Influence of on-site sanitation on groundwater quality from large diameter wells

Field work was conducted to understand the possible influence of on-site sanitation on groundwater quality, estimate minimum bore well volumes of water to be purged from large wells to obtain representative aquifer samples, and unravel possible trends in variability in physico-chemical water quality of water samples from wells lying between the hills and discharge areas. We applied a chemical pollution risk assessment of wells modified from microbial risk assessment to enable the quantification of risk to chemical pollution of wells from on-site sanitation. For two wells, 3.2 well volumes were purged with progressive sampling and physico-chemical quality of purged and recovered water. Physico-chemical quality of samples was analyzed for wells lying be-tween hills and discharge areas. Commonly identified contamination sources are sullage drains, pit latrines, waste dumps and septic tanks, and 14 out of the 15 wells (93%) studied at risk to pollution. Anion concentrations and EC values increased during purging and were stable and high during recovery indicating inflow of water with high ion content possibly from identified risks to chemical anion pollution. Stabilised physical water quality (EC and pH) during recovery indicate representative aquifer samples from large diameter wells in the shallow aquifer of the area may be possible after removing at least three well volumes of water. Apart from Nitrate, average concentrations of anions and EC were high in water samples from wells in the low-lying area. Results of pump test and sampling between the hills and discharge areas, we, respectively, conclude that: the wells are hy-draulically connected to contamination sources and the observed groundwater quality may due to effluents from identified contamination sources, and dissolution of minerals and increased infiltration of contaminants may be responsible for high electrical conductivity and anion concentration in the low-lying discharge areas of the study area.