Ankit Singh, Suneet Naithani, Harshit Agarwal and Yash Kumar Shah
Abstract
Uncontrolled urban expansion and unsewered informal settlements along the Rispana and Bindal rivers in Dehradun, Uttarakhand, continuously discharge raw sewage into local stream channels. This study evaluates the subsurface transport kinetics and migration risk of Fecal Coliform (FC) within the unconfined Doon Gravel aquifer system against Bureau of Indian Standards (BIS IS 10500:2012) and World Health Organization (WHO) drinking water guidelines. Using MODFLOW-2005 for three-dimensional groundwater flow and MT3D-USGS via Model Muse, numerical solute transport simulations were executed over days, months and 1, 3, 5, 10 year projection horizons. Pre-monsoon surface water quality monitoring establishes severe microbial contamination, with Fecal Coliform concentrations reaching 54,000 MPN/100 ml and Biological Oxygen Demand (BOD) reaching 50 mg/l. Subsurface modeling demonstrates a strong spatial decoupling between surface sewage discharge and deep groundwater vulnerability. Setting the first-order biological inactivation rate to ë = 0.10 day-1, corresponding to a field half-life (t/) of 6.93 days, causes advective microbial mass input from riverbed seepage to reach dynamic decay equilibrium within 30 to 45 days. Consequently, spatial plume contours remain identical across 1, 3, 5, 10 year simulation periods. Vertical cross-sections confirm that viable coliform transport remains confined to shallow vadose and perched saturated zones (<18 m depth). The model serves as an uncalibrated conceptual screening tool due to unavailable private tubewell screen coordinates and missing deep piezometric sampling. Findings indicate that while shallow hand-pumps near slum settlements face direct microbial hazards, deeper municipal production aquifers (> 35 m) remain shielded by natural subsurface die-off processes.