Field Studies In a Sandy Tropical Environment to Inform Groundwater Protection Efforts

The vadose zone plays a critical role in groundwater protection. Understanding shallow moisture fluxes is important in developing management practices to reduce groundwater contamination. There is little documented about the shallow hydrology in sandy constructed fill areas, although these have become more common, particularly in coastal regions in Asia. This work presents results from a field study of shallow subsurface moisture fluxes at a constructed fill site on the coast of Singapore to gain an improved understanding of unsaturated flow dynamics under tropical climate forcings. Soil moisture and soil temperature measurements were taken at three depths: 0.10 m, 0.35 m and 0.50 m, and soil matric pressure was measured at 0.35 m and 0.50 m below the ground surface. Meteorological conditions, including surface air temperature, solar radiation, precipitation, relative humidity, and wind speed, were also recorded at the field site. The soil matric pressure measurements show a diurnal signal that is consistent with changes driven by evaporative demand. During drier periods in Singapore, the soil moisture and soil temperature measurements have a clear pattern that is consistent with thermally-driven moisture fluxes, demonstrating that temperature gradients can impact the shallow unsaturated hydrology in a humid tropical climate. Wetting front advance in this environment is determined primarily by the precipitation depth of a particular event, which is equal to the infiltration depth for the period recorded. The rate of advance of the wetting front is sensitive to the soil moisture at the start of the precipitation event, with faster wetting fronts corresponding to wetter initial soil conditions. This indicates that gravity-driven fluxes are more important than fluxes due to pressure gradients following infiltration events. These results contribute to our understanding of groundwater vulnerability in this under-studied environment and can be used to inform stormwater management and groundwater protection practices.