Purpose and Scope

The purpose of this study is to document data collection and provide insight into to the hydrogeology at the NRF. Geologic, geophysical, and aquifer test data were collected and analyzed to determine lithologic and hydraulic properties of the ESRP aquifer. Additionally, water samples were collected and analyzed for inorganic, organic, stable isotope, and radionuclide constituents for the newly drilled well and to provide water-quality data after well construction. The scope of this report presents results of the drilling, construction, geophysical logging, aquifer testing, and water sampling for wells NRF-17 and NRF-18.

Hydrogeologic Setting

The INL is located within the west-central part of the ESRP (fig. 1). The ESRP is a northeast-trending structural basin about 200 miles (mi) long and 50–70 mi wide. Formation of the ESRP was caused by the passage of the North American Plate over the Yellowstone hot spot (Pierce and Morgan, 1992). The ESRP is subject to continuing basaltic volcanism and subsidence because disruption to the crust resulted in increased heat flow (Blackwell and others, 1992) and emplacement of a dense, mid-crustal sill (Shervais and others, 2006). The subsiding ESRP basin was filled with interbedded terrestrial sediments and late Pliocene to Pleistocene basalt, 0.6–1.2 mi thick (Whitehead, 1992). The basaltic rocks and sedimentary deposits make up the ESRP aquifer.

The ESRP is composed mostly of olivine tholeiite basalt flows, which erupted as tube-fed, inflated, pahoehoe flows that constitute more than 85 percent of the subsurface volume of the ESRP at the INL (Anderson and Liszewski, 1997). The distribution of basalt flows is controlled by topography, rate of effusion, and duration of eruption. Near-vent flows are thinner than distal flows, and accumulations of thin flows have a larger volume of high conductivity zones than the same volume of thick flows (Anderson and others, 1999).

The part of the Snake Plain aquifer that underlies the ESRP is one of the most productive aquifers in the United States (U.S. Geological Survey, 1985, p. 193). Groundwater in the ESRP aquifer generally moves from northeast to southwest, eventually discharging to springs along the Snake River downstream from Twin Falls, Idaho—about 100 mi southwest of the INL (Whitehead, 1992). Water moves through basalt fracture zones at the tops, bases, and sides of basalt flows. Infiltration of surface water, groundwater pumping, geologic conditions, and seasonal fluxes of recharge and discharge locally affect the movement of groundwater (Garabedian, 1986). Recharge to the ESRP aquifer is primarily from infiltration of applied irrigation water, streamflow, precipitation, and groundwater inflow from adjoining mountain drainage basins (Ackerman and others, 2006; Rattray, 2018).

Previous investigations have determined groundwater flow characteristics and hydraulic properties of the ESRP aquifer within and near INL. The 2021 water-table elevation ranges from about 4,560 to 4,410 ft (Treinen and others, 2024, fig. 9); at wells NRF-17 and NRF-18, the elevation of the water table is about 4,462 ft. The depth to water below land surface in wells completed in the ESRP aquifer ranges from about 286 ft in the northern part of the INL to more than 990 ft in the southeastern part. The May 2024 depth to water is 392 ft BLS as measured in wells NRF-17 and NRF-18 is about 392 ft BLS at both locations. Water in the ESRP aquifer primarily moves through interflow and fracture zones in the basalt. A large proportion of groundwater moves through the upper 200–800 ft of basaltic rocks (Mann, 1986, p. 21). Twining and Maimer (2019) estimated transmissivity for the upper part of the aquifer to be 2.0–540,000 feet squared per day (ft²/d). Anderson and others (1999) reported a range of hydraulic conductivity at the INL of 0.01–32,000 feet per day (ft/d). The hydraulic conductivity of rocks underlying the aquifer varies from 0.002 to 0.03 ft/d (Mann, 1986, p. 21). Localized tracer tests at the INL have shown vertical and horizontal transport rates as high as 60 and 150 ft/d, respectively (Nimmo and others, 2002; Duke and others, 2007).

Previous Investigations

The USGS INL Project Office provides completion reports for special studies addressing drilling, borehole geophysical data, and monitor well testing at the INL. Several recent reports include work done for wells located around the NRF. A list of all the reports published by the USGS on project work completed at the INL can be found in Fisher (2022). Notable publications related to the purpose and scope of this study include Bartholomay and others (2002) and Twining and others (2016, 2010).

Bartholomay and others (2002) examined the chemical and radiochemical constituents in water samples collected from 13 wells near the NRF in 2000. Water samples were analyzed for naturally occurring constituents and anthropogenic contaminants. A total of 52 samples were collected from the 13 monitoring wells. The routine samples contained detectable concentrations of total cations and dissolved anions, and nitrite plus nitrate as nitrogen. Most of the samples also contained detectable concentrations of gross alpha- and gross beta-particle radioactivity and tritium.

Twining (2016) collected borehole deviation data for wells constructed within the ESRP aquifer. The well completion summary report examined deviation data collected through gyroscopic and magnetic methods and presented correction factors for water-level observations at well sites in and near INL. Borehole well deviation survey data were considered for 177 wells completed within the ESRP aquifer, but not all wells had deviation survey data available. Of the 57 wells with gyroscopic deviation surveys, correction factors for 16 wells ranged from 0.20 to 6.07 ft and inclination angles (SANG) ranged from 1.6 to 16.0 degrees. To remove the effects of well deviation, the USGS INL Project Office applies a correction factor to water-level data when a well deviation survey indicates a change in the reference elevation of greater than or equal to 0.2 ft at the water surface.

Twining and others (2010) provided drilling, construction, geophysical log data, and lithologic logs for borehole NRF-16 using a report format similar to that of the current report. Data collected during drilling, geophysical logging, aquifer testing, and sampling were summarized. Borehole NRF-16 was initially cored to a depth of 425 ft BLS and water samples and geophysical data were collected. Two consecutive single-well aquifer tests were conducted after final construction to estimate hydraulic properties, averaged transmissivity and hydraulic conductivity were 4.8×10³ ft²/d and 9.9 ft/d, respectively. Water samples were analyzed for metals, nutrients, total organic carbon, volatile organic compounds, semi-volatile organic compounds, herbicides, pesticides, polychlorinated biphenyls, and radionuclides. All chloride, nitrate, and sulfate concentrations were less than background concentrations. Concentrations in water samples for most of the organic compounds and radionuclides were less than the reporting limits and reporting levels.

Monitor Well NRF-17 Construction

USGS mobilized equipment to well NRF-17 on August 14, 2023. Steel well casing, 12.0-in.-and 8.0-in.-diameter, were previously installed during 2020 (Trcka and Twining, 2023a). On August 15, 2023, the USGS started drilling using an 8.0-in.-diameter drill bit attached to a 7.5-in.-diameter stabilizer. At about 76 ft BLS, the 8.0-in. bit stopped advancing as it became partially lodged inside the 8.0-in. steel casing, so the steel casing was removed along with the drill bit. The 12.0-in. steel casing, installed to a depth of 20 ft BLS, remained in place after the 8.0-in. casing was removed.

Starting on August 17, 2023, the USGS reassembled a similar 8.0-in.-diameter drill bit and 7.5-in.-diameter stabilizer and drilled to a depth of 372 ft BLS over the course of 3 days. On August 22, 2023, the USGS ran a video log to confirm that the well was open before installing 6.0-in.-diameter steel well casing. The well casing would not advance past 160 ft BLS and was removed to clear an obstruction. On September 5, 2023, the USGS drilled using a 10.0-in.-diameter hammer bit to 230 ft BLS before the hammer started having operational issues and was changed out for a tri-cone style drill bit. On September 7, 2023, the 10.0-in. tri-cone drill bit completed reaming to a depth of 376 ft BLS. Before the tri-cone bit was removed, the borehole was cleared for 30 minutes to remove any residual cuttings using a combination of drilling foam and supply water. On September 11, 2023, a video was run showing that the drilled section was clear from obstructions, so 6.0-in. well casing was set to 371 ft BLS. A 10.0-in. grout basket was installed on the bottom casing string to hold the bentonite hole plug and prevent it from falling past 371 ft BLS.

Starting on September 11, 2023, 8,000 pounds of bentonite hole plug were poured from land surface, hydrated over 2 days, and tagged near 50 ft BLS. About 2,800 pounds of neat cement were added on September 12, 2023, from 50 ft to land surface to complete the annular seal (fig. 3).

Starting on September 12, 2023, the USGS drilled using a 5.9-in. tri-cone drill bit and 5.5-in. stabilizer and competed reaming from 371 to 461 ft BLS. The well was developed for about 90 minutes after drilling. Well development continued for a few hours the next day (September 13, 2023). On September 14, 2023, the drill stem was removed, and a video run showed that the hole was open to 461 ft BLS. The video showed that the previously cored section remained open down to a depth of about 540 ft BLS. After discussion with the NRF, the USGS placed approximately 1,000 pounds of bentonite hole plug to seal the corehole section from 461 to 540 ft BLS. The section of borehole was tagged using a weighted measurement line to confirm the total well depth as 461 ft BLS. The USGS mobilized equipment over to well NRF-18 on September 18, 2023.

On September 25, 2023, the USGS installed a submersible pump, pump wire, and measurement line (fig. 3). Well NRF-17 was configured with a submersible pump and motor attached to a 1.3-in.-diameter stainless-steel discharge and 1.0-in.-diameter stainless-steel open-ended measurement line. The submersible pump suction was placed at 420.5 ft BLS. The final construction of well NRF-17 includes (1) 12.0-in.-diameter carbon steel casing extending from land surface to 20 ft BLS; (2) 6.0-in.-diameter carbon steel casing extending 2 ft above land surface to 371 ft BLS; and (3) 5.9-in., inside-diameter open borehole from 371 to 461 ft BLS (fig. 3). Surface completion includes a 4-ft-diameter concrete pad complete with a brass survey marker, and a locking wellhead (table 1).

Monitor Well NRF-18 Construction

USGS mobilized equipment to well NRF-18 on September 18, 2023 and started to drill with an 8.0-in. bit attached to a 7.5-in.-diameter stabilizer. Drilling occurred over 2 days to a completion depth of 461 ft BLS. Well NRF-18 was developed for 3 hours after drilling stopped and until there was evidence of good formation flow on September 19, 2023. The upper borehole section, which includes 12.0-in. casing to 15 ft BLS and 8.0-in. casing to 380 ft BLS, was completed as part of the prior drilling activity and did not require any modification (Trcka and Twining, 2023b).

On September 20, 2023, a video was run to confirm the borehole condition and showed the bottom of the drilled 8.0-in. borehole free from obstruction to a depth of 461 ft BLS. The video also showed that the previously cored section was only partially filled with drill cuttings and was open between 461 ft and 539 ft BLS. About 825 pounds of bentonite hole plug was added and 200 pounds of silica sand (6-9 mesh size) to bring the well bottom up to the design depth of 450 ft BLS. Well NRF-18 depth was tagged using a weighted measurement line while adding material and later confirmed by borehole video. The 6.0-in. stainless-steel well screen assembly was installed on September 21, 2013, with the bottom cap landing at 450 ft BLS. The well screen assembly included a threaded K-packer adapter, 39 ft of blank 6.0-in., stainless-steel blank casing, and 60 ft of 6.0-in., stainless-steel well screen (20-slot wire wrap; fig. 4).

Starting on September 27, 2023, the USGS installed a submersible pump and measurement line. Well NRF-18 was configured with a submersible pump and motor attached to a 1.3-in.-diameter, stainless-steel discharge and 1.0-in.-diameter, stainless-steel, open-ended measurement line. The submersible pump suction was placed at 410.6 ft BLS. The final construction of well NRF-18 includes (1) 12.0-in.-diameter carbon steel casing extending from land surface to 15 ft BLS; (2) 8.0-in.-diameter carbon steel casing extending just above land surface to 380 ft BLS; (3) 6.0-in.-diameter, stainless-steel blank casing attached to a K-packer assembly extending from 351 to 390 ft BLS; and (4) 6.0-in.-diameter, stainless-steel, wire-wrap, 20-slot well screen extending from 390 to 450 ft BLS with a capped bottom (fig. 4). Surface completion includes a 4-ft-diameter concrete pad complete with a brass survey marker, and a locking wellhead (table 1).

Geophysical Data

Geophysical data were collected immediately following core drilling during 2020 and after final construction of each monitoring well during 2023. Table 2 summarizes the borehole geophysical data collected from wells NRF-17 and NRF-18. Geophysical data presented in figures 5 and 6 (including natural gamma, neutron, and gamma-gamma density) were collected during 2020 and represent the cored section prior to reaming and casing in wells NRF-17 and NRF-18 (table 2). Additional geophysical log data, acoustic televiewer (ATV), was collected in 2023 following reaming for well NRF-17 and well NRF-18. Select geophysical data (neutron, natural gamma, and gamma-gamma) collected in 2020 following core drilling were shown in figures where source log resolution can be affected by a larger borehole diameter.

Geophysical log data collected for wells NRF-17 and NRF-18 include natural gamma, neutron, gamma-gamma dual density, gyroscopic deviation, mechanical 3-arm caliper, acoustic caliper, and ATV (table 2). Additionally, borehole videos were collected to view the conditions of the open borehole during construction and after final completion. Geophysical log data were collected and saved as electronic files in the form of physical measurement and depth and processed using WellCAD software. Processed data were used to infer changes in geologic material along with lithologic descriptions from core. Borehole geophysical data can be accessed through the USGS GeoLog Locator (U.S. Geological Survey, 2019).

Geologic Data

The surficial geology at wells NRF-17 and NRF-18 was characterized as sparsely vegetated loess. Surface sediment was not cored; however, drill cuttings from above the first basalt contact were described as unconsolidated soil consisting mostly of fine-to-coarse sand with a brief occurrence of large boulders, cobbles, and gravel at well NRF-17 and mostly of fine-grained loess material at well NRF-18. Surface sediment extends to about 48 ft BLS at well NRF-17 and to about 6 ft BLS at well NRF-18.

Sediment layer descriptions were noted in core logs (Trcka and Twining, 2023a, 2023b) and lithologic descriptions were included in geophysical logs for wells NRF-17 (fig. 5) and NRF-18 (fig. 6). Excluding surface sediment, core descriptions were compared against geophysical data and suggest sediment layer thickness ranges from 1 to 7 ft for the constructed well section from 48 to 461 ft BLS in well NRF-17 and from 2 to 5 ft for the constructed well section from 6 to 461 ft BLS in well NRF-18. Only one of five sediment layers in well NRF-17 were recovered during coring for the constructed section described (fig. 5). No sediment was recovered from well NRF-18 for the section described (fig. 6). The sediment recovered from well NRF-17 starts near 216 ft BLS and is described as fine sand (Trcka and Twining, 2023a). Other sediment layers were either washed out or not recovered during coring. Excluding the 48 ft of surface sediment in well NRF-17 and 6 ft in well NRF-18, sediment layers described below the first basalt contact only account for a small percentage (less than 4 percent) of the overall thickness in both wells (figs. 5 and 6).

Basalt layers were described for wells NRF-17 and NRF-18 (Trcka and Twining, 2023a, 2023b). The texture of basalt varied between aphanitic, diktytaxitic, and porphyritic and generally ranged from medium to dark gray in color. Lithologic descriptions of basalt layers suggest variable thickness of units and indicate typical basalt variations of fractured upper and lower crust and more massive interiors along with varying degrees of vesiculation. Basalt constituted about 96 percent of the general lithology, excluding surface sediment, for the constructed section in wells NRF-17 and NRF-18.

Geophysical Logs

Borehole geophysical data were collected using wireline logging tools operated and owned by the USGS INL Project Office. Geophysical data generally were collected coming up out of the borehole unless otherwise described in table 2. Borehole video files were recorded using wireline camera equipment, also owned and operated by the USGS INL Project Office. The log type, logging tool identifier, logging depth, date and time of log, and sensor uncertainty are provided in table 2. The USGS calibrates geophysical logging equipment and sensors annually or as needed.

Neutron Logs

Neutron logs are a general indicator of hydrogen content and, when combined with natural gamma logs for sediment location, can be used to identify perched groundwater. The neutron detector continuously records induced radiation produced by bombarding surrounding material (casing, formation, and fluid) with fast neutrons (energies greater than 10⁵ electron volts) from a sealed neutron source, which collide with surrounding atomic nuclei until they are captured (Keys, 1990, section 5, p. 95). The neutron probe used by the USGS INL Project Office has an americium/beryllium neutron source and a helium-3 detector that counts slow (thermal) neutrons (those that have energies less than 0.025 electron volts). The neutron logs were collected through drill casing after core drilling to capture borehole conditions before reaming to a larger diameter (table 2).

Neutron logs indicate no evidence of perched groundwater in unsaturated media, located above the regional aquifer or from land surface down to about 392 ft BLS in wells NRF-17 and NRF-18. Neutron logs shown for well NRF-18 (fig. 6) were collected inside drill stem filled with drilling fluid, resulting in an overall lower neutron count compared to the neutron log collected for well NRF-17 (fig. 5). The neutron logs correlated well with ATV logs and lithologic logs collected for wells NRF-17 and NRF-18 (figs. 7 and 8). Zones with low hydrogen content correlated with dense and massive basalt, whereas zones with high hydrogen content correlated with fractured and vesicular basalt for saturated media. Relative change in neutron porosity, generated from neutron measurements, were used to confirm areas of higher and lower hydrogen content in wells NRF-17 and NRF-18. In well NRF-17, the 2 primary fractured areas that contribute to groundwater flow are present near 407 and 418 ft BLS, respectively (fig. 7); however, most of the lithology within the water column was described as dense basalt, as confirmed by neutron measurements. In well NRF-18, fractured and vesicular basalt constitute most of the water column. Based on neutron measurements and estimated porosity, pumping production capacity for well NRF-18 is likely to be substantially higher than that observed for well NRF-17.

Single-Well Aquifer Test Procedures

During aquifer tests for wells NRF-17 and NRF-18, fluid pressure head, fluid temperature, air temperature, and barometric pressure were measured continuously. The fluid pressure head, Ψ_w+atm, and fluid temperature were measured using an absolute (non-vented) Solinst Levelogger suspended on a direct read cable and placed inside a 1-in., stainless-steel, water-level line before testing (fig. 9). The Model 3001 Solinst Levelogger (model F65/M20; Solinst, Georgetown, Ontario, Canada) has a manufacturer-stated, full-scale range and accuracy of 65.6 and ±0.03 ft, respectively. The barometric pressure, Ψ_atm, and air temperature were measured with a Model 3001 Solinst Barologger, F5/M1.5, with an operating temperature range of –20–80 degrees Celsius (°C) and stated accuracy of ±0.01 pounds per square inch (lb/in²). The barrologger was suspended on a cable about 10 ft down inside the well casing and dataset to collect at 5-minute intervals over the duration of both aquifer tests (figs. 9, 10). During the aquifer test, barometric pressure ranged from 12.37 to 12.34 lb/in² at well NRF-17 and from 12.29 to 12.32 lb/in² at well NRF-18; air temperature ranged from 15.8 to 14.9 °C at well NRF-17 and from 14.6 to 14.7 °C at well NRF-18 (fig. 10).

The compensated fluid pressure head, $\text{ψ}$, was obtained by subtracting the fluid pressure head, ${\text{ψ}}_{w+atm}$, from the barometric pressure, ${\text{ψ}}_{atm}$ (eq. 1). Compensated fluid pressure head computation was done using the software provided by Solinst and calculated using the following equation:

Manual water-level measurements were collected using an electric tape during each aquifer test. The electric tape probe was lowered down the same 1-in., stainless-steel measurement line as the datalogger used to measure fluid pressure head change during the test (fig. 9). The manual water-level measurements were not collected throughout the entire test, but attempts were made to collect early and late time measurements as time permitted to supplement compensated fluid pressure readings. Compensated fluid pressure readings, after the submersible pump was turned off (recovery data), were collected but not analyzed. Additionally, no manual water-level measurements were collected after the pump was turned off.

The pumping rate, Q, associated with well discharge was monitored periodically using an inline flowmeter sensor (fig. 9). The pumping rate reported in well NRF-17 decreased during the first 30 minutes of the test from 4.4 to 3.1 gal/min where it remained relatively constant for the remainder of the test, averaging 3.3 gal/min (fig. 11A). The pumping rate reported in well NRF-18 increased from 29.3 gal/min to 32.0 gal/min over the first 30 minutes of the test and remained relatively constant during the remainder of the test, averaging 31.0 gal/min (fig. 11B).

Analysis of Single-Well Aquifer Test Data

Single-well aquifer test results from monitoring well NRF-17 were analyzed using the Cooper-Jacob method of curve fitting (Cooper and Jacob, 1946) to estimate transmissivity. The Cooper-Jacob method (Cooper and Jacob, 1946), a simplification of the Theis solution, assumes that the pumping well fully penetrates a confined, homogeneous, and isotropic aquifer of infinite extent. Hydrologic conditions at well NRF-17 depart greatly from the Theis (1935) model given that the well partially penetrates an unconfined heterogeneous anisotropic aquifer. The Cooper-Jacob method was used, regardless of the differences between field conditions and theory, for its simplicity. In an analysis of single-well tests, Halford and others (2006) reported that the Cooper-Jacob method (Cooper and Jacob, 1946) is well-suited for estimating hydraulic properties.

The Cooper-Jacob method estimates transmissivity by fitting a straight line to drawdown in the pumping well on an arithmetic axis compared to time on a log-arithmetic axis. Transmissivity, T, is determined from the slope of the straight line using the following equation:

Single-well aquifer test results for well NRF-18 were analyzed using a specific-capacity method to estimate transmissivity (Ackerman, 1991). This method uses linear regression to estimate transmissivity (in feet squared per day) near the well from specific capacity (in gallons per minute per foot; fig. 12). The specific-capacity method uses a modified Theis equation (Theis and others, 1963, p. 332, eq. 1) and assumes constant values for the storage coefficient and the effective well radius to estimate transmissivity. Because of the near instantaneous drop in water level to pumping, the specific-capacity method was used to estimate hydraulic properties at well NRF-18. Both aquifer test methods, the Cooper-Jacob and the specific-capacity methods, have been applied during previous studies and provide a reasonable comparison of hydraulic properties estimated for wells drilled in fractured basalt at the INL.

Specific-capacity is an expression of the productivity of a well and is commonly expressed as the ratio of the pumping rate, Q, in gallons per minute, to the total measured drawdown, $\Delta S$, in feet (eq. 3). This specific-capacity method uses linear regression to estimate transmissivity (T) through the following equations:

The drawdown in the well, s, at any given time, t,$$is determined by subtracting the compensated pressure head at time t, $\psi \left(t\right),$ from the initial compensated pressure head before pumping, ${\psi }_0$. Drawdown as a function of time was calculated using the following equation:

Estimations of horizontal hydraulic conductivity, K, were based on the aquifer thickness, b, rather than the water column height. Halford and others (2006) reported that, in most cases, using aquifer thickness as the divisor gave the best estimates of horizontal hydraulic conductivity for unconfined aquifers with partial penetration. Aquifer thickness was approximated at 850 ft BLS from electrical resistivity surveys and data collected from limited deep boreholes at the INL (Whitehead, 1986, sheet 2; Ackerman and others, 2010). Horizontal hydraulic conductivity (K) was calculated using transmissivity (T) and aquifer thickness (b) in the following equation:

Hydraulic Property Estimates

Transmissivity and hydraulic conductivity were estimated from two single-well aquifer tests performed at wells NRF-17 and NRF-18. Hydraulic property estimates were estimated using an analysis method selected based on individual well response to pumping at a semi-constant rate (Q). Drawdown measurements that occurred during each test were reported from manual water-level measurements and compensated fluid-pressure measurements.

Figure 13A shows manual water-level measurements taken in well NRF-17 during the first 13 minutes of the test and compensated fluid pressure measurements taken throughout the duration of the 180-minute test. Because of a measurement line obstruction, manual water-level measurements were discontinued after 13 minutes; however, compensated fluid-pressure measurements continued and were used for aquifer test analysis. The source of the manual water-level line obstruction cannot be confirmed; however, the obstruction may have been related to the electric tape probe tip encountering pump wire or a pipe coupler below the 1-in. measurement line (fig. 3).

Log cycle drawdown ($\Delta s$ in eq. 2), used to estimate hydraulic properties for well NRF-17, was approximated at 13.1 ft using compensated fluid pressure measurements over one log cycle (1 and 10 minutes; fig. 13B). Measured water-level data collected during the pumping test ranged from 390.48 ft BLS (pre-test) to 409.89 ft BLS, or 19.41 ft, at about 13 minutes into test (Zingre, 2024). Compensated fluid pressure measurements and manual water-level measurements were similar and suggest stable drawdown after about the first 15 minutes into the test at well NRF-17.

Well NRF-18 manual water-level measurements and compensated fluid pressure measurements were taken throughout the duration of the 180-minute test (fig. 14). Average drawdown ($\Delta s$), used to estimate hydraulic properties for well NRF-18, was estimated at 0.57 ft for manual water-level and at 0.56 ft using compensated fluid pressure measurements (fig. 14). Manual water-level measurements collected during the test ranged from 391.16 ft BLS (pre-test) to 391.71 ft BLS (Zingre, 2024). Compensated fluid pressure measurements and manual water-level measurements show similar results and suggest that the reported water-level drop that occurred at the onset of pumping occurred immediately (less than 1 minute afterwards) and remained stable throughout the remainder of the test. Once the pump was shut off, the well recovered immediately and the water level in the well rebounded above the pre-test water level. The immediate recovery response is related to the high production capacity of the well.

Transmissivity for well NRF-17 was estimated using the Cooper-Jacob method (eq. 2), which involves linear regression of a subset of water-level drawdown data to fit a semi-log model and calculate a semi-log slope (fig. 13B). Transmissivity for well NRF-18 was estimated using the specific-capacity method (eq. 4). Compensated pressure head measurements are affected by transducer settings prior to the beginning of the test; therefore, they are relative values and not reflective of actual depth below water.

Using the compensated transducer data that accounts for barometric pressure, values of transmissivity and hydraulic conductivity for well NRF-17 were 8.81 ft²/d and 1.04×10^-2 ft/d, respectively (eqs. 2 and 6). Well NRF-17 drawdown ($\Delta s$) was estimated at 13.1 ft for the log cycle between 1 minutes and 10 minutes (fig. 13B). The calculations of these hydraulic properties, as defined by eqs. 2 and 6, are shown as follows:

Using compensated transducer data that accounts for barometric pressure, the estimated values of transmissivity and hydraulic conductivity for well NRF-18 were 4.77×10³ and 5.61 ft/d, respectively (eqs. 4 and 6). Drawdown ($\Delta s$) during the well NRF-18 pumping test was estimated by taking the difference between the initial elevation and the pumping elevation of water (fig. 14) and estimated at 0.56 ft based on compensated transducer data. The calculations of these hydraulic properties are shown as follows:

A comparison between the estimated transmissivity for wells NRF-17 and NRF-18 and the transmissivity values determined from past aquifer tests conducted at wells near the NRF shows reasonable agreement (fig. 2; table 4). The estimated average transmissivity values at wells NRF-17 NRF-18 are within the wide range of transmissivity values from previous aquifer tests (Ackerman, 1991; Twining and others, 2016, 2018), which range from less than 3.0×10⁰ ft²/d to greater than 5.4×10⁵ ft²/d (table 4). The average hydraulic conductivity values estimated for wells NRF-17 and NRF-18 are within the range of values reported in the literature for similar rock types. Freeze and Cherry (1979) reported hydraulic conductivity values for permeable basalt ranging from 5.7×10^-2 to 5.7×10³ ft/d. The hydraulic conductivity of the ESRP aquifer at or near the INL have been reported from about 1.0×10^-2 to 3.2×10⁴ ft/d (Anderson and others, 1999).

Sample Collection Methods

Water-quality samples were collected from well NRF-17 (on November 7, 2023) and well NRF-18 (on November 8, 2023) from a 0.25-in.-diameter, stainless-steel sample port mounted on piping at the wellhead after the well was purged with a submersible pump and field measurements were stable. Field measurements of water temperature, pH, specific conductance, dissolved oxygen, and alkalinity were collected before sampling (Bartholomay and others, 2021; table 5).

Water-quality samples were processed in the field according to protocols for the analyses requested. Samples analyzed for chemical constituents by the USGS National Water Quality Laboratory (NWQL) were placed in containers and preserved in accordance with laboratory requirements specified by Bartholomay and others (2021, app. 1). Containers and preservatives were supplied by the NWQL and underwent rigorous quality-control procedures (Pritt, 1989, p. 75) to minimize sample contamination. Samples requiring field filtration were filtered through a disposable 0.45-micrometer cartridge pre-rinsed with at least 2 liters of deionized water. Water samples analyzed for radionuclides were collected and preserved in accordance with laboratory requirements specified by Bodnar and Percival (1982) and Bartholomay and others (2021, app. A) before being sent to the Radiological and Environmental Sciences Laboratory (RESL). Water samples for stable isotopes of oxygen and hydrogen, and chlorofluorocarbon (CFC-11) concentrations were collected in bottles provided by the NWQL and shipped to the USGS Reston Stable Isotope Laboratory (RSIL) in Reston, Virginia, for analysis. Water samples for tritium were collected and shipped to the USGS Menlo Park Research Laboratory in California for analysis.

Analytical Methods

Analytical methods used by the USGS for selected organic, inorganic, and radionuclide constituents are described by Goerlitz and Brown (1972), Thatcher and others (1977), Wershaw and others (1987), Fishman and Friedman (1989), Faires (1993), Fishman (1993), McCurdy and others (2008), Rose and others (2016), and Foreman and others (2021). Analytical methods used for selected isotopic constituents were summarized by Busenberg and others (2000). A discussion of procedures and methods used by the RESL for the analysis of radionuclides in water is provided by Sill and Sill (1994), U.S. Department of Energy (1995), and Bodnar and Percival (1982).

Guidelines for Interpretation of Analytical Results

The analytical results for water-quality samples from wells NRF-17 and NRF-18 are reported with stated laboratory uncertainties, minimum reporting limits (MRLs), or method detection limits (MDLs) depending on the analytical technique. Analytical results are interpreted in the context of historical results and established background concentrations, as available. The guidelines for interpreting analytical results are based on an extension of a method proposed by Currie (1984) that is given in Bartholomay and others (2020).

Concentrations of radionuclides are reported with an estimated sample standard deviation, s, which is obtained by propagating sources of analytical uncertainty in measurements. McCurdy and others (2008) provided details on interpreting radiological data used by the USGS. In this report, radionuclide concentrations less than 3s are considered to be less than the “reporting level.”

Concentrations of inorganic and organic constituents are reported with reference to MDL or MRL, respectively. Detection limits for inorganic constituents were determined using the detection and quantification calculation (DQCALC) procedure as described in U.S. Geological Survey (2015) and the reporting limits for organic constituents were determined following the procedures described in Foreman and others (2021).

Previous work has identified several recharge sources to the ESRP aquifer including regional groundwater, tributary valley groundwater, tributary valley surface water, irrigation return water, geothermal water, and precipitation (Rattray, 2018). For the purposes of this report, recharge sources are broadly defined as “regional groundwater” that originates from the ESRP aquifer to the northeast of the INL, and ‘tributary valley groundwater’, which originates from the tributary valleys to the west and northwest of INL. Each of these recharge sources have unique chemical and isotopic signatures. Wells NRF-17 and NRF-18 are located along the northwestern area of the INL (fig. 1) and the general water chemistry and recharge sources can be approximated based on this geographic location (Rattray and Paces, 2023). Recharge sources to the Northwest INL area include surface water from the Big Lost River during higher-than-average precipitation years and groundwater from the Little Lost River Valley and the Big Lost River Valley (Rattray, 2018, 2019).

The following chemistry data sections include select results as well as how these chemical signatures relate to potential groundwater recharge sources at wells NRF-17 and NRF-18 during this sampling event in autumn 2023. Some water-chemistry results at the INL are compared to background values for the ESRP aquifer in general. Finally, water-quality results are compared with known industrial activities that historically have introduced chemical and radiochemical constituents to the aquifer.

Inorganic Chemistry Data

Water samples collected from wells NRF-17 and NRF-18 were sent to the NWQL and the USGS Redox Laboratory (chromium, lead, and manganese only) to be analyzed for inorganic dissolved concentrations of (1) cations of calcium, magnesium, potassium, silica, and sodium; (2) anions of bromide, chloride, fluoride, and sulfate; and (3) trace elements of aluminum, antimony, arsenic, barium, beryllium, boron, cadmium, chromium (total and speciation), cobalt, copper, iron (total and speciation), lead, lithium, manganese, mercury, molybdenum, nickel, selenium, silver, strontium, thallium, tungsten, uranium, vanadium, and zinc.

Selected sample results from samples collected at wells NRF-17 and NRF-18 (reported respectively) indicated that calcium concentrations were 56.4 and 59.8 mg/L, potassium concentrations were 6.2 and 2.90 mg/L, silica concentrations were 25.3 and 24.0 mg/L, chloride concentrations were 37.6 and 27.5 mg/L, and sulfate concentrations were 39.2 and 34.2 mg/L, respectively (table 5). In addition to total and dissolved iron (Fe) and chromium (Cr) results from the NWQL, Fe and Cr were measured at the USGS Redox laboratory and include total Fe and Cr as well as Fe(II) and Cr(VI), two oxidation states of these elements (table 5). The notable results from these split samples sent to the USGS Redox Laboratory were that the total Cr concentration of 21.3 micrograms per liter (µg/L) for well NRF-17 was much greater than the mean background Cr concentration of less than 5 µg/L for the tributary valley groundwater and eastern regional groundwater (Bartholomay and Hall, 2016). The total, dissolved Cr concentration at well NRF-18 (8.40 µg/L) is closer to the median for these sources of potential recharge water to the NRF (Redox Laboratory), table 5. Both wells have Cr(VI) that dominates the speciation of the total dissolved Cr concentrations for both wells, at 21 µg/L (well NRF-17) and 8.40 µg/L (well NRF-18; table 5).

Based on the inorganic constituent results, the water chemistry at wells NRF-17 and NRF-18 is typical of tributary valley groundwater as supported by the lithium concentration (less than 5 µg/L) and silica concentrations (Bartholomay and Hall, 2016; Rattray, 2018). However, the sulfate, nitrate, and chloride concentrations suggest that a source, potentially from industrial wastewater disposal activities at the NRF, is mixing with recharge water from the west and northwest tributary groundwater sources to produce the groundwater observed at these wells. Wells NRF-17 and NRF-18 are located west of the mixing line of water recharge types (regional groundwater and tributary valley groundwater) as described in Fisher and others (2012) and Rattray (2018); however, intermittent recharge from the Big Lost River, variable recharge from the Little Lost River, and Little Lost River valley groundwater recharge are likely influencing the chemical composition of the groundwater at this location. The water chemistry supports a mixture of groundwater from tributary valley groundwater recharge, local groundwater recharge potentially affected by industrial activities, and surface-water recharge (Rattray, 2019). The biggest difference between the inorganic constituent results between wells NRF-17 and NRF-18 is in the aluminum and chromium concentrations (table 5).

Nutrient samples were collected and sent to the NWQL to be analyzed for dissolved concentrations of ammonia as nitrogen (N), nitrite as N, nitrate plus nitrite as N, and orthophosphate as phosphorus (P; table 5). The nitrate plus nitrite as (N) concentrations for both wells (wells NRF-17 and NRF-18) were 1.97 mg/L, which is higher than the mean concentration determined for the eastern regional groundwater source. Locally, at the NRF facility, wastewater-influenced groundwater was observed as being related to industrial activities there.

Organic Chemistry Data

Water-quality samples collected from wells NRF-17 and NRF-18 were analyzed at the NWQL for volatile organic compounds (VOCs). All of the sixty VOC analytes measured had concentrations less than laboratory MRL values (table 5) and the Environmental Protection Agency (EPA) maximum contaminant levels (MCLs; U.S. Environmental Protection Agency, 2022). These results are consistent with results from nearby wells (Treinen and others, 2024).