GEOCHEMICAL ANALYSIS OF A SMALL URBAN FLOODPLAIN: FOURCHE CREEK BOTTOMS IN LITTLE ROCK, ARKANSAS

Today most people live in heavily populated urban environments. Urbanization is vital to human development by providing government, industry, trade, economic development, goods and services, and infrastructure, but it also has profound impacts on the natural landscape and environment. The environmental changes in urban areas include modification of watershed hydrologic response, chemical makeup of soils, and transport of elements into waterways. The chemistry of urban soils is affected as increases of trace metals and other organic and inorganic contaminants enter the air and fall to the ground. As these elements accumulate in urban soils, increased stormwater runoff then filters these pollutants into local watersheds and their interconnected wetlands, streams, and rivers. Runoff from seventy-three percent of Little Rock’s surface area empties into Fourche Creek, then its urban wetlands, before it is further transported to the Arkansas River. Previous studies have revealed that the Fourche Creek wetlands mitigate flooding and remove contaminants from the water column. In June 2017, we began to investigate the impacts of urban activities and flooding in the wetlands on the geochemical composition of floodplain sediment in the Fourche Creek Bottoms. To complete this study we collected sediment samples along transects of Fourche Creek at three locations, beginning at the water's edge and traversing a distance between seventy to one hundred feet into the floodplain. Sediments were dried, homogenized, and sieved for grain size distribution. We then performed leaching experiments at a 10:1 deionized water to sediment ratio to determine cation, anion and the trace element concentrations adsorbed to the surface, which could easily be mobilized. Results show that overall the study area is composed of fine to coarse grain sand, and homogenous grain size distribution at each location. This is most likely due to the size of the study area and the quantity of each size fraction present in the creek water during a flood event. Concentrations of cations, anions, and trace elements varied with distance from the creek and location within the wetlands. Elevated concentrations of Al, Fe, Mn, Zn, Br, Sr, Ni, and B from the sediment reveal impacts from urban activities (i.e. runoff, mining, vehicle traffic).