Fluid upwelling and alluvial controls on spring localization: An example from Sri Lanka

In water-scarce regions, perennial springs can be a valuable source of drinking water. However, to identify unreported springs and shallow water upwelling zones, it is essential to understand the factors that control spring localization. In a crystalline basement, as in Sri Lanka, without a sedimentary cover, faults and fractures provide the only far-reaching fluid pathways and springs commonly emerge at fault/fracture intersections. While surveying cold and hot water springs in Sri Lanka, it was observed that all springs probed were located at the edge of alluvium. In order to gain insight into this relationship, we performed a topographic and geomorphological analysis was conducted utilizing remote sensing, geological and soil maps, and geological mapping in the field. The results of our analysis of 27 springs indicate that their localization is controlled by fault intersections, non-permeable clay in the alluvium and laterite, and the chemically weathered surface of the bedrock. Furthermore, the constant discharge rates observed over the years and isotope analysis suggest that the springs are part of a tens-of-kilometer-wide regional groundwater system. Based on these results, we propose a conceptual model in which water rises at fault intersections from depth until it reaches the base of the alluvium where up to several meters thick clay with low to zero permeability further inhibits vertical flow forcing the water to spread laterally. Along the alluvium clay boundary with the more permeable weathered bedrock, the water continues its path to the surface. The localization of springs differs from that of fault intersection by tens of meters, with the potential for mixing between shallow and deep groundwater. This observed effect of alluvium and their contact boundaries on spring localization has not been reported for Sri Lanka. Consequently, discharge rates may be significantly increased if the fault intersections are specifically targeted by shallow drilling.