USING “LEGACY SEDIMENTS” TO DETERMINE PAST LAND USE CHANGES AND FUTURE SEDIMENT RELEASE IMPACTS ON DOWNSTREAM ECOSYSTEMS: YELLOW BREECHES CREEK WATERSHED, CUMBERLAND COUNTY, PA

Over the last 250 years, thousands of mill dams in south central PA have accumulated sediments and included pollutants (nutrients and trace elements) due to changes in land-use over time. Ironically these so-called “legacy sediments” (Walter and Merritts, 2008) are currently being released back into streams by dam removal to improve stream ecology. Depending on surrounding land use, these sediments can contain elevated levels of nutrients, organic compounds, and/or bio-available trace elements. Here we present comparative geochemical and sedimentological data at two legacy sediment sites within the Yellow Breeches Creek (YBC) watershed, Cumberland County, PA. The Eaton-Dikman Pond (EDP) site is contained within forested land use and the Wittlinger Dam (WD) site within agricultural land use.

Thirty-one sediment cores were collected from the 91,000 m² EDP site, and ten cores from the 15,000 m² WD site. Using core depths and GIS, total volume of legacy sediments at the two sites is estimated. Magnetic susceptibility, inorganic and organic carbon, and grain size analyses reveal two very different stratigraphies. The EDP site has large volumes of sand with iron ore slag and charcoal derived from forest clear cutting for the local iron industry in the late 18th and early 19th centuries. These sand bodies are between overlying silty clays and soil horizons and underlying basal wetland hydric soils and sandstone and metavolcanic bedrock. The WD site contains exclusively silty clay on sandstone colluvium and underlying carbonates. XRD analyses of core facies mirror bedrock sources. XRF analyses show significant differences in nutrient and trace elements. Total phosphorus is double at WD over EDP confirming agricultural inputs. REE's are enriched over average shale in both sites. Transition metals (Cd, Co, Cu, Ni, Pb, Zn) are 2-3 times more enriched at WD than EDP especially in the upper half of the cores suggesting airborne as well as agricultural sources of pollution.

Weekly water samples were collected at both sites to determine the dissolved and suspended load release relative to discharge after dam removal. Preliminary data indicate that the majority of stream load release is caused by episodic discharge events and stream bank slumping. These data are compared to an 11-year record from another site on the YBC.