Authors:

Abstract

The radial distance over which a particular land-use has the major influence on the groundwater quality is known as the radius of influence (ROI). For this study, total of 39 open well sampling sites were selected across the 13 DSDs in Colombo district. Landsat 8 OLI/TIRS image was acquired and supervised classification was employed to classify land-uses into four major categories. From each well, buffer zones were created at 100 m interval from 0 to 500 m. Following that the intersect tool was applied at each buffer zone to calculate the areas of intersecting land-uses. Simple regression analysis between higher percentage land-use and parameters were conducted to find the optimum radius of influence. The results of groundwater quality revealed that COD, BOD₅, DO and pH levels in some wells were not meeting the standard levels of 15, 3, 6 mg L^-1 and 6-8.5 respectively. Most of the wells with higher COD and BOD₅ have higher percentage of impervious areas. Highest co- efficient for COD, 0.23 (p < 0.05) at 200 m radius and for BOD₅, 0.05 (p < 0.05) at 200 m radius were obtained. Low pH values were observed in intense agricultural areas and at 500 m radius, highest co-efficient of 0.037 (p < 0.05) was obtained. Correlations between COD and BOD5 concentrations with impervious areas were low (R²; 11% and 21% respectively) even at significant level of 0.05 but correlation between pH and vegetated areas was comparatively higher (R²; 40.9%). The study suggests that sustainable land-use management practices should be carried out to protect the groundwater resources.

Keywords: Landsat, GIS, Groundwater quality, ROI, Remote sensing