Arsenic-contaminated aquifers are currently estimated to affect ~150 million people around the world. However, the full extent of the problem remains elusive. This is also the case in Pakistan, where previous studies focused on isolated areas. Using a new data set of nearly 1200 groundwater quality samples throughout Pakistan, we have created state-of-the-art hazard and risk maps of arsenic-contaminated groundwater for thresholds of 10 and 50 μg/liter. Logistic regression analysis was used with 1000 iterations, where surface slope, geology, and soil parameters were major predictor variables. The hazard model indicates that much of the Indus Plain is likely to have elevated arsenic concentrations, although the rest of the country is mostly safe. Unlike other arsenic-contaminated areas of Asia, the arsenic release process in the arid Indus Plain appears to be dominated by elevated-pH dissolution, resulting from alkaline topsoil and extensive irrigation of unconfined aquifers, although pockets of reductive dissolution are also present. We estimate that approximately 50 million to 60 million people use groundwater within the area at risk, with hot spots around Lahore and Hyderabad. This number is alarmingly high and demonstrates the urgent need for verification and testing of all drinking water wells in the Indus Plain, followed by appropriate mitigation measures.

INTRODUCTION

The trace element arsenic (As) is found throughout Earth’s crust and hydrosphere (1). In particular, arsenic can strongly affect groundwater quality through natural geogenic leaching processes from host rocks and sediments (2–5). The general geochemical conditions that lead to mobilization of arsenic into groundwater are characterized by one or more of the following features: reducing (6–8) environments, arid oxidizing environments with elevated pH (1, 9, 10), geothermal activity (11, 12) and/or oxidative weathering of sulfide minerals (13, 14). Aquifers within Holocene sediments are particularly susceptible to arsenic enrichment due to the sediments’ limited time of exposure to groundwater flushing such that the sediments continue to hold a relative abundance of mobilizable arsenic within its grains (15, 16). Arsenic concentrations can also increase due to a low hydrological gradient, resulting in sluggish groundwater flow (1), as well as a strongly arid environment that leads to evaporative concentration (11, 17).

Regular consumption of water containing high concentrations of arsenic can have adverse health effects, including skin disorders, lung cancer, and cardiovascular diseases (18, 19). In actuality, arsenic-contaminated water is one of the most serious global health threats, with currently estimated 150 million people relying on arsenic-contaminated groundwater (5). The permissible concentration of arsenic in drinking water set by the World Health Organization (WHO) is 10 μg/liter, whereas the guideline in Pakistan is 50 μg/liter (20).

To determine where best to apply the limited resources for groundwater testing, geostatistical modeling can identify areas likely to be affected by arsenic contamination by finding statistically significant relationships measured arsenic concentrations and environmental predictors (21–25). As opposed to indicator kriging, such an approach takes into account the relevant physical processes of contaminant release and accumulation. This also has the advantage of being able to use spatially continuous predictor data sets to identify areas of high arsenic hazard, where groundwater quality data are lacking. Although this method can efficiently predict the occurrence of contamination on a large scale, it is generally ineffective at the scale of individual wells due to small-scale aquifer heterogeneities that are undetectable at the surface. Winkel et al. (25) explored the use of three-dimensional (3D) geological information in modeling, which, although more accurate, showed that models based on 2D geological information can effectively predict elevated concentrations of arsenic in groundwater.

Here, we investigate and model the distribution of arsenic in Pakistan, which faces critical water quality challenges. Although microbial contamination presents the most immediate health threat and causes one-third of all deaths in the country (20, 26), arsenic and other toxic metals pose a significant health hazard through chronic exposure (27–30). While the full health effects of arsenic in Pakistan are not yet known, various studies over the past decade have uncovered arsenic-related skin disorders (29, 31) and high levels of arsenic in blood and hair samples (32, 33) from people living in predominantly rural areas with high exposure to arsenic in groundwater. Food crops in the Sindh (34, 35) and Punjab (own measurements) provinces also indicate a potentially severe health threat due to plant uptake of arsenic via irrigation water extracted from shallow Holocene aquifers.

Numerous small-scale local studies, generally at the village level, have reported high arsenic concentrations in groundwater up to hundreds of micrograms per liter, primarily in the provinces of Punjab and Sindh (20, 30, 36–41). However, a lack of resources in the country has prevented the comprehensive evaluation of arsenic in groundwater (30). Considerable arsenic contamination has also been reported in other South and East Asian countries, for example, India, Bangladesh, Cambodia, and Vietnam (42–46). Shallow small-scale and family-based hand and motorized pumps have long been a major source of drinking water in the Indus Plain and are as widespread in Pakistan as in those other arsenic-affected regions of Asia. Higher-volume pumping with tube wells became popular throughout Pakistan in the 1960s and is used primarily not only for irrigation but also for municipal water supplies (47).

Pakistan is characterized by the flat-lying Indus Plain in the east; the Himalaya, Karakoram, and Hindu Kush mountain ranges in the north; hill regions in the northwest; and the Baluchistan plateau in the west. With the exception of the temperate northwest, the climate is semiarid to arid. The most significant aquifers of the country are found in the Indus Plain, which is composed of up to 300 m of Quaternary alluvial deposits and permeable soils low in organic content (48), with groundwater yields in the range of 100 to 300 m3/hour to 150-m depth (49). Windblown sands generally dominate in the neighboring desert regions (groundwater yields of 10 to 50 m3/hour), and permeable gravels of limited extent can be found in the northwest (49). Because of its abundant water resources and fertile soils, the Indus Plain of Pakistan hosts extensive agricultural production and a population of over 100 million people, including the major cities of Karachi, Islamabad, Lahore, and Hyderabad (Fig. 1). On account of a highly arid climate in the Indus Plain, extensive irrigation uses groundwater resources and a widespread canal system that distributes water from the Indus River and its main tributaries across the adjacent plains.

Fig. 1 Arsenic concentrations measured in Pakistan groundwater. Arsenic exceeds the WHO guideline of 10 μg/liter in large parts of the Indus plain. The green to brown coloring illustrates the topography. The Indus River and its major tributaries as well as the major cities are indicated. The samples were collected for this study (n = 1184) between 2013 and 2015.

The morphology and age of flat-lying, Holocene fluvial sediments along the Indus River and tributaries are similar to those of the well-known arsenic-affected areas of the Ganges/Brahmaputra Rivers in India and Bangladesh (5), the Red River in Vietnam (45), and the Mekong River in Cambodia and Vietnam (46). A chemically reducing environment generally dominates in the aquifers along these rivers, which is generally due to an abundance of organic material along with a limited supply of oxygen, and results in the desorption of arsenic from iron oxy(hydr)oxides. Depleted oxygen levels can come about, for example, due to an impermeable near-surface silt and/or clay layer that prevents contact of the aquifer with the atmosphere.

The aquifers of the Indus Plain, however, are generally unconfined and have hydraulic connectivity with the surface (48, 50). This results, for example, in a strong connection between surface water of the Indus Basin Irrigation System and the underlying aquifer (37, 51). Since the introduction of widespread irrigation, the water table has risen significantly with accompanying waterlogging and groundwater salinization (37).

Rather than being a detailed geochemical investigation, this study focuses on risk determination based on our new groundwater quality data set and has produced the first-ever statistically based arsenic hazard model and health risk map for Pakistan. Furthermore, the main geochemical conditions of arsenic release were assessed in conjunction with various environmental variables.