QUANTIFYING THE IMPACT OF URBANIZATION ON THE FOURCHE CREEK WATERSHED, LITTLE ROCK, AR

The Fourche Creek Watershed is an urbanized drainage basin in Central Arkansas, serving as the primary drainage basin for most of metropolitan Little Rock. Urban environments like Little Rock can contribute contaminants that influence surface water quality including metals, sewage, and pesticides. The goal of this project is to introduce a novel method of quantifying the impact of urbanization on these streams using a mixing model based on the ⁸⁷Sr/⁸⁶Sr of the endmembers rainwater, groundwater, and surface water. With the ⁸⁷Sr/⁸⁶Sr mixing model, we used a multi-pronged approach including collected field parameters (pH, conductivity, temperature, dissolved oxygen concentration, turbidity), the geochemistry of the waters, land use/land cover analysis, and contaminant load calculations throughout the Fourche Creek Watershed to discern the inputs into Fourche Creek and its tributaries. We sampled seven times from July 2019 to June 2020 for cations, anions, trace elements, and Sr isotopes throughout the watershed. Leaching experiments were performed on samples of local geologic units with deionized water to simulate groundwater rock interactions. GIS was used to delineate drainage basins of the tributaries of the Fourche Creek Watershed and quantify percent urban land in each basin. This investigation determined that Ca and HCO₃ concentrations increased during warmer months and decreased during cooler months. The estimated yearly load of As and Sr in this watershed is 121.4 and 9296.5 kg, respectively. Sr concentrations either increase or remain the same downstream; however, the ⁸⁷Sr/⁸⁶Sr of surface water decreases downstream from 0.71284 to 0.71060. The various geologic units provide different ⁸⁷Sr/⁸⁶Sr values for groundwater feeding into these urban streams mostly underlain by the Jackfork Shale (⁸⁷Sr/⁸⁶Sr = 0.71484) and Arkansas Novaculite (⁸⁷Sr/⁸⁶Sr = 0.70598). With Central Arkansas’ projections to urbanize outlying regions by 2030, understanding how urban expansion is affecting streams in this drainage basin could provide insight into protecting water resources by quantifying the anthropogenic impact on surface waters.