Occurrence, sources, and spatial distribution of fluoride in the Ganga alluvial aquifer, India.

Groundwater contamination throughout India is a global concern as it feeds more than a billion people. Of all the contaminants, fluoride (F) is one of the most widespread and well documented since its toxic nature pose serious threats to human health. In India, groundwater F concentrations have been extensively studied over the past decades. These studies have generally concluded that the groundwater F concentrations are typically higher than the drinking water standard for human health. Here, we present the occurrence, distribution, and sources of groundwater F in the Kanpur Nagar and Kanpur Dehat districts covering ~ 6000 km(2) of the area in the central part of the Ganga Basin. The result revealed significant spatial variability in dissolved F concentration ranging between 0.2 and 5.2 mg/L (average 0.9 +/- 0.7 mg/L, n = 172, 1 SD), which is beyond the drinking water guideline (0.5-1.5 mg/L) of the Indian Standards. We find that 31% of groundwater sampled have F content below the optimal requirement of 0.5-1.0 mg/L causing dental caries problems. The F levels only exceeded the safe drinking water limit of 1.5 mg/L in 8% of the groundwater sampled mostly in the urban regions. Fluoride distribution shows a closer resemblance with the spatial distribution pattern of electrical conductivity, and total dissolved solids demonstrate that F in the shallow alluvial aquifers is largely derived from geogenic sources. This is further confirmed by a strong positive correlation (r = 0.91, p < 0.05) observed between chloride-normalized concentration of F and the sum of geogenic elements ( summation operatorLi, Rb, Sr, Ba). We additionally performed health risk assessments, which revealed that children are most vulnerable to dental caries (commonly known as tooth decay) and dental fluorosis problems. As F concentrations show large spatial variability in the studied aquifer, we suggest that uniform application of a single de-fluoridation and fluoridation technology on an aquifer or sub-aquifer scale without a detailed well-designed groundwater F survey will have an adverse health impact on local residents as optimal level of F in drinking water may not be compromised.