HIGH RESOLUTION HEAT TRACING OF GROUNDWATER DISCHARGE TO A CONTAMINATED REACH OF NINEMILE CREEK, NEW YORK

Recent advances in methods for measuring and interpreting stream and streambed temperature records have enabled the use of heat as a tracer of surface water-groundwater interactions. Advantages of heat tracing include the ease and low cost of taking temperature measurements and that groundwater flux to streams can be quantified at a high spatial resolution. We installed temperature data loggers in a 30-m reach of Ninemile Creek, New York, to determine the amplitude of the daily temperature signal at multiple locations and depths. We then used an analytical heat transport model to directly calculate daily seepage fluxes from temperature records. A rating curve was developed between the average daily seepage fluxes and point-in-time streambed temperature measurements. We mapped the streambed temperature in the study area and determined seepage fluxes at a high spatial resolution by applying our rating curve to the streambed temperature measurements. We calculated a total groundwater discharge of 330 Lm^-2day^-1 to the 30-m study area. The rating curve, representing the relationship between measured streambed temperatures and calculated seepage fluxes, has a high correlation coefficient (R² = 0.71), indicating the relationship is valid and can be extrapolated to the point-in-time temperature measurements made at the 30-m study site. Contaminated groundwater at the study site is highly saline relative to the stream water. The specific conductance of water at the streambed interface positively correlated with the computed groundwater discharge rate, showing that estimates of high groundwater flux were consistent with observations of high salinity. We found the groundwater discharge rates in our study area were highly variable in space and that groundwater enters the creek in specific, well-defined locations. Heat tracing allowed for the characterization of groundwater discharge at high spatial resolution (i.e. every 3 m²) over a small spatial area (i.e. 350 m²).