No abstract

Drinking shallow groundwater with naturally elevated concentrations of arsenic is causing widespread disease in many parts of South and Southeast Asia. In the Bengal Basin, growing reliance on deep (>150 m) groundwater has lowered exposure. In the most affected districts of Bangladesh, shallow groundwater concentrations average 100 to 370 μg L−1, while deep groundwater is typically < 10 μg L−1. Groundwater flow simulations have suggested that, even when deep pumping is restricted to domestic use, deep groundwater in some areas of the Bengal Basin is at risk of contamination. However, these simulations have neglected the impedance of As migration by adsorption to aquifer sediments. Here we quantify for the first time As sorption on deeper sediments in situ by replicating the intrusion of shallow groundwater through injection of 1,000 L of deep groundwater modified with 200 μg L−1 of As into a deeper aquifer. Arsenic concentrations in the injected water were reduced by 70% due to adsorption within a single day. Basin-scale modelling indicates that while As adsorption extends the sustainable use of deep groundwater, some areas remain vulnerable; these areas can be prioritized for management and monitoring.

Storms dominate solute export budgets from catchments and drive hydrogeochemical changes in the near-stream environment. We captured near-stream hydrogeochemical dynamics during an intense storm (Hurricane Sandy, October 2012), by instrumenting a riparian-hyporheic zone transect of White Clay Creek in the Christina River Basin Critical Zone Observatory with pressure transducers, redox probes, and pore water samplers. In the floodplain aquifer, preferential vertical flow paths such as macropores facilitated rapid infiltration early in the storm. Water table rose quickly and promoted continuous groundwater discharge to the stream. Floodplain-hillslope topography controlled poststorm aquifer drainage rates, as the broad, western floodplain aquifer drained more slowly than the narrow, eastern floodplain aquifer adjacent to a steep hillslope. These changes in groundwater flow drove heterogeneous geochemical responses in the floodplain aquifer and hyporheic zone. Vertical infiltration in the floodplain and hyporheic exchange in the streambed increased DOC and oxygen delivery to microbially active sediments, which may have enhanced respiration. Resulting geochemical perturbations persisted from days to weeks after the storm. Our observations suggest that groundwater-borne solute delivery to streams during storms depends on unique interactions of vertical infiltration along preferential pathways, perturbations to groundwater geochemistry, and topographically controlled drainage rates.

[1] The goal of simulation of aquifer heterogeneity is to produce a spatial model of the subsurface that represents a system such that it can be used to understand or predict flow and transport processes. Spatial simulation requires incorporation of data and geologic knowledge, as well as representation of uncertainty. Classical geostatistical techniques allow for the conditioning of data and uncertainty assessment, but models often lack geologic realism. Simulation of physical geologic processes of sedimentary deposition and erosion (process-based modeling) produces detailed, geologically realistic models, but conditioning to local data is limited at best. We present an aquifer modeling methodology that combines geologic-process models with object-based, multiple-point, and variogrambased geostatistics to produce geologically realistic realizations that incorporate geostatistical uncertainty and can be conditioned to data. First, the geologic features of grain size, or facies, distributions simulated by a process-based model are analyzed, and the statistics of feature geometry are extracted. Second, the statistics are used to generate multiple realizations of reduced-dimensional features using an object-based technique. Third, these realizations are used as multiple alternative training images in multiple-point geostatistical simulation, a step that can incorporate local data. Last, a variogram-based geostatistical technique is used to produce conditioned maps of depositional thickness and erosion. Successive realizations of individual strata are generated in depositional order, each dependent on previously simulated geometry, and stacked to produce a fully conditioned three-dimensional facies model that mimics the architecture of the processbased model. We demonstrate the approach for a typical subsea depositional complex.

Nearly half of the world population inhabits coastal environments, and for these communities, groundwater is commonly the primary resource for drinking water. The highly dynamic nature of these settings dictates enhanced vulnerability to both natural and anthropogenic environmental stresses, leading to water scarcity and degradation (Michael et al., 2017). For coastal groundwater resources, the greatest threat is most often salinization due to seawater intrusion. Generally, there are two processes that can introduce seawater into coastal aquifers. The first is lateral saltwater intrusion (SWI) in which the position of the subsurface freshwater-saltwater interface moves landward due to reduction in the land-sea hydraulic gradient

Across South Asia, millions of villagers have reduced their exposure to high‐arsenic (As) groundwater by switching to low‐As wells. Isotopic tracers and flow modeling are used in this study to understand the groundwater flow system of a semi‐confined aquifer of Pleistocene (>10 kyr) age in Bangladesh that is generally low in As but has been perturbed by massive pumping at a distance of about 25 km for the municipal water supply of Dhaka. A 10‐ to 15‐m‐thick clay aquitard caps much of the intermediate aquifer (>40‐ to 90‐m depth) in the 3‐km2 study area, with some interruptions by younger channel sand deposits indicative of river scouring. Hydraulic heads in the intermediate aquifer below the clay‐capped areas are 1–2 m lower than in the high‐As shallow aquifer above the clay layer. In contrast, similar heads in the shallow and intermediate aquifer are observed where the clay layer is missing. The head distribution suggests a pattern of downward flow through interruptions in the aquitard and lateral advection from the sandy areas to the confined portion of the aquifer. The interpreted flow system is consistent with 3H‐3He ages, stable isotope data, and groundwater flow modeling. Lateral flow could explain an association of elevated As with high methane concentrations within layers of gray sand below certain clay‐capped portions of the Pleistocene aquifer. An influx of dissolved organic carbon from the clay layer itself leading to a reduction of initially orange sands has also likely contributed to the rise of As.

Elevated arsenic in Bengal Basin aquifers threatens human health. Most deep (>150 m) groundwater in Pleistocene aquifers is low in arsenic; however higher concentrations have been reported in the southwest border region. Here, we establish that this extensive arsenic contamination at depth is not associated with well failure. A combination of geochemistry and flow modeling constrains the factors that contribute to arsenic contamination at depth in this region. Deep groundwater in the affected area is younger (2.0 ± 0.6 kyr) than deep, low‐arsenic groundwater elsewhere (12.0 ± 4.0 kyr) based on radiocarbon. Stratigraphic data indicate pre‐Holocene deposition of the contaminated aquifers, but few low‐permeability strata. Numerical modeling indicates that this stratigraphic anomaly permits a natural flow system that transports shallow groundwater to depth. Thus, in areas lacking low‐permeability layers, arsenic contamination can occur in pre‐Holocene aquifers and is probably not an early sign of future deep contamination in regions with interbedded low‐permeability strata.