A ¹³⁷CS INVENTORY OF A SMALL AGRICULTURAL WATERSHED IN LANCASTER COUNTY, PA, WITH IMPLICATIONS FOR SEDIMENT AND NUTRIENT LOADS TO THE CHESAPEAKE BAY

Within Pennsylvania's Chesapeake Bay watershed, Lancaster County is a hotspot for high sediment and nutrient yields. Other high-yield regions within Pennsylvania's Lower Susquehanna River region include York, Adams, Cumberland, Dauphin and Lebanon Counties. These areas contribute 40 to 45 % of the nutrient and sediment loads despite comprising only 20% of Pennsylvania's Chesapeake Bay watershed. Why these low-relief Piedmont terrains should have such high sediment and nutrient loads is a paradox. To unravel this requires an understanding of possible sediment and nutrient sources, the processes causing erosion, and delivery mechanisms to downstream waterways. Current models for the Lower Susquehanna watershed suggest that agricultural sources account for nearly 90% of the needed nutrient and sediment load reductions, despite little data to validate these claims.

Here we use a ¹³⁷Cs inventory to document the relative contributions of sediment from two landscape sources in the Big Spring Run watershed (8 km²): agricultural slopes and stream banks. An inventory of fallout ¹³⁷Cs activity from two hill slope transects adjacent to Big Spring Run yields average erosion rates of 1.8 t/ha/yr and 0.3 t/ha/yr, both of which are significantly less than the presumed county-wide rate of 4 t/ha/yr (8 t/acre/yr). These ¹³⁷Cs inventory rates agree with erosion rates we calculate from the revised universal soil loss equation using GIS interpretations of aerial photographs flown over the past 60 years. This study indicates a dramatic reduction in soil erosion rates from 25 t/acre/yr in 1940 to <5 t/acre/yr from 1988 to 2005. Although the lower value still indicates soil mobility, we do not know how much of it reaches the stream. However, the average contribution of sediment supplied to Big Spring Run from bank erosion can be deduced from mass balance calculations of ¹³⁷Cs data, which shows that roughly 50% of the sediment supply from this watershed can be attributed to bank erosion. Also, geochemical studies reveal that these bank sediments contain 400-800 ppm of phosphorus and 1500-3000 ppm of nitrogen. We estimate that significant quantities of erosive, nutrient-rich “legacy sediment” are stored in stream valleys throughout the region, which accumulated in abundant mill dam reservoirs constructed in the 17^th-19^th Centuries.