Geophysical Insight Into Multi-Scale Groundwater-Surface Water Exchange Processes

Abstract:

The most pressing challenges groundwater/surface water exchange research involve the integration of data collected at disparate spatial scales, especially upscaling of point-scale observations. Physical and chemical point measurements in a streambed are inherently difficult to integrate to the reach and watershed scale, whereas tracer experiments at larger scales obscure key fine-scale dynamics. Geophysical methods can help bridge these scales by remotely and directly sense physical properties, such as temperature and electrical conductivity, which may indicate exchange processes and controlling geologic characteristics of the critical zone. Therefore, novel applications of surface geophysical techniques can provide a process-based understanding of groundwater/surface water exchange when paired with physical measurements. Here, we provide multiple examples of the broad applicability of surface geophysical methods to connect point-scale measurements to the larger systems. First, we present comprehensive geophysical/thermal/hydrodynamic data collected at the upper Delaware River (PA, USA) in an effort to inform management of the endangered dwarf wedgemussel, and at the Quashnet River (Cape Cod, MA, USA) to describe preferential native brook trout habitat (see image for active heating example). Second, 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 solute exchange within streambed less-mobile pore space. Finally, moving outside of the stream domain, we show data from Palmyra Atoll in the central Pacific where surface electromagnetic and borehole monitoring techniques have been integrated to begin a long-term record of fresh groundwater response to climate change. Together, these studies illustrate the important role that surface geophysics plays in obtaining information regarding groundwater-surface water processes that are difficult to capture with conventional sampling and hydrologic measurements.