Groundwater Recharge Study in Jaffna Peninsula -Sri Lanka: An assessment using stable isotope technique

Abstract

Jaffna peninsula in northern Sri Lanka consists of different aquifer systems such as shallow karstic, alluvial, coastal sand and lagoon. Investigating groundwater recharge mechanism in different aquifers is important for future groundwater extraction to provide safe water for the people in the area. Stable isotope (18O & 2H) composition of surface water, groundwater and rain was measured to interpret the recharge mechanism of groundwater in Jaffna Peninsula in northern Sri Lanka. Samples were collected and field water parameters were measured during October 2012 and March 2013 representing before and after monsoon periods respectively. Then average values of ∂18O and ∂2H of rainfall collected over a period of 04 months (Nov. 2012 – Feb. 2013) are -5.86 % and -34.68 % represented by the Local Meteoric water Line (LMWL): ∂ 2H = 8.5 ∂ 18O + 15.0 (r2 = 0.99). Falling of stable isotopic data of surface water and some of shallow water samples along the evaporation line in the ∂ 2H vs. ∂ 18O plot and having the slope less than 8.5 indicates that the surface and some shallow waters have subjected to evaporation in both periods. The ∂ 18O and 2H in shallow water (water level ~ 1 to 10 m) varies from -7.30 to -2.53 % and -45.4 to -19.64 % respectively in October 2012 and the same water shows ∂ 18O and ∂ 2H from -7.25 to -1.46 % and -47.37 to -9.55 % respectively in March 2013. Two groundwater recharge processes: recharged by direct rain and also by evaporated surface waters are observed during the study period. The groundwater having isotope values more depleted than the average isotope values of rain occurred during the study period; indicate their recharge would be due to a different rain condition experienced before the study period. Groundwater dating tool like 3H analysis would be used for further confirmation of this phenomenon. The Electrical Conductivity (EC) of shallow groundwater varies from 403.0 to 7780.0 μS/cm in pre monsoon period and after the monsoon, the variation of EC is from 256.0 to 5730.0 μS/cm. The northeast monsoon rain has diluted the dissolved ions in groundwater. Sea water mixing with groundwater can be observed in few locations at the coastal and lagoonal belt. They have EC values ranging from 2820.0 to 7780.0 μS/cm in pre monsoon period and from 2090.0 to 5730.0 μS/cm in post monsoon period. The other source to increase the salinity of groundwater in the peninsula (1000-2500 μS/cm) would be dissolution of geoganic materials. The groundwater with isotope values around the average rain isotope values (∂18O = - 5.86% and ∂2H = 34.68 %) always indicates fresh groundwater with low EC, in the peninsula due to direct rain recharge. Their EC values vary from 403-1512 μS/cm in pre monsoon period and from 273-999 μS/cm in post monsoon period. Sea water is available inside the peninsula at about 30 m deep levels. Fresh groundwater recharged by rain, is mixed with above sea water and discharges at northern coast of the peninsula. The tendency of mixing of sea water with fresh waters in the peninsula is observed in deep levels. Mixing of sea water starts at about 15 m deep levels. Present study reveals that long-term monitoring of isotope indices of different waters could facilitate precise assessment of the groundwater recharge mechanism in the Jaffna peninsula.