Seawater intrusion has become one of the major problems in most of the coastal regions of the world. The unconfined aquifer system, lying between Mahanadi River and Devi River in the Jagatsinghpur District of Odisha, having thickness of about 50 meter was investigated. Agriculture is the main water intensive activity in the district. Due to its geographical situation, the district is congenial and prone to occurrence of various natural calamities like flood, cyclone or both and even drought. The Jagatsinghpur deltaic region belongs to quaternary formation which comprises of recent alluvium sediments of flood plain deposits of the Mahanadi River and the Devi River in upper part of the study area. The deposits mainly consist of gravels, sand (fine to coarse grain), silt and clay (red, yellow and black). This coastal tract acts as a good zone of groundwater availability as very large thickness of sediments of varying sizes get deposited in this part of the study area. Groundwater exploration from the unconfined coastal aquifer of the Mahanadi delta has been estimated using the three-dimension groundwater flow model (MODFLOW). The model study area covering 1668km² is gridded into 9394 cells with 77 rows (I=77) and 122 columns (J=122) and each cell consists of 600m × 600m square blocks. The groundwater flow model has been calibrated and validated using 4 years (2004-07) groundwater water level data. The model domain is classified into 3 hydraulic and specific yield zones for this single unconfined aquifer system. The horizontal hydraulic conductivity (K_h) of ZONE I, ZONE II and ZONE III are 40 m/day , 42 m/day and 45 m/day respectively, whereas the corresponding vertical conductivity (K_v) of three zones are 4 m/day, 4.2 m/day and 4.5 m/day respectively. The different specific yield values 0.05, 0.06 and 0.07 for three respective zones I, II, III have been taken during the calibration of groundwater model. The groundwater flows from North West (NW) direction to South East (SE) direction. Most of the groundwater draft in the area is done for irrigation purposes during non-monsoon period. It has been estimated that 19 million cubic meters (MCM) of more groundwater is extracted as compared to annual groundwater recharge.

Key words: Seawater intrusion, Groundwater recharge, MODFLOW, unconfined aquifer, Mahanadi delta