Groundwater Pumping For Irrigation Research Articles

A framework for parameter estimation, sensitivity analysis, and uncertainty analysis for holistic hydrologic modeling using SWAT+

Abstract. Parameter sensitivity analysis plays a critical role in efficiently determining main parameters, enhancing the effectiveness of the estimation of parameters and uncertainty quantification in hydrologic modeling. In this paper, we demonstrate an uncertainty and sensitivity analysis technique for the holistic Soil and Water Assessment Tool (SWAT+) model coupled with new gwflow module, spatially distributed, physically based groundwater flow modeling. The main calculated groundwater inflows and outflows include boundary exchange, pumping, saturation excess flow, groundwater–surface water exchange, recharge, groundwater–lake exchange and tile drainage outflow. We present the method for four watersheds located in different areas of the United States for 16 years (2000–2015), emphasizing regions of extensive tile drainage (Winnebago River, Minnesota, Iowa), intensive surface–groundwater interactions (Nanticoke River, Delaware, Maryland), groundwater pumping for irrigation (Cache River, Missouri, Arkansas) and mountain snowmelt (Arkansas Headwaters, Colorado). The main parameters of the coupled SWAT+gwflow model are estimated utilizing the parameter estimation software PEST. The monthly streamflow of holistic SWAT+gwflow is evaluated based on the Nash–Sutcliffe efficiency index (NSE), percentage bias (PBIAS), determination coefficient (R2) and Kling–Gupta efficiency coefficient (KGE), whereas groundwater head is evaluated using mean absolute error (MAE). The Morris method is employed to identify the key parameters influencing hydrological fluxes. Furthermore, the iterative ensemble smoother (iES) is utilized as a technique for uncertainty quantification (UQ) and parameter estimation (PE) and to decrease the computational cost owing to the large number of parameters. Depending on the watershed, key identified selected parameters include aquifer specific yield, aquifer hydraulic conductivity, recharge delay, streambed thickness, streambed hydraulic conductivity, area of groundwater inflow to tile, depth of tiles below ground surface, hydraulic conductivity of the drain perimeter, river depth (for groundwater flow processes), runoff curve number (for surface runoff processes), plant uptake compensation factor, soil evaporation compensation factor (for potential and actual evapotranspiration processes), soil available water capacity and percolation coefficient (for soil water processes). The presence of gwflow parameters permits the recognition of all key parameters in the surface and/or subsurface flow processes, with results substantially differing if the base SWAT+ models are utilized.

The changing impact of rural electrification on Indian agriculture

Rural electrification policies in the developing world primarily focus on household power, often at the cost of electricity supply to other productive sectors of the economy. We examine the consequences of this imbalance in rural electrification policy priority on agricultural development in India. Electric pumping of groundwater for irrigation is a major driver of India’s agricultural growth. However, the government of India shifted its rural electrification focus towards universal household electrification starting early 2000s. Using a newly constructed panel-dataset spanning three decades, we find that districts electrifying after the policy change experience much lower gains in electrified groundwater irrigation. On average, electrifying 100 additional rural households is associated with an increase of two additional electrified wells among newly electrified districts – eight times lower compared to 16 electrified wells per 100 electrified households among districts electrified pre-policy change. Our estimates imply that newly electrified districts would have witnessed nearly 20% more irrigated cropland in the dry season if rural electrification policy priorities had not shifted away from agriculture. These results highlight the need to complement household electrification with powering income-generating sectors of the rural economy.

Natural and anthropogenic drivers of the lost groundwater from the Ganga River basin

Ganga river basin is the most populated and among the worst water-stressed river basins in the world. The basin contributes to 40% of India’s gross domestic product. Despite the Ganga basin’s cultural, heritage, and economic importance, the interplay among the crucial factors that make the basin one of the global groundwater depletion hotspots is not well understood. Using observations from wells and Gravity Recovery and Climate Experiment satellites and simulations from a hydrological model, here we show that the Ganga river basin has lost 226.57 ± 25.22 km3 groundwater during 2002–2016, which is about 20 times the storage capacity of the largest (Indira Sagar) reservoir in India. A significant (p-value < 0.05) decline (∼11%) in the summer monsoon (June–September) during 1951–2016, severe and frequent droughts (2009, 2014, 2015), and groundwater pumping for irrigation have contributed to groundwater depletion. However, the non-renewable groundwater abstraction is the most significant (relative contribution = 80%) contributor to the groundwater depletion in the basin. Renewable groundwater pumping contributed to only 20% of the total groundwater depleted during the 2002–2016 period. Severe and frequent droughts in the basin pose a double whammy of reducing groundwater recharge and increasing withdrawal. Changes in cropping patterns, groundwater metering, and improved water use efficiency are needed to reduce the non-renewable groundwater abstraction for irrigation, which is crucial for water sustainability in the basin.

Assessing Stream-Aquifer Connectivity in a Coastal California Watershed

We report the results of field and laboratory investigations of stream-aquifer interactions in a watershed along the California coast to assess the impact of groundwater pumping for irrigation on stream flows. The methods used include subsurface sediment sampling using direct-push drilling, laboratory permeability and particle size analyses of sediment, piezometer installation and instrumentation, stream discharge and stage monitoring, pumping tests for aquifer characterization, resistivity surveys, and long-term passive monitoring of stream stage and groundwater levels. Spectral analysis of long-term water level data was used to assess correlation between stream and groundwater level time series data. The investigations revealed the presence of a thin low permeability silt-clay aquitard unit between the main aquifer and the stream. This suggested a three layer conceptual model of the subsurface comprising unconfined and confined aquifers separated by an aquitard layer. This was broadly confirmed by resistivity surveys and pumping tests, the latter of which indicated the occurrence of leakage across the aquitard. The aquitard was determined to be 2–3 orders of magnitude less permeable than the aquifer, which is indicative of weak stream-aquifer connectivity and was confirmed by spectral analysis of stream-aquifer water level time series. The results illustrate the importance of site-specific investigations and suggest that even in systems where the stream is not in direct hydraulic contact with the producing aquifer, long-term stream depletion can occur due to leakage across low permeability units. This has implications for management of stream flows, groundwater abstraction, and water resources management during prolonged periods of drought.

Numerical groundwater flow modeling for managing the Gabes Jeffara aquifer system (Tunisia) in relation with oasis ecosystems

The Gabes Jeffara aquifer system, in southeastern Tunisia, is essentially recharged by rainfall infiltration and by groundwater inflow from the Intercalary Continental aquifer in the northwest. The increase in groundwater pumping for irrigation in recent decades has induced a serious decrease in groundwater levels, depletion of springs and degradation of oasis ecosystems. A multidisciplinary study was carried out to better understand the behavior of oasis ecosystems and aquifer systems and to provide tools and recommendations for water resources management. An important part of the study was devoted to developing a hydrodynamic flow model of the Jeffara aquifer system, which can be used as a future groundwater management tool considering different recharge or exploitation scenarios. This model was built with Processing Modflow, gathering data on geology, hydrogeology, hydrology, rainfall, piezometry, withdrawals and spring flow rates. The model was calibrated in steady state with reference to the piezometric levels measured in 1970 and in transient state for the period 1972–2014, using records from more than 200 wells and piezometers. The analysis of current and future water consumption was carried out with existing data, processed satellite images and farmer surveys. This analysis was used to define water demand scenarios combined with scenarios of decreased groundwater withdrawal, reinforced groundwater recharge and use of alternative water resources. The scenarios tested with the model show that the situation will be critical in less than 25 years without radical measures to reduce groundwater withdrawals by at least 60 MCM/year.

Assimilating GRACE Into a Land Surface Model in the Presence of an Irrigation‐Induced Groundwater Trend

AbstractAssimilating terrestrial water storage observations from the Gravity Recovery and Climate Experiment (GRACE) mission into land surface models (LSMs) provides an opportunity to disaggregate and downscale GRACE information to finer scales and improve water component estimates in LSMs. However, the performance of GRACE data assimilation (GRACE‐DA) is limited by the lack of representation of human activities in most LSMs. To simultaneously improve GRACE‐DA and reduce the uncertainties in the modeled anthropogenic processes, we assimilate mascon‐based GRACE terrestrial water storage into the Noah‐Multiparameterization LSM that includes groundwater extraction for irrigation. Simulations with and without GRACE‐DA and with and without groundwater pumping for irrigation are performed to study the isolated and combined effects of groundwater irrigation and GRACE‐DA on water and energy fluxes over the High Plains Aquifer (HPA). The results reveal that the DA‐only simulation may erroneously distribute increments across water storage components and affect the related fluxes through biased feedbacks, while the irrigation‐only simulation may overestimate groundwater decline due to shortcomings in the irrigation and groundwater parameterizations. Assimilating GRACE when irrigation is simulated produces the best overall performance for water storage trends over the northern HPA. For the southern HPA, GRACE assimilation with irrigation performs similarly to irrigation‐only simulation for water storage components and evapotranspiration. GRACE assimilation also improves results in nonirrigated regions and can potentially alleviate the overestimation of groundwater trends in regions with greater irrigation uncertainties. This study highlights the potential to advance hydrological data assimilation in the context of anthropogenic water consumption and land‐atmosphere interactions.

Delineation of critical regions for mitigation of carbon emissions due to groundwater pumping in central Punjab

The expansion of groundwater use has not only resulted in a speedy decline in the groundwater table but also added to the problem of carbon emissions (C-emissions) in Punjab. Estimates of C-emissions from groundwater pumping for irrigation in central Punjab indicated that during the last 15 years (1998–2013), the groundwater levels had fallen by 8.89 m; pump set density (per 1000 ha) increased by 45%; groundwater use increased by 140 Mm3; energy requirements increased by 4845.1 MkWh (90%), all this resulted into an increase in C-emissions by 984.1 kiloton (30%). The relationship between carbon emissions and groundwater depth showed that for each meter fall of groundwater level in central Punjab, C emissions will increase by 2.67 g/m³. The present level of groundwater exploitation in Bhawanigarh, Dharmkot, Dhuri, Malerkotla I, Malerkotla II, Nabha, Nakodar, Sangrur, Samana, Sherpur and Sunam is considered as threat to the environment and should be targeted to reduce C-emissions.

The water–food–energy nexus in Pakistan: a biophysical and socio-economic challenge

Abstract. We draw on previous work examining historical trends, likely future water use and food availability in Pakistan and extend the analysis to consider interactions with hydropower generation and the energy demand in food production due to pumping of groundwater for irrigation. Business-as-usual scenarios suggest growing demands for groundwater and energy use for food production as population grows rapidly. However, groundwater use is already unsustainable in many areas, and energy supply is failing to keep up with demand. Quantifying material linkages between water, food and energy provides a means to explore biophysical constraints. Characterising institutional constraints is equally important, as they can be significant barriers to effective stewardship of water, energy and food resources. The experience in Pakistan reinforces this finding, and we discuss the implications for hydrologists.

Impact of irrigated agriculture on groundwater resources in a temperate humid region

The groundwater irrigation expansion, and its multiple potential impacts on the quantity and quality of water resources, is not just restricted to areas that are water limited. In this study we present the seasonal impacts irrigation practices can have on groundwater resources in a temperate humid region, where the average annual rain/PET ratio is 1.0. In this system the irrigation expansion is solely supported by groundwater pumping, but despite this only 5 boreholes are monitored for hydraulic head data. In this study, we compensate the scarce hydrophysical dataset by incorporating environmental tracers (major ions, δ18O, δ2H and δ13C) and dating tracers (3H, CFC, SF6 and 14C). Results indicate that at 9 of the 15 irrigation sites investigated, groundwater pumping for irrigation has induced the mixing of recent groundwater (up to <1year) with older waters. The origin of the older waters was from either the deeper marl aquifer, or the shallow sand-clay aquifer (SCB) that has a 14C mean residence time (MRT) of up to 9700years. Secondly, although high nitrate loads in infiltrating waters were being diverted via the artificial subsurface drainage system, increases in fertiliser loads have resulted in higher NO3 concentrations in younger groundwater (NO3: 9–45mg/L, MRT <20years), compared with older groundwater (NO3≤9mg/L, MRT>20years). The changes in flow pathways, induced by irrigation, also results in seasonal declines in groundwater NO3 concentrations due to mixing with older waters. In temperate humid areas, such evaluations of the seasonal evolution of water residence time, mixing process, and agrochemical contaminants are an important contribution to real water resources management in irrigated catchments.

Assessment and Mitigation of Greenhouse Gas Emissions from Groundwater Irrigation

Irrigation with groundwater consumes considerable energy as well as water resources across the world. Using a case study from Indian Punjab, this article emphasizes how a continued and massive use of groundwater for irrigation has reduced groundwater levels and increased carbon emissions. Estimates of C emissions from groundwater pumping for irrigation in Punjab indicate that over a period of 14 years (1998–2012), groundwater use has increased by 23%; groundwater levels have fallen by 5.47 m; energy requirements have increased by 67% resulting in increase in C emissions by 110%. Emissions rates have increased from 33 to 55 g m−3 of groundwater used, and 43.2 to 78 g-C kg−1 of grain. Thus, groundwater management is not only important to ensure sustainability of the finite resource but also is vital to control environmental consequences of groundwater use for irrigation. Copyright © 2016 John Wiley & Sons, Ltd.

Groundwater modeling as a precursor tool for water resources sustainability in Khatt area, UAE

Groundwater is the main source of irrigation in the Khatt area located in the northeastern part of the United Arab Emirates. The excess pumping of groundwater for irrigation during the last three decades has decreased the average groundwater flow by one tenth. Water resources in the region are of strategic national importance. Therefore, the main objective of this paper is to establish an accurate groundwater budget model that will enable assessment of water availability in the Khatt Basin. The well-known software MODFLOW is used to simulate groundwater in the study area. Several simulations for different periods were conducted including a steady state calibration simulation for 1969, a transient calibration simulation from 1970 to 1986, and a verification simulation that represents current conditions. Additionally, future scenarios were simulated till 2035. Results show that the current extraction rates are not sustainable and need to be reduced by at least 25 % to mitigate the present severe groundwater depletion and to achieve sustainable development in the area.

Tools to Estimate Groundwater Levels in the Presence of Changes of Precipitation and Pumping

Central Wisconsin has the greatest density of high capacity wells in the state, most of which are used for agricultural irrigation. Irrigated agriculture has been growing steadily in the region since the 1950’s, when irrigation systems and high capacity wells became inexpensive and easy to install. Recent low lake and river levels have increased concerns that unregulated groundwater pumping for irrigation will undermine the availability of groundwater to support surface waters and domestic uses. Some research has quantified the magnitude of groundwater level declines due to irrigation pumping, but no studies have identified its relation to climatic precipitation changes. Changes in precipitation can appear to exacerbate or mask the effect of groundwater pumping. In this study, four groundwater monitoring wells and five climate stations were examined for shifts in groundwater levels and precipitation changes. Through statistical analysis, significant precipitation increases were identified in the southern part of the study area which averaged 2.7 mm per year, but no significant change was determined for the northern portion. Bivariate analysis identified water level declines within the region in the years 1974, 1992 and 1999 for irrigated land covers. Multiple regression analysis explained, predicted and quantified the interaction between precipitation and pumping. Wells located in areas with many high capacity wells showed a decline in water levels of up to 1.28 meters. In the southern portion of the study area where increases in precipitation occurred, this decline was thought to be masked. Results for one region (Plover) agreed with a previously published calibrated groundwater model, which demonstrates that this statistical method may be used to separate the impact of groundwater pumping from changing precipitation, even where observation well data are not widely available.

A persistent scatterer interpolation for retrieving accurate ground deformation over InSAR‐decorrelated agricultural fields

AbstractInterferometric synthetic aperture radar (InSAR) is a radar remote sensing technique for measuring surface deformation to millimeter‐level accuracy at meter‐scale resolution. Obtaining accurate deformation measurements in agricultural regions is difficult because the signal is often decorrelated due to vegetation growth. We present here a new algorithm for retrieving InSAR deformation measurements over areas with severe vegetation decorrelation using adaptive phase interpolation between persistent scatterer (PS) pixels, those points at which surface scattering properties do not change much over time and thus decorrelation artifacts are minimal. We apply this algorithm to L‐band ALOS interferograms acquired over the San Luis Valley, Colorado, and the Tulare Basin, California. In both areas, the pumping of groundwater for irrigation results in deformation of the land that can be detected using InSAR. We show that the PS‐based algorithm can significantly reduce the artifacts due to vegetation decorrelation while preserving the deformation signature.

Examples of deformation-dependent flow simulations of conjunctive use with MF-OWHM

Abstract. The dependency of surface- and groundwater flows and aquifer hydraulic properties on deformation induced by changes in aquifer head is not accounted for in the standard version of MODFLOW. A new USGS integrated hydrologic model, MODFLOW-OWHM, incorporates this dependency by linking subsidence and mesh deformation with changes in aquifer transmissivity and storage coefficient, and with flows that also depend on aquifer characteristics and land-surface geometry. This new deformation-dependent approach is being used for the further development of the integrated Central Valley hydrologic model (CVHM) in California. Preliminary results from this application and from hypothetical test cases of similar systems show that changes in canal flows, stream seepage, and evapotranspiration from groundwater (ETgw) are sensitive to deformation. Deformation feedback has been shown to also have an indirect effect on conjunctive surface- and groundwater use components with increased stream seepage and streamflows influencing surface-water deliveries and return flows. In the Central Valley model, land subsidence may significantly degrade the ability of the major canals to deliver surface water from the Delta to the San Joaquin and Tulare basins. Subsidence can also affect irrigation demand and ETgw, which, along with altered surface-water supplies, causes a feedback response resulting in changed estimates of groundwater pumping for irrigation. This modeling feature also may improve the impact assessment of dewatering-induced land subsidence/uplift (following irrigation pumping or coal-seam gas extraction) on surface receptors, inter-basin transfers, and surface infrastructure integrity.

Quantifying water and energy budgets and the impacts of climatic and human factors in the Haihe River Basin, China: 1. Model and validation

We have developed an operational model to simulate water and energy fluxes in the Haihe River Basin (231,800 km(2) in size) for the past 28 years. This model is capable of estimating water and energy fluxes of irrigated croplands and heterogeneous grids. The model was validated using actual evapotranspiration (ETa) measured by an eddy covariance system, measured soil moisture in croplands, groundwater level measurements over the piedmont plain and runoff observations in a mountainous catchment. A long-term time series of water and energy balance components were then simulated at a daily time step by integrating remotely sensed information and meteorological data to examine the spatial and temporal distribution and changes in water and energy fluxes in the basin over the past 28 years. The results show that net radiation (R-n) in the mountainous regions is generally higher than that in the plain regions. ETa, in the plain regions is higher than that in the mountainous regions mostly because of higher air temperature and larger areas of irrigated farmland. Higher sensible heat flux (H) and lower ETa in the urban areas are possibly due to less vegetation cover, an impervious surface, rapid drainage, and the heat island effect of cities. During the study period, a water deficit continuously occurred in the plain regions because of extensive pumping of groundwater for irrigation to meet the crop water requirements. Irrigation has led to significant groundwater depletion, which poses a substantial challenge to the sustainability of water resources in this basin. (C) 2015 Elsevier B.V. All rights reserved.

Pond-Derived Organic Carbon Driving Changes in Arsenic Hazard Found in Asian Groundwaters

Microbially mediated reductive processes involving the oxidation of labile organic carbon are widely considered to be critical to the release of arsenic into shallow groundwaters in South and Southeast Asia. In areas where there is significant pumping of groundwater for irrigation the involvement of surface-derived organic carbon drawn down from ponds into the underlying aquifers has been proposed but remains highly controversial. Here we present isotopic data from two sites with contrasting groundwater pumping histories that unequivocally demonstrate the ingress of surface pond-derived organic carbon into arsenic-containing groundwaters. We show that pond-derived organic carbon is transported to depths of up to 50 m even in an arsenic-contaminated aquifer in Cambodia thought to be minimally disturbed by groundwater pumping. In contrast, in the extensively exploited groundwaters of West Bengal, we show that pond-derived organic carbon is transported in shallow groundwater to greater depths, in excess of 100 m in the aquifer. Intensive pumping of groundwaters may potentially drive secular increases in the groundwater arsenic hazard in this region by increasing the contribution of bioavailable pond-derived dissolved organic carbon drawn into these aquifer systems and transporting it to greater depths than would operate under natural flow conditions.

Hydrogeochemical and Isotopic Investigations on Groundwater Salinization in the Datong Basin, Northern China1

Abstract: Analyses of major elements, environmental isotope ratios (δ18O, δ2H), and PHREEQC inverse modeling investigations were conducted to understand the processes controlling the salinization of groundwater within the Datong Basin. The hydrochemical results showed that groundwater with high total dissolved solid (TDS) concentrations was dominated by sodium bicarbonate (Na‐HCO3), sodium chlorite (Na‐Cl), and sodium sulfate (Na‐SO4) type waters, whereas low‐TDS groundwater from near mountain areas was dominated by calcium bicarbonate (Ca‐HCO3) and magnesium bicarbonate (Mg‐HCO3) type waters. The characterization of the major components of groundwater and PHREEQC inverse modeling indicated that the aluminosilicate hydrolysis, cation exchange, and dissolution of evaporites (halite, mirabilite, and gypsum) governed the salinization of groundwater within the Datong Basin. The environmental isotope (δ18O, δ2H) and Cl−/Br− ratios revealed the impact of fast vertical recharge by irrigation returns and salt‐flushing water on the groundwater salinization. According to the analyses of major hydrochemical components and PHREEQC inverse modeling, evaporite dissolution associated with irrigation and salt‐flushing practice was probably the dominant controlling factor for the groundwater salinization, especially in the central part of the basin. Therefore, groundwater pumping for irrigation and salt‐flushing should be controlled to protect groundwater quality in this area.

China’s water–energy nexus: greenhouse-gas emissions from groundwater use for agriculture

China is the world’s largest emitter of greenhouse gases (GHGs) and the agricultural sector in China is responsible for 17–20% of annual emissions and 62% of total freshwater use. Groundwater abstraction in China has increased rapidly from 10 km3 yr−1 in the 1950s to more than 100 km3 yr−1 in the 2000s, such that roughly 70% of the irrigated area in northern China is now groundwater-fed. Pumping of water for irrigation is one of the most energy consuming on-farm processes; however, to date this source of GHG emissions in China and elsewhere has been relatively neglected. We derive the first detailed estimate of GHG emissions from groundwater pumping for irrigation in China, using extensive village survey data from 11 provinces, broadly representative of the situation during the mid-2000s. The 11 provinces cover roughly half of China’s irrigated cropland and we upscale to the national level using government statistics for the remaining 20 provinces. Our results show emissions of 33.1 MtCO2e, just over half a per cent of the national total. Groundwater abstraction represents an important source of GHG emissions that has been rapidly increasing and which at present is largely unregulated. Water scarcity in China is already driving policies to improve water conservation. These results suggest that significant potential exists to promote the co-benefits of water and energy saving in order to meet national planning targets.

Influence of irrigation practices on arsenic mobilization: Evidence from isotope composition and Cl/Br ratios in groundwater from Datong Basin, northern China

Summary Environment isotopes (δ18O and δ2H) and Cl/Br ratios in groundwater have been used to trace groundwater recharge and geochemical processes for arsenic contamination in Datong Basin. The arsenic concentrations of groundwater samples ranged from 0.4 to 434.9 μg/L with the average of 51.2 μg/L, which exceeded China’s drinking water standard (10 μg/L). All the groundwater samples are plotted on or close to the meteoric water line of the δ18O vs. δ2H plot, indicating their meteoric origin. The relationship between δ18O values and Cl/Br ratios and Cl concentrations demonstrate that leaching and mixing are the dominant processes affecting the distribution of high arsenic groundwater in this area. The observed non-linearity in the trend between δ18O and arsenic concentration is due to combined effects of mixing and leaching. The similarity of the trend in Cl/Br ratios and δ18O values for high arsenic groundwater demonstrate that extensive leaching of irrigation return and salt flushing water flow could be the dominant process driving arsenic mobilization in the groundwater system. Moreover, the long term irrigation practice can cause the drastic change of the biogeochemical and redox condition of in the aquifer system, which in turn promotes the mobilization of arsenic. Therefore, groundwater pumping for irrigation in this area of waterborne endemic arsenic poisoning should be under strict control to protect groundwater quality in this area.

Arsenic in South Asia Groundwater

AbstractWithin the deltas of South Asia, widespread consumption of groundwater containing dangerous levels of arsenic adversely impacts tens of millions of people in Bangladesh, West Bengal, India, Cambodia, Vietnam, and Myanmar. This massive health crisis is the product of a confluence of processes initiated by the erosion of arsenic‐bearing minerals in the Himalaya, transport of sediments containing arsenic‐bearing iron oxides down the Brahmaputra–Ganges, Mekong, Irrawaddy and Red river systems, and deposition as deltaic sediments. Upon burial, dissolution of these arsenic‐bearing iron‐oxide minerals is triggered by reaction with organic carbon, releasing arsenic to the aquifer and producing aqueous concentrations that can exceed 1000 μg/L, 100 times the World Health Organization drinking water standard. These waters are the primary drinking water source for the majority of the people living in these low‐lying regions of Asia. Although groundwater contamination has likely persisted for millennia on the Asian deltas, consumption of these waters became widespread in the 1970s as groundwater was tapped to avoid water‐borne diseases in surface water bodies. Groundwater pumping for irrigation and other land use changes, including widespread rice cultivation, are now impacting the distribution of arsenic in (presently) unpredictable ways. Further, irrigation with arsenic‐rich waters is decreasing rice yields and providing an additional pathway of human exposure. In general, deeper groundwater contains low levels of arsenic. Efforts must continue to focus on protecting this resource for human consumption.