SALTWATER INTRUSION MODELING IN JAGATSINGHPUR COASTAL AQUIFER SYSTEM, ODISHA

Abstract

The present study dealt with saltwater intrusion modeling in a coastal aquifer system in the Mahanadi river delta region near the coast of Bay of Bengal. The aquifers of Mahanadi delta are characterized as shallow aquifers (< 50 m) and deeper aquifers (> 50 m). Electrical conductivity (EC) of groundwater varied from fresh of 146 μS/cm (NW of the Mahanadi delta) to a saline of 33900 μS/cm (close to sea coast) with cation dominance in the order Na+ > Ca2+ > Mg2+ > K+ and anion dominance of Cl- > HCO-3 > SO4 2-. The hydro chemical facies changed from Ca-Mg-Na-HCO3 type to Na-Cl type along the groundwater flow direction due to ion exchange processes. A strong positive correlation (r > 0.9) between Cl- with EC, Na+, Mg2+, Ca2+, SO4 2- and K+ was observed, indicating the influence of seawater on coastal aquifer. The ionic ratios (Na+/Cl-, HCO-3/Cl-, Mg2+/Ca2+, SO4 2-/Cl-, Ca2+/(HCO-3/SO4 2-)) also suggested that the groundwater is affected by seawater intrusion. Stable isotope compositions (δ18O and δ2H) varied from -1.86 0/00 to -6.87 0/00 for δ18O and from -10.79 0/00 to -45.42 0/00 for δ2H, implying the mixing of saline water and fresh groundwater in the coastal region of the Mahanadi delta. PHREEQC code was used to measure the saturation index of carbonate end minerals such as calcite and dolomite. The saturation index of calcite and dolomite minerals with < 1 is an indication of presence of fresh groundwater in Upper deltaic formation. This was supported by Ca-rich water with Mg2+ /Ca2+ ratio < 1. The proportion of seawater in groundwater was estimated to vary from 0% in the Upper delta formation to 72% in the Lower delta formation of the Mahanadi delta (close to sea coast), which is due to inland intrusion of seawater. A conceptual diagram was prepared to visualize the chemical variation in this coastal aquifer system. Shallow well close to Bay of Bengal showed less concentration of Na+ and Cl- content with low EC. The deep well close to the sea coast, showed high ionic concentration of Na+ and Cl- with an indication of seawater intrusion. An appropriate groundwater flow model was developed to estimate seawater intrusion and understand the factors of groundwater dynamics in the Jagatsinghpur coastal district. The hydraulic head varied from 0.7 m to 15 m above mean sea level (MSL) with an average head of 6 m in the low-lying coastal region, where the groundwater is mainly abstracted (overdraft) for agricultural activities. Aquifer parameters were estimated using Parameter Estimation ii Technique (PEST) module in Visual MODFLOW. The horizontal hydraulic conductivity and the specific yield values in the area were observed to vary from 40 m/day to 45 m/day and 0.05 to 0.07 respectively. The model has been calibrated for two years (2004 and 2005) by using these parameters, followed by validation of four years (2006 to 2009). The calibrated numerical model was used to calculate net recharge and groundwater balance and to infer interaction between river and coastal unconfined aquifer system and responses during different seasons. The net groundwater recharge to coastal aquifer thus estimated, showed variations from 247.89 to 262.63 million cubic meter (MCM) for year 2006 and 2007. Similarly, the inflow from river boundary was calculated as 34 MCM during pre- and post monsoon period. In monsoon time, the costal aquifer discharged around 23 to 27 MCM of water to river system. Water level contour of this coastal aquifer was also studied, which showed high and low hydraulic gradient in the upper and lower part of the deltaic region respectively. The low hydraulic gradient and slugish movment of groundwater in the lower part of the study area was due to influence of seawater. Because of low net recharge, seawater ingress has been inferred. Other human factors such as, growing urbanization and industrialization also led to decline in the hydraulic head. Hence, appropriate recharge methods need to be adopted to reduce the seawater intrusion in the affected areas.