GROUNDWATER CONTRIBUTIONS TO HEADWATER STREAMS ON FRACTURED ROCK IN THE NORTH CAROLINA PIEDMONT AND BLUE RIDGE

Baseflow is generally assumed to homogenously accrete into headwater channels through flow from soil and porous bedrock, but on crystalline rocks there may be discrete up-welling and down-welling zones associated with fractures. Despite the prevalence of fractured crystalline rocks in the Appalachians and Piedmont of the eastern United States, little work has been done to document and understand groundwater-stream interactions in fractured rock environments.

At three sites in the North Carolina Piedmont and Blue Ridge provinces, groundwater and first-order streams were monitored for temporal and longitudinal temperature and water quality patterns. Stream temperatures at all sites have strong diurnal and seasonal fluctuations, while streambed sediments show smaller diurnal variability. Near-stream piezometers and wells show no diurnal temperature fluctuations, and seasonal fluctuations lag air temperature changes by 1-7 months or are absent. These lags generally increase with depth. In response to rainfall events, a shallow well in a discharge zone at one site (Bent Creek) showed temperature perturbations within 18-20 hours, suggesting upwelling from deeper flow zones. At another site (Allison Woods), rainfall perturbed groundwater temperatures in piezometers screened 1-2.4 m below land surface, but not in wells screened 2.1-7 m below land surface, suggesting groundwater recharge. There is a general trend towards downstream heating in the summer, but several temperature probes deviate from this trend, and synoptic surveys show that some areas with depressed temperatures have elevated specific conductance. These results suggest that there are distinct groundwater upwelling areas within the streambed. At one site (Deep Creek), seasonal variation in stream water isotopes suggest that baseflow is sourced in water <5 years old.

Some streams also have ephemeral reaches that correspond with debris jams and sediment wedges, where all baseflow infiltrates into the stream bed. In other reaches, the streams flow on bedrock with fine alluvium banks and hyporheic exchange may be quite limited. Ongoing work aims to understand the relative importance of hyporheic exchange versus fracture systems in setting the patterns of groundwater-surface water interactions in Piedmont and Blue Ridge headwater streams.