GEOCHEMISTRY OF LEGACY SEDIMENTS BEHIND STRODE’S MILL DAM: IMPLICATIONS FOR WATER QUALITY IN BRANDYWINE CREEK, PA

Strode’s Mill was built in the early 18th century on Plum Run near West Chester, PA.  The dam captured the flow of a 679 hectare watershed that included, over time, farmland, orchards, and the southern half of the growing urban center of West Chester. The watershed also includes soils developed over bedrock consisting of felsic and mafic gneisses, metadiabase, and serpentinite. Maps and aerial photos show that the mill dam was breached by 1935, by which time the sediments had built up to near the top of the 3-m dam. Plum Run quickly cut an incised channel deep into the sediments. Eroding legacy sediments are now a major input into Plum Run, a tributary to Brandywine Creek, and are implicated in Plum Run’s designation as an impaired quality stream.

Our study was designed to compare the chemical composition of legacy sediments exposed by erosion at several locations behind the dam, including portions fed by different tributaries to Plum Run; sediments at upstream locations; and rocks and soils present within the watershed.  Our measurements in sediments were at 10 to 20 cm intervals in 2-m high paired vertical profiles separated by a few meters. Our measurements were made in situ in the field using handheld X-ray fluorescence and gamma ray spectrometers. These methods are likely less accurate than laboratory analyses but their speed (about 2 minutes per analysis) allowed us to investigate many more sample locations and spatial relationships. Elements that could be consistently determined include: K, Fe, Mn, Cr, Co, Ni, Cu, Mo, Zn, Pb, Rb, Sr, Ba, Ti, Zr, U and Th.

Our results for vertical profiles show relatively little chemical variation and no consistent trends or patterns with depth, and there is little compositional correlation between paired profiles. However, average compositions of profiles from different locations behind the dam vary systematically and form arrays that tend to be on, or parallel to, mixing lines between mafic and felsic gneiss. Stream sediments at an upstream location contain high concentrations of metals: 660 ppm Cr, 13000 ppm Co, 5000 ppm Ni & 580 ppm Cu. Eroding serpentinite or soils associated with them might be implicated though known outcrop patterns are not consistent with this. Elements that are often of environmental concern, such as As, Pb, and Hg, are typically in low concentrations.