ASSESSING THE IMPACT OF LAND USE CHANGE FROM LATE 18TH TO EARLY 20TH CENTURY MILL DAM SEDIMENT GEOCHEMISTRY AND ITS POTENTIAL EFFECTS ON THE CHESAPEAKE BAY

A recent chemostratigraphic study of two legacy sediment deposits in Cumberland County, PA with distinctly different land use characteristics reveals a significant increase in P and trace metals of Cu, Ni, Zn, Co, and Pb over baseline soil compositions suggesting anthropogenic inputs from agricultural practices over the last century (Niemitz and other, Geology, 2012). Here we expand our study to include two new but well documented legacy sediment deposit associations. In Lancaster County, PA we compare deposits in the mixed forested-agricultural West Branch Little Conestoga Creek and the predominately agricultural Big Beaver Creek watersheds. In Baltimore County, MD we compare the predominately forested upper reach of Gunpowder Falls Creek with an urban impacted site on the lower reach of Little Gunpowder Falls Creek 5 km above Cunninghill Cove in upper Chesapeake Bay.

Similar to the Cumberland County study, the Lancaster County sites and forested deposits of northwestern Baltimore County site exhibit increasing trends in P/Al, Co/Al, Cu/Al, Pb/Al, and Zn/Al ratios however the trends all show significant step increases at depth rather than a more continuous increase up core. In contrast the magnitude of the increases in diagnostic elemental ratios is only 2-3 times greater in the legacy sediment mass than in the pre-settlement hydric soils compared to background soils. Ni/Al ratios are constant throughout all cores. The urban site shows no distinctive trends in any elemental ratios compared to background soils. These new studies suggest that 1) the ability to separate anthropogenic from background elemental inputs decreases toward more intensely farmed and urban areas in the Piedmont, 2) mixtures of land use within a sub-watershed tend to subdue trends, and 3) our urban site consists of mostly reworked sediment (legacy and otherwise) from upstream sources. The magnitude of the elemental ratios and the dominantly sandy soils in the urban site may indicate that much of the finer grained sediment, which sequesters more trace metals, is already in Chesapeake Bay.