Saline Water Intrusion Along the River Benthera and its Impact on Irrigation Water

Abstract

Saline water intrusion is a major issue in the present era, and Benthara River has beenaffected with serious saline water intrusion along the river. The present study was conductedto identify sea water intrusion and its impact on irrigation system in the Benthera waterscheme. In addition to that study objectives were to assess salinity levels in surface waterwith precipitation and tidal waves. The study area along the river is covered with mangrovesand paddy. The research was conducted in the Ambalangoda irrigation area, Galle district inthe southern province of Sri Lanka. Study area was located in the river Bentota and the leftbank of the Benthara river basin irrigation scheme. Continuous monitoring was conductedover six months from January to June 2012. Five sampling points along the river from sea toten kilometers (10 km) upstream were identified, based on two kilometers (2 km) interval inbetweensampling points. Further, three samples from three levels that is surface, mid streamand deeper level were taken from each collecting point. Laboratory analyses of chemicalwater quality parameters were observed.

The results revealed the pH and Electrical Conductivity (EC) of the river water are affectedby sea water intrusion and other anthropogenic influences. Throughout the monitoring periodEC values had not changed due to precipitation except with heavy rain. The study helped toprepare maps depicting salinity levels in the river using GIS software. It shows that most ofthe saline water goes through the river bed. Sea water intrusion occurs during afternoons ofthe time and EC values in February remained at the highest level of 32,900 micro Siemensper centimeter (μs/cm). The pH range varies from 6 to 7.5 in the morning and from 6 to 8 inthe afternoon. Electrical Conductivity values of surface water in the study area were up to32,900 μs/cm in the morning and 37,300 μs/cm in the afternoon. Chloride levels hadsignificantly differed between the collecting point near the mouth of the river and theconcluding point. Highest value of Chloride was 19,800 milligrams per liter (mg/l) in themorning and 20,692 mg/l in the afternoon.