When water is pumped slowly from saturated sediment-water inteface sediments, the more highly connected, mobile porosity domain is prefferentially sampled, compared to less-mobile pore spaces. Changes in fluid electrical conductivity (EC) during controlled downward ionic tracer injections into interface sediments can be assumed to represent mobile porosity dynamics, which are therefore distinguished from less-mobile porosity dynamics that is measured using bulk EC geoelectrical methods. Fluid EC samples were drawn at flow rates similar to tracer injection rates to prevent inducing preferential flow. The data were collected using a stainless steel tube with slits cut into the bottom (USGS MINIPOINT style) connected to an EC meter via c-flex or neoprene tubing, and drawn up through the system via a peristaltic pump. The data were compiled into an excel spreadsheet and time corrected to compare to bulk EC data that were collected simultaneously and contained in another section of this data release. Controlled, downward flow experiments were conducted in Dual-domain porosity apparatus (DDPA). Downward flow rates ranged from 1.2 to 1.4 m/d in DDPA1 and at 1 m/d, 3 m/d, 5 m/d, 0.9 m/d as described in the publication: Briggs, M.A., Day-Lewis, F.D., Dehkordy, F.M.P., Hampton, T., Zarnetske, J.P., Singha, K., Harvey, J.W. and Lane, J.W., 2018, Direct observations of hydrologic exchange occurring with less-mobile porosity and the development of anoxic microzones in sandy lakebed sediments, Water Resources Research, DOI:10.1029/2018WR022823.

Quantification of mobile/less-mobile porosity dynamics at the sediment/water interface is critical to predicting contaminant storage, release, and transformation processes. Zones in groundwater flow-through lakes where lake water recharges the aquifer can strongly control aquifer water quality. Less-mobile porosity has previously been characterized in aquifers using flow path scale (10's of m+) tracer injections which are analyzed using numerical models. Methodology was recently developed to couple geoelectric measurements (bulk electrical conductivity, EC), which are directly sensitive to less-mobile ionic tracer exchange processes, with pumped fluid EC tracer data over time. If the fluid EC concentration history is assumed to reflect the more mobile porosity exchange processes, these paired fluid and bulk EC measurements can be used to quantify less-mobile porosity exchange in discrete cm-scale packets of sediment at the interface between surface and groundwater. For this study, tracer experiments were conducted in multiple rate-controlled downward flow experiments over several days. Although the bed was composed predominantly of highly permeable sands and gravels, which is not an intuitive sediment texture for less-mobile porosity, embedded cobbles created areas of less-mobile flow zones proximal to large cobbles. These experimental findings are described in detail in the associated publication: Briggs, M.A., Day-Lewis, F.D., Dehkordy, F.M.P., Hampton, T., Zarnetske, J.P., Singha, K., Harvey, J.W. and Lane, J.W.(2018), Direct observations of hydrologic exchange occurring with less-mobile porosity and the development of anoxic microzones in sandy lakebed sediments, Water Resources Research, DOI:10.1029/2018WR022823.

In June 2017, U.S. Geological Survey (USGS) collected geophysical measurements to help map variations in electrical properties to infer shallow flowpaths and storage zones influenced by residual spilled unconventional oil and gas (UOG). Two survey profiles were collected, each including dipole-dipole and Wenner-Schlumberger configurations. For each survey a total of 56 electrodes spaced 1.0 meter (m) apart were used. During the ERT measurement, current is injected through two current electrodes and voltage is measured sequentially across multiple pairs of potential electrodes; the known current and the measured voltages are used to determine the apparent resistivity of the subsurface. Inverse modeling of ERT survey results provide profiles of resistivity that can be interpreted for subsurface layers. This data release provides the raw ERT data and output from inversion.

In order to test hypotheses about groundwater flow under and into estuaries and the Atlantic Ocean, geophysical surveys, geophysical probing, submarine groundwater sampling, and sediment coring were conducted by U.S. Geological Survey (USGS) scientists at Cape Cod National Seashore (CCNS) from 2004 through 2006. Coastal resource managers at CCNS and elsewhere are concerned about nutrients that are entering coastal waters via submarine groundwater discharge, which are contributing to eutrophication and harmful algal blooms. The research carried out as part of the study described here was designed, in part, to help refine assumptions required by earlier versions of models about the nature of submarine groundwater flow and discharge at CCNS. This study was conducted in four phases, with a variety of field techniques and equipment employed in each phase. Phase 1 consisted of continuous resistivity profiling (CRP) surveys of the entire study area conducted in 2004. Phase 2 consisted of CRP ground-truthing via resistivity probe measurements and submarine groundwater sampling from hydraulically-drive piezometers using a barge in the Salt Pond/Nauset Marsh area in 2005. Phase 3 consisted of supplemental detailed CRP surveys in the Salt Pond/Nauset Marsh area in 2006. Finally, Phase 4 consisted of sediment coring and porewater extraction in the Salt Pond/Nauset Marsh area later in 2006 to supplement the 2005 sampling.

Electromagnetic (EM) geophysical methods provide information about the bulk electrical conductivity of the subsurface. EM data has been widely used to investigate aquifers and geologic structures. In the following study, the United States Geological Survey conducted a boat-towed, waterborne transient electromagnetic (FloaTEM) survey to examine conductivity within the subsurface of the main Delaware River channel and the Leipsic River. The Leipsic River flows through an estuary into the Delaware Bay. Subsurface conductive zones, when viewed in the context of the regional conceptual model and other data, can help determine the likely groundwater location of the freshwater/saltwater interface within the Delaware River, as well as key hydrogeological layers such as the Lower Potomac-Raritan-Magothy Aquifer within the Northern Atlantic Coastal Plain Aquifer System, and their connectivity with the riverbed. Permeable aquifers could provide a hydraulic connection between surface water and inland groundwater. Therefore, changes to river water salinity could have an accelerated impact on water pumped from wells inland that are connected via these permeable aquifers. The FloaTEM system transmits a primary electrical current through a transmitter loop (Tx) wire. This creates a static primary magnetic field. Then, the current in the TX loop is subsequently turned off, resulting in secondary electrical currents being induced in the earth. These induced electrical currents decay with time, and this rate of decay in the secondary electrical field is a function of the bulk conductivity of the subsurface material. As the secondary electrical field decays, a secondary magnetic field is induced and measured at a receiver (Rx) loop towed behind the Tx loop. The Rx loop measures the decay of the secondary magnetic field as a function of time (dB/dt). Measured dB/dt decay curves can be inverted to recover the depth-dependent resistivity structure of the earth. FloaTEM surveys were conducted downstream from Wilmington, DE on 8/26/2020 and 8/27/2020. Data from 8/26/2020 were collected around the Augustine Wildlife Area boat ramp, and data on 8/27/2020 were collected near the Collins Landing boat ramp. FloaTEM surveys were again conducted downstream from Wilmington, DE on 8/25/2021 and 8/26/2021. Data from 8/25/2021 were collected upstream of the 2020 surveys around the Pennsville public boat ramp, while data on 8/26/2021 were collected near the Collins Landing boat ramp and covered a similar area as the 2020 data. Data collected in 2021 also included a section of the Delaware River further upstream near Philadelphia PA, collected on 8/24/2021 and made use of the Fort Mifflin boat ramp. A final back and forth profile in the Leipsic River within the Bombay Hook National Wildlife Refuge (estuary) was gathered on 8/27/21, and used the Port Mahon Boat Launch as the starting/ending point. Surface water specific conductance data were also collected during portions of the surveys.

This dataset was generated during the precision testing of three water-quality sondes before picking one to use for field deployment of high frequency ground-water quality monitoring. Precision is important because the authors wanted to try and minimize calibration drift corrections between site visits. A laboratory experiment was conducted for the three sondes to simultaneously measure at hourly intervals with a setup of standard solution circulating past the sondes to simulate field conditions. The electrical conductivity experiment lasted 33 hours, the pH experiment lasted 13 hours, and the DO experiment failed (no data).

The electrical conductivity of the earth is used to help infer lithological and pore fluid properties. Various geophysical methods can provide estimates of the distribution of below ground electrical conductivity, with each method having certain limitations. This data release presents raw and processed results from land-based and water-based frequency domain electromagnetic induction (EMI) data collected from August 23, 2017 to August 28, 2017. The raw data consist of .csv files from the Geophex GEM-2 unit. Data were primarily collected by walking with the instrument at approximately 1 m off the ground in horizontal coplanar (ski flat) mode. A survey along a section of the Little Wind River in a kayak (with about 0.3 m of elevation above the water surface) was also collected.

This data release consists of three different types of data: including direct current (DC) resistivity profiles, frequency domain electromagnetic (FDEM) survey data, and global navigation satellite system (GNSS) coordinate data of the geophysical measurement locations. A data dictionary is included along with the data and defines all of the table headings, definitions, and units. Earthen dams are common on lakes and ponds, but characteristics of these structures such as construction history, composition, and integrity are often unknown for older dams. Geophysical surveying techniques provide a non-invasive method of mapping their lithology and structure. In particular, DC resistivity and FDEM methods can, when properly processed, provide the information necessary to construct a lithologic model of an earthen dam without having to trench or core through the shell of the dam itself. In September, 2016 the U.S. Geological Survey (USGS) conducted geophysical surveys at Bob Kidd Lake, an 81-hectare lake, in northwestern Arkansas to help determine the composition of the earthen dam and guide any potential geotechnical investigations. A series of DC resistivity surveys were conducted along, parallel, and perpendicular to the axis of the crest of the dam to identify the soil-bedrock interface and any variations in the composition of the earthen dam. A dense survey using a multi-frequency electromagnetic sensor was used to map the shallow materials comprising the dam at a higher resolution. Resistivity measurements were made by transmitting a known current through two electrodes (transmitter) and measuring the voltage potential across two other electrodes (receiver). The multiple channels on the resistivity meter allow for voltage measurements to be made at 10 receivers simultaneously following a current injection. The configuration of the transmitter relative to the receiver(s) is referred to as an array. For these surveys, a Reciprocal Schlumberger array was used, which positions the transmitting pair of electrodes toward the center of the array and the receiving pairs extending away from the transmitter (Loke, 2000; Zonge and others, 2005). The electrical resistance was calculated by dividing the measured voltage by the applied current. The apparent resistivity was determined by multiplying the electrical resistance by a geometric factor. Apparent resistivity is not the true resistivity, but rather a volume-averaged estimate of the true resistivity distribution, because a homogeneous, isotropic subsurface is assumed. To estimate the true resistivity of the heterogeneous and/or anisotropic subsurface, the apparent resistivity data were processed using an inverse modeling software program. The FDEM method complements the two-dimensional (2-D) DC resistivity method and was used to extend the depth of subsurface characterization obtained with resistivity profiles. The FDEM method uses multiple current frequencies to measure bulk electric conductivity values (the inverse of resistivity values) of the earth at different depths (Lucius and others, 2007). For this project FDEM data were collected with a GEM-2, a broadband, multifrequency, fixed-coil electromagnetic induction unit (Geophex, 2015). In addition to the geophysical surveys a concurrent Global Navigation Satellite System (GNSS) survey was conducted using a Real Time Kinematic system (RTK). All electrode locations on the DC resistivity profiles, all measurement locations in the FDEM survey, as well as a point-cloud survey were collected and are included in the dataset. These data were used to geo-reference the geophysical data and may be used to create a Digital Elevation Model (DEM) of the dam surface.

Streambed instruments that measure electrical resistivity were constructed and installed along the main channel in San Antonio Creek and along tributary streambeds in order to provide insight into intermittent streamflow not recorded by traditional stream gaging stations. These instruments can record high-resolution temporal and geographic responses of streamflow to precipitation in intermittent streams. Streambed electrical resistivity sensors (SERS) and stream temperature, intermittency, and conductivity sensors (STICs) are small, cost-effective, instruments that can be installed (and re-installed) along stream reaches to measure stream intermittency. These instruments employ water contact electrodes to record wet and dry periods—dry periods with no streamflow are characterized by no or small electrical signals, whereas wet periods with streamflow are characterized by large electrical signals due to the presence of water. SERS are constructed by repurposing commercial temperature sensors to record electrical conductivity by removing the sensor thermistor and replacing it with water contact electrodes (Blasch, and others, 2002). STICs serve the same function as SERS but are constructed by repurposing optical sensors so that a temperature and conductivity signal can be recorded (Chapin, and others, 2014). Twenty one SERS and four STICS were constructed and installed in protective casings at bed level along the main channel in San Antonio Creek, and in tributary streams in order to provide insight into stream intermittency. The SERS and STICs were in use between August 2015 and February 2019; the period of record for each instrument varied depending on the date of installation and removal.

The bulk electrical conductivity of the subsurface was indirectly measured with electromagnetic imaging (EMI) by using induced secondary electromagnetic signals generated by subsurface electrical conductors in response to transmitted electromagnetic energy (Zohdy and others, 1974). Electromagnetic induction data were collected using a DUALEM-421 (DualEM, Inc.) mounted on an inflatable stand-up paddle board about 15 centimeters above the water surface. The DUALEM-421 uses 3 transmitter-receiver coil spacings (4-, 2-, and 1-meters) and 2 orientations (vertical dipole, and horizontal dipole). Larger coil spacings interrogate a larger/deeper sampling volume than smaller coil separations. REFERENCE: U.S. Geological Survey, Techniques of Water-Resources Investigations, Book 2, Chapter D1, Zhody, A. A. R., Eaton , G. P., and Mabey, D. R. https://doi.org/10.3133/twri02D1