THE IMPLICATIONS OF CLIMATE CHANGE ON STREAM FLOW AND LEGACY SEDIMENT REMOBILIZATION: YELLOW BREECHES CREEK, CUMBERLAND COUNTY, PA

Thousands of late 18th- early 20th century mill dams in the Piedmont and Valley and Ridge provinces of the Middle Atlantic States have accumulated sediments and included pollutants with changes in land-use over time. These deposits are commonly called “legacy sediments” (Walter and Merritts, 2008). More recent dam removal is releasing these sediments back into streams. Depending on the duration of land use, these sediments can contain elevated levels of nutrients, organic compounds, and bioavailable trace elements. Concurrently there is statistically valid evidence that flow in non-impacted New England streams has risen in response to climate change over the last century (Collins, 2009, Armstrong, 2010). Here we present preliminary evidence to address two converging hypotheses: 1) that non-impacted stream flow in the Middle Atlantic is also increasing as climate changes over time and 2) that remobilization of legacy sediments may increase over time due to the increase in the magnitude and frequency of storm events. We test our hypotheses using a long term flow record on the Yellow Breeches Creek in south central PA and the phase geochemistry of trace metals in suspended sediments above and below legacy sediment deposits during high and low flow events with comparisons to the trace metal content of a variety of legacy sediment lithostratigraphic facies. A sequential extraction technique on pre-dam hydric soil, typical legacy clay, and legacy soil reveals Zn to be dominant in adsorbed and organic phases especially in the legacy clay. Total trace metals are equally present in the three phases of the hydric soil with Cu (adsorbed), V (hydroxide), and Zn (organic) most dominant. Zn, V, and Pb dominate the legacy soil. At low flow conditions, with the possible exception of adsorbed Zn, trace metal concentrations show no significant increases below legacy sediment deposits. In high flow conditions where legacy sediments are most vulnerable to remobilization, we find a 1500-fold increase in organic Zn and 200-fold increase in hydroxide V and Pb loads for a 6-fold increase in stream flow. This suggests that the legacy sediment deposits contribute most if not all of their trace metal load during storm events. The relative contribution of legacy sediment metals during storm events versus base flow over time is discussed.