INTEGRATING SURFACE GEOPHYSICAL METHODS INTO MULTI-SCALE INVESTIGATIONS OF GROUNDWATER/SURFACE WATER EXCHANGE

Some of the most pressing research challenges related to groundwater/surface water exchange involve the integration of data collected at disparate spatial scales and the upscaling of point-scale observations. Physical and chemical point measurements in a streambed are inherently difficult to integrate to the reach/watershed scale, whereas larger-scale tracer experiments obscure fine-scale processes. More useful predictive relations need to be developed before the disparate scale problem can be overcome. Geophysical methods can remotely and directly sense physical properties, such as temperature and electrical conductivity, which may indicate exchange processes and controlling geologic characteristics. Therefore, novel applications of surface geophysical techniques can provide a process-based understanding of groundwater/surface water exchange, particularly when paired with traditional physical and chemical characterizations. To illustrate these concepts, we present comprehensive geophysical/thermal/hydrodynamic data recently collected at the upper Delaware River (PA, USA) in an effort to inform management of the endangered dwarf wedgemussel (Alasmidonta heterodon), and at the Quashnet River (Cape Cod, MA, USA) to describe preferential brook trout (Salvelinus fontinalis) habitat. The remote sensing capability of electrical methods can be used to describe exchange dynamics of less mobile compartments of streambed transient storage (i.e., bound pore water), which are functionally invisible to traditional chemical sampling. We present a centimeter-scale electrical technique that, when combined with co-located chemical sampling, can be applied to the streambed to quantify preferential flow and mass transfer within less-mobile pore space. Outside of the stream domain, we show data from Palmyra Atoll in the central Pacific where surface geophysical and borehole monitoring techniques have been integrated to begin a long-term record of fresh groundwater response to climate change. This group of studies illustrates the broad applicability of surface geophysical methods to connect point-scale measurements to the larger systems in which they are made, and obtain information regarding processes that are difficult to capture with conventional sampling and hydrologic measurements.