Purpose and Scope

This report describes a gain/loss assessment conducted by the U.S. Geological Survey (USGS), in cooperation with the U.S. Air Force Civil Engineer Center (AFCEC), to determine whether downward vertical hydraulic gradients at Pine Lake and Little Pine Lake are present, indicating a losing condition from the lake to aquifer and a greater likelihood of PFAS contamination of the unconfined aquifer, or if vertical gradients are upward, indicating a gaining condition and a lesser likelihood of PFAS contamination of domestic wells. From August 2020 to May 2021, continuous water level and water temperature monitoring were performed in each lake and in temporary groundwater piezometers at each lake to determine hydraulic gradients. Qualitative permeability testing by vertical hydraulic profiling methods is also described to provide general hydrogeologic information about the aquifers underlying these lakes.

Hydrologic Setting

Vertical Hydraulic Profiling

To assess general heterogeneity with depth below Pine Lake and Little Pine Lake, vertical hydraulic profiling was conducted using a qualitative depth interval pumping method. A Geoprobe SP16 drive-point sampler was driven into the subsurface to a specific depth interval at which a screen was exposed. The interval was then pumped of three casing volumes (casings being the rods above the SP16 sampler screen), and water levels were monitored manually until recovery. Recovery to static water levels was assumed to be 0.03 feet (ft) or less of change in 10 minutes. The sampler was then repositioned to another depth in the same hole and the process was repeated as many as five times per hole.

The vertical hydraulic profiling method used in this study is considered a qualitative measure of the general permeability of the sediments surrounding the test interval. If an interval is pumped dry or did not recover within an arbitrarily chosen time (45 minutes to 1 hour was used in this study), the permeability is likely low and the lithology is likely a clay, silt, or very fine sand. If the interval sustained pumping and recovered to static water levels, the sediments at that depth are likely permeable sand or gravel. Recovered water level hydraulic heads at different test intervals can then be assessed together to observe the direction(s) of the vertical hydraulic gradients at that location. This method has proven effective for analyzing hydrogeology and groundwater-surface water exchange at other locations in the Kirkwood-Cohansey aquifer system where clay subunits were present (Fiore and others, 2018). For more quantitative approaches, such as those to obtain hydraulic conductivity, aquifer transmissivity, or other values, longer duration tests with continuously monitored water levels and accurate pump rates would be required for each test interval to obtain more specific drawdown and recovery data.

Vertical hydraulic profiling tests at Pine Lake were performed on June 26, 2020. Tests at Little Pine Lake were performed on July 8 and July 29, 2020. Descriptions of the test intervals at Little Pine Lake and Pine Lake are provided in table 1. Four depth intervals were tested at Pine Lake within one hole (PL-GP), and eight depth intervals were tested at Little Pine Lake among two holes (LPL-GP1 and LPL-GP2).

Results of the vertical hydraulic profiling are discussed in the context of the hydrogeologic framework by Fiore (2020) in the vicinity of the lakes (figs. 3, 4). Information from the hydrogeologic framework was supplemented with existing publicly available driller logs available in the NJDEP DataMiner database (New Jersey Department of Environmental Protection, 2021), as well as interpretations from Stanford (2013). All additional lithology and borehole logs used in this report are available in the USGS GeoLog Locator database (U.S. Geological Survey, 2021), though no logs were collected specifically for this study.

Continuous Monitoring of Water Levels

Hydraulic gradients between the surface water in the lakes and the adjacent groundwater were determined by continuous water-level monitoring data. Lower groundwater levels than lake levels would indicate surface water is flowing into the groundwater system, thus indicating a losing condition and the potential for PFAS contamination of the aquifer. Conversely, when groundwater levels are higher than nearby lake levels, it would indicate groundwater is flowing into the lake, and thus a gaining condition and lower potential for PFAS contamination of the aquifer. Both instances assume hydraulic connection between the groundwater and surface water.

Owing to seasonal and punctuated variations in water levels in lakes and groundwater, continuous water-level monitoring better assesses hydraulic gradients when compared to discrete water level synoptic gaging events. Annual highs for water levels in New Jersey typically occur in the spring following low evapotranspiration of shallow groundwater through the winter months. Annual low water levels typically occur in the fall, following high evapotranspiration through the summer months. Therefore, higher springtime water levels may produce a gaining condition and lower fall water levels may produce a losing condition within the same surface water body depending on season. Water levels may also vary with changes in the duration and intensity of individual precipitation events. Furthermore, continuous monitoring allows for consideration of dam effects, such as unscheduled lowering of the spillway gate at Little Pine Lake, which would increase lake levels and potentially cause a losing condition when the gate is closed.

Three temporary piezometers were installed at each of the two lakes to monitor groundwater levels (fig. 5). Each piezometer was screened at a different depth so vertical gradients could be assessed. Piezometers and their depths are listed in table 1. At Little Pine Lake, all three piezometers were continuously monitored. At Pine Lake, only the shallowest piezometer was continuously monitored and the remaining two had discrete measurements only as the rods used in their installation were too narrow for the transducers. However, the lack of continuous data at these piezometers did not affect the results of the hydrograph interpretations, as discussed later. One surface water gage site was installed per lake (fig. 5; table 2), which consisted of a transducer inserted through a slotted PVC pipe fixed to a dock piling (at Little Pine Lake) and a concrete wall (at Pine Lake). The North American Vertical Datum of 1988 (NAVD 88) altitudes at each piezometer and surface water gage location were determined by levelling their measuring point altitude from a reference Global Navigation Satellite System receiver. Each piezometer and surface water gage was visited every 10 weeks throughout the continuous deployment to check for sensor drift and to download data from the recorders. At each visit, discrete water-level measurements were made manually at all piezometer and surface water sites with a calibrated steel tape using techniques described by Cunningham and Schalk (2011). These measurements were used to make corrections for instrument drift during data analysis. Each site was instrumented with an In Situ Level Troll 500 vented pressure transducer recording at intervals of 15 minutes. Water-level data are available in the USGS National Water Information System (NWIS) database (U.S. Geological Survey, 2020).

At Little Pine Lake, piezometer LPL-PZ-23 was installed in the same drive-point hole used for test intervals LPL-GP2-42 and LPL-GP2-52. Piezometer LPL-PZ-10 was installed in the same drive-point hole (LPL-GP1) used for the five additional test intervals at that location. Piezometer LPL-PZ-05 was installed by hand below the lakebed. At Pine Lake, piezometer PL-PZ-29 was installed in the same drive-point hole as the test intervals at that location. Piezometer PL-PZ-16 was also installed using a drive-point, but no intervals were tested in that drive-point hole. Piezometer PL-PZ-06 was installed by hand below the lakebed.

Continuous Monitoring of Temperature

Continuous monitoring of groundwater temperatures can provide insights into gain/loss on the basis of the diurnal fluctuations of water temperature at different depths (Stonestrom and Constantz, 2003). Deeper groundwater generally remains at a near-constant temperature and is expected to have muted, negligible temperature fluctuations throughout the day. Conversely, surface water temperature fluctuates in a similar pattern to air temperature and is generally highest midday and lowest before sunrise. In the case of a gaining condition, shallower groundwater would exhibit similarly minimal variations in temperature as deeper groundwater because groundwater is flowing upwards from depth. In the case of a losing condition, the shallower groundwater would instead be imprinted by the higher diurnal temperature variations of surface water as the downward hydraulic gradient transports heat from the lake into the aquifer system. Water temperature should be considered a secondary line of evidence to indicate gain/loss compared to water levels, which are the primary indicator.

The transducers used to monitor water levels also include an internal thermistor to monitor temperature, so water levels and temperature data are collected simultaneously in the same gages and piezometers. Temperature calibration procedures followed USGS protocols from Wilde (2006) to ensure the transducer readings were within the USGS established error of ±0.02 degrees Celsius (°C) of difference from standard. The transducer thermistors were checked using a five-point temperature check prior to installation and following retrieval. A five-point temperature check tests the thermistor through a range of temperatures from 40 to 0 °C. A two-point temperature comparison was completed in the field around the middle of the deployment period, checking transducer thermistor temperatures against a National Institute of Standards and Technology calibrated thermistor at 0 and 40 °C water brought to the field. Temperature comparisons were also checked for in situ groundwater as well. Temperature data are available in the USGS National Water Information System database (U.S. Geological Survey, 2020).

Vertical Hydraulic Profiling

The spatial distribution of vertical hydraulic profiling results from Little Pine Lake are displayed in figure 6 and within the regional hydrogeologic context in figure 7. Eight intervals were tested among two holes at Little Pine Lake (table 1). One interval, LPL-GP1-51, was able to be pumped and measured to recovery, and is the only test interval at Little Pine Lake that did not indicate aquifer material with low permeability. Two intervals, LPL-GP2-52 and LPL-GP2-42, were pumped dry, indicating low permeability aquifer material. Four intervals (LPL-GP1-37, LPL-GP1-27, LPL-GP1-17, LPL-GP1-09) did not recover to static water-level conditions following pumping, which also indicates low permeability. Pump equipment issues prevented full pumping of interval LPL-GP1-37, but the water levels did not recover following its partial development, indicating that this interval was composed of low permeability aquifer material Interval LPL-GP1-60 was not tested because the pre-pumping water level in the interval was too deep to be pumped with a peristaltic pump. The presumed cause for the deep water level is low permeability aquifer material surrounding the screen preventing water from entering, but this explanation cannot be confirmed.

Based on their screen altitudes in the context of the hydrogeologic framework, intervals LPL-GP2-52, LPL-GP2-42, LPL-GP1-60, and LPL-GP1-37 were likely screened within the Manasquan-Shark River confining unit that underlies the Kirkwood-Cohansey aquifer system. Results of the hydraulic profiling are consistent with this placement. LPL-GP1-51 is also situated in this unit, but the higher permeability indicated by its ease of pumping and subsequent recovery to static water levels is more representative of an aquifer permeability, thus this interval may have been screened in a lens of sandier material than its surroundings. Interval LPL-GP1-27 was situated in a mixed clayey, silty, and sandy unit that Fiore (2020) and Sugarman and others (2018) also consider part of the Manasquan-Shark River confining unit, and based on its lack of recovery to static water levels, LPL-GP1-27 was likely situated in a more silty and clayey unit at this location. Although the intervals LPL-GP1-17 and LPL-GP1-09 did not recover, the framework places these depths within the Kirkwood-Cohansey aquifer system. The lack of recovery may have been caused by residual screen clogging left over from testing in finer-grained units at deeper intervals, incomplete screen development during pumping, and (or) the interval being situated in a silty or clayey lens subunit common in the Kirkwood-Cohansey aquifer system.

Water levels were measured in the intervals that did not recover and are available in NWIS, but they have limited usefulness in this assessment because of being collected in nonstatic, unrecovered conditions. The sole static water level during testing at Little Pine Lake was from LPL-GP1-51 at an altitude of 65.77 ft above NAVD 88. The water-level elevation of Little Pine Lake was not recorded at the at the time of this measurement.

Water-Level Monitoring

Hydrographs of the continuous water-level data from Little Pine Lake are plotted on figure 8, along with a precipitation hyetograph with data from the National Oceanic and Atmospheric Administration (NOAA) Philadelphia-Mt. Holly weather station (National Oceanic and Atmospheric Administration, 2021) located about 13 mi west of Little Pine Lake. Results of water-level monitoring at Little Pine Lake indicate gaining conditions were present throughout the monitoring period. Water level altitudes in Little Pine Lake were consistently lower than water level altitudes in the piezometers (fig. 8A), which indicates a gaining condition, with prevailing flow moving from the aquifer and into the lake. Similar fluctuations in continuous groundwater levels from all piezometers and surface water levels in Little Pine Lake in response to stressors such as precipitation events indicate the piezometers were likely hydraulically connected to the lake and screened in similar aquifer material.

In general, hydraulic gradients at Little Pine Lake were low. The highest groundwater-level altitudes in the deepest piezometer LPL-PZ-23 were rarely greater than 1 ft higher than the Little Pine Lake surface water level; the only instances greater than 1 ft occurred on August 31 and October 30–November 5. Shallow- and intermediate-depth piezometers LPL-PZ-05 and LPL-PZ-10 were typically about 0.5 ft higher than Little Pine Lake. The difference of water level altitudes between LPL-PZ-05 and LPL-PZ-10 was also less than 0.03 ft from August 2020 through mid-fall (fig. 8B), a negligible difference for water-level differences between two sites, thus the water-level altitudes in LPL-PZ-05 and LPL-PZ-10 can be considered approximately equal during this time period. However, the difference became greater later in the fall and through the winter, when LPL-PZ-10 levels were as much as 0.14 ft higher than LPL-PZ-05, which is more strongly indicative of an upward gradient.

Section A-A′ during the approximate lowest (fig. 9A) and highest (fig. 9B) water levels recorded during the monitoring period (September 8, 2020, and April 1, 2021, respectively) is shown in figure 9. During the highest conditions on April 1, water-level altitudes and potentiometric surfaces indicate a flow direction into the lake (fig. 9B). During the lowest conditions on September 8, water-level altitudes in LPL-PZ-05 and LPL-PZ-10 are approximately equal (65.96 and 65.95 ft, respectively). Given the spatial difference between the deeper screen altitude of LPL-PZ-10 compared to LPL-PZ-05, water-level contours on figure 9A indicate the potential for a slight outward direction of flow in the subsurface below the lake from the center toward the edges. This flow may only be a localized effect where shallow groundwater flows outward but is discharged back into the lake, as is drawn on the cross section. Regardless, the water-level altitude in Little Pine Lake was lower than the water-level altitudes in both piezometers and overall indicative of a gaining condition. However, the uncertainty associated with these low hydraulic gradients require additional data to be collected in order to fully assess this hydrology.

The instrumentation to monitor the lake levels was located upstream of the piezometers, so assuming that the flow in Little Pine Lake was always from north to south without reversals of flow direction caused by a buildup of water behind the berm on the downstream end, the lake level near the piezometers may have been slightly lower than at the transducer location. Thus, the gaining condition may be more pronounced. However, because of the presence of the berm downstream of the monitoring equipment, the potential for higher water levels from accumulation behind the berm may cause losing conditions to occur near the berm. Water that is lost to the aquifer could then potentially cause PFAS contamination if that water flows away from the lake, and (or) that water could flow around the berm and discharge back into surface water on the downstream Big Pine Lake side.

Temperature Monitoring

Temperature monitoring data for Little Pine Lake are shown in figure 10. Diurnal temperature fluctuations in Little Pine Lake were negligible or nonexistent. The surface water would be expected to have strong fluctuations, but the water column above the surface water monitor may have been too deep to see these effects considering the transducer was installed just above the lakebed. However, gaining conditions indicated by water-level data are consistent with the lack of diurnal fluctuations in groundwater. The results of the temperature data are typical of general seasonal effects, with the highest temperatures in late summer and lowest temperatures in winter, except for LPL-PZ-23 with a high in late fall/early winter and a low in summer. The lake level had the greatest seasonal fluctuations and the groundwater piezometers had increasingly smaller seasonal fluctuations with depth, as would be expected.

Vertical Hydraulic Profiling

The spatial distribution of vertical hydraulic profiling results for Pine Lake are displayed in figure 11 and within the regional hydrogeologic context in figure 12. Four intervals were tested at Pine Lake: PL-GP-45, PL-GP-38, PL-GP-27, and PL-GP-16. The deepest interval, PL-GP-45, was the only interval that failed to recover after pumping, which indicates low permeability and is consistent with the clayey subunit in Kirkwood-Cohansey aquifer system within which the hydrogeologic framework places these depths. The hydrogeologic framework also places interval PL-GP-38 in a silty sand unit, but that interval recovered to static water levels after pumping and likely has a moderate permeability. PL-GP-16 and PL-GP-27 also recovered to a static water level after pumping, consistent with the lack of fine-grained material at those depths.

Altitudes of the recovered water levels indicate an overall upward hydraulic gradient at the borehole location. However, the Pine Lake surface water-level altitude was at least 1.5 ft higher than the highest groundwater level during this time—the lake level is above the lake bottom at piezometer PL-PZ-06, where the lake bottom altitude is 28.42 ft above NAVD 88, and the highest water level altitude during testing was 26.93 ft in PL-GP-38. Therefore, Pine Lake was likely experiencing a losing condition during hydraulic testing. The upward gradient in these piezometers is more likely representative of the upward vertical component of the overall outward flow exiting the lake into the aquifer, which is also illustrated by the water-level data collected in the piezometers described in the Water-Level Monitoring section.

The Kirkwood-Cohansey aquifer system has a greater thickness near Pine Lake owing to its location farther downdip, where the aquifer thickens to more than 150 ft at certain locations, compared to the generally less than 80-ft thickness of the aquifer system near Little Pine Lake (Fiore, 2020). Nearby domestic well E201213312 also has a reported 6-ft-thick clayey unit from 27 to 33 ft depth below land surface, which is a similar depth to the test interval PL-GP-27. Vertical hydraulic profiling tests did not indicate any evidence of this unit, which may pinch out north of the testing location or thin to a thickness with a negligible effect on hydrology. The driller log for E201213312 also describes a black color for this clay unit. A high organic carbon content could cause the color to be black, which is of interest to this study as high organic carbon sediments likely cause sorption of PFAS.

Water-Level Monitoring

Hydrographs of the continuous water-level data from Pine Lake are plotted on figure 13, along with a precipitation hyetograph of data from the NOAA Philadelphia-Mt. Holly weather station located about 29 mi west of Pine Lake. Surface water-level altitudes ranged from approximately 28.7 to 30.2 ft, and groundwater-level altitudes ranged from about 26.0 to about 29.7 ft. Results of water-level monitoring at Pine Lake indicate losing conditions were present throughout the monitoring period. The water-level altitude in Pine Lake was constantly higher than the groundwater-level altitudes in the piezometers (fig. 13), which indicates an overall downward vertical gradient from the lake into the aquifer. Continuous water-levels from PL-PZ-06 had similar fluctuations to Pine Lake in response to stressors such as precipitation events, so PL-PZ-06 was likely hydraulically well-connected to the lake. Because no continuous data were collected in PL-PZ-16 and PL-PZ-29, the monitoring data cannot indicate their hydraulic connectivity. However, connectivity can be inferred by the results of the vertical hydraulic profiling discussed earlier.

Pine Lake and PL-PZ-06 had about 0.3–0.5 ft water-level difference during the monitoring period. Discrete water levels in PL-PZ-16 and PL-PZ-29 were typically more than 3 and 2 ft deeper than the lake level, respectively. PL-PZ-16 had a lower water-level altitude than PL-PZ-29, consistent with the results of vertical hydraulic profiling. Although these heads would be typical of an upward gradient, the higher lake level is still indicative of an overall losing condition. The apparent upward vertical gradient from PL-PZ-29 to PL-PZ-16 is likely the vertical component of outward and upward flow away from laterally upgradient and (or) more central portions of the lake, particularly in the shallow portion of the overall groundwater flow system along the edge of the lake, as depicted on the hydrologic cross section through these piezometers (fig. 14). The approximately 1-ft differences in water-level altitude between PL-PZ-16 and PL-PZ-29 were greater than water-level altitudes in drive-point test intervals of similar depths (PL-GP-16 and PL-GP-27, respectively), which had a water-level difference of less than 0.1 ft. This discrepancy likely resulted from the test intervals being nested in the same location, but the piezometers in separate boreholes with PL-PZ-29 is located about 10 ft in the upstream direction of PL-PZ-16. The higher head difference likely indicates a strong lateral component imprinted on the overall downward hydraulic flow gradient from a more north-to-south direction, representing incoming flow from farther upgradient at Pine Lake. Flow paths that occur at depths below the observation data available, especially from the center portion of the lake outward toward the edges, likely have an even stronger outward gradient for which the upward and outward component occurs farther from the lake in contrast to the upward and outward component at the shallower depths represented by the piezometer data. These deeper flow paths likely flow through the silty sands below Pine Lake, for which the vertical hydraulic profiling results have shown to not retard groundwater flow to a measurable extent.

The surface water monitoring instrumentation at Pine Lake was located downstream of the piezometers, so the surface-water altitude assessed at the monitoring location may be slightly lower than the surface-water altitude closer to the piezometers. If so, the downward gradients and losing conditions at Pine Lake would likely be larger. Based on the evidence for these losing conditions at Pine Lake, there is a strong possibility that PFAS in Pine Lake has entered the underlying aquifer.

Temperature Monitoring

Results of temperature monitoring at Pine Lake (fig. 15) showed negligible or nonexistent diurnal fluctuations in surface water and groundwater. Observations of minimal fluctuations in surface water temperature, together with the strong downward gradient evident from the water-level data, indicates inconclusive usefulness of the temperature data. The surface water instrumentation was installed in a location where the flow was visibly higher, which may have caused mixing and contributed to the lack of diurnal fluctuation. Groundwater conditions at the surface water sensor location may have also been different that groundwater conditions near the piezometers, and similar to Little Pine Lake, the transducer may have been installed too deep below the lake surface to be affected by air temperature. Similarly, the shallowest piezometer PL-PZ-06 may have been screened too deep below the lake bottom to retain an unattenuated diurnal temperature signal from above. The highest seasonal temperatures occurred in summer and the lowest occurred in winter, as would be expected.