Holly A. Michael

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

Arsenic in Asia Millions of humans in South and Southeast Asia are exposed to potentially dangerous levels of the carcinogen arsenic via their drinking water every day. Although high arsenic levels are a known problem, a growing demand for drinking water drives the continued construction of new groundwater wells in these regions. Fendorf et al. (p. 1123) review chemical and hydrological factors controlling the release of arsenic in groundwater in South and Southeast Asia, which include the distribution of arsenic in groundwater aquifers used for drinking water and irrigation. Despite incomplete sampling and characterization of these factors across these regions, several key directions for improvements to water quality are presented. 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.

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 worlds largest groundwater contamination problems.

Arsenic migration to deep groundwater in Bangladesh influenced by adsorption and water demand

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.

Characterizing submarine groundwater discharge: A seepage meter study in Waquoit Bay, Massachusetts

[1] A seepage meter study was performed in Waquoit Bay on Cape Cod, Massachusetts to characterize the amount, pattern, and origin of submarine groundwater discharge. Measurements from grids of 40 seepage meters provide a detailed representation of groundwater flux in both space and time. At the head of the bay, a distinct band of high, saline discharge was observed between 25 and 45 m from the shoreline. Slug tests indicated no pattern of permeability to explain the band of discharge, and the band was not observed offshore of an island where freshwater discharge is negligible. Experiments using clusters of seepage meters showed large variability in discharge at the meter scale and similar temporal variation throughout the domain, reflecting tidal influence primarily near shore. The small-scale variability challenges the assumption of locally homogeneous flow used in many models, and the band of discharge contradicts predictions that total outflow is largely fresh and decreases monotonically from shore. INDEX TERMS: 1829 Hydrology: Groundwater hydrology; 4235 Oceanography: General: Estuarine processes; 1894 Hydrology: Instruments and techniques; 4825 Oceanography: Biological and Chemical: Geochemistry. Citation: Michael, H. A., J. S. Lubetsky, and C. F. Harvey, Characterizing submarine groundwater discharge: A seepage meter study in Waquoit Bay, Massachusetts, Geophys. Res. Lett., 30(6), 1297, doi:10.1029/ 2002GL016000, 2003.

Saltwater‐freshwater mixing dynamics in a sandy beach aquifer over tidal, spring‐neap, and seasonal cycles

The biogeochemical reactivity of sandy beach aquifers is closely linked to physical flow and solute transport processes. Thus, a clearer understanding of the hydrodynamics in the intertidal zone is needed to accurately estimate chemical fluxes to the marine environment. A field and numerical modeling study was conducted over a 1 year timeframe to investigate the combined effects of tidal stage, spring-neap variability in tidal amplitude, and seasonal inland water table oscillations on intertidal salinity and flow dynamics within a tide-dominated, microtidal sandy beach aquifer. Measured and simulated salinities revealed an intertidal saline circulation cell with a structure and cross-sectional mixing zone area that varied over tidal, spring-neap, and seasonal time scales. The size of the circulation cell and area of the mixing zone were shown for the first time to be most affected by seasonal water table oscillations, followed by tidal amplitude and tidal stage. The intertidal circulation cell expanded horizontally and vertically as the inland water table declined, displacing the fresh discharge zone and lower interface seaward. Over monthly spring-neap cycles, the center of the circulation cell shifted from beneath the backshore and upper beachface to the base of the beach. Salinity variations in the intertidal zone over semidiurnal tidal cycles were minimal. The dynamics of the circulation cell were similar in simulations with and without a berm. The highly transient nature of intertidal salinity over multiple time scales may have important implications for the types and rates of chemical transformations that occur in groundwater prior to discharge to the ocean.

Combining geologic-process models and geostatistics for conditional simulation of 3-D subsurface heterogeneity

[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 variogram-based 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 process-based model. We demonstrate the approach for a typical subsea depositional complex.

An Arsenic Forecast for China

A map of possible arsenic contamination in Chinas aquifers may guide mitigation efforts. [Also see Report by Rodríguez-Lado et al.] About 140 million people worldwide drink groundwater containing unsafe levels of arsenic (1). Chronic exposure to this tasteless, odorless poison leads to health effects such as skin lesions and cancer. In China, pollution is pervasive and anthropogenic groundwater contamination has attracted attention (2). Naturally-occurring arsenic is perhaps less widespread, yet equally dangerous to those exposed. Though the problem has been known for decades (3) and mitigation is ongoing (4), estimates of the exposed population differ widely (5, 6). On page 866 of this issue, Rodríguez-Lado et al. (7) assess the probability of the occurrence of unsafe arsenic levels in Chinas groundwater and identify at-risk areas where data are sparse. They suggest that more than 19 million Chinese may be drinking water above the World Health Organization guideline of 10 µg/liter. Such predictive models could guide action toward minimizing the impact of this widespread threat to human health.

Hydrologic dynamics and geochemical responses within a floodplain aquifer and hyporheic zone during Hurricane Sandy

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.

Behavior in a Spatially Explicit Groundwater Resource: Evidence from the Lab

This research uses laboratory experiments to examine how hydrogeologic properties of groundwater models influence decision making. The results reveal that pumping rates are highest when the underlying model is such that the future costs of groundwater use are broadcast evenly to all users, as a majority of participants behave myopically. There is less myopic behavior when the groundwater dynamics are governed by spatially explicit models, where the private cost of groundwater use is high relative to external costs. These results suggest that models used to simulate common-pool resource dynamics play an important role in determining both economic predictions and behavioral outcomes. Copyright 2012, Oxford University Press.