Impact of Limestone Quarrying on Runoff and Stream Transit Times in a Tropical Watershed, Trinidad and Tobago

ABSTRACT Limestone quarrying is a major economic activity across the globe. In small islands like Trinidad and Tobago, quarrying has also encroached on many peri‐urban areas leading to health and ecological issues. While the air quality and sediment transport implications of limestone quarries are well described, the effects on streamflow and runoff processes remain poorly understood. Using a paired catchment approach (natural and quarried watersheds), we measured rainfall characteristics and the hydrograph response to examine the changes in runoff generation, while a geochemical approach using stable water isotope and ions was used to estimate the stream transit time and source water contributions. The runoff response of the quarried watershed was substantially different from the natural watershed. The mean R/P (0.13) and peak discharge rates in the quarried watershed were two—three times higher than the natural watershed, while the response lag (26.3 min) and lag to peak (179.4 min) in the quarried watershed were significantly shorter than the natural watershed. The mean transit time (MTT) in the quarried watershed was 1.0 year and was substantially shorter than the MTT of 1.8–2.0 years in the natural watershed. The young water fraction in the quarry was 11.1% and was greater than the 4.7% observed in the natural watershed. These observations suggest that limestone quarry development resulted in changes to runoff and streamflow processes. The data suggest that limestone quarrying led to the higher contributions from surface or shallow subsurface flow paths during storm events while also reducing the overall catchment storage and contributions to streamflow from groundwater/deep subsurface sources. While the study sheds important light on limestone quarry impact on hydrological processes, a more detailed and long term investigation is needed where the use of other geochemical tracers and identification of more endmembers will provide more information on the processes.