Modelling groundwater pollution from septic soak pits (India)

To study the impact of septic tanks and soak pits on groundwater quality, an understanding of the fate and transport of contaminants resulting from septic effluent in groundwater is needed, which in turn is governed by various physical, chemical, and biological processes. The study included preparation of conceptual and numerical groundwater model in Visual MODFLOW Flex 6.0 for simulation of contaminant transport. The computational model was setup to represent the basic high-level depiction of hydrogeological system of the study area, along with the definition of species parameters that were modelled for the study. This high- level computational model was then converted to a numerical model by defining various model properties as well as the boundary conditions. The study included 10-year period simulations under steady state conditions for transport through soil medium of four major contaminants present in septic tank effluent, viz., Nitrate, Ammonia, Phosphate, and Faecal Coliform Bacteria. The flow results were obtained from model in terms of observed head and water table. The contaminant transport results were obtained in terms of contour maps. From the obtained results it was inferred that the contaminants released in the effluent from on-site septic tanks do pose a potential threat to the quality of regional groundwater and hence their effect should be thoroughly evaluated before planning the construction of on-site wastewater collection, treatment, and disposal systems.