Occurrence and Distribution of PFAS in Sampled Source Water of Public Drinking-Water Supplies in the Surficial Aquifer in Delaware, 2018; PFAS and Groundwater Age-Dating Results

Open-File Report 2021-1109
Prepared in cooperation with the Delaware Geological Survey and Delaware Department of Natural Resources and Environmental Control
By:

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Abstract

The U.S. Geological Survey, in cooperation with the Delaware Department of Natural Resources and Environmental Control and the Delaware Geological Survey, conducted a groundwater-quality investigation to (1) describe the occurrence and distribution of PFAS, and (2) document any changes in groundwater quality in the Columbia aquifer public water-supply wells in the Delaware Coastal Plain between 2000 and 2008 and between 2008 and 2018. Thirty public water-supply wells located throughout the Columbia aquifer of the Delaware Coastal Plain were sampled from August through November 2018. Groundwater collected from the wells was analyzed for the occurrence and distribution of 18 per- and polyfluorinated alkyl substances (PFAS) as well as groundwater age. Descriptive statistical analyses were performed to assess PFAS analytical results within the well network and the combined perfluorooctanoic acid (PFOA) and perfluorooctane sulfonate (PFOS) concentrations were compared to the U.S. Environmental Protection Agency’s (EPA) health advisory level (HAL) for informational purposes only and not for evidence of compliance or noncompliance with Federal regulations. The EPA’s HAL is a health-based reference level for public drinking water as supplied to customers and is not applied to source (raw) water. Groundwater-age data were compared for sites sampled in 2000, 2008, and 2018 to document any changes.

All samples were analyzed for 18 PFAS using EPA Method 537 (modified). Forty-four percent of the analyzed PFAS were detected in the study well network. Sixteen of the sampled wells have one or more PFAS detections, and as many as eight different PFAS were found in a single sample. Wells with a higher number of PFAS detected (five or more) were in New Castle and Sussex Counties. The PFAS most frequently detected were PFOA, with 47 percent detection; perfluorohexanoic acid (PFHxA), with 33 percent detection; and PFOS and perfluorohexane sulfonate (PFHxS), with 27 percent detection each. PFAS concentrations were below 1,000 parts per trillion (ppt). Two wells exceeded the EPA’s lifetime-drinking water health advisory level of 70 ppt for combined concentrations of PFOA and PFOS.

The average age of groundwater entering the screens of the supply wells sampled in 2018 ranged from 8.2 to 45.8 years, with a median groundwater age of 25.7 years. Groundwater age was positively correlated with well depth and negatively correlated with dissolved oxygen. Groundwater age and PFAS concentrations were negatively correlated in the Columbia aquifer. Data from the 23 resampled wells indicate a significant positive difference in the average modeled groundwater-sample-age results. The average groundwater age from samples collected in 2018 was generally 5 years older than the average groundwater age from samples collected in 2008. The same pattern was found during cycle two (2008) of this study, where the 2008 groundwater age was on average 7 years older than the samples collected in 2000. The distribution of groundwater sample ages among the 17 trend wells and during the three study cycles (2000, 2008, and 2018) indicates that sample-age medians were statistically different from zero; well-water sample-age data show a slight increase in groundwater sample age.

Acknowledgments

This project was funded under a cooperative agreement between the Delaware Geological Survey (DGS) and the U.S. Geological Survey (USGS), in support of the Delaware Department of Natural Resources and Environmental Control (DNREC). The author would like to thank DNREC employees for their excellent help, cooperation, and support in providing the database of public water-supply wells and assisting in the selection of replacement wells for this study. The author would also like to thank USGS employees Deborah A. Bringman and Michael S. Brownley, who provided office and field technical support for this project, as well as Meghan N. Petenbrink and Alexander M. Soroka who provided significant help with data analysis and manuscript preparation, and Irene Fisher for her per- and polyfluorinated alkyl substances expertise. In addition, thank you to all the well owners and water managers who allowed access to their property to sample the wells for this study.

Introduction

Groundwater is the primary drinking water source in Delaware. Previous studies have documented the influence of human activities on the quality of this important resource through the detection of low concentrations (<1 microgram per liter [µg/L] or parts per billion [ppb]) of pesticides, volatile organic compounds, and other manmade chemicals in supply wells. These chemicals were detected in source waters of 30 Delaware public water-supply wells (fig. 1) sampled in previous monitoring studies completed in 2000 (Ferrari, 2002), 2008 (Reyes, 2010), and in this 2018 study. All 30 wells sampled are screened in the Columbia surficial aquifer (well-depth range was between 37 and 139 feet below land surface). Although concentrations of these organic compounds were generally low, all water samples had at least one compound detected, and most samples had multiple detections. These results demonstrate the influence of human activities on the quality of this important drinking water resource.

As part of the Source Water Assessment and Protection Program, which was created by Congress as part of the Safe Drinking-Water Act Amendments of 1996, State of Delaware officials have a continued interest in monitoring and in observing broad changes in the water quality of source waters since previous sampling events (2000 and 2008) detected low-level contamination in supply well samples. As part of this new sampling effort, per- and polyfluoroalkyl substances (PFAS) were included. PFAS, which are a large group of manmade chemicals that include perfluorooctanoic acid (PFOA), perfluorooctane sulfanate (PFOS), and hexafluoropropylene oxide dimer acid (HFPO-DA or Gen X), have come to the attention of researchers and risk managers because of their persistence, toxicity, and number of adverse effects in humans and laboratory animals (U.S. Environmental Protection Agency, 2018). Although there are many chemicals included in the PFAS group and the U.S. Environmental Protection Agency (EPA) has not established a maximum contaminant level (MCL) for all of them, the EPA did establish a lifetime drinking-water health advisory level (HAL) of 70 parts per trillion (ppt; or 0.07 ppb) for PFOA and PFOS in May 2016, as these have been the most extensively produced and studied PFAS (U.S. Environmental Protection Agency, 2016a). Both chemicals are potentially of concern to human health in the Coastal Plain aquifer and warrant continued study of their and other PFAS distribution in groundwater. Although combined concentrations of PFOA and PFOS were compared to the EPA’s HAL, these comparisons were made for informational purposes and not for evidence of compliance or noncompliance with Federal regulations. The EPA HAL is a health-based reference level for public drinking water as supplied to customers and is not applied to source (raw) water. Groundwater-age data were collected as part of this study by averaging the laboratory sulfur hexafluoride (SF6) ages estimated from each of two samples collected from each water-supply well and were compared for sites sampled in 2000, 2008, and 2018 to document any groundwater-age changes.

Drinking-water wells throughout the State in urban, agricultural, and forested land
                     use areas.
Figure 1. Drinking-water wells distribution and Delaware land use.

Description of Study Area

The study area falls entirely in the Delaware portion of the Atlantic Coastal Plain Physiographic Province and all wells sampled are screened in the Columbia aquifer (Benson and others, 1986; Reyes, 2010). The Columbia aquifer consists primarily of sands and gravels of fluvial and marginal marine origin and is an important drinking water resource in the Atlantic Coastal Plain of Delaware. Since the Columbia aquifer is largely unconfined, it is susceptible to contamination from spills, unmanaged wastes, and the application of agricultural chemicals on or near the land surface.

Agriculture is the predominant land use in Delaware with corn, soybeans, and small grains as the major crops. In 2017, approximately 37 percent of the available land area was used for farming (U.S. Department of Agriculture, 2017). Most of the agricultural activity in Delaware takes place in Kent and Sussex Counties and south of the Chesapeake and Delaware (C&D) Canal in New Castle County. The primary land uses surrounding this relatively shallow public-supply well network in the Coastal Plain of Delaware include agriculture, and low-density urban and suburban areas associated with small towns and communities (fig. 1).

PFAS General Description

What are PFAS?

Per- and polyfluoroalkyl substances (PFAS) are a large group (more than 4,000) of man-made, highly persistent chemicals that have been in use since the 1940s. PFAS are found in a wide range of consumer and industrial products such as cookware, pizza boxes, carpets, stain repellants, and firefighting foams, as well as other common products (fig. 2). PFAS are released into the air, soil, and water by PFAS manufacturing and processing facilities (such as metal plating operations), historical use in firefighting foams, and regular everyday use by consumers. Owing to their widespread use, persistence in the environment, and long half-lives, most people in the United States have been exposed to PFAS. There is evidence that because of their long half-lives and toxicity, continued exposure above threshold levels for specific PFAS may lead to adverse health effects (U.S. Environmental Protection Agency, 2016a; Agency for Toxic Substances and Disease Registry, 2018). PFOA and PFOS have been the most extensively produced and studied of these chemicals. Both chemicals are very persistent in the environment and human body, and can bioaccumulate in living organisms (U.S. Environmental Protection Agency, 2017).

There are a variety of ways in which people can be exposed to PFAS and at different levels of exposure, for instance

  • Food ingestion and food packaging

  • Daily exposure with commonly used items such as, but not limited to, carpets, cookware, rain gear, and so forth

  • Occupational exposure

  • Drinking water

Firefighting foam, dishware, raincoat, food packaging, and pizza boxes are examples
                        of products that use per- and polyfluoroalkyl substances (PFAS).
Figure 2. Examples of products that use per- and polyfluoroalkyl substances (PFAS). Photographs from unsplash.com.

PFAS Effects on the Human Body

There is evidence that exposure to PFAS can lead to adverse health outcomes in humans or animals as they can accumulate within the body. Some studies have indicated that PFAS can cause reproductive, developmental, liver, kidney, and immunological adverse effects, as well as cancer and other health issues (U.S. Environmental Protection Agency, 2016c, 2017). The EPA established a drinking-water HAL of 70 ppt for combined concentrations of PFOA and PFOS (U.S. Environmental Protection Agency, 2016b). This HAL was established to provide information on the health risks of these chemicals and to allow for appropriate actions to protect consumers (U.S. Environmental Protection Agency, 2014). The State of Delaware adopted EPA’s 2016 HAL for combined concentrations of PFOA and PFOS as screening values for hazardous substances in the Delaware Natural Resources and Environmental Control (DNREC) Remediation Section that same year (2016).

Study Methods

This study was developed to obtain information on the spatial occurrence and distribution of PFAS in the unconfined aquifer in the Delaware Coastal Plain. Source water samples collected for this purpose were analyzed for PFAS and gaseous SF6 to determine groundwater ages.

Study Sampling Network

The study well network consisted of 30 drinking-water wells distributed throughout the State of Delaware and screened in the Columbia aquifer. Its purpose was to observe source groundwater quality across a network of previously sampled wells (Ferrari, 2002; Reyes, 2010), and identify and quantify the spatial and temporal changes in water quality in Delaware’s Columbia aquifer public-drinking water-supply wells between 2000, 2008, and 2018. All groundwater samples were collected from the source water to supply wells prior to any filtering or treatment and are therefore representative of the available groundwater resource rather than drinking water. The network (fig. 1; table 1) of 30 wells was sampled from August to November 2018, and the samples were analyzed for 18 PFAS using the modified EPA Method 537 (U.S. Environmental Protection Agency, 2009; table 2) and groundwater age.

Table 1.    

Well-construction data for sampled public water-supply wells screened in the Columbia aquifer in Delaware and modeled ages of groundwater sampled during the 2000, 2008, and 2018 sampling periods.

[USGS, U.S. Geological Survey; DGS, Delaware Geological Survey; DNREC, Delaware Natural Resources and Environmental Control; Equip. blank, a sample collected in which blank water is processed through the equipment used for environmental sample collections. It is similar to a field blank, but done in a controlled environment such as a laboratory; Resampled, 23 public water-supply wells sampled in 2008 and 2018; Resampled, trend, 17 public water-supply wells used as trend wells, sampled in 2000, 2008, and 2018; Replacement, 7 public water-supply wells sampled only in 2018; --, no data]

USGS site identification number DGS local well number Sampling periods Well description DNREC permit number County Year well constructed Casing material Depth of well (feet) Diameter of well (inches) Depth to top of screen (feet) Depth to bottom of screen (feet) Average age in 2000 (years) Average age in 2008 (years) Average age in 2018 (years)
394100075334501 Cd52-40 2018 Replacement 235991 New Castle 2011 Steel 80 12 58 76 -- -- 13.0
393928075440202 Db11-27 2000, 2008, 2018 Resampled, trend 10004 New Castle 1956 Steel 66 10 41 62 8.5 7.3 15.0
393916075440802 Db11-28 2000, 2008, 2018 Resampled, trend 10003 New Castle 1956 Unknown 62 10 31 62 7.0 5.8 25.0
393739075394202 Dc31-15 2000, 2008, 2018 Resampled, trend 10434 New Castle 1960 Unknown 76 17 50 70 7.5 7.6 14.0
390538075325101 DE-KE 187731 2018 Replacement 187731 Kent 2002 Plastic 55 4 48 55 -- -- 14.9
383101075141001 DE-SU 56105 2018 Replacement 56105 Sussex 1984 Plastic 128 12 88 128 -- -- 34.2
391747075364202 Hc34-03 2000, 2008, 2018 Resampled, trend 10068 Kent 1948 Steel 100 16 80 95 13.0 18.0 22.5
391060075282801 Ie42-03 2000, 2008, 2018 Resampled, trend 85022 Kent 1991 Steel 70 16 49 64 12.0 16.4 18.6
390703075371801 Jc33-12 2018 Replacement 102056 Kent 1994 Plastic 79 8 59 79 -- -- 45.8
385522075251802 Le55-09 2000, 2008, 2018 Resampled, trend 31756 Kent 1974 Steel 91 10 71 91 18.0 23.9 37.9
385448075341801 Md11-04 2000, 2008, 2018 Resampled, trend 65911 Kent 1986 Unknown 70 10 50 70 11.0 21.7 13.2
384818075354101 Nc25-37 2000, 2008, 2018 Resampled, trend 72714 Sussex 1988 Steel 63 12 40 63 15.0 18.1 27.1
384819075190101 Ng21-03 2000, 2008, 2018 Resampled, trend 71704 Sussex 1987 Plastic 111 4 91 111 20.5 25.2 28.1
384856075151101 Ng25-04 2000, 2008, 2018 Resampled, trend 97993 Sussex 1994 Plastic 139 8 99 139 18.5 33.7 38.6
384526075091601 Ni51-32 2000, 2008, 2018 Resampled, trend 55833 Sussex 1984 Plastic 139 16 85 135 21.0 27.1 26.4
384428075355701 Oc15-11 2000, 2008, 2018 Resampled, trend 10319 Sussex 1955 Unknown 119 12 100 119 13.0 25.0 28.2
384139075230101 Of42-01 2000, 2008, 2018 Resampled, trend 10325 Sussex 1948 Unknown 120 6 UNKNOWN 120 19.5 26.7 32.4
384428075135501 Oh12-07 2008, 2018 Resampled 181528 Sussex 2001 Plastic 118 4 108 118 -- 22.4 34.6
384322075051101 Oi25-18 2000, 2008, 2018 Resampled, trend 93955 Sussex 1993 Plastic 38 4 23 38 9.0 8.0 8.2
384326075050801 Oi25-19 2000, 2008, 2018 Resampled, trend 93496 Sussex 1992 Unknown 37 4 27 37 9.0 14.7 17.7
383823075382101 Pc22-06 2008, 2018 Resampled 74465 Sussex 1988 Steel 103 16 63 103 -- 19.5 30.4
383815075271001 Pe23-185 2000, 2008, 2018 Resampled, trend 72060 Sussex 1987 Plastic 120 4 100 110 18.5 34.6 44.8
383732075191301 Pg31-12 2008, 2018 Resampled 35113 Sussex 1975 Plastic 73 12 53 73 -- 16.1 20.7
383729075101601 Ph35-25 2008, 2018 Resampled 63104 Sussex 1986 Unknown 58 8 58 73 -- 21.3 28.5
383914075080501 Pi12-11 2008, 2018 Resampled 75500 Sussex 1988 Plastic 68 4 58 68 -- 18.1 16.1
383713075085501 Pi32-15 2008, 2018 Resampled 10653 Sussex 1972 Unknown 90 6 75 85 -- 21.8 26.3
383346075340301 Qd21-42 2018 Replacement 79306 Sussex 1989 Steel 100 10 75 100 -- -- 21.3
382805075330301 Rd22-01 2000, 2008, 2018 Resampled, trend 66041 Sussex 1986 Plastic 60 2 52 60 15.0 25.5 26.3
382755075341501 Rd31-24 2018 Replacement 10665 Sussex 1976 Plastic 98 4 79 89 -- -- 24.4
382807075070701 Ri23-15 2018 Replacement 186732 Sussex 2002 Plastic 80 6 55 65 -- -- 22.9
390918075291701 USGS Office at Dover, Delaware 2000, 2008, 2018 Observation, Equip. blank Unknown Kent 2000 Unknown Unknown Unknown Unknown Unknown -- -- --

Table 2.    

Information for per- and polyfluorinated alkyl substances (PFAS) for which unfiltered groundwater samples from public water-supply wells were analyzed in the Columbia aquifer in Delaware in 2018 under the modified U.S. Environmental Protection Agency Method 537 (U.S. Environmental Protection Agency, 2009).

[NWIS, National Water Information System; ppt, parts per trillion; RDL, reportable detection limit; MDL, method detection limit]

PFAS name PFAS abbreviation NWIS parameter code Parameter unit RDL MDL
6:2 Fluorotelomer sulfonate 6:2 FTS 53593 ppt 20 6.6
8:2 Fluorotelomer sulfonate 8:2 FTS 53594 ppt 20 6.6
Perfluorobutane sulfonate PFBS 53588 ppt 20 5.4
Perfluorobutanoic acid PFBA 53577 ppt 20 5.5
Perfluorodecane sulfonate PFDS 53591 ppt 20 6
Perfluorodecanoic acid PFDA 53583 ppt 20 6.1
Perfluorododecanoic acid PFDoA 53585 ppt 20 5
Perfluoroheptanoic acid PFHpA 53580 ppt 20 7.4
Perfluorohexane sulfonate PFHxS 53589 ppt 20 5.6
Perfluorohexanoic acid PFHxA 53579 ppt 20 3.5
Perfluorononanoic acid PFNA 53582 ppt 20 8.7
Perfluorooctane sulfonamide PFOSA 53592 ppt 20 3.4
Perfluorooctane sulfonate PFOS 53590 ppt 20 6
Perfluorooctanoic acid PFOA 53581 ppt 20 3.3
Perfluoropentanoic acid PFPeA 53578 ppt 20 7.5
Perfluorotetradecanoic acid PFTeDA 53587 ppt 20 2.7
Perfluorotridecanoic acid PFTrDA 53586 ppt 20 3.8
Perfluoroundecanoic acid PFUnA 53584 ppt 20 2.5

To the extent possible, the 2018 well network was the same as in fall of 2000 and 2008. In the event a previously sampled well was no longer in use, U.S. Geological Survey (USGS) personnel worked with DNREC personnel to identify a suitable replacement well. Replacement wells were selected based on the following DNREC established criteria: (1) well located within a 1-mile radius with similar well construction, (2) used as a public drinking-water supply, and (3) screened in the unconfined aquifer.

Sampling Collection and Analysis

Wells were sampled and analyzed for PFAS and groundwater age. Field parameters (appendix 1; table 1.1) including water temperature, specific conductance, pH, dissolved oxygen, and alkalinity were determined in the field following the protocols outlined in U.S. Geological Survey (variously dated). All groundwater samples were collected from the raw-sample tap prior to any filtering or treatment and are therefore representative of the available groundwater resource rather than drinking water. Wells were purged to remove standing water in the casing (generally three well volumes) before samples were collected. Purging continued until dissolved oxygen (±0.3 milligrams per liter [mg/L]), pH (±0.1 units), specific conductance (±3 percent), water temperature (±0.2 degrees Celsius [°C]), and turbidity (±10 percent) stabilized. Direct collection of PFAS water-quality samples from the spigot was collected into two 250-milliliter (mL) bottles or one 500-mL bottle using elbow-length gloves. Another subset of samples was collected into 1-liter bottles for dissolved gases and SF6 analyses for groundwater age. Bottles were chilled to maintain a temperature of 4 °C during shipment to the laboratory.

PFAS analyses were performed by Maxxam Analytics, a subcontract lab from RTI Laboratories, Inc., following U.S. Environmental Protection Agency Method 537 for determination of selected perfluorinated alkyl acids in drinking water by solid phase extraction and liquid chromatography/tandem mass spectrometry protocol (U.S. Environmental Protection Agency, 2009). Concentrations of gaseous SF6 were determined at the USGS Groundwater Dating Laboratory, Chlorofluorocarbon/Dissolved Gas Laboratory in Reston, Virginia, to determine groundwater age (Plummer and Busenberg, 1999; Plummer and Friedman, 1999; Plummer and Busenberg, 2000; Busenberg and others, 2001).

Data Analysis

Descriptive statistical analyses were used to assess PFAS analytical results within the well network. Combined PFOA and PFOS concentrations were compared to the EPA’s HAL. This comparison was made for informational purposes only and not for evidence of compliance or noncompliance with Federal regulations because this health-based reference level is for public drinking water as supplied to customers and is not applied to source (raw) water.

Explanatory and nonparametric statistical tests were used to characterize water chemistry differences of groundwater-age data between the three sampling events (2000, 2008, and 2018), to identify changes in groundwater quality, and to identify patterns in groundwater chemistry during the new 10-year study period (2008 to 2018). Twenty-three of the wells sampled in 2008 were available for resampling and used for comparison between the 2008 and 2018 samples. Of the 23 resampled wells, 17 wells were sampled in 2000, 2008, and 2018; these wells were used for statistical trend analysis. PFAS censored data (less than or undetected at the method detection limit; table 3) were set to nondetects before any analyses or ranks and in the figures. PFAS concentrations with E values indicated that PFAS concentrations were detected below the reporting detection limit but above the method detection limit, and these were set as detections. All statistical tests were evaluated at the 95-percent confidence level (a=0.05). The R statistical program (ver. 4.0.2) was used to perform all the statistical analyses. All water-chemistry data collected for this report are available through the National Water Information System (NWIS) web page (https://waterdata.usgs.gov/nwis/) and ScienceBase (Reyes, 2021).

Table 3.    

Concentrations and detections of per- and polyfluoroalkyl substances (PFAS) in sampled public water-supply wells in the Columbia Aquifer in Delaware, sampled in 2018 using the modified U.S. Environmental Protection Agency Method 537.

[PFAS descriptions and list of analytical compounds are provided in table 2. A total of 30 public water-supply wells were sampled in 2018. USGS, U.S. Geological Survey; DGS, Delaware Geological Survey; ppt, parts per trillion; Environmental, groundwater sample from the public water-supply well; E, value is between method detection limit and reporting detection limit; <, less than, undetected at the method detection limit; HAL, EPA health advisory level for combined concentrations of perfluorooctanoic acid (PFOA) and perfluorooctane sulfonate (PFOS); ND, nondetected PFAS]

USGS site identification number DGS local well number Sample type PFAS concentrations (ppt) Number of PFAS detected per well HAL (70 ppt)a
PFBA PFPeA PFHxA PFHpA PFOA PFNA PFDA PFUnA PFDoA PFTrDA PFTeDA PFBS PFHxS PFOS PFDS PFOSA 6:2-FtS 8:2-FtS
394100075334501 Cd52-40 Environmental 34 39 45 25 57a <20 <20 <20 <20 <20 <20 <20 22 20a <20 <20 <20 <20 7 77a
393928075440202 Db11-27 Environmental E13 E14 E15 E12 23 <20 <20 <20 <20 <20 <20 E11 E9.4 <20 <20 <20 <20 <20 7 23
393916075440802 Db11-28 Environmental E12 E16 E16 E12 22 <20 <20 <20 <20 <20 <20 <20 E9 E13 <20 <20 <20 <20 7 35
393739075394202 Dc31-15 Environmental <20 <20 <20 <20 29 <20 <20 <20 <20 <20 <20 <20 <20 <20 <20 <20 <20 <20 1 29
390538075325101 DE-KE 187731 Environmental <5.5 <7.5 <3.5 <7.4 <3.3 <8.7 <6.1 <2.5 <5 <3.8 <2.7 <5.4 <5.6 <6 <6 <3.4 <6.6 <6.6 0 ND
383101075141001 DE-SU 56105 Environmental <5.5 <7.5 <3.5 <7.4 <3.3 <8.7 <6.1 <2.5 <5 <3.8 <2.7 <5.4 <5.6 <6 <6 <3.4 <6.6 <6.6 0 ND
384139075230101 Georgetown 1 Environmental E5.8 <20 E5.6 <20 E11 <20 <20 <20 <20 <20 <20 <20 E12 23 <20 <20 <20 <20 5 34
391747075364202 Hc34-03 Environmental <20 <20 E5.5 <20 E9.7 <20 <20 <20 <20 <20 <20 E7.2 <20 E10 <20 <20 <20 <20 4 19.7
391060075282801 Ie42-03 Environmental <20 <20 <20 <20 E5.9 <20 <20 <20 <20 <20 <20 <20 <20 <20 <20 <20 <20 <20 1 5.9
390703075371801 Jc33-12 Environmental <20 <20 E5 <20 <20 <20 <20 <20 <20 <20 <20 <20 <20 <20 <20 <20 <20 <20 1 ND
385522075251802 Le55-09 Environmental <5.5 <7.5 <3.5 <7.4 <3.3 <8.7 <6.1 <2.5 <5 <3.8 <2.7 <5.4 <5.6 <6 <6 <3.4 <6.6 <6.6 0 ND
385448075341801 Md11-04 Environmental <7 <4.1 <6.4 <7.1 <7.4 <4.9 <4.1 <4.3 <6.8 <6.9 <6.7 <5.1 <5.2 <5.2 <7.2 <6.6 <5.9 <5.9 0 ND
384818075354101 Nc25-37 Environmental <20 <20 <20 <20 <20 <20 <20 <20 <20 <20 <20 <20 <20 <20 <20 <20 <20 <20 0 ND
384819075190101 Ng21-03 Environmental <5.5 <7.5 <3.5 <7.4 <3.3 <8.7 <6.1 <2.5 <5 <3.8 <2.7 <5.4 <5.6 <6 <6 <3.4 <6.6 <6.6 0 ND
384856075151101 Ng25-04 Environmental <5.5 <7.5 <3.5 <7.4 <3.3 <8.7 <6.1 <2.5 <5 <3.8 <2.7 <5.4 <5.6 <6 <6 <3.4 <6.6 <6.6 0 ND
384526075091601 Ni51-32 Environmental <20 <20 <20 <20 <20 <20 <20 <20 <20 <20 <20 <20 <20 <20 <20 <20 <20 <20 0 ND
384428075355701 Oc15-11 Environmental <5.5 <7.5 <3.5 <7.4 E5.1 <8.7 <6.1 <2.5 <5 <3.8 <2.7 <5.4 <5.6 <6 <6 <3.4 <6.6 <6.6 1 5.1
384428075135501 Oh12-07 Environmental <5.5 <7.5 <3.5 <7.4 <3.3 <8.7 <6.1 <2.5 <5 <3.8 <2.7 <5.4 <5.6 <6 <6 <3.4 <6.6 <6.6 0 ND
384322075051101 Oi25-18 Environmental 20 67 44 E12 28 <20 <20 <20 <20 <20 <20 E15 E16 21 <20 <20 <20 <20 8 49
384326075050801 Oi25-19 Environmental <20 <20 <20 <20 29 <20 <20 <20 <20 <20 <20 <20 32 21 <20 <20 <20 <20 3 50
383823075382101 Pc22-06 Environmental <20 <20 E5.6 <20 E5.4 <20 <20 <20 <20 <20 <20 E5.5 <20 <20 <20 <20 <20 <20 3 5.4
383815075271001 Pe23-185 Environmental <5.5 <7.5 <3.5 <7.4 <3.3 <8.7 <6.1 <2.5 <5 <3.8 <2.7 <5.4 <5.6 <6 <6 <3.4 <6.6 <6.6 0 ND
383732075191301 Pg31-12 Environmental <20 <20 <20 <20 <20 <20 <20 <20 <20 <20 <20 <20 <20 <20 <20 <20 <20 <20 0 ND
383729075101601 Ph35-25 Environmental <20 <20 <20 <20 <20 <20 <20 <20 <20 <20 <20 <20 <20 <20 <20 <20 <20 <20 0 ND
383914075080501 Pi12-11 Environmental <5.5 <7.5 <3.5 <7.4 <3.3 <8.7 <6.1 <2.5 <5 <3.8 <2.7 E7.1 <5.6 <6 <6 <3.4 <6.6 <6.6 1 ND
383713075085501 Pi32-15 Environmental <20 <20 <20 <20 <20 <20 <20 <20 <20 <20 <20 <20 <20 <20 <20 <20 <20 <20 0 ND
383346075340301 Qd21-42 Environmental <7 <4.1 <6.4 <7.1 <7.4 <4.9 <4.1 <4.3 <6.8 <6.9 <6.7 <5.1 <5.2 <5.2 <7.2 <6.6 <5.9 <5.9 0 ND
382805075330301 Rd22-01 Environmental <5.5 <7.5 <3.5 <7.4 E6 <8.7 <6.1 <2.5 <5 <3.8 <2.7 <5.4 <5.6 <6 <6 <3.4 <6.6 <6.6 1 6
382755075341501 Rd31-24 Environmental <5.5 E8.5 E9.8 <7.4 E11 <8.7 <6.1 <2.5 <5 <3.8 <2.7 <5.4 E9.2 E18 <6 <3.4 <6.6 <6.6 5 29
382807075070701 Ri23-15 Environmental E15 32 35 E17 40a <8.7 <6.1 <2.5 <5 <3.8 <2.7 100 130 59a <6 <3.4 <6.6 <6.6 8 99a
Number of wells with PFAS detections 6 6 10 5 14 0 0 0 0 0 0 6 8 8 0 0 0 0 16 2
a

Detections are above the health advisory level.

Quality-Control Sampling

Quality-control samples including equipment and field blanks, sequential replicate samples, and laboratory spikes, were collected to evaluate and estimate potential contamination bias and measure variability from water-quality data-collection processes following protocols described in Koterba and others (1995). An equipment blank was collected prior to sampling; seven field blanks and three replicates for PFAS were collected during field activities at selected wells (Reyes, 2021); all 30 wells have replicate samples for SF6 and dissolved gases for groundwater-age estimation. Field collection procedures for quality-control samples were established using the USGS National Field Manual (U.S. Geological Survey, variously dated) and in a manner consistent with procedures for the acquisition of environmental samples.

Equipment and field blanks were collected to estimate the accuracy of concentrations and to ensure that sample collection and processing did not result in contamination. No PFAS were detected in equipment or field blanks, indicating that selected equipment, cleaning, sampling, and handling procedures are sufficient to provide data that reflect environmental conditions.

Replicate field samples measure the combined precision of sampling and laboratory analysis procedures. All the replicate samples had results similar to or consistent (within a relative percent difference of 30 percent) with their respective environmental samples.

Spikes were analyzed to determine the extent of degradation of the analyte concentration during sample processing and analysis, recovery bias, and variability (Koterba and others, 1995). In this study, samples were spiked in the laboratory with a known quantity of PFAS. Spike data are available upon request from the USGS Maryland-Delaware-D.C. Water Science Center in Baltimore, Maryland, Water-Quality Data Section (https://www.usgs.gov/centers/md-de-dc-water/about).

PFAS Results

All 30 wells were analyzed for 18 PFAS using the modified EPA Method 537 (U.S. Environmental Protection Agency, 2009; table 2). More than half (16) of the sampled wells had one or more PFAS detections, and as many as eight PFAS were detected in two wells (Ri23-15 and Oi25-18; table 3). The spatial distribution of PFAS detections across Delaware is shown in figure 3. Wells with higher numbers of compounds detected (five or more) were located in New Castle and Sussex Counties. Forty-four percent of the analyzed PFAS (8 of 18) were detected in the study well network, with individual compound concentrations detected raging from E5.0 to 130 ppt. The four most frequently detected PFAS were PFOA with 47 percent detection, PFHxA with 33 percent detection, and PFOS and PFHxS with 27 percent detection each (table 3).

Delaware Atlantic coastal plain with the scattered locations of sampled wells and
                     per- and polyfluoroalkyl substances detections.
Figure 3. Location of public water-supply wells sampled in 2018 and per- and polyfluoroalkyl substances (PFAS) detection distribution in the Columbia aquifer in Delaware. EPA, U.S. Environmental Protection Agency; HAL, health advisory level; ppt, parts per trillion; PFOA, perfluorooctanoic acid; PFOS, perfluorooctane sulfonate.

Most of the PFAS detections were in oxic environments (dissolved oxygen levels greater than or equal to 1 mg/L), although higher concentrations of PFAS compounds (greater than or equal to 100 ppt) were observed in low-oxygen environments (fig. 4).

Majority of the PFAS detections (below or greater than the reportable detection limit)
                     were found in dissolved oxygen concentrations greater than 2 mg/L; majority of high-concentration
                     PFAS samples were found in dissolved oxygen concentrations less than 1 mg/L.
Figure 4. Distribution of per- and polyfluoroalkyl substances (PFAS) concentration by dissolved oxygen concentration in the Columbia aquifer, 2018. Refer to table 2 for PFAS descriptions and list of analytical compounds. Values plotted at 0 are nondetects, as discussed in the text.

Two wells (Cd52-40 in New Castle County and Ri23-15 in Sussex County), out of the 14 wells with combined PFOS and PFAS detections, were above the EPA drinking-water HAL of 70 ppt (fig. 5). Both wells are treated to remove PFAS from their system.

Two wells sampled contained combined PFOA and PFOS concentrations higher than the
                     U.S. Environmental Protection Agency drinking water health advisory level of 70 ppt.
Figure 5. Comparison between combined concentrations of perfluorooctane sulfonate (PFOS) and perfluorooctanoic acid (PFOA) and the U.S. Environmental Protection Agency (EPA) drinking water health advisory level. Values plotted at 0 are nondetects, as discussed in the text.

Groundwater Age Results

Water that infiltrates the landscape and percolates downward toward the water table becomes recharge to the aquifer system. As additional recharge continues to enter the aquifer, older recharge is pushed deeper by the newer recharge, resulting in a trend of increasing groundwater age with depth. Groundwater age represents the average age of water withdrawn from the well, as well screens typically span several feet of aquifer sediment, thus integrating waters of various ages. Dissolved-gas analyses also were performed on samples of each well as part of the age-dating analysis to determine the average recharge temperature of the water in the aquifer (appendix 2; table 2.1). Recharge dates for groundwater samples were estimated based on measured concentrations of SF6, which is a common chemical refrigerant that has been released into the environment. The reported age in this study was calculated by averaging the laboratory SF6 ages estimated from each of two samples collected from each well (appendix 3; table 3.1).

The average groundwater age among the 30 wells sampled in 2018 ranged from 8.2 to 45.8 years, with a median of 25.7 years. Refer to table 1 for well-construction and groundwater-age data. The same relations from previous study years were observed; groundwater ages are positively correlated (R2=0.6024, p-value<0.0004) with well depth (fig. 6A) and negatively correlated (R2= –0.4293, p-value <0.018) with dissolved oxygen concentrations (fig. 6B) in the Columbia aquifer. The relation of groundwater-age data and total PFAS concentrations is shown in figure 7. This groundwater age-concentration relation supports the fact that PFAS are as result of anthropogenic activities and that these contaminants are relatively novel.

Significant-positive correlation between well depth and the average modeled groundwater
                     sample age, and a significant-negative correlation between dissolved oxygen concentration
                     and the average modeled groundwater sample age.
Figure 6. Relation of average modeled groundwater sample age to (A) well depth, and (B) dissolved oxygen concentrations in the Columbia aquifer in Delaware, 2018.
Significant-negative correlation between PFAS concentrations and the average modeled
                     groundwater sample age.
Figure 7. Relation of average modeled groundwater sample age to per- and polyfluoroalkyl substances (PFAS) concentrations in the Columbia aquifer in Delaware, 2018. Refer to table 2 for PFAS descriptions and list of analytical compounds. Values plotted at 0 are nondetects, as discussed in the text.

Comparison of the average modeled groundwater sample-age data (fig. 8A) from the 23 wells sampled in 2018 indicates that the average groundwater age was significantly different than the 2008 sample-age data; 2018 sample ages were, on average, 5 years older than 2008 sample ages. A similar pattern was found during cycle two (2008) of this study, where the 2008 average modeled groundwater age was, on average, 7 years older than the samples collected in 2000. The distribution of the groundwater sample ages among the 17 trend wells and during the three study cycles (fig. 8B) indicates that sample-age medians were statistically different than zero (p-value<0.00124). Well-water sample-age data show a slight increase in groundwater sample age. Groundwater age represents a mixture of older and young groundwater because well screens in public-supply wells are relatively long and pumping enhances the mixing of waters of disparate groundwater ages (Dunkle and others, 1993). The specific reason for the differences in groundwater ages is unknown, although Gholam and others (2006) indicated that this age difference could reflect a higher proportion of water drawn from slow-moving storage, possibly as a result of pumping for long periods of time, or a pumping increase that caused more water from older, deeper, or more distant sources to be drawn into the well.

Data show that 2018 sampled wells average groundwater age is significantly older than
                     the 2008 sampled wells average groundwater data. Average groundwater age during the
                     three study cycles shows a significant-positive trend.
Figure 8. Average modeled groundwater sample age (A) comparison over the 10-year study period from the 23 sampled wells in the Columbia aquifer in Delaware between 2008 and 2018, and (B) distribution over the three sampling cycles in the Columbia aquifer in Delaware, 2000, 2008, and 2018. IQR, interquartile range.

Summary

This study’s results mark a baseline for per- and polyfluorinated alkyl substances (PFAS) concentration in the Columbia aquifer. Many findings from this study suggest the need to continue active monitoring for these anthropogenic contaminants. There was a widespread distribution of PFAS detections throughout Delaware, as well as a significant number of detections per wells and the variety of compounds detected, including the higher concentrations and high number of detections in younger waters and the results of the U.S. Environmental Protection Agency’s health advisory level exceedances for perfluorooctanoic acid (PFOA) and perfluorooctane sulfonate (PFOS) in source water samples. These findings show the susceptibility of this aquifer for these and other anthropogenic contaminants which may significantly affect the drinking water sources in the Columbia aquifer as a result of the aquifer’s characteristics (surficial aquifer; good soil drainage; and sandy, transmissive aquifer sediments) and the persistence of these compounds. Comparison of the average modeled groundwater ages indicates that the 2018 samples were significantly older than the 2008 and 2000 samples. Among the resampled wells, the average groundwater age difference was 7 years from 2000 to 2008 and 5 years from 2008 to 2018.

References Cited

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Appendix 1. Groundwater-quality data for sampled public water-supply wells in the Columbia aquifer in Delaware, sampled in 2018—Field parameters.

Table 1.1.    

Groundwater-quality data for sampled public water-supply wells in the Columbia aquifer in Delaware, sampled in 2018—Field parameters.

[A total of 30 public water-supply wells were sampled in 2018. USGS, U.S. Geological Survey; DGS, Delaware Geological Survey; °C, degrees Celsius; mm Hg, millimeters of mercury; mg/L, milligrams per liter; µS/cm, microsiemens per centimeter at 25 °C; pH is given in standard units; CaCO3, calcium carbonate; --, no data]

USGS site identification number DGS local well number Field parameters
Water temperature (°C) Air temperature (°C) Air pressure (mm Hg) Specific conductance, field (µS/cm) Dissolved oxygen (mg/L) pH (field, standard units) Alkalinity (mg/L as CaCO3)
394100075334501 Cd52-40 15.6 26 -- 394 6.9 5.54 19.5
393928075440202 Db11-27 14.8 11.5 775.8 389 6.8 5.41 15.5
393916075440802 Db11-28 17.9 12.5 773 637 2.7 5.46 23
393739075394202 Dc31-15 14.8 29 769 381 8.2 5.54 --
390538075325101 DE-KE 187731 12.9 11 766.8 191 8.3 5.575 10.9
383101075141001 DE-SU 56105 15.25 2 776 268 0.01 5.77 21
384139075230101 Georgetown 1 17.4 32 768 533 0.42 5.14 17.7
391747075364202 Hc34-03 16 24 772 250 5.1 5.45 10.4
391060075282801 Ie42-03 15.9 26.5 767 192 2.2 5.555 11.4
390703075371801 Jc33-12 14.9 12.5 767.5 258 0.48 7.045 125.7
385522075251802 Le55-09 14.3 7 767 266 0.63 5.685 10.9
385448075341801 Md11-04 17.6 29 -- 333 4.1 5.77 19.5
384818075354101 Nc25-37 17.3 29 761.5 175 3.2 5.46 6.8
384819075190101 Ng21-03 15.6 19.5 765.5 197 8.9 6.87 3.1
384856075151101 Ng25-04 14.3 -- 765.5 119 2.3 6.045 6.2
384526075091601 Ni51-32 16.64 28.5 768.5 187 4.24 5.48 9.6
384428075355701 Oc15-11 15.9 22 767.2 201 6.2 5.295 7.8
384428075135501 Oh12-07 13.9 -- 767.6 121 6.9 5.52 6.2
384322075051101 Oi25-18 16.1 22.5 769 245 3.1 5.61 24.1
384326075050801 Oi25-19 15.1 21 763 292 2.1 5.47 18.9
383823075382101 Pc22-06 17.02 31 769.8 191 6 4.57 0
383815075271001 Pe23-185 15.8 14 764 89.5 0.4 5.995 38.7
383732075191301 Pg31-12 16.9 29 770 125 8.7 5.19 8.3
383729075101601 Ph35-25 15.8 26 768.9 92 4 5.55 8.1
383914075080501 Pi12-11 14.6 11.1 767 155 4.9 5.5 8.6
383713075085501 Pi32-15 15.6 24 768.9 162 6.3 5.44 9.2
383346075340301 Qd21-42 16 14.5 771.5 128 4.1 5.04 4.4
382805075330301 Rd22-01 15.5 9 769.4 113 0.85 5.5 22.3
382755075341501 Rd31-24 14.3 4 -- 143 4.3 5.375 14
382807075070701 Ri23-15 15.425 7 -- 130 0.49 5.86 25.6

Appendix 2. Groundwater-quality data for sampled public water-supply wells in the Columbia aquifer in Delaware, sampled in 2018—Age dating, dissolved-gas data.

Table 2.1.    

Groundwater-quality data for sampled public water-supply wells in the Columbia aquifer in Delaware, sampled in 2018—Age dating, dissolved-gas data.

[A total of 30 public water-supply wells were sampled in 2018. USGS, U.S. Geological Survey; DGS, Delaware Geological Survey; °C, degrees Celsius; mg/L, milligrams per liter; CH4, methane; CO2, carbon dioxide; N2, nitrogen; O2, oxygen; Ar, argon]

USGS site identification number DGS local well number Sampling date (MM/DD/YYYY) Field temperature (°C) Recharge elevation (feet) Dissolved-gas data
Concentration (mg/L) Partial pressures at field temperatures (atmospheres)
CH4 CO2 N2 O2 Ar CH4 CO2 N2 O2 Ar
394100075334501 Cd52-40 9/26/2018 16.65 31 0.0000 52.8474 19.5414 3.6531 0.6791 0.000000 0.027771 0.9460 0.0771 0.01048
394100075334501 Cd52-40 9/26/2018 16.65 31 0.0000 55.2778 19.4356 3.7599 0.6767 0.000000 0.029048 0.9409 0.0794 0.01044
393928075440202 Db11-27 10/18/2018 14.80 79 0.0000 68.8482 19.4053 3.5948 0.6840 0.000000 0.034145 0.9080 0.0731 0.01017
393928075440202 Db11-27 10/18/2018 14.80 79 0.0000 69.3144 19.4602 3.7539 0.6861 0.000000 0.034376 0.9105 0.0764 0.01020
393916075440802 Db11-28 10/18/2018 17.80 71 0.0048 91.8770 20.5688 1.5426 0.7056 0.000184 0.050008 1.0163 0.0333 0.01113
393916075440802 Db11-28 10/18/2018 17.80 71 0.0043 87.5226 20.2808 1.0831 0.6978 0.000167 0.047638 1.0021 0.0234 0.01101
393739075394202 Dc31-15 9/26/2018 14.85 69 0.0000 47.7767 17.7711 5.6729 0.6491 0.000000 0.023732 0.8323 0.1155 0.00966
393739075394202 Dc31-15 9/26/2018 14.85 69 0.0000 46.3701 17.7760 5.7130 0.6493 0.000000 0.023034 0.8325 0.1163 0.00966
390538075325101 DE-KE 187731 11/19/2018 12.40 38 0.0000 22.0549 18.3485 1.1274 0.6611 0.000000 0.010122 0.8196 0.0218 0.00934
390538075325101 DE-KE 187731 11/19/2018 12.40 38 0.0000 25.5813 18.2641 0.8505 0.6565 0.000000 0.011740 0.8158 0.0164 0.00928
383101075141001 DE-SU 56105 12/11/2018 15.20 45 0.2299 51.9034 24.0309 0.2993 0.6773 0.008374 0.026069 1.1328 0.0061 0.01015
383101075141001 DE-SU 56105 12/11/2018 15.20 45 0.2345 52.7459 24.1872 0.2713 0.6780 0.008541 0.026492 1.1402 0.0056 0.01016
384139075230101 Georgetown 1 8/29/2018 17.40 100 0.0093 168.7824 21.2382 0.2148 0.6858 0.000354 0.090758 1.0420 0.0046 0.01074
384139075230101 Georgetown 1 8/29/2018 17.40 100 0.0088 163.6883 21.0437 0.2231 0.6827 0.000337 0.088018 1.0325 0.0048 0.01069
391747075364202 Hc34-03 9/25/2018 15.76 47 0.0000 38.5056 21.5703 0.2418 0.6648 0.000000 0.019683 1.0275 0.0050 0.01008
391747075364202 Hc34-03 9/25/2018 15.76 47 0.0000 39.8315 21.7603 0.2152 0.6684 0.000000 0.020361 1.0365 0.0045 0.01013
391060075282801 Ie42-03 8/15/2018 15.90 100 0.1554 44.7366 20.6919 0.2708 0.6684 0.005749 0.022968 0.9881 0.0056 0.01016
391060075282801 Ie42-03 8/15/2018 15.90 100 0.1478 47.2036 20.2884 0.2280 0.6564 0.005469 0.024235 0.9689 0.0047 0.00998
390703075371801 Jc33-12 10/17/2018 14.90 65 0.0068 3.6587 23.4469 0.3115 0.8119 0.000247 0.001820 1.0991 0.0063 0.01209
390703075371801 Jc33-12 10/17/2018 14.90 65 0.0068 3.4303 23.7118 0.3039 0.8183 0.000247 0.001707 1.1115 0.0062 0.01219
385522075251802 Le55-09 11/14/2018 14.30 25 0.0220 22.1631 23.5053 0.2382 0.6907 0.000786 0.010818 1.0894 0.0048 0.01016
385522075251802 Le55-09 11/14/2018 14.30 25 0.0208 25.2027 23.4349 0.2300 0.6897 0.000741 0.012302 1.0862 0.0046 0.01015
385448075341801 Md11-04 9/6/2018 17.60 100 0.0000 70.9495 19.5759 0.2325 0.6915 0.000000 0.038384 0.9639 0.0050 0.01087
385448075341801 Md11-04 9/6/2018 17.60 100 0.0000 69.9906 19.4861 1.6293 0.6916 0.000000 0.037865 0.9595 0.0350 0.01087
384818075354101 Nc25-37 8/22/2018 17.30 100 0.0000 64.0824 21.8024 2.1495 0.7288 0.000000 0.034353 1.0678 0.0460 0.01139
384818075354101 Nc25-37 8/22/2018 17.30 100 0.0000 65.6991 21.8657 1.5268 0.7300 0.000000 0.035220 1.0709 0.0327 0.01141
384819075190101 Ng21-03 11/1/2018 15.40 19 0.0000 8.9095 18.8242 0.2538 0.6807 0.000000 0.004503 0.8907 0.0052 0.01024
384819075190101 Ng21-03 11/1/2018 15.40 19 0.0000 9.4851 19.5802 0.2304 0.6814 0.000000 0.004794 0.9265 0.0047 0.01025
384856075151101 Ng25-04 11/1/2018 14.20 9 0.0020 19.6182 22.5871 0.3021 0.7716 0.000072 0.009545 1.0449 0.0061 0.01133
384856075151101 Ng25-04 11/1/2018 14.20 9 0.0021 18.4492 22.6336 0.3042 0.7767 0.000075 0.008976 1.0471 0.0061 0.01140
384526075091601 Ni51-32 8/21/2018 16.64 100 0.0000 35.2573 19.2657 1.8738 0.6591 0.000000 0.018522 0.9325 0.0396 0.01017
384526075091601 Ni51-32 8/21/2018 16.64 100 0.0006 38.1656 20.0639 2.7172 0.6844 0.000024 0.020050 0.9711 0.0574 0.01056
384428075355701 Oc15-11 9/10/2018 15.90 100 0.0000 49.4550 21.0576 3.7781 0.7356 0.000000 0.025391 1.0056 0.0786 0.01118
384428075355701 Oc15-11 9/10/2018 15.90 100 0.0000 52.5790 21.0302 4.6456 0.7362 0.000000 0.026995 1.0043 0.0966 0.01119
384428075135501 Oh12-07 10/31/2018 13.90 11 0.0000 12.3962 20.0780 2.4075 0.7103 0.000000 0.005974 0.9235 0.0481 0.01036
384428075135501 Oh12-07 10/31/2018 13.90 11 0.0000 11.8774 20.2001 2.0207 0.7150 0.000000 0.005724 0.9291 0.0403 0.01043
384322075051101 Oi25-18 9/27/2018 16.10 14 0.0005 66.6818 18.5353 0.6754 0.6599 0.000019 0.034449 0.8884 0.0141 0.01007
384322075051101 Oi25-18 9/27/2018 16.10 14 0.0006 60.1163 18.6686 0.2564 0.6603 0.000021 0.031057 0.8948 0.0054 0.01008
384326075050801 Oi25-19 9/27/2018 15.05 16 0.0005 69.1981 20.2120 0.2518 0.6969 0.000017 0.034591 0.9502 0.0051 0.01041
384326075050801 Oi25-19 9/27/2018 15.05 16 0.0009 67.4162 20.4884 0.2444 0.6925 0.000033 0.033701 0.9631 0.0050 0.01035
383823075382101 Pc22-06 8/16/2018 17.00 100 0.0000 26.4196 22.7365 5.3385 0.7136 0.000000 0.014034 1.1076 0.1135 0.01109
383823075382101 Pc22-06 8/16/2018 17.00 100 0.0000 29.6218 22.4266 5.4670 0.7090 0.000000 0.015735 1.0925 0.1162 0.01101
383815075271001 Pe23-185 10/29/2018 15.80 45 0.0208 22.9528 22.6964 0.2746 0.7867 0.000767 0.011748 1.0819 0.0057 0.01193
383815075271001 Pe23-185 10/29/2018 15.80 45 0.0175 20.8462 22.5136 0.2620 0.7805 0.000645 0.010669 1.0732 0.0054 0.01184
383732075191301 Pg31-12 8/27/2018 16.90 100 0.0000 18.1929 19.2523 6.4418 0.6980 0.000000 0.009634 0.9362 0.1367 0.01082
383732075191301 Pg31-12 8/27/2018 16.90 100 0.0000 19.6172 19.0474 4.7741 0.6873 0.000000 0.010388 0.9263 0.1013 0.01066
383729075101601 Ph35-25 10/3/2018 15.80 10 0.0022 17.0579 27.2156 0.3027 0.8748 0.000083 0.008730 1.2973 0.0063 0.01327
383729075101601 Ph35-25 10/3/2018 15.80 10 0.0026 21.0300 26.8148 0.3144 0.8732 0.000098 0.010763 1.2782 0.0065 0.01325
383914075080501 Pi12-11 10/24/2018 14.60 21 0.0000 20.5907 18.4821 0.2571 0.6686 0.000000 0.010147 0.8615 0.0052 0.00990
383914075080501 Pi12-11 10/24/2018 14.60 21 0.0000 17.9845 16.9586 0.2280 0.6138 0.000000 0.008863 0.7905 0.0046 0.00909
383713075085501 Pi32-15 10/3/2018 15.60 9 0.0000 30.9825 18.3921 0.9265 0.6667 0.000000 0.015758 0.8735 0.0192 0.01007
383713075085501 Pi32-15 10/3/2018 15.60 9 0.0000 30.5532 18.4681 0.2690 0.6648 0.000000 0.015540 0.8771 0.0056 0.01004
383346075340301 Qd21-42 10/16/2018 16.00 35 0.0000 33.7830 20.2067 2.0274 0.6984 0.000000 0.017399 0.9667 0.0423 0.01064
383346075340301 Qd21-42 10/16/2018 16.00 35 0.0000 34.9999 20.4046 2.6175 0.7061 0.000000 0.018025 0.9762 0.0546 0.01075
382805075330301 Rd22-01 10/25/2018 15.40 49 0.0050 64.7183 22.1146 0.2301 0.7361 0.000184 0.032711 1.0464 0.0047 0.01108
382805075330301 Rd22-01 10/25/2018 15.40 49 0.0045 66.2592 21.6313 0.2385 0.7248 0.000163 0.033490 1.0235 0.0049 0.01091
382755075341501 Rd31-24 11/15/2018 14.30 49 0.0059 52.4434 21.5068 0.2743 0.7226 0.000210 0.025598 0.9968 0.0055 0.01063
382755075341501 Rd31-24 11/15/2018 14.30 49 0.0055 52.5961 21.2735 0.2540 0.7182 0.000195 0.025673 0.9860 0.0051 0.01057
382807075070701 Ri23-15 12/11/2018 15.40 4 0.0775 42.7689 20.1309 0.2630 0.6914 0.002836 0.021617 0.9525 0.0054 0.01040
382807075070701 Ri23-15 12/11/2018 15.40 4 0.0752 41.7015 20.2346 0.3057 0.6932 0.002753 0.021077 0.9574 0.0063 0.01043

Appendix 3. Groundwater-quality data for sampled public water-supply wells in the Columbia aquifer in Delaware, sampled in 2018—Age dating with sulfur hexafluoride data.

Table 3.1.    

Groundwater-quality data for sampled public water-supply wells in the Columbia aquifer in Delaware, sampled in 2018—Age dating with sulfur hexafluoride data.

[A total of 30 public water-supply wells were sampled in 2018. USGS, U.S. Geological Survey; DGS, Delaware Geological Survey; NOAA, National Oceanic and Atmospheric Administration; °C, degrees Celsius; mL, milliliters; fMol/L, fentomoles per liter; --, no data]

USGS site identification number Sample number DGS local well number Sampling date (MM/DD/YY) Sulfur hexafluoride (SF6) data
NOAA scale (fMol/L) Excess air (mL) Recharge temperature (°C) Elevation (feet) Estimated date of recharge1 Estimated age of groundwater (year)1 Comments
394100075334501 1 Cd52-40 09/26/18 2.1993 2.8 13.8 31 2002.5 16.2 --
394100075334501 2 Cd52-40 09/26/18 2.8895 2.8 13.8 31 2009.0 9.7 --
393928075440202 1 Db11-27 10/18/18 2.2615 2.4 12.7 79 2003.0 15.8 --
393928075440202 2 Db11-27 10/18/18 2.5839 2.4 12.7 79 2004.5 14.3 --
393916075440802 1 Db11-28 10/18/18 1.4374 3.4 12.7 71 1993.0 25.8 --
393916075440802 2 Db11-28 10/18/18 1.6702 3.4 12.7 71 1994.5 24.3 --
393739075394202 1 Dc31-15 09/26/18 1.8148 1.0 13.7 69 2002.0 16.7 --
393739075394202 2 Dc31-15 09/26/18 2.3839 1.0 13.7 69 2007.5 11.2 --
390538075325101 1 DE-KE 187731 11/19/18 2.2728 1.6 13.6 38 2005.0 13.9 --
390538075325101 2 DE-KE 187731 11/19/18 2.0348 1.6 13.6 38 2003.0 15.9 --
383101075141001 1 DE-SU 56105 12/11/18 0.7104 3.9 15.1 45 1985.0 33.9 --
383101075141001 2 DE-SU 56105 12/11/18 0.6437 3.9 15.1 45 1984.5 34.4 --
384139075230101 1 Georgetown 1 08/29/18 0.7349 3.7 14.2 100 1986.0 32.7 --
384139075230101 2 Georgetown 1 08/29/18 0.7951 3.7 14.2 100 1986.5 32.2 --
391747075364202 1 Hc34-03 09/25/18 1.6734 3.6 15.6 47 1996.5 22.2 --
391747075364202 2 Hc34-03 09/25/18 1.6693 3.6 15.6 47 1996.0 22.7 --
391060075282801 1 Ie42-03 08/15/18 2.1818 3.5 15.7 100 2000.5 18.1 --
391060075282801 2 Ie42-03 08/15/18 2.0580 3.5 15.7 100 1999.5 19.1 --
390703075371801 1 Jc33-12 10/17/18 0.2823 4.1 6.5 65 1975.0 43.8 --
390703075371801 2 Jc33-12 10/17/18 0.1484 4.1 6.5 65 1971.0 47.8 --
385522075251802 1 Le55-09 11/14/18 0.4087 4.0 14.3 25 1980.5 38.4 --
385522075251802 2 Le55-09 11/14/18 0.4997 4.0 14.3 25 1981.5 37.4 --
385448075341801 1 Md11-04 09/06/18 2.4578 2.3 12.1 100 2005.0 13.7 --
385448075341801 2 Md11-04 09/06/18 2.6963 2.3 12.1 100 2006.0 12.7 --
384818075354101 1 Nc25-37 08/22/18 1.5370 4.7 12.3 100 1991.5 27.1 --
384818075354101 2 Nc25-37 08/22/18 -- 4.7 12.3 100 1952.0 -- Bottle cracked-not analyzed
384819075190101 1 Ng21-03 11/01/18 1.0538 2.2 12.8 19 1990.5 28.3 --
384819075190101 2 Ng21-03 11/01/18 1.1624 2.2 12.8 19 1991.0 27.8 --
384856075151101 1 Ng25-04 11/01/18 0.5413 4.2 8.9 9 1980.5 38.3 --
384856075151101 2 Ng25-04 11/01/18 0.4791 4.2 8.9 9 1980.0 38.8 --
384526075091601 1 Ni51-32 08/21/18 1.2251 3.2 15.6 100 1992.0 26.6 --
384526075091601 2 Ni51-32 08/21/18 1.3179 3.2 15.6 100 1992.5 26.1 --
384428075355701 1 Oc15-11 09/10/18 1.3177 3.1 10.0 100 1991.5 27.2 --
384428075355701 2 Oc15-11 09/10/18 1.1335 3.1 10.0 100 1989.5 29.2 --
384428075135501 1 Oh12-07 10/31/18 0.6699 2.5 11.0 11 1984.5 34.3 --
384428075135501 2 Oh12-07 10/31/18 0.6414 2.5 11.0 11 1984.0 34.8 --
384322075051101 1 Oi25-18 09/27/18 2.6700 2.0 14.1 14 2007.5 11.2 --
384322075051101 2 Oi25-18 09/27/18 3.3588 2.0 14.1 14 2013.5 5.2 --
384326075050801 1 Oi25-19 09/27/18 2.4008 3.6 13.5 16 2001.5 17.2 --
384326075050801 2 Oi25-19 09/27/18 2.2765 3.6 13.5 16 2000.5 18.2 --
383823075382101 1 Pc22-06 08/16/18 0.9893 6.8 16.1 100 1987.0 31.6 --
383823075382101 2 Pc22-06 08/16/18 1.2720 6.8 16.1 100 1989.5 29.1 --
383815075271001 1 Pe23-185 10/29/18 0.2262 3.7 7.9 45 1974.0 44.8 --
383815075271001 2 Pe23-185 10/29/18 0.2120 3.7 7.9 45 1974.0 44.8 --
383732075191301 1 Pg31-12 08/27/18 1.5463 1.6 11.2 100 1997.0 21.7 --
383732075191301 2 Pg31-12 08/27/18 1.8065 1.6 11.2 100 1999.0 19.7 --
383729075101601 1 Ph35-25 10/03/18 1.7933 7.6 6.5 10 1989.5 29.3 --
383729075101601 2 Ph35-25 10/03/18 2.0591 7.6 6.5 10 1991.0 27.8 --
383914075080501 1 Pi12-11 10/24/18 1.9002 1.1 16.5 21 2003.5 15.3 --
383914075080501 2 Pi12-11 10/24/18 1.7669 1.1 16.5 21 2002.0 16.8 --
383713075085501 1 Pi32-15 10/03/18 1.2219 1.5 13.1 9 1992.5 26.3 --
383713075085501 2 Pi32-15 10/03/18 1.1681 1.5 13.1 9 1992.5 26.3 --
383346075340301 1 Qd21-42 10/16/18 2.0305 3.2 12.5 35 1997.5 21.3 --
383346075340301 2 Qd21-42 10/16/18 2.0272 3.2 12.5 35 1997.5 21.3 --
382805075330301 1 Rd22-01 10/25/18 1.6356 4.7 12.3 49 1992.5 26.3 --
382805075330301 2 Rd22-01 10/25/18 1.6343 4.7 12.3 49 1992.5 26.3 --
382755075341501 1 Rd31-24 11/15/18 1.8067 4.3 12.5 49 1994.5 24.4 Wrong cap
382755075341501 2 Rd31-24 11/15/18 -- 4.3 12.5 49 1952.0 -- Broken in shipping; not analyzed
382807075070701 1 Ri23-15 12/11/18 1.7413 3.4 13.5 4 1996.0 22.9 --
382807075070701 2 Ri23-15 12/11/18 1.7504 3.4 13.5 4 1996.0 22.9 --
1

Model ages corrected for excess air.

Conversion Factors

International System of Units to U.S. customary units

Multiply By To obtain
Length
centimeter (cm) 0.3937 inch (in.)
meter (m) 3.281 foot (ft)
kilometer (km) 0.6214 mile (mi)
Area
square kilometer (km2) 0.3861 square mile (mi2)
Volume
liter (L) 0.2642 gallon (gal)
Mass
gram (g) 0.03527 ounce, avoirdupois (oz)
kilogram (kg) 2.205 pound avoirdupois (lb)
Pressure
kilopascal (kPa) 0.009869 atmosphere, standard (atm)
atmosphere, standard (atm) 760 millimeters of mercury (mmHg)

Temperature in degrees Celsius (°C) may be converted to degrees Fahrenheit (°F) as follows:

°F = (1.8 × °C) + 32.

Temperature in degrees Fahrenheit (°F) may be converted to degrees Celsius (°C) as follows:

°C = (°F – 32) / 1.8.

Datum

Vertical coordinate information is referenced to the North American Vertical Datum of 1988 (NAVD 88).

Horizontal coordinate information is referenced to North American Datum of 1983 (NAD 83).

Supplemental Information

Specific conductance is given in microsiemens per centimeter at 25 degrees Celsius (µS/cm at 25 °C).

Concentrations of chemical constituents in water are given in either milligrams per liter (mg/L) or as part per trillion (ppt).

Abbreviations

DNREC

Delaware Department of Natural Resources and Environmental Control

EPA

U.S. Environmental Protection Agency

HAL

health advisory level

HBSL

health-based screening level

MCL

maximum contaminant level

NWIS

National Water Information System

PFAS

per- and polyfluorinated alkyl substances

PFOA

perfluorooctanoic acid

PFOS

perfluorooctane sulfonate

ppb

parts per billion

ppt

parts per trillion

SMCL

secondary maximum contaminant level

USGS

U.S. Geological Survey

For additional information, contact:

Director, Maryland-Delaware-D.C. Water Science Center

U.S. Geological Survey

5522 Research Park Drive

Catonsville, MD 21228

or visit our website at https://www.usgs.gov/centers/md-de-dc-water/

Publishing support provided by the West Trenton Publishing Service Center

Suggested Citation

Reyes, B., 2021, Occurrence and distribution of PFAS in sampled source water of public drinking-water supplies in the surficial aquifer in Delaware, 2018; PFAS and groundwater age-dating results: U.S. Geological Survey Open-File Report 2021–1109, 27 p., https://doi.org/10.3133/ofr20211109.

ISSN: 2331-1258 (online)

Study Area

Publication type Report
Publication Subtype USGS Numbered Series
Title Occurrence and distribution of PFAS in sampled source water of public drinking-water supplies in the surficial aquifer in Delaware, 2018; PFAS and groundwater age-dating results
Series title Open-File Report
Series number 2021-1109
DOI 10.3133/ofr20211109
Year Published 2021
Language English
Publisher U.S. Geological Survey
Publisher location Reston, VA
Contributing office(s) Maryland-Delaware-District of Columbia Water Science Center
Description Report: vii, 27 p.; Data Release; Database
Country United States
State Delaware
Online Only (Y/N) Y
Additional Online Files (Y/N) N
Google Analytic Metrics Metrics page
Additional publication details