USING SOIL GEOCHEMISTRY TO MAP HISTORIC AND LATE HOLOCENE FLOODPLAINS, FOUR MILE CREEK, OHIO

Stream systems have been altered significantly from their natural state due to human influences such as land use change and channelization. Many stream channels in the Midwest are isolated from their floodplain, which reduces nutrient and sediment storage and degrades habitat. Stream restoration projects aim to reestablish the natural conditions of the stream and connection to the floodplain is vital to this goal. To better plan and conduct stream restoration, the floodplain that was active in the recent geologic past should be mapped to delineate the channel migration zone. In locations with a history of landscape alteration, channel migration zones cannot be delineated readily from older floodplains using traditional indicators such as surface roughness and subtle scarps of fluvial terraces. In this study, soil geochemistry was used to map time-stratigraphic overbank deposits in the Four Mile Creek watershed. Over time, the concentration of mobile elements in soil decreases due to dissolution in infiltrating water while the relative concentration of immobile elements increases. The relative time since deposition of a series of soil profiles can be determined using a weathering ratio of immobile to mobile elements.

Samples were collected at depths of 0-10, 20-30, and 60-70 cm in a grid from the modern channel to the uplands, oven dried, and analyzed using a portable XRF in the laboratory. The mapping zone is approximately 0.2 km² with a total of 163 sampling sites and 467 samples collected. Kriging geostatistics were used to interpolate geochemical data across the landscape. The weathering ratio values were then compared to a soil chronosequence within the watershed constrained with radiocarbon ages of channel deposits to give approximate ages to the overbank deposits. Samples taken at 0-10 cm were influenced by modern land use, while the clearest trends were observed from samples taken at 60-70 cm. The 500-year floodplain extends approximately 250 m from the modern channel and the late Pleistocene fluvial terrace is approximately 600 m from the modern channel. These findings indicate how the stream behaved prior to human influence. Future work will further constrain the geochemical evolution of soil in the study area with time using additional radiocarbon ages to provide more refined ages of overbank deposits.