Preliminary groundwater assessment and water quality study in the shallow aquifer system in the Attanagalu Oya Basin

The Attanagalu Oya basin is situated between two major river basins, Kelani and Maha Oya in the Western Province of Sri Lanka, with an extent of 727 km2. It contains four streams: Diyaeli Oya, Attanagalu Oya, Uruwala Oya and Kimbulapitiya Oya, which discharge into the Negombo Lagoon as Dandagamuwa Oya. A study was conducted in the Attanagalu Oya basin in order to assess the groundwater potential of the area and to identify the water quality of the shallow aquifer system of the basin. The available qualitative and quantitative data on groundwater were collected and compiled, and a groundwater database was developed for the basin. The total basin area was divided into five major divisions and shallow groundwater monitoring networks were designed for each. Representative shallow dug wells were selected for the monitoring network and 100 water samples were collected from each division. Samples were analyzed for 17 physical and chemical parameters. In addition, 10 water samples were collected from surface and groundwater bodies in the paddy cultivated areas and analyzed for pesticides. Another 10 water samples from the industrialized area were analyzed for lead and 10 from urban areas were analyzed for bacteria. Based on the data collected, geochemical maps were prepared for the entire basin. These indicated that the pH values of the shallow groundwater in some parts of the Attanagalu Oya basin were very low, varying from 4 to 8.5. High electrical conductivity (EC) values were reported in the coastal area. Bacterial contamination was reported in groundwater sources in the urban areas. No pesticide contamination was detected in any of the water samples collected in paddy cultivated areas. The occurrence of lead in shallow groundwater was reported within the range of 0.01-0.02 ppm. The major aquifer types in the basin are river alluvium, coastal sand and fractured basement hard rocks. The existing northeast-southwest trending lineaments were identified as promising areas for groundwater development.