Eric A. White

Continuous resistivity profiling to delineate submarine groundwater discharge—examples and limitations

Aquifer-ocean interaction, saline intrusion, and submarine groundwater discharge (SGD) are emerging topics in hydrology and oceanography with important implications for water-resource management and estuarine ecology. Although the threat of saltwater intrusion has long been recognized in coastal areas, SGD has, until recently, received much less attention. It is clear that SGD constitutes a major nutrient flux to coastal waters, with implications for estuarine ecology, eutrophication, and loss of coral reefs; however, fundamental questions regarding SGD remain unanswered: What are the spatial and temporal distributions of SGD offshore? How do seasonal and storm-related variations in aquifer recharge affect SGD flux and nutrient loading? What controls do aquifer structure and heterogeneity impose? How are SGD and saline recirculation related? Geophysical methods can provide insights to help answer these questions and improve the understanding of this intriguing and environmentally relevant hydrologic pheno...

Temperature data for study of shallow mountain bedrock limits seepage-based headwater climate refugia, Shenandoah National Park, Virginia

A combination of long-term daily temperature records and depth to bedrock measurements were used to parameterize one-dimensional models of shallow aquifer vertical heat transport in Shenandoah National Park, VA, USA. Spatially discontinuous roving water surface and bank temperatures surveys were performed with a handheld thermal infrared camera in September and December 2015 along the main channel of a headwater stream supporting coldwater-dependent brook trout (Salvelinus fontinalis). We also installed vertical arrays of thermal data loggers to estimate bulk thermal diffusivity of the saturated alluvium at two stations in the upper trout section. The methods are fully documented in the associated journal article, Briggs, M.A., J.W. Lane, C.D. Snyder, E. White, Z.C. Johnson, D.L. Nelms, and N.P. Hitt, 2017, Shallow mountain bedrock limits seepage-based headwater climate refugia, Limnologica, http://dx.doi.org/10.1016/j.limno.2017.02.005. This Data Release includes temperature measurements collected as part of the study. The directory RAW_DATA contains the measured temperature time series at streambed, stream, and air locations as described in the local read.me file. The OUTPUT directory contains the processed temperature time series and VFLUX2 calculations of thermal diffusivity (Ke) from streambed data, and annual temp signal amplitude/phase lag from stream/air data are listed.

Raw Data from Continuous Resistivity Profiles Collected May 24-25, and July 28, 2011 in Biscayne Bay and a Cooling Canal System, Miami-Dade County, Florida

The salinity of groundwater at the Florida Power & Light Company (FPL) Turkey Point Nuclear Plant in southeastern Florida is being evaluated. The power plant contains a large cooling canal system with warm water; which has salinities elevated above typical, natural surface water in southeastern Florida, circulating within the canals in the uppermost part the highly permeable karst carbonate Biscayne aquifer. The canal system is a closed system that does not discharge directly to fresh or marine surface water bodies. However, exchange between the canal system and groundwater has been postulated based on existing hydrologic information. A pilot study was conducted to determine if Continuous resistivity profiles could be used to determine the vertical and horizontal extent of the hypersaline water from the cooling canal system. Continuous resistivity profiles were collected May 24-25, and July 28, 2011 in some of the cooling canals of the Florida Power and Light Company Turkey Point power plant and in Biscayne Bay near the power plant.

USE OF ELECTROMAGNETIC INDUCTION METHODS TO MONITOR REMEDIATION AT THE UNIVERSITY OF CONNECTICUT LANDFILL: 2004-2011

Time-lapse geophysical surveys using frequency-domain electromagnetics (FDEM) can indirectly measure time-varying hydrologic parameters such as fluid saturation or solute concentration. Monitoring of these processes provides insight into aquifer properties and the effectiveness of constructed controls (such as leachate interceptor trenches), as well as aquifer responses to natural or induced stresses. At the University of Connecticut landfill, noninvasive, electromagnetic induction (EMI) methods were used to monitor changes in subsurface electrical conductivity that were related to the landfill-closure activities. After the landfill was closed, EMI methods were used to monitor changes in water saturation and water quality. As part of a long-term monitoring plan to observe changes associated with closure, redevelopment, and remediation of the former landfill, EMI data were collected to supplement information from groundwater samples collected in wells to the south and north of the landfill. In comparison to single-point measurements that could have been collected by conventional installation of additional monitoring wells, the EMI methods provided increased spatial coverage, and were less invasive and therefore less destructive to the wetland north of the landfill. To monitor effects of closure activities on the subsurface conductivity, EMI measurements were collected from 2004 to 2011 along discrete transects north and south of the landfill prior to, during, and after the landfill closure. In general, the results indicated an overall decline in subsurface electrical conductivity with time and with distance from the former landfill. This decline in electrical conductivity indicated that the closure and remediation efforts reduced the amount of leachate that originated from the landfill and that entered the drainages to the north and south of the landfill.

Integrated Use of Surface Geophysical Methods for Site Characterization — A Case Study in North Kingstown, Rhode Island

A suite of complementary, non-invasive surface geophysical methods was used to assess their utility for site characterization in a pilot investigation at a former defense site in North Kingstown, Rhode Island. The methods included frequency-domain electromagnetics (FDEM), ground-penetrating radar (GPR), electrical resistivity tomography (ERT), and multi-channel analysis of surface-wave (MASW) seismic. The results of each method were compared to each other and to drive-point data from the site. FDEM was used as a reconnaissance method to assess buried utilities and anthropogenic structures; to identify near-surface changes in water chemistry related to conductive leachate from roadsalt storage; and to investigate a resistive signature possibly caused by groundwater discharge. Shallow anomalies observed in the GPR and ERT data were caused by near-surface infrastructure and were consistent with anomalies observed in the FDEM data. Several parabolic reflectors were observed in the upper part of the GPR profiles, and a fairly continuous reflector that was interpreted as bedrock could be traced across the lower part of the profiles. MASW seismic data showed a sharp break in shear wave velocity at depth, which was interpreted as the overburden/bedrock interface. The MASW profile indicates the presence of a trough in the bedrock surface in the same location where the ERT data indicate lateral variations in resistivity. Depths to bedrock interpreted from the ERT, MASW, and GPR profiles were similar and consistent with the depths of refusal identified in the direct-push wells. The interpretations of data collected using the individual methods yielded non-unique solutions with considerable uncertainty. Integrated interpretation of the electrical, electromagnetic, and seismic geophysical profiles produced a more consistent and unique estimation of depth to bedrock that is consistent with ground-truth data at the site. This test case shows that using complementary techniques that measure different properties can be more effective for site characterization than a single-method investigation.

Combined use of frequency‐domain electromagnetic and electrical resistivity surveys to delineate the freshwater/saltwater interface near saline lakes in the Nebraska Sand Hills, Nebraska, USA

We investigate the use of frequency-domain electromagnetic (FDEM) and electrical resistivity (ER) surveys for rapid and detailed characterization of the direction of lake-aquifer fluxes and the configuration of salt plumes generated from saline lakes. This methodology was developed and applied at several lakes in the Nebraska Sand Hills, Nebraska, in an area with both freshwater and saline lakes hydraulically connected to the freshwater surficial aquifer. The FDEM survey was conducted by mounting the instrument on a fiberglass cart towed by an all-terrain vehicle. The towed FDEM surveys covered about 25 km per day and served as a reconnaissance method for choosing locations for the more quantitative and detailed ER surveys. Around the saline lakes, areas with high electrical conductivity are consistent with the regional direction of ground-water flow. Lower electrical conductivity was measured around the freshwater lakes with anomalies correlating to a paleovalley axis inferred from previous studies. The efficacy of this geophysical approach is attributed to: (1) significant contrast in electrical conductivity between freshwater and saltwater, (2) near-surface location of the freshwater/saltwater interface, (3) minimal cultural interference, and (4) relative homogeneity of the aquifer materials.