1 EFFECT OF THE INDIAN OCEAN TSUNAMI ON GROUNDWATER QUALITY IN COASTAL AQUIFERS IN EASTERN SRI LANKA

Changes in water quality of a sand aquifer on the east coast of Sri Lanka due to the December 26, 2004 tsunami and subsequent disturbance due to well pumping and flushing by precipitation were investigated. Two closely spaced tsunami affected transects, spanning the ocean and an interior lagoon across a 2 km wide land strip were monitored from October, 2005 to September, 2006. Water samples were collected from 15 dug wells and 20 piezometers, from the disturbed and undisturbed sites respectively to evaluate the temporal and spatial trends in water quality. The EC values observed from the undisturbed area showed a significant decrease (3000 to 1200 µS/cm) with the rain from November 2005 to March 2006, while the values in the disturbed area appeared to have stabilized without further decline through the same period. The concentration range of EC, Ca, K, Na, alkalinity, total hardness and sulphate were higher in the disturbed site than in the undisturbed site. PHREEQC modeling showed that the mixed sea water fraction is higher in the disturbed site than in the undisturbed site, and this is likely due to the movement of the disturbed plume by water extraction through pumping and extensive well cleaning after the tsunami, causing forced diffusion and dispersion. No arsenic contamination was observed as all observed arsenic concentrations were below 10 µg/L. For the sites investigated, there are clear indications of only a slow recovery of the aquifer with time in response to the onset of the monsoon