Determining surface and groundwater interaction for an entrenched coastal stream based on streambed temperature time-series analysis utilizing several techniques
Scotch, Chester G.
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Surface water exchange with groundwater has become an increasingly active area of investigation since the 1980’s, as researchers have recognized them as a hydrologic continuum. Wide ranges of hydrologic setting have been investigated, though very few studies have investigated these exchanges in coastal streams in semi-arid environments. This study’s objective is to improve the understanding of groundwater-surface water interaction in a coastal low-flow streambed, characterized by relatively high clay contents, by implementing a combination of analytical, mathematical, statistical, and geophysical methods. Thermal responses resulting from heat transfer due to conduction (no groundwater movement) and advection (by groundwater transport) are analyzed in a streambed characterized by low hydraulic gradients and conductivity sediments with possibly diffusive and small-scale flow paths. These characteristics provide a challenge when attempting to quantify surface and groundwater fluxes utilizing traditional methods. A new approach to separate heat advection from conduction through decomposition of temperature time-series data is proposed. The estimates provided by the numerical and analytical solutions are consistent and indicate that groundwater upwelling is occurring in the streambed during the summer and winter periods at an average of 9 mm d-1 and 3.5 mm d-1, respectively. However, there were discrepancies in specific discharge with depth, indicating multi-dimensional flow in the hyporheic zone. The decomposition method results suggest it may not be applicable to fine-textured coastal stream sediments. Resistivity results provided a good first order approximation of groundwater discharge and serves as a reliable validation tool for thermal methods. The overall results of this study confirm that thermal methods are capable of quantifying surface and groundwater interaction in a coastal low-flow stream. Because coastal streams flow into environmentally and economically sensitive bays and estuaries that serve as key ecosystems and breeding grounds for a large variety of species along coastal areas, improving scientific understanding of groundwater discharge is of significance since it can serve as a transport mechanism for contaminants into these environments. Further research should be conducted to quantify multi-dimensional flows in the hyporheic zone.