Regional distribution and geochemical controls on fluoride variations in the aquifers of Indus Basin, Punjab, Pakistan

Abstract

Chronic exposure from drinking well-water with naturally high concentrations of fluoride (F- ) has serious health consequences in several regions across the world including South Asia, where the rural population is particularly dependent on untreated groundwater pumped from private wells. Understanding F - distribution and its variation is critical for developing effective mitigation strategies for F groundwater contamination. For addressing this matter an extensive campaign to test 21,444 wells was conducted in the Punjab plains of Pakistan by relying primarily on field kits because this is the only way to realistically cover enough villages across the area of 250,000 km2 study area to document the scale of the problem and shed light on the underlying mechanisms. Groundwater samples were collected from a subset of 345 wells for laboratory analysis of F and other constituents. A handful of sites showing contrasting levels of F in groundwater were also drilled to determine if the composition of aquifer sediment differed between these sites and for the reason aquifer sediments might provide additional clues to the processes that regulated groundwater F- levels in the region. The laboratory data show that the field kits correctly classified 90% of the samples relative to the World Health Organization guideline for drinking water of 1.5 mg/L F- . Overall 9% of 21,444 wells contained >1.5 mg/L F- . The kit data indicate that 9% of wells across a region extending from the Indus to the Sutlej rivers were elevated in F relative to this guideline. Field data indicate an association between the proportion of well-water samples with F- > 1.5 mg/L and electric conductivity (EC) > 1.5 mS/cm across six floodplains and six intervening doabs. Although in the overall floodplains and doab wells the proportion wells exceeding F guideline of F- > 1.5 mg/L was not different 9% in both geomorphological units, but the groundwater of doabs show higher EC values 25% two times higher than floodplains (13%). pH values were much different in floodplains and doabs. At the broad-scale on geomorphological unit level, doabs, and floodplains the association of high F- with high EC is striking but becomes less consistent at finer scales. The geomorphological units most affected by high F in groundwater are the Thal and Chaj doab wells exceeding >1.5 mg/L F- . The least affected units are Chenab and Jhelum. Widespread salinization of Punjab aquifers attributed to irrigation may have contributed to higher F levels in groundwater of the region. Regional distribution and Geochemical Controls on Fluoride variations in the Aquifers of Indus Basin, Punjab, Pakistan. 2 Historical conductivity data suggest salinization has yet to be reversed despite changes in water resources management. The average TDS values presented by Greenman et al. 1967 in the region of Chaj and Rachna doab show generally similar patterns, particularly in the lower parts of Chaj and Rachna doabs in shallow wells and Bari doab in the deep wells. Where the high TDS values are consistent with our data from present study. Our detailed laboratory analysis shows that groundwater F - in the Punjab plains is controlled by geochemical processes such as coprecipitation of calcite and fluorite, cation exchange. Common characteristics of groundwater with elevated levels of fluoride are low Ca2+ concentrations and elevated bicarbonate (HCO3 - > 500 mg/L) and sodium (Na+ > 200 mg/L) in high F groundwater suggest regulation by fluorite. This could be through either the lack of precipitation or the dissolution of fluorite regulated by the loss of Ca2+ from groundwater due to precipitation of calcite and/or ion exchange with clay minerals. Elevated levels of Mn2+ in a subset of samples prevented the loss of Ca2+ from groundwater and therefore allowed precipitation of fluorite to maintain lower F concentrations. The combination of local hydrological and geochemical factors that regulate Ca2+ in groundwater by calcite precipitation seem to be more important instead. F - content in the aquifer sediments does not show association with groundwater F concentration in Punjab, an important implication of our observation is that F content in the sediments does not affect the local scale heterogeneity, this question might be resolved by the detailed studies related to processes along the groundwater flow path