Groundwater Modelling Around Wetland Patches in Gurupura Sub-Basin, Karnataka, India

Abstract

Numerical groundwater flow models solve the distribution of hydraulic head and describe flow whereas numerical transport models solve the distribution of solute concentration due to advection, dispersion and chemical reactions. In the present study an attempt is made to formulate groundwater flow and transport modelling in and around wetlands of Gurupura basin in Karnataka state of India. The study intended to simulate the response of an unconfined, shallow, tropical coastal aquifer comprising of wetlands using SEAWAT. The numerical simulation of groundwater flow was carried out by building a MODFLOW model of the basin and the transport parameters are assigned to execute the MT3DMS model. Finally, the SEAWAT model which is a coupled version of MODFLOW and MT3DMS designed to simulate three dimensional, variability density groundwater flow and multi-species transport, is developed. The model is calibrated from August 2011 to August 2013 using observed groundwater heads and TDS data obtained from 27 observation wells. The data from VES (Vertical Electrical Sounding) and pumping tests conducted in the study area are used for aquifer characterization. The model is validated for 2013-2015. The model performance is encouraging except for monsoon months (June to September), while evaluating with three techniques R 2 , RMSE and NSE. Overall, the model performance is satisfactory with NSE≥0.5. The spatial distribution of simulated groundwater map shows presence of groundwater at a higher level in the areas around wetlands in the study area, even during peak summer months (April and May). The sensitivity analysis conducted shows that the aquifer is sensitive to specific yield, hydraulic conductivity and recharge rate. The simulations of solute transport model reveals the presence of TDS concentrations in and around the wetland regions during winter and summer seasons, but within safe range. The groundwater budget was estimated for the aquifer using groundwater mass balance simulation package ‘ZONEBUDGET’. This analysis shows that during the period of maximum potential position (August), the component of groundwater contributing to wetland is 4.5% of total outflow. During dry season with minimum potential head, the groundwater contribution to wetland is 1.4% of total outflow. Rest of the outflow contributes to river discharge and pumping of wells. Hence, the presence of water in the wetland during the non-monsoon months is established by the contribution of only groundwater, in the study area. The prognostic simulations conducted for 20 years period (2015-2035) confirms the safety of aquifer, both from quantity and quality perspective.