GEOCHEMISTRY AND SEDIMENT OF AN URBAN KARST SYSTEM

Valley Creek Basin, in eastern Chester County, southeastern Pennsylvania, is an urban karst system. Four springs within the Elbrook Formation have been monitored for suspended sediment, base flow water chemistry, and trace metals in sediment and water. The suspended sediment concentration data has shown a fairly constant low level over a five month period. The concentrations mostly range from 0.5 to 2.0 mg/L and the variation falls within the noise range. The low suspended sediment values suggest that the karst system is not receiving a heavy load of urban sediments in the recharge area. The alkalinity data (HCO3-) shows concentration ranges of 50 mg/L at three of the four sites, and they have average alkalinities between 250 and 300 mg/L. The fourth site has a higher alkalinity range of 125 mg/L. This variability in alkalinity is typical of karst systems although the relatively high concentrations suggest the water has been in contact with the rock matrix sufficiently to equilibrate. This system is most likely made of interconnected cracks and fractures. Trace metal analyses for Cr, Co, Ni, Cu, Zn, As, Cd, Sn, and Pb were obtained for base flow water samples and bedload sediment collected at the spring mouths. The water samples had low concentrations of metals, less than 10 ppb. As expected the metal concentrations found in the sediment were higher, mostly around 5 to 150 g/kg with a few higher concentrations for Zn and for Pb. The higher metal concentrations in sediment are because metals adsorb onto clay size particles and accumulate. Nonetheless, the metal concentrations on sediment were relatively low, and similar to sediment from a karst spring in a rural setting. This suggests that there is a lack of build up of metals in this urban karst system, perhaps because of short travel paths.