Three main lithological units can be distinguished in Sri Lanka's Proterozoic crust based on its lithological, geochronological, and geochemical characteristics. The study area falls under Wanni Complex (formerly the western Vijayan Complex) (Dissanayake & Chandrajith 2018). Since the entire study area belongs to the Wanni complex, there may be some other factors for the separation of clusters like land use types or soil types. Soil types belonging to sampling sites of three clusters are tabulated in Table 7. Figure 5 shows that soil types belonging to cluster 1 and cluster 2 are different. Cluster 3 sampling sites have all soil types of clusters 1 and 2 except Solodized solonetz and solonchaks. Since the lithological characteristics are same for the cluster 1 and cluster 2, having different types of soils might be a possible causative factor for the variation of groundwater quality.
Soil types belong to sampling sites of three clusters (Ministry of Lands 2016a, 2016b)