Saltwater Intrusion and Its Impact on Koggala Lagoon and Associated Waters, Southern Coast of Sri Lanka

Saltwater intrusion is a major coastal management problem which leads to the destruction of fishery, breeding grounds and habitat. Several dykes and water regulatory systems have been implemented to prevent salt water intrusion and to regulate water level in the catchment area in the Koggala lagoon. The groyne built at the lagoon mouth helped to keep the outlet open throughout the year for better mixing and water exchange, but it has been a matter of concern for local resource users due to salinity increase in the lagoon. Therefore, the study was focused in salt water intrusion into lagoon and upstream areas and its impact on water quality and drinking water wells of surrounding areas. Monthly mean salinity of the lagoon varied strongly from 20 psu to 34.5 psu in relation to the monsoon rains. During the drought, due to the water level difference between lagoon and the canal created a salt water intrusion into upstream areas through the sluice gates, even the gates were closed. During the period of high salinity (February and March) of the lagoon, surface and bottom salinity levels of the upsteam area were 16 and 22 psu respectively due to saltwater intrusion through the anicut at Warabokka. Moreover, the water exchange and mixing of the lagoon is governed by the gravitational circulation. However during the dry season, tide plays a major role for salt water intrusion and mixing creating well mixed condition. Thus, the lagoons can be classified as a partially mixed estuary. The faunal and the floral compositions of the lagoon had changed markedly due to salt water intrusion. Present analysis indicated that the salinity, dissolved oxygen content and chlorophyll-a content mostly affected the abundance and diversity of zooplankton in the Koggala lagoon. Construction of barrier across the rockfill groyne may minimize the salt water intrusion.