Elevated arsenic concentrations in groundwater of the Upper Indus Plain of Pakistan across a range of redox conditions.

Groundwater of the Ravi River floodplain is particularly elevated in arsenic (As) on both sides of the Pakistan-India border. To understand this pattern, 14 sites were drilled to 12-30 m depth across floodplains and doabs of Pakistan after testing over 20,000 wells. Drill cuttings were collected at 1.5 m intervals, 132 of which were sand overlain by 77 intervals of clay and/or silt. Radiocarbon dating of clay indicates deposition of the aquifer sands tapped by wells 20-30 kyr ago. Most (85 %) of the sand samples were gray in color, indicating partial reduction to Fe(II) oxides, whereas most (92 %) of the clay and/or silt samples were orange. Associations between groundwater electrical conductivity, dissolved Fe, sulfate, and nitrate suggest that wells can be elevated (>10 µg/L) in As in the region due to either reductive dissolution of Fe oxides, evaporative concentration, or alkali desorption. In the Ravi floodplain, 47 % of 6445 wells tested contain >10 µg/L As compared to only 9 % of 14,165 tested wells in other floodplains and doabs. The As content of aquifer sands in the Ravi floodplain of Pakistan averages 4 ± 4 mg/kg (n = 66) and is higher than the average of 2 ± 2 mg/kg (n = 51) for aquifer sands outside the Ravi. Synchrotron spectroscopy and column-based speciation indicate predominance of As(V) over As(III) in both aquifer sands and groundwater. Whereas multiple processes may be responsible for elevated levels of As in groundwater across the region, spatial heterogeneity in groundwater As concentrations in the Ravi floodplain seems linked to variations in As concentrations in aquifer sands. Regulation by the solid phase may limit variations in groundwater As over time in response to natural and human-induced changes in hydrology. This means spatial heterogeneity could be taken advantage of to lower the exposure across the region with more testing and targeted drilling.

Characterization and role of derived dissolved organic matter on arsenic mobilization in alluvial aquifers of Punjab, Pakistan.

Biogeochemical mobilization of arsenic in groundwater depends on the presence of dissolved organic matter (DOM) that likely promotes the As release, i.e., reductive dissolution, complexation, competition, and electron shuttling. We investigated the role of DOM in As release, along with its complete characterization, in the Indus plain of Pakistan, one of the worst arsenic impacted zones in the South Asian region. In total, 60 groundwater and 15 soil samples, collected at six sites from north to south within the flood plain of the Ravi River, Lahore, Pakistan were investigated. Arsenic concentration ranged from 9.61 µg/L to 386 µg/L in the groundwater samples (high As observed in areas close to the river). Dissolved organic carbon (DOC) in 29 groundwater samples ranged between 0 and 10.1 mg-C/L. A moderately positive correlation of As with DOC and Fe in the northern part of the study area suggest the reductive dissolution of FeOOH associated with dissolved organic matter (DOM). The reductive dissolution plays an essential role for As enrichment in the area evidenced by the lower concentrations of SO42-, NO3-, and PO34-and a non-correlative pattern with As. In contrast, a positive correlation of As with PO34-, DOC, and HCO3- in the southern part indicate competitive desorption behind the As release. Fluorescence excitation-emission matrix intensity data of DOM indicate the maximum presence of humic-like substances in the northern part that gradually shifts to aromatic, fulvic and protein type towards the southern part. Specific ultraviolet absorbance and fluorescence index display aromatic and terrestrial (allochthonous) sources of DOM near the riverbank and mixed (both allochthonous and autochthonous) source away from the river. The positive correlations of As with DOC and fluorescence intensity also attest that DOM played a vital role in the As mobilization in groundwater of the study area.

High levels of fluoride contamination in groundwater of the semi-arid alluvial aquifers, Pakistan: evaluating the recharge sources and geochemical identification via stable isotopes and other major elemental data.

Hydrogeochemical methods were integrated to delineate the geochemical factors controlling fluoride (F-) contamination in groundwater at four sites in the districts of Lahore (Samada) and Kasur (Sari Chimba, Kot Maiga, and Chah Fatehwala) in Panjab province of Pakistan. Hydrochemical data and stoichiometric ratios indicate Na-Cl and Na-HCO3 as the dominant water types with silicate weathering influencing overall hydrogeochemistry of the study area. The groundwater F- concentrations ranged between 0.54 mg/L and 17.5 mg/L, with more than 70% samples having F- concentrations above the World Health Organization (WHO) provisional drinking water guideline (1.5 mg/L). Saturation indices determined that 100% samples were saturated with respect to calcite and 96% samples were undersaturated with respect to fluorite, indicating the influence of calcite precipitation on fluoride enrichment. A positive correlation was observed between fluoride with pH, Na+, and HCO3-, confirming that high fluoride concentrations were the result of weathering of silicate minerals and the exchange of OH- on clay surface under the alkaline pH conditions. The isotopic values of δ18O and δ2H in groundwater ranged from 9.14 to - 5.51‰ and 56.57 to - 39.5‰, respectively. The stable isotope data indicated the meteoric origin of groundwater with some evaporative effect, which is partly influencing groundwater quality such as high pH and salinity, as a result facilitating anion exchange (OH- for F-) on clays surface. The research indicates that the groundwater quality of the study area is not recommendable for drinking due to its high total dissolved solids (TDS) and elevated fluoride concentrations.

Field testing of over 30,000 wells for arsenic across 400 villages of the Punjab plains of Pakistan and India: Implications for prioritizing mitigation.

Most of the rural population of 90 million in Punjab province in Pakistan and Punjab state in India drinks, and cooks with, untreated water drawn from shallow wells. Limited laboratory testing has shown that groundwater in the region can contain toxic levels of arsenic. To refine this assessment, a total of 30,567 wells from 383 villages were tested with a field kit in northern Punjab province of Pakistan and western Punjab state of India. A subset of 431 samples also tested in the laboratory show that 85% of wells were correctly classified by the kit relative to the World Health Organization guideline of 10 µg/L for arsenic in drinking water. The kit data show that 23% of the tested wells did not meet the WHO guideline for arsenic but also that 87% of households with a well high in arsenic live within 100 m of a well that meets the WHO guideline. The implication is that many households could rapidly lower their exposure if the subset of safe wells could be shared. In a follow-up conducted one year later in five villages where 59% of wells were elevated in arsenic, two-thirds of households indicated that they had switched to a neighboring well in response to the testing. The blanket testing of millions of wells for arsenic in the region should therefore be prioritized over much costlier water treatment and piped water supply projects that will take much longer to have a comparable impact.

Vertical mixing with return irrigation water the cause of arsenic enrichment in groundwater of district Larkana Sindh, Pakistan.

Stable isotopes ratios (‰) of Hydrogen (δ2H) and Oxygen (δ18O) were used to trace the groundwater recharge mechanism and geochemistry of arsenic (As) contamination in groundwater from four selected sites (Larkana, Naudero, Ghari Khuda Buksh and Dokri) of Larkana district. The stable isotope values of δ2H and δ18O range from 70.78‰ to -56.01‰ and from -10.92‰ to -7.35‰, relative to Vienna Standard for Mean Ocean Water (VSMOW) respectively, in all groundwater samples, thus indicating the recharge source of groundwater from high-salinity older water. The concentrations of As in all groundwater samples were ranged from 2 µg/L to 318 µg/L, with 67% of samples exhibited As levels exceeding than that of World Health Organization (WHO) permissible limit 10 µg/L and 42% of samples expressed the As level exceeding than that of the National Environmental Quality Standard (NEQS) 50 µg/L. The leaching and vertical mixing with return irrigation water are probably the main processes controlling the enrichment of As in groundwater of Larkana, Naudero, Ghari Khuda Buksh and Dokri. The weathering of minerals mostly controlled the overall groundwater chemistry; rock-water interactions and silicate weathering generated yielded solutions that were saturated in calcite and dolomite in two areas while halite dissolution is prominent with high As area.

Hydrogeochemical and isotopic evaluation of groundwater with elevated arsenic in alkaline aquifers in Eastern Punjab, Pakistan.

Geochemical investigation was carried out for delineating factors responsible for the mobilization of arsenic (As) from aquifer material into the groundwater. Four sites along Ravi River, (Samada, Sarai Chimba, Kot Maiga and Chah Fatehwala), were selected based on the blanket survey. Groundwater-rock interaction and evaporation were the key phenomena controlling groundwater chemistry, as shown by the hydrogeochemical data. Groundwater was predominantly Na-Cl type, with other principle facies being Na-HCO3, Na-Ca-HCO3 and Ca-Mg-Cl. The groundwater As concentration ranged between below detection level (2 µg/L) to 548 µg/L with 59% samples exceeding the World Health Organization (WHO) guidelines for As in drinking water (10 µg/L) and 31% having higher concentrations than the National Environmental Quality Standard (NEQS, 50 µg/L). Moderate to high concentrations of SO4-2 averaged at 244 mg/L and moderate NO3- concentrations averaged at 8 mg/L, together with alkaline pH (7.3-8.8) and high Eh values (113-402 mV) suggest partial oxidizing nature of the aquifers. The values for δ 18O and δ 2H in groundwater varied between -9.14 and -5.51‰, and -56.57 to -39.5‰ respectively, and suggests meteoric origin of the groundwater with some evaporative loss. This effect could be partly responsible for elevated levels of pH and salinity in groundwater. Based on geochemical and isotopic composition of groundwater, desorption of As from metal surfaces under alkaline environment might be the factor causing As enrichment in study area.