ASSESSMENT OF SALT WATER INTRUSION IN THE SOUTH WESTERN COASTAL AQUIFERS OF KHULNA BY USING VISUAL MODFLOW

Abstract

The objective of the study is to investigate the salinity intrusion status of groundwater using visual MODFLOW in the study area (Khulna) and salinity transport scenario in the coastal aquifers. The model domain covers an area of approximately 350 km² in 40m X 40m grid size and defined by three hydro-stratigraphic layers. To set up model fluid transfer boundary of Northern and Southern sides‘ ground water levels from observed wells and surface water level of various ponds were used. For Eastern and Western boundaries water level of the river Shibsa and Rupsha were used respectively. The observed groundwater levels from the wells were used as a boundary condition for deeper aquifer. Model base represents impermeable boundary and model top represents recharge boundary. Beside this the concentration of different rivers was used as mass transfer boundary condition for upper shallow aquifer. Other than upper shallow aquifer no concentration boundary was used. Recharge due to infiltration of rainfall in the catchment area was estimated to the range between 8% to 12% of rain fall. The value of Kx and Ky is assigned 2.5x10-4 cm/s to the entire aquifer. In this case the Kx and Ky are the same indicating that the assigned property values are horizontally and vertically isotropic. In these three layer models, layer 1 represents the upper aquifer and layer 3 represents the lower aquifer. Layer 2 represents the aquitard separating the upper and lower aquifers. In the model for contamination transport (salinity) MT3DMS engine was used and also upstream finite difference solution method with the implicit GCG solver. It provides a stable solution to the contaminant transport model in a relatively short period of time. The distribution of salinity was simulated for a period of twenty years in terms of chloride concentration. Through the representation of the salinity model in MODFLOW the salinity concentration in the sub-surface layers appears to increase by about 3.75 times in a span of 20 years due to the occurrence of surface water-groundwater interaction. Present salinity concentration has a value of nearly 800-2200 mg/L while in another two years in 2020 the concentration is expected to reach a maximum of approximately 3200 mg/L. The salinity model developed in MODFLOW was checked for stability using Courant number and it was found that with the increase in time step Courant number also increases which marks the stability of the model. The model was further validated using field data collected in 2013 for the study area and the model data for 2013 in the same location. The salinity data from the field and the model has striking similarities.