T256 Urban Geochemistry

Friday, 30 October 2020
10:00 AM - 2:00 PM
Meeting Rooms

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

Paper number: T256-2
Program: Technical Sessions
Time: 10:15 AM – 10:30 AM

Authors

Abstract

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.

Geological Society of America Abstracts with Programs. Vol. 52, No. 6

doi: 10.1130/abs/2020AM-357206

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