FLUSHING THE SAPROLITE: BASEFLOW AND QUICKFLOW AGE VARIABILITY IN THE STREAMFLOW OF A DEEPLY-WEATHERED PIEDMONT WATERSHED

Catchments in the Piedmont physiographic province draining to the Chesapeake Bay are characterized by deep profiles of weathered crystalline bedrock. These catchments can have sustained baseflow even during long periods of drought, while also responding quickly to rainfall events. Precipitation flows through these catchments along a network of shifting flow pathways generating a distribution of water ages in streams. While flow pathways are difficult to observe at catchment scale, the distribution of water ages can be inferred from the study of environmental tracers, providing insights into how catchments transport water and associated solutes. Here we present on-going research from a small forested watershed, Pond Branch in Baltimore County, Maryland, aimed at better understanding how the fast and slow pathways in these catchments interact over multiple time scales, and how response dynamics and transit times are related to catchment structure. The work employs a combination of high frequency water sampling for stable water isotopes, dissolved gas sampling, watershed modeling and geophysical surveys. Geophysics results indicate that tens of meters of saprolite underlie the hillslope in the study watersheds, and that the bedrock topography is inverted with respect to surface topography, reaching higher elevations under the stream, and lower under adjacent ridges. Dissolved gas sampling indicates that the mean age of baseflow is on the order of 5 years, while high frequency isotope sampling and transport modeling suggests that the quickflow component of discharge is composed of a mixture of event and pre-event water mobilized from the valley bottom and riparian zone, which make up only 2% of the total catchment area. These results suggest that in this and similar watersheds uplands and lowlands integrate hydrologic inputs over very distinct ranges of timescales. Slowly-varying older baseflow is overprinted by higher-frequency dynamics generated in a relatively small area of the watershed. This set of dynamics suggests that the catchment-scale implications of short-term studies of water quality should be assessed with caution, and long term changes in bay watershed management will be necessary to realize improvements to bay water quality due to the volume of stored groundwater that must first be flushed.