TWO CONTRASTING FALL LINE WATERSHEDS IN NW DELAWARE AND SE PENNSYLVANIA

Streamflow discharge in watersheds is considered a mixture of baseflow from groundwater discharge plus transient increases from precipitation events. Baseflow within urban watersheds shows decreased infiltration due to impervious land cover, with flashy response to runoff from precipitation events. We examined two contrasting watersheds in NW Delaware and SE Pennsylvania (White Clay Ck. and Darby Ck., respectively). Hourly data for discharge, water temperature, and specific conductance from the USGS stations #01478650 (White Clay Ck. in Newark, DE) and #01475510 (Darby Ck. near Darby, PA) were augmented with daily to bi-weekly grab sample measurements of anion concentrations, isotope ratios of H and O in water, and Cl isotope ratios of chloride. Catchment areas above these stations were 179 sq. km and 97 sq. km, respectively. Land cover percentages in the White Clay Ck. watershed were roughly 37% developed, 33% agriculture, and 30% forested; those in the Darby Ck. watershed were about 58% developed, 20% forested, and the remaining 22% ]grasslands, bare soil and water. Hourly precipitation data for White Clay Creek watershed were retrieved from the Delaware Environmental Observing System.

The observed contrasts in behavior between these watersheds are summarized as follows: White Clay Ck. has higher baseflow discharge and lower specific conductance than Darby Ck.; Darby Ck. water shows strong increases in specific conductance during winter months, indicating likely input of road salt, whereas such increases are minor or negligible in White Clay Creek owing to its protected status. Higher average specific conductance of Darby Ck. water most likely indicates input from treated wastewater effluent and untreated sewer discharge into the creek. Decreases in specific conductance are correlated with precipitation events in both streams during warmer periods. The isotopic variability of water in both streams has a similar annual range (e.g., d¹⁸O from about -9 ‰ to -6 ‰). During and after precipitation events, relative amplitudes of discharge peaks at White Clay Ck. were smaller than those of Darby Creek, whereas the relaxation times of discharge peaks were shorter at Darby Creek, most likely reflecting the high percentage of impervious land cover in the Darby Creek watershed.