Hillside Quarry Impacts on Streamflow and Stormflow response in a Tropical Watershed: A Geochemical and Hydrometric Investigation

Throughout the Caribbean, hillside quarrying has become a common practice. While these activities remove large sections of the critical soil zone, very little work has been done on how hillside quarrying impacts rainfall runoff response and catchment water storage. We hypothesised that the removal of the critical soil zone during hillside quarrying will increase the timing and magnitude of streamflow response to storm events due to its close proximity to the river, while also reducing the overall storage of the watershed. The aim of this study is to understand the landuse impacts on rainfall runoff response and catchment storage. A paired catchment study between the 3.6 km² Acono (forested) and the adjacent 3.6 km² Don Juan (quarried) watersheds in Trinidad and Tobago was conducted using a hydrometric and geochemical approach. Direct measurements of rainfall and streamflow and bi-weekly water sample collections for geochemistry and stable isotopes of ¹⁸O and ²H from rainfall, baseflow, soils, springs and groundwater were done. Fraction of young water (Fyw) an inverse transit time proxy was computed along with the mean transit time distributions (MTTDs) by sine wave fitting were used as important descriptors of runoff generation and the catchment storage. The quarried watershed had higher streamflow levels during the wet season than the forested watershed. However, during the dry season there is a reversal.. The quarried watershed responded faster to rainfall events with a lag time between 1–3 hours with a higher peak rate of streamflow versus a lag time of 2-4 hours in the forested watershed with a lower peak rate of flow. In the upper quarried watershed 18.4 % of the stream water were younger than 0.46 years and 20.3% were younger than 0.55 years in the lower portion of the catchment. In the upper forested catchment 5.2 % of the stream water was younger than 2.71 years whereas 4.7% of the stream water was younger than 3.04 years in the lower catchment. The data suggest that the quarry leads to the faster delivery of water during storm events while also reducing the overall storage in the catchment. With an anticipated increase in hillside quarrying, this study provides important information for land use and water resource managers.