Abstract:
Groundwater is often a forgotten resource in Sri Lanka, which is the hidden prime resource that is the
only source of freshwater in the Jaffna peninsula. Saltwater intrusion is alarmingly increasing making
freshwater at risk in the Peninsula. Therefore this study was conducted to assess the aquifer
vulnerability at adjacent to the Vadamaradchi lagoon area in Jaffna using a simple modeling
technique. Electrical conductivity (EC) of groundwater was measured in 42 dug wells from March to
June 2014. The DRASTIC hydrogeologic vulnerability ranking method uses a set of seven hydro geologic parameters which are assigned a rate and a weight to classify the vulnerability of the aquifer.
The parameters are depth to groundwater, net recharge rate, the aquifer media, the soil media,
topography, impact of the vadose zone and the hydraulic conductivity of the aquifer. From March to
June 2014, the average EC values are 3.8 +4.0, 4.0 +4.4, 6.8 +7.5 and 7.3 +8.3 mS cm-1
recorded
respectively. Calculated DRASTIC Index value is modified by EC rating and weight to assess the
potential risk of groundwater to salinization in the study area i.e. Modified DRASTIC Index value.
Average Modified DRASTIC Index value is 172 that deviate by +3.6 ranging from 170 – 184 and
classified as ‘High’ in vulnerability. Rainfall plays a significant role at recharge of groundwater and
also influences solute transport in underground via pours medium. Therefore vulnerability of
Modified DRSTIC Index values adjusted and/or validated by different rainfall rating by expected
probability of rainfall return period in Jaffna. One year return period of rainfall average index is 177;
whereas for two years, the return period of rainfall average index is 181. For 10 years return period of
rainfall average index become much higher, 185; and for 25 years it increased to 189. Risk index
analyzed with different rainfall probability return period and increased return period showed a
positive correlation indicating risk.
