Groundwater Pollution Vulnerability by Drastic Method in Ja-Ela Divisional Secretorial Division, Sri Lanka

Abstract

Groundwater contamination occurs when man-made products such as gasoline, oil, road salts and chemicals get into the groundwater and cause it to become unsafe and unfit for human use. Materials from the land's surface can move through the soil and end up in the groundwater. Ja-Ela Divisional Secretariat Division is one of most populated Division in Gampaha District at Western Province Sri Lanka. The land area is 64 km² and it is occupied by 210,294 people with 59,067 house units in 2011. This is high industrialized area occupied by 403 industries in 2017 and bounded to environment sensitive areas like Dandugama River, Muthurajawela wetland and several threaten marshy lands. The main objective of this study is to assess pollution vulnerability of the groundwater in Ja-Ela DSD by using the most commonly used overlay and index method; the DRASTIC method which is developed by United States Environmental Protection Agency (USEPA). Assessment of groundwater vulnerability was done by using a modified DRASTIC model. Evaluation of groundwater vulnerability was done by using computer programs based on Geographical Information System (GIS) in order to facilitate data management and spatial analysis. The most important mappable factors that regulate the groundwater potential are depth of water, net recharge, aquifer media, soil media, topography, impact of vadose zone and hydraulic conductivity of the aquifer. Assessment of DRASTIC Index involves multiplying each parameter weight by its rating corresponding to its study area and summing the total. The monthly average rain fall in this area was 190mm and four main soil types were existed. The land elevation was ranged from sea level to 55m and the depth to groundwater table was varied between 0 to 13m. DRASTIC indexes were ranged from 70 to 195 and the mean DRASTIC index value was 147. According to obtained vulnerability map, negligible vulnerable zones; <80 was exhibited by 0.047% of geographical area; low vulnerable zones ranging from 80 to 120 with 19.6% geographical area; moderate vulnerable zones ranging from 120 to 160 with 46.4 % geographical area; high vulnerable zones ranging from 160 to 200 with 33.2 % geographical area and extreme vulnerable zone with greater DRASTIC index value higher than 200 with 0.6% area from total land extend of Ja-Ela DSD. Generally this was higher potential for pollution. The water quality assessment was done by using 30 sample points. The pH, COD, PO₄^-3 level, SO₄-² level and NO₃^-1 level were analyzed. The validation of DRASTIC method was done with comparing water quality values with DRASTIC index value. According to the validation of DRASTIC method with using statistical analysis, measurements, the pH level, SO₄-² level and NO₃^-1 level were identified as good indicators. The PO₄^-3 SO₄^-2 and NO₃^-1 concentrations of 30 sample points were varied from 0 to 1.5mg/L, 0-64mg/L and 0.4-4.4mg/L respectively. The pH level was acidic and it was ranged from 3.5 to 6.3. The COD level is very less among all points. The DRASTIC index value was shown a positive strong relationship with above mentioned parameters. The results provide important information for the local authorities and decision making personals for effective management of ground water resource.