into the saline part of the coastal aquifer. This study aims to investigate the hydrological and environmental impacts of such injection using groundwater flow and solute transport numerical models, showing the fresh-saline water interface (FSI) response at different injection rates, depths, and distances from shore. Moreover, this study investigates the recovery evolution of the aquifer after injection stops. We also analyze the hydrological response when pumping saline groundwater (SGW) (for desalination) or freshwater (for water supply) simultaneously with brine injection. Results show that brine injection creates a high salinity plume that pushes the FSI landward and salinizes the aquifer. After 20 years of injecting 5 million m3 y-1, 17 million m3 of freshwater are lost due to salt contamination. It is also shown that by injecting further offshore and in shallower depths, the impact on the FSI and the aquifer is reduced. Furthermore, pumping SGW simultaneously with brine injection negates the brine plume effect on the FSI and results in a more stable interface. It is noted that aquifer recovery is a long process and even after 100 years of recovery, the aquifer is not fully rehabilitated. This paper shows for the first time the hydrological implications of brine injection into the saline part of the coastal aquifer and demonstrates its potential utility for desalination plants in protecting the environment.

Submarine groundwater discharge data at meter scale (223Ra, 224Ra, 226Ra, 228Ra and 222Rn) in Indian River Bay (Delaware, US).

Submarine groundwater discharge (SGD) was sampled at high-spatial resolution in Indian River Bay, DE, USA, in July 2016 to characterize the spatial variability of the activity of the radium and radon isotopes commonly used to estimate SGD. These data were part of an investigation into the methods and challenges of characterizing SGD rates and variability, especially in the coastal aquifer transition from freshwater to saltwater (Hydrogeological processes and near shore spatial variability of radium and radon isotopes for the characterization of submarine groundwater discharge (Duque et al., 2019)). Samples were collected with seepage meters and minipiezometers to obtain sufficient volumes for analytical characterization. Seepage meter samples (for 223Ra, 224Ra, 226Ra, and 228Ra) were collected at two-hour intervals over a semi-diurnal tidal cycle from 30 seepage meters. Samples for 222Rn characterization were collected with a minipiezometer from 25 cm below the bay bed at each seepage meter location. All samples were analyzed with standard and state of the art procedures.

Megacity pumping and preferential flow threaten groundwater quality.

Many of the world's megacities depend on groundwater from geologically complex aquifers that are over-exploited and threatened by contamination. Here, using the example of Dhaka, Bangladesh, we illustrate how interactions between aquifer heterogeneity and groundwater exploitation jeopardize groundwater resources regionally. Groundwater pumping in Dhaka has caused large-scale drawdown that extends into outlying areas where arsenic-contaminated shallow groundwater is pervasive and has potential to migrate downward. We evaluate the vulnerability of deep, low-arsenic groundwater with groundwater models that incorporate geostatistical simulations of aquifer heterogeneity. Simulations show that preferential flow through stratigraphy typical of fluvio-deltaic aquifers could contaminate deep (>150 m) groundwater within a decade, nearly a century faster than predicted through homogeneous models calibrated to the same data. The most critical fast flowpaths cannot be predicted by simplified models or identified by standard measurements. Such complex vulnerability beyond city limits could become a limiting factor for megacity groundwater supplies in aquifers worldwide.

Investigation of Seepage Meter Measurements in Steady Flow and Wave Conditions.

Water exchange between surface water and groundwater can modulate or generate ecologically important fluxes of solutes across the sediment-water interface. Seepage meters can directly measure fluid flux, but mechanical resistance and surface water dynamics may lead to inaccurate measurements. Tank experiments were conducted to determine effects of mechanical resistance on measurement efficiency and occurrence of directional asymmetry that could lead to erroneous net flux measurements. Seepage meter efficiency was high (average of 93%) and consistent for inflow and outflow under steady flow conditions. Wave effects on seepage meter measurements were investigated in a wave flume. Seepage meter net flux measurements averaged 0.08 cm/h-greater than the expected net-zero flux, but significantly less than theoretical wave-driven unidirectional discharge or recharge. Calculations of unidirectional flux from pressure measurements (Darcy flux) and theory matched well for a ratio of wave length to water depth less than 5, but not when this ratio was greater. Both were higher than seepage meter measurements of unidirectional flux made with one-way valves. Discharge averaged 23% greater than recharge in both seepage meter measurements and Darcy calculations of unidirectional flux. Removal of the collection bag reduced this net discharge. The presence of a seepage meter reduced the amplitude of pressure signals at the bed and resulted in a nearly uniform pressure distribution beneath the seepage meter. These results show that seepage meters may provide accurate measurements of both discharge and recharge under steady flow conditions and illustrate the potential measurement errors associated with dynamic wave environments.

Spatial and temporal variations of groundwater arsenic in South and Southeast Asia.

Over the past few decades, groundwater wells installed in rural areas throughout the major river basins draining the Himalayas have become the main source of drinking water for tens of millions of people. Groundwater in this region is much less likely to contain microbial pathogens than surface water but often contains hazardous amounts of arsenic--a known carcinogen. Arsenic enters groundwater naturally from rocks and sediment by coupled biogeochemical and hydrologic processes, some of which are presently affected by human activity. Mitigation of the resulting health crisis in South and Southeast Asia requires an understanding of the transport of arsenic and key reactants such as organic carbon that could trigger release in zones with presently low groundwater arsenic levels.