Changes in stream temperatures in response to restoration of groundwater discharge and solar heating in a culverted, urban stream

SummaryBoone Creek is a mountainous headwater stream that lies within an urbanized environment in northwestern North Carolina. The primary source of thermal pollution in Boone Creek is the urban infrastructure, which affects stream temperatures through (1) heated runoff, which creates temperature surges and (2) the elimination of groundwater–surface water interactions. In this study, we use a Monte Carlo thermal mixing model to predict the thermal impact of removing a 700-m-long culvert. Our thermal mixing model balances stream discharge and temperatures with surface-heat exchange parameters and restored baseflow. We calculate the daily-average groundwater discharge velocity at 15 locations in the stream using signal decay in streambed temperatures, and utilize a Monte Carlo implementation of the heterogeneous baseflow field in the thermal mixing model. We also calculate surface-heat exchange per unit area for conditions upstream and downstream of the existing culvert and utilize those values in the thermal mixing model. Our modeled temperatures suggest a decrease in summer stream temperatures downstream of the culvert that average 1.35 °C and 1.17 °C for upstream and downstream surface-heat exchange conditions, respectively. The results of our study have implications for the balance between baseflow and the urban infrastructure in any high-gradient urban stream that experiences similar thermal effects.