GROUNDWATER FLOWPATH CONTROLS ON SEEPAGE TEMPERATURE AND FUTURE COLD WATER STREAM HABITAT

As surface-water temperatures increase, climate refugia driven by groundwater connectivity are expected to enable cold-water fish species to survive. Additionally, salmonids are known to seek out groundwater discharge zones for spawning activity. Hydrogeophysical methods, including heat tracing, are enhancing understanding of coupled stream ecology and groundwater hydrology throughout the river corridor. We present two case studies regarding native brook trout (Salvelinus fontinalis), a cold-water fish that is challenged by poor water quality and rising temperatures. First, we demonstrate the utility of passive seismic measurements to evaluate bedrock depth along a headwater stream in Shenandoah National Park, VA, USA. The bedrock contact adjacent to zones of known brook trout habitat was found to average 2.6 m beneath land surface, and numerical models predicted strong sensitivity of shallow groundwater temperature due to the downward conduction of surface heat. Annual stream temperature dynamics lagged local air temperature by ~ 25-40 d, similar to predicted groundwater at 2.6 m. This lag suggests strong thermal exchange with the alluvial aquifer, for which the thermal buffering capacity will be reduced over time in a warming climate. Next we explore the interplay among subsurface geology, groundwater discharge, and preferential fish habitat along a 2-km reach of the Quashnet River, Cape Cod, Massachusetts, USA. Zones of focused groundwater discharge through the sandy streambed contributing to an approximate 110 L/s gain in streamflow over the lower 1 km of the reach. Novel actively-heated high spatial resolution fiber-optic distributed temperature sensors installed within a zone of strong groundwater seepage indicate true vertical flow to at least the 0.6 m streambed depth, indicative of regional groundwater discharge. The active-heating approach allows quantitative evaluation of streambed water fluxes well below the depth of diel surface signal extinction. Deeper regional groundwater flowpaths are predicted to show less near-term sensitivity to climate warming compared to shallow local flowpaths, therefore the Quashnet River may provide long-term habitat for native brook trout.