SPRING MONITORING IN AN URBAN KARST SYSTEM

Valley Creek Basin located in eastern Chester County, southeastern Pennsylvania, is an urban karst system. Four springs within the Elbrook Formation have been monitored for suspended sediment, baseflow water chemistry, and storm response. The alkalinity concentrations in monthly samples were variable (more than 100 mg/L annual variation). This variability in alkalinity is typical of karst systems with periodic flushing along fast flow paths. The carbonate mineral SI's were at or slightly undersaturated (0 to –0.5), suggesting that this system is most likely made of solutionally enlarged fractures, with mixing between conduit and matrix flow. The suspended sediment concentration data has shown fairly constant low levels over the past year. The concentrations mostly range from 0.5 to 4.0 mg/L. The low suspended sediment values suggest that the karst system is not receiving a heavy load of urban sediments in the recharge area. Furthermore, trace metal analysis of baseflow water samples and spring mouth sediment showed only low concentrations.

The response to storms within the system was rapid, on the order of 1-3 hours. Recovery to baseflow conditions for both conductivity and water level was also rapid, generally within 3-6 hours. This contrasts with studies of storm response in rural karst settings. For example, a rural spring in central PA took 1-2 days from the start of the precipitation event until the increase in the water stage and the decrease in conductivity. The water stage response was on the order of 10-100 centimeters higher in the rural setting than in the urban springs. The maximum water stage increase at the urban springs were typically less than 10 centimeters, with the more commercialized springs showing only ≤ 5 cm increase. The urban setting thus influenced spring response by limiting the capture area and reducing infiltration. The reduced infiltration leads to smaller storm response and less contaminant input, and the smaller capture area leads to rapid storm response.