Groundwater Samples

Groundwater wells were sampled on May 31 and June 1, 2022, to determine the water-quality characteristics of the underlying aquifer. Water samples were collected from outdoor faucets at private homes because they were easily accessible, and the water supplied there is typically not connected to whole-house water treatment systems and should therefore be representative of groundwater. Before sampling, volunteer homeowners were asked whether their outdoor faucets were treated or untreated, and all responded that the water was untreated to the best of their knowledge. A total of 12 samples collected from groundwater wells were analyzed for physical properties (pH and specific conductance), inorganics (acid neutralizing capacity, nitrate plus nitrite nitrogen [nitrate], and chloride), perchlorate, metals, and GRO and BTEX compounds, and sample-bottle blanks and trip blanks were collected as part of the study (tables 1, 3, 4, 5, and 6; Mueller and others, 2015). Water from 6 of the 12 wells was also analyzed for low-level lead concentrations and lead isotopes (Schachter and Stuntebeck, 2024). The two upgradient wells and four spatially distributed downgradient wells were selected for the additional lead-isotope analysis.

USGS National Field Manual (NFM) procedures were used to measure field values and collect groundwater samples (U.S. Geological Survey, variously dated). Upon arriving at each home, an outside faucet was turned on and water was purged through a hose while preparing sampling equipment. Teflon-lined polyethylene sample tubing that was precleaned following NFM procedures was then connected to the outdoor faucet with a threaded, nylon connector while the other end of the tubing was connected to a flowthrough chamber with an attached multiparameter, water-quality sonde (figs. 2 and 3). Water was discharged through the flowthrough chamber at rates ranging between about 2.6 and 7.5 gallons per minute while parameters from the water-quality sonde were recorded (temperature, pH, dissolved oxygen, specific conductance, and turbidity) until the values stabilized. The field manual indicates purging three or more well volumes of standing water before sample collection, but because the wells were presumed to be in use by homeowners every day, parameter stabilization was substituted as the sampling threshold.

After the parameters stabilized, sample tubing was disconnected from the flowthrough chamber and pumped water was dispensed into prelabeled bottles provided by the laboratories inside a clean sample-processing chamber (fig. 3). To avoid environmental contamination, a clean hands/dirty hands sampling technique was used. This sampling technique included using tubing cleaned according to the NFM at every location, new gloves, and a new sample-processing chamber.

Samples analyzed for physical properties and inorganics were collected in 1-quart and 60-milliliter (mL) high-density polyethylene (HDPE) bottles, whereas samples for perchlorate analysis were collected in 200-mL HDPE bottles. Samples for metals analysis were collected in 250-mL HDPE bottles and then acidified in the field with 1.25 mL of 0.5-percent nitric acid. Samples for GRO and BTEX analysis were collected in preacidified (hydrochloric acid), 40-mL, septum-capped glass bottles filled to the top with no air space, per laboratory instructions. Samples analyzed for low-level lead and lead isotopes were collected in 1-liter Teflon bottles. In a clean laboratory environment, concentrated Optima trace-metal-grade nitric acid was added to the low-level lead and lead-isotope sample bottles at about 1 mL per 50 mL of sample water.

Surface-Water Samples

Streamflow samples were collected during base flow conditions and during a rainfall-runoff event to characterize the surface-water resources within and near the airport. Base flow samples were collected on June 13 and July 19, 2022, from three of the four surface-water stations (station AR does not have base flow). On June 8–9, 2022, multiple samples were collected at each of the four monitoring stations during a rainfall-runoff event generated by about 1 inch (in.) of rainfall (U.S. Geological Survey, 2023). A total of 26 samples were analyzed for perchlorate and metals, 22 samples were analyzed for GRO and BTEX compounds, and 11 samples were analyzed for low-level lead concentrations and lead isotopes (tables 7 and 8; app. 1, tables 1.1 and 1.2; Schachter and Stuntebeck, 2024).

Each surface-water monitoring station contained instrumentation to measure the water level in the stream and an automated sampler (fig. 4A) to pump water samples into 350-mL cylindrical glass bottles via Teflon-lined tubing (fig. 4B). Autosampler pumping was triggered either remotely via cellular communications or by rate of water-level change in the stream. The glass sample bottles were collected from the automated samplers within 30 hours of being pumped. Bottles were labeled, placed into a cooler with ice, and transported back to a USGS laboratory facility for processing and preservation.

Samples from the automated sampler were selected for perchlorate and metals analyses from the larger subset of total collected samples to target the rising limb, near peak, and falling limb of the rainfall-runoff hydrograph, resulting in five samples per station. Selected samples were shaken vigorously and poured from the glass bottles into the prepared laboratory bottles described in the “Groundwater Samples” section.

GRO and BTEX compounds can volatilize, so all samples analyzed for GRO and BTEX compounds were manually collected by dipping a 1-liter glass beaker directly into the stream. From this beaker, preacidified, septum-capped bottles supplied by the laboratory were then filled completely and capped, leaving no air space for volatilization. Four samples were collected per station, mostly near the peak and on the falling limb of the rainfall-runoff hydrograph.

Low-level lead and lead-isotope samples were collected during rainfall runoff concurrently with some of the GRO and BTEX samples to represent near-peak and recession limbs of the hydrograph. Samples were collected by dipping a laboratory-cleaned Teflon bottle directly into the stream and then capping and placing it in a cooler with ice to be acidified, as described in the “Groundwater Samples” section, to minimize contamination risk.

Base flow samples were collected for perchlorate and metals analyses by triggering a sample from the automated sampler. Base flow samples for GRO and BTEX compounds, low-level lead concentration, and lead-isotope analysis were collected using the same methods as described previously for rainfall-runoff sample collection.

Shallow Groundwater Samples

Shallow groundwater samples were collected on May 23–24, 2022, from temporary wells installed at seven locations within and near the airport. A rotary hammer was used to drive a 1-ft, stainless-steel (types 304 and 316), wire-wound, screened drive point (0.0001-in. screen spacing) attached to 5-ft sections of 1 1/4-in.-diameter galvanized steel pipe to depths ranging from 5 to 15 ft. The drive point and the galvanized stainless-steel pipes were rated for potable water (ASTM International, 2016, 2023). Depth to water in the temporary wells ranged from 2 to 11 ft. At location Z1, despite its proximity to a flowing intermittent stream, no water was detected, so no water samples were collected. Six shallow groundwater samples were analyzed for physical properties, perchlorate, metals, and GRO and BTEX compounds (tables 9 and 10). Three spatially distributed samples were analyzed for low-level lead concentrations and lead isotopes (Schachter and Stuntebeck, 2024).

Methods used to collect shallow groundwater samples followed those for homeowner groundwater wells, except a peristaltic pump was used to extract water from slightly above the bottom of the well pipe into the flowthrough chamber where a multiparameter probe recorded physical properties of temperature, pH, oxygen, conductivity, and turbidity. After three or more well volumes were purged and parameter values had stabilized, pumped water was dispensed into prepared laboratory bottles inside a clean field sample-processing chamber using clean tubing and clean gloves, as described in the “Groundwater Samples” section.

Soil Samples

Soil samples were collected from seven sites on May 23 and 24, 2022, near where the shallow groundwater samples were collected. Samples were collected at two depth horizons—1 and 2 ft deep—by digging a small hole with a metal shovel. In total, 14 soil samples were analyzed for soil moisture content, perchlorate, metals, and GRO and BTEX compounds and 4 spatially distributed samples were analyzed for low-level lead concentrations and lead isotopes (tables 11 and 12; Schachter and Stuntebeck, 2024).

Soil samples for metals, perchlorate, and low-level lead and lead-isotope analysis were collected with a modified plastic sample bottle to minimize any potential metals contamination from the metal shovel. Soil samples for metals analysis were dispensed into a 250-mL HDPE bottle (about one-fourth full, per laboratory instructions). Soil samples for perchlorate analysis were dispensed into two 150-mL glass jars (one filled amber jar and one half-filled clear jar). Soil samples for low-level lead and lead isotope analysis were dispensed into 30-mL Nalgene polypropylene plastic jars.

Soil samples for GRO and BTEX analysis were collected by inserting a laboratory-provided sampling device horizontally into the soil profile and extracting a 1/2-in. by 3/4-in. cylindrical-shaped core of material. The soil cores were then dispensed into laboratory-prepared 40-mL glass vials containing methanol as a preservative.

Groundwater

Perchlorate was detected at concentrations greater than the laboratory reporting limit (0.2 µg/L) in samples from 6 of the 12 groundwater sampling sites; concentrations ranged from less than 0.2 to 1.5 µg/L and the median concentration was about 0.3 µg/L (table 6). Of the six detections, five were in Highwood Circle Estates and one was upgradient from the airport. The highest concentration of 1.5 µg/L measured in Highwood Circle Estates (MW–9) was the only groundwater sample that exceeded the Wisconsin public health enforcement standard of 1 µg/L (Wisconsin Department of Natural Resources, 2023b).

Surface Water

The highest perchlorate concentrations among the four surface-water sampling stations were observed at station AR on the southeastern side of the airport with a maximum of 2.2 µg/L (fig. 5; table 8; app. 1, table 1.2). The laboratory reporting limit was 0.2 µg/L for four samples, but for the other three samples, the reporting limit was 1 µg/L. These varying reporting limits were the result of the laboratory completing a dilution because of sample matrix interference. All five perchlorate concentrations at station AR were higher than every concentration greater than reporting limits at the other three surface-water stations. The highest concentration at station AR was detected just before the peak of the June 8–9, 2022, rainfall-runoff event; conversely, the highest concentrations at station NF were measured during base flow sampling. The lowest reported concentrations were measured at station TP west of the airport.

Shallow Groundwater and Soil

All six shallow groundwater samples had perchlorate concentrations less than the laboratory reporting limit (table 10). The laboratory reporting limit was 0.2 µg/L for one sample, but for the other five samples, the reporting limit was 1 µg/L. These varying reporting limits were the result of the laboratory completing a dilution because of sample matrix interference. All but 1 of the 14 soil samples had perchlorate levels less than the reporting limit (table 12), which varied from 1.8 micrograms per kilogram (µg/kg) to 2.6 µg/kg because different masses of soil samples were used for each analysis. The shallow soil sample from site Z3 had a reported concentration of 0.7 µg/kg.

Perchlorate Implications

Perchlorate concentrations were greater than the Wisconsin public health enforcement standard of 1 µg/L (Wisconsin Department of Natural Resources, 2023b) for 1 groundwater sample and 4 surface-water samples (of 44 total samples). The highest of these measurements, 2.2 µg/L at surface-water station AR, was about 15 to 100 times lower than those detected in other studies that evaluated the effects of repeated fireworks displays to surrounding water resources. For example, after annual Fourth of July fireworks displays from 1998 to 2009 at Mount Rushmore National Memorial, Hoogestraat and Rowe (2016) detected perchlorate concentrations of as much as 54 µg/L in the nearby surface waters and as much as 38 µg/L in groundwater. Andrews and others (2021) determined that 50 years of annual fireworks displays in Evart, Mich., resulted in a maximum concentration of perchlorate in monitoring wells of 240 µg/L.

Higher concentrations of perchlorate at station AR relative to the other surface-water stations, coupled with the proximity of station AR to the fireworks launched in July 2021 (fig. 1), indicate that fireworks were a likely source of perchlorate in the surface drainage at the southeastern side of the airport. Detectable concentrations 1 year after the fireworks launch were consistent with prior research demonstrating that perchlorate can be persistent in the environment for years (Massachusetts Department of Environmental Protection, 2007).

Concentrations of perchlorate measured at the other three surface-water stations distant from the fireworks launch location and at one groundwater station upgradient from the fireworks launch location indicate that perchlorate sources other than fireworks were in the airport study area. A potential source of perchlorate in the airport study area (other than fireworks) could be from fertilizer. Most of the land use in the Pheasant Branch drainage basin upstream from Schneider Road is used to grow crops to support livestock production (U.S. Department of Agriculture, National Agricultural Statistics Service, 2024). Although research indicates that some amount of perchlorate occurs naturally in the environment, most perchlorate not from rocket fuels or fireworks is sourced from commercial fertilizers derived from rock deposits and minerals (Susarla and others, 1999). Synthetic fertilizers have also been determined to have some amount of perchlorate present, possibly from chemicals used in the manufacturing process (Susarla and others, 1999).

Groundwater

All metals detected in groundwater samples were less than drinking water enforcement standards, except for arsenic (U.S. Environmental Protection Agency, 2009; Wisconsin Department of Natural Resources 2023b; tables 5 and 6). One sample at groundwater well MW–8 had an arsenic concentration of 10.5 µg/L, greater than the EPA MCL of 10 µg/L. Several metals (beryllium, chromium, molybdenum, selenium, silver, thallium, and titanium), including some that are commonly associated with fireworks, were not detected in any groundwater wells.

No spatial pattern in groundwater concentrations of metals commonly associated with fireworks was discernable. For example, barium was detected in similar concentrations in groundwater wells upgradient from the airport (mean of 63.3 µg/L) and downgradient in Highwood Circle Estates (mean of 47.0 µg/L). Additionally, potassium was detected at all groundwater wells, and the highest potassium concentration (3.28 mg/L at MW–Z1) was detected upgradient from the airport.

Surface Water

Metals concentrations in surface-water samples varied among constituents and collection locations, but some patterns were evident (tables 7 and 8; fig. 6; app. 1, tables 1.1 and 1.2). Beryllium, lithium, selenium, silver, and thallium concentrations were less than laboratory detection levels for all samples. For most other metals, the highest maximum and mean concentrations relative to other surface-water stations were measured at station SC upstream from the airport. The next highest concentrations for most metals were measured at station NF, whereas concentrations at station TP were generally lowest among all surface-water stations.

Metals concentrations from station AR were generally more variable than at the other stations. For some metals such as cobalt, iron, manganese, and titanium, median concentrations at station AR were comparatively low and more like those measured at station TP. However, median concentrations at station AR for many of the remaining metals were more like those from station NF.

Shallow Groundwater

Metals in shallow groundwater samples had a distinct pattern with respect to sampling location (tables 9 and 10). Concentrations for 20 of 24 detected metals were highest at two sites, A3 and Z3 (fig. 7). In addition, concentrations of selenium, lead, and lithium were greater than detection limits only at these two shallow groundwater sampling sites. Only arsenic, sodium, potassium, and boron had no distinct pattern with respect to sampling location. Antimony, silver, and thallium were not detected at any shallow groundwater sites.

Use of stainless-steel drive points and galvanized-iron pipes could have contributed to higher concentrations of some of the metals measured in the shallow groundwater samples. Specifically, iron, zinc, nickel, chromium, manganese, and molybdenum are all known to be metallic components of these materials (ASTM International, 2016, 2023). However, the methods used to collect shallow groundwater samples were similar for all sites. Thus, the higher concentrations of these metals detected at sites A3 and Z3 are not fully attributable to the use of these materials. Additionally, higher concentrations of many other metals were also measured at these two shallow groundwater sites.

Soil

All metals analyzed—except selenium and silver—were detected at concentrations greater than the LOD within the soil samples (tables 11 and 12). Overall, metals concentrations between locations had more variability than between the samples collected at 1-ft and 2-ft depths at a single location. For 13 of the 25 metals detected, concentrations were highest at site Z1, on the western side of the airport, more than half a mile away from the 2021 fireworks launch site. Generally, the next highest metals concentrations were detected at sites A3 and Z3, including fireworks-associated metals such as arsenic, copper, and lithium. Metals concentrations for 20 of the 27 metals were lowest at site A1, on the northern side of the airport.

Overall, most metal concentrations in soils within and near the airport were similar to or lower than the global averages reported by Kabata-Pendias (2011). Antimony, arsenic, and thallium did have average concentrations in soil greater than the global average as reported by Kabata-Pendias. Concentrations of lead were less than the global average soil concentration (27 mg/kg), about an order of magnitude lower than what is defined as contaminated soils (Kabata-Pendias, 2011).

Metals Implications

No pattern was detected that would identify the 2021 fireworks display at the airport as a contributing source of fireworks-associated metals to groundwater wells or surface water on and near the airport. Groundwater samples had no discernable spatial pattern in concentrations of metals commonly associated with fireworks, and many metals were detected at or near their highest concentrations at sites upgradient from the airport and the fireworks launch site. Among surface-water stations, the highest metals concentrations were most frequently measured at station SC north of the airport, indicating an alternate source of metals to the surface waters within and near the airport other than the 2021 fireworks show at the airport.

A discernable pattern of metals concentrations was detected in the shallow groundwater samples collected within and near the airport. Concentrations of most metals in shallow groundwater samples were highest at sites A3 and Z3, including those commonly associated with fireworks such as barium, copper, lithium, and titanium. As shown in figure 1, the fireworks were launched near sampling location A2. On the evening of July 23, 2021, at about 9 p.m. central daylight time, wind speed at the airport was reported to be about 5 miles per hour from the south-southwest (National Weather Service, 2021). Assuming the fireworks were launched with a near-vertical alignment, debris could have landed near sampling location Z3 or farther northeast into the agricultural field. Because groundwater and surface water flow generally to the southeast in this area, fireworks debris (and associated metals) landing near Z3 could have been transported via overland runoff or in shallow groundwater flow across the airport property to sampling site A3 in the year between the launch and sample collection.

Surface-water station AR did not demonstrate the same pattern of high fireworks-associated metals concentrations relative to other surface-water stations, despite being the drainage point for the basin where fireworks were launched and landed. Metals, unlike perchlorate, possibly washed away in surface-water runoff or infiltrated to soil or shallow groundwater in the year after the fireworks display. Hoogestraat and Rowe (2016) also detected no increase in metals concentrations at the groundwater and surface-water sites that had high concentrations of perchlorate after annual Fourth of July fireworks displays at Mount Rushmore National Memorial. Fireworks could have contributed some metals to surface-water runoff at station AR, but the signal was not as obvious as in the shallow groundwater.

Organics Implications

Toluene was the only organic compound measured in this study that had concentrations exceeding laboratory reporting limits. Toluene was only detected in surface water, and only in rainfall-runoff samples at stations SC and NF on Pheasant Branch upstream and downstream from the airport, respectively. Toluene was not detected in base flow samples, indicating that it was washed from the landscape during the rainfall-runoff event rather than contributed to the stream by groundwater. Although no toluene was detected in drinking water wells in this study, for reference, the Wisconsin drinking water standard is 800 µg/L—nearly 100 times greater than the maximum concentration detected in surface water (Wisconsin Department of Natural Resources, 2023b).

The potential spillage (and subsequent washoff during rainfall-runoff events) of aviation fuels and degreasers (Sulej and others, 2012) could be a source of toluene in the airport study area. The highest concentration was measured at station NF downstream from the airport near most of the aircraft support buildings. The nearest airport building was less than 100 ft away from the NF sampling location.

Concentrations of toluene were also detected at station SC upstream from the airport, indicating that airport activities were not the only possible source of toluene. Generalized agriculture and developed (urbanized) land uses were not likely significant sources because the TP basin also had substantial agriculture and development, but measured concentrations of toluene there were all less than laboratory detection limits. Fuels or oils deposited on roads by vehicles were also not a likely significant source: the SC and TP basins contain roads but concentrations were only detected at station SC. Localized activities upstream from the airport involving machinery or maintenance of machinery could be an additional source of toluene measured in the study area.

Groundwater

Of the 12 groundwater samples, 6 were tested for low-level lead concentrations and lead isotopes. Lead was detected at low concentrations in all six samples, ranging from 0.0433 to 0.956 µg/L (table 6; Schachter and Stuntebeck, 2024). No distinct spatial pattern was detected among lead concentrations in groundwater, upgradient or downgradient from the airport. The minimum and maximum concentrations of lead in groundwater were detected at the two wells upgradient from the airport; all lead concentrations in samples from Highwood Circle Estates fell within that range.

Isotopic ratios of lead in groundwater samples revealed a mix of lead sources (fig. 8A). Three groundwater samples had isotopic signatures matching background atmospheric lead deposition, two samples had isotopic signatures similar to Avgas 100LL, and one sample had an intermediate signature between Avgas 100LL and background atmospheric lead deposition (similar to lead-based paint). Of the two samples with signatures close to Avgas 100LL, one was in Highwood Circle Estates and one was upgradient from the airport.

Surface Water

Of the 26 surface-water samples, 11 were tested for low-level lead and lead isotopes. Similar to the pattern observed with other metals, the highest concentrations of lead were measured at station SC; the median concentration was 0.911 µg/L (table 8; app. 1, table 1.2). Surface-water stations AR and NF had similar concentrations of lead with median concentrations of 0.251 and 0.219 µg/L, respectively, and surface water at station TP had the lowest median concentration of lead of 0.112 µg/L. Lead concentrations were lower during base flow conditions than during the rainfall-runoff event for each of the stations that had base flow (SC, NF, and TP).

The isotopic signatures of lead measured in surface-water samples indicate sourcing from a mixture of background atmospheric lead deposition and Wisconsin galena lead ore (fig. 8B). Most of the lead signatures in surface-water samples were more similar to background atmospheric lead deposition than to Wisconsin galena lead ore, except for two samples taken at station TP during the rainfall-runoff event, which more closely matched Wisconsin galena lead ore.

Shallow Groundwater

Three of the six shallow groundwater samples were tested for low-level lead and lead isotopes. Low-level lead concentrations in shallow groundwater samples were measured to be 0.860 µg/L at site A1, 0.041 µg/L at site S1, and 46.8 µg/L at site A3 (table 10).

Lead-isotope ratios for the sample taken at site S1 (upgradient from the airport) were most similar to background atmospheric lead deposition or lead-based paint, whereas isotope ratios from sites A1 and A3 on airport property seem to be a mix of background atmospheric lead deposition and Wisconsin galena lead ore (fig. 8C). Although the concentration of lead in shallow groundwater at site A3 was relatively high and that site is near the fireworks launch location, the source of lead in this sample could not be directly attributed to fireworks because reference isotopic signatures of any potential lead in fireworks were not available for comparison.

Soil

Of the 14 soil samples, 4 were tested for low-level lead and lead isotopes, with concentrations ranging from 5.42 to 21.3 mg/kg (table 12). Lead concentrations in soil samples were within the expected range of natural soils; the global mean soil lead content has been determined to be about 27 mg/kg (Kabata-Pendias, 2011).

All four soil samples tested had lead-isotope ratio signatures that were a mix of background atmospheric lead deposition and Wisconsin galena lead ore but more similar to background atmospheric lead deposition (fig. 8D). Little was different between soil samples collected on or off airport property with respect to lead-isotope ratios. Although the concentration of lead in the soil at site A3 was relatively high and that site is near the fireworks launch location, the source of lead in this sample could not be directly attributed to fireworks because reference isotopic signatures of any potential lead in fireworks were not available for comparison.

Potential Sources of Lead

Attributing lead in environmental samples to specific sources based on isotopic signatures is complex (Tuccillo and others, 2023). First, each sample is not necessarily homogeneous. For example, a surface-water sample may contain water sourced from base flow and rainfall runoff or a groundwater sample may contain water from varying sources (see “Characterizing Groundwater Wells” section). Each contributing water source could also have multiple sources of lead because of a mixing of lead in the environment. Therefore, each isotopic ratio represents a blend of lead sources; the most abundant source in the sample most strongly affects the resultant ratio. Second, the isotopic signatures for reference samples in figure 8A–D represent an average of several samples, but a relatively wide range of ratios can be identified for some of the sources, and thus, substantial overlap exists in isotopic signatures among some lead sources (Wang and others, 2022). This section describes the most likely sources of lead in water and soil samples collected within and near the airport from the information available within the scope of this study.

Ratios of stable lead isotopes in most water and soil samples collected within the airport study area indicate that lead was primarily sourced from background atmospheric lead deposition or Wisconsin galena lead ore. Of the 24 samples analyzed for lead isotopes, all 11 surface-water samples, all 4 soil samples, 2 of the 3 shallow groundwater samples, and 3 of the 6 groundwater samples had isotopic ratios of lead matching some combination of these sources. These sources are not unique to the airport study area; samples collected from areas distant from the airport (for example, elsewhere in Wisconsin) could have similar source contributions because most lead measured in the environment is from atmospheric deposition of airborne particulate lead or local geologic sources (Sherman and others, 2015; Wisconsin Geological and Natural History Survey, 2024).

The two shallow groundwater samples from sites A1 and A3 had lead-isotope ratios most similar to Wisconsin galena lead ore. One potential source for lead ore in the shallow groundwater on the eastern edge of the airport property is the former Middleton Sportsmen’s Club (Wisconsin Department of Natural Resources, 2023a). Until 2002, activities at the former Middleton Sportsmen’s Club included trap and skeet shooting, where lead shot was dispersed into the environment around the shooting range. However, this site has undergone extensive cleanup under Wisconsin Department of Natural Resources supervision, and the general direction of groundwater flow in this area is away from the airport and toward this property to the southeast of the airport. Thus, it is unlikely that the lead measured in these shallow groundwater samples was sourced from activities at the former Middleton Sportsmen’s Club.

Of the 24 samples, 4 (3 groundwater samples and 1 shallow groundwater sample) had isotopic signatures of lead distinctly different from those described previously. Two groundwater samples (one in Highwood Circle Estates [MW–10] and one upgradient from the airport [MW–Z1]) had isotopic signatures of lead most closely matching the Avgas 100LL used at the airport. Two samples (one groundwater sample [MW–3] and one shallow groundwater sample [S1]) had isotopic signatures most closely matching lead-based paint. The lead in groundwater at site MW–3 could be from an upgradient source of lead-based paint, or it could be sourced from some combination of background atmospheric lead deposition and Avgas 100LL. The lead in the shallow groundwater at site S1 could be sourced from lead-based paint because this sample was collected near an older home.