Printed on recycled paper

Groundwater Chloride Concentrations in Domestic Wells 

and Proximity to Roadways in Vermont, 2011–2018

Introduction

There is growing evidence that deicing salt, in the form of 

sodium chloride, is affecting groundwater across New England 

(Mullaney and others, 2009; Lindsey and Rupert, 2012; Medalie, 2013; Pieper and others, 2018), which is often observed as 

increased chloride concentrations in groundwater (Torizzo and 

others, 2016; Robinson and others, 2017). Substantial increases 

were observed in sodium and chloride across the glacial aquifers 

of the United States and particularly in New England (Lindsey 

and Rupert, 2012; Medalie, 2013) during the past 30 to 40 years. 

The U.S. Geological Survey (USGS), in cooperation with the 

Vermont Department of Health (VDH), analyzed the concentrations of chloride in groundwater samples collected from 

4,319 domestic wells across Vermont between 2011 and 2018. 

Ninety wells were sampled twice to analyze temporal trends, 

and the remaining 4,229 wells were sampled once to evaluate 

geographical patterns.

There are multiple possible sources that can contribute 

chloride to groundwater. These include fertilizers 

commonly used in agriculture, animal waste from 

animal feeding operations, irrigation practices, evapotranspiration, landfill leachates, 

septic systems, and road deicing compounds applied to roadways or stored in 

salt sheds (Granato and others, 2015). 

Although the determination of the 

source of chloride to groundwater is 

outside the scope of this study, Heindel 

(1982) found an association between 

road deicing practices in Williston

and chloride in groundwater wells located 

within 100 meters (m) of roadways.

Increases in chloride in groundwater 

can increase the chloride-to-sulfate (Cl/SO4)-

mass ratio (Stets and others, 2018), which can raise 

the potential for water to become more corrosive and to 

leach lead and copper from plumbing. The Cl/SO4 ratio and 

alkalinity, or the ability of water to neutralize acid, play major 

roles in determining the potential to promote galvanic corrosion (PPGC)—an indicator of water corrosivity (Belitz and 

others, 2016).

Groundwater in Vermont has been characterized as potentially corrosive based on the PPGC and the Langelier saturation index (Belitz and others, 2016). Although these conditions are due in part to the natural chemistry of groundwater, 

increasing chloride concentrations are likely contributing to 

increased corrosivity of New England groundwater. Furthermore, 77 percent of homes in Vermont were built before 1989 

(U.S. Census Bureau, 2014) but it was not until 1986 that the 

Safe Drinking Water Act (SDWA) restricted the maximum 

allowable lead content in plumbing fixtures to be a weighted 

average of 8 percent in piping and 0.2 percent in solder and flux 

(Tiemann, 2017). When coupled with increasing chloride levels, 

corrosivity has the potential to cause metals, such as lead, to 

leach from water system components and possibly cause adverse 

human health effects.

Photograph by the U.S. Geological Survey

Highlights

• Chloride concentrations in groundwater were analyzed

from approximately 4,319 (or 4 percent) of private

wells in Vermont.

• Approximately 1 percent of groundwater samples had

chloride concentrations above the U.S. Environmental

Protection Agency’s secondary maximum contaminant

level (SMCL) of 250 milligrams per liter (mg/L), with a

maximum of 3,262 mg/L.

• More than 50 percent of groundwater samples had chloride concentrations below the laboratory reporting level

(RL) of 5 mg/L.

• Groundwater in wells closer to a paved road had substantially higher concentrations of chloride than wells

farther away and were also five times more likely to

exceed the SMCL.

• Only Chittenden and Grand Isle Counties

had chloride concentrations that exceeded

both the RL and SMCL at higher than 

average rates.

• High chloride concentrations

were generally in water from wells

closer to roads in high population

and urban areas; however, these

factors did not always explain high

concentrations of chloride, such as in

Grand Isle County.

U.S. Department of the Interior

U.S. Geological Survey

Open-File Report 2019–1148

April 2020
2

Photograph by the U.S. Geological Survey

Methods

Data Collection

The VDH compiled chloride concentration data in water 

from domestic wells and springs between 2011 and 2018 

throughout Vermont. These samples were voluntarily provided 

by the homeowners, analyzed by the VDH Laboratory, reported 

to well owners, and stored in a drinking water quality database. 

These results have never been analyzed in aggregate. Of the 

5,972 records available, 4,319 were determined to be water 

samples from untreated drinking water systems and were successfully georeferenced (fig. 1). The data include single chloride 

concentrations from 4,229 wells to determine the spatial distribution across the State. Ninety wells with two samples each—

one taken before the median sample date of September 21, 2015, 

and one taken after—were used for temporal analysis.

Spatial

The dataset (Levitt and Larsen, 2019) was checked by the 

USGS and the VDH for duplicate samples taken at a single site 

on the same day or at a single site on multiple dates and only sites 

with single sample dates were used when comparing the spatial 

variables (n=4,229; fig. 1). For each sample, the VDH computed 

the distance to the nearest paved road, percent urban and agricultural land use and population density of each well location (VCGI, 

2018), and 5-year tract-level household and family data (U.S. 

Census Bureau, 2018). Location information was then removed 

from the well data to keep well addresses anonymous.

Quality Control

Water samples were analyzed for chloride concentrations by 

the VDH Laboratory, which is certified by the State and the U.S. 

Environmental Agency (EPA). The laboratory uses EPA method 

300.0 for anions in drinking water (Pfaff, 1993) using an ion 

chromatography system to determine chloride concentrations. The 

standard sample sequence used by EPA method 300.0 includes 

analysis of samples for calibration, quality control, ongoing calibration checks, laboratory reagent blanks, fortification (spikes), 

and laboratory replicates to measure the accuracy and precision of 

results. Laboratory analysis shows that concentrations in replicate 

samples for chloride (fig. 2) have a relative percent difference of 

less than 10 percent.

Analysis of Data

Because a large part (60 percent) of the chloride concentration data were reported as less than 5 milligrams per liter 

(mg/L)—the reporting level (RL) in this study—nonparametric 

statistical methods (rank correlation and rank sum tests) and summary statistics were used to compare concentrations to land use 

and demographic variables. The Wilcoxon rank sum test (Helsel 

and Hirsch, 2002) was used to determine whether there was a statistically significant difference in chemical concentrations within 

and outside the 100-m buffer zone. For this study, a p-value of less 

than 0.01 was used to indicate a statistically significant difference.

Summary statistics and the percentage of domestic wells 

with chloride concentrations that exceeded the RL and the 

secondary maximum contaminant level (SMCL) in Vermont 

were also compared by county. A total of eight statistics were 
3

73° 72°

45°

44°

43°

0 15 30 KILOMETERS

0 10 20 30 MILES

Base from Vermont Department of Health digital data, 2018

Transverse Mercator projection

North American Datum of 1983

State planar feet Vermont FIPS 440

ME

VT

NH

MA

RI

CT

Map area

EXPLANATION

Well sampled only once—For this study, 

4,229 wells were sampled once

Well sampled twice—For this study, 

90 wells were sampled twice

U.S. highway

State highway

Shaded areas are delineations of counties

Figure 1. Location of wells sampled for chloride in Vermont from 2011 to 2018.
4

computed for each county (not including minimum, which was 

<5 mg/L for all counties): median, 75th, 90th, 95th, 99th, maximum (100th) percentiles, percent >RL, and percent >SMCL. To 

qualitatively compare the counties by overall chloride concentration, counties were divided into groups by chloride level. 

The chloride level (table 1) was defined as low when zero to 

two summary statistics were above the statewide value, moderate when three to five were above the statewide value, and high 

when more than six summary statistics were above the statewide 

value. This characterization allowed for further analysis of the 

distribution of chloride concentrations in domestic wells across 

the State, including how the concentrations may relate to road 

proximity, land use, and population density.

For this study, an equal-area (65-square-kilometers [km2

]) 

grid approach (Belitz and others, 2010) was used to characterize maximum chloride concentrations. The resulting map (fig. 3) 

was overlain with paved roadways to help visualize spatial patterns of chloride concentrations and high-chloride-concentration 

hotspots. In combination with the summary statistics and correlations related to geography, this method allows for statewide 

comparison of chloride concentrations and proxies for road proximity, such as population density and urban land cover. It also 

highlights areas of the State that are unsampled.

On the temporal scale, an increase, decrease, or no change was 

determined by subtracting the first chloride concentration at each 

well from the second for the 90 wells sampled before and after the 

median date of September 15, 2015. A change, no matter the magnitude, was considered an increase or decrease when the calculated 

difference was nonzero. When concentrations from both sampling 

dates at any well were censored (<RL), the result was considered 

“no change.” If the concentration at a well changed from below the 

reporting level (<RL) to above the reporting limit (>RL), this was 

an increase; if the converse was true, it was a decrease. These three 

groups (increase, decrease, and no change) were then compared to 

variables related to well setting and sample timing.

Figure 2. Replicate sample 

analysis of chloride concentrations 

for samples collected in Vermont 

from 2011 to 2018. The reporting level 

for chloride is 5 milligrams per liter 

(mg/L).

0.01

0.1

1

10

100

5 50 500

Relative percent difference

Chloride concentration, in mg/L
5

Results

Spatial

Equal-area grid analysis (fig. 3) of the 4,229 wells sampled 

once shows that high chloride concentrations in wells are widely 

distributed across the State but are generally closer to roads and 

urban centers. Unsampled grid cells (shown in white) are areas 

that could be candidates for future sampling efforts. Wilcoxon 

rank sum test results showed that wells closer than 100 m to 

roads had significantly (p<0.001) higher concentrations of chloride than those wells more than 100 m from roads (fig. 4).

Summary statistics and the percent of samples with concentrations of chloride greater than the RL (table 1) illustrate 

the differences among the counties. Thirteen out of 14 counties 

have at least one domestic well with a concentration of chloride 

higher than the EPA SMCL of 250 mg/L (U.S. Environmental 

Protection Agency, 2019). Chittenden and Grand Isle Counties 

had higher concentrations of chloride than statewide concentrations and are the only two counties with a greater percentage 

of wells with concentrations above both the RL and the SMCL 

than statewide percentages. Concentrations of chloride in wells 

in Chittenden County are related to distance to road but in 

Grand Isle County are not (table 2). In the counties with the 

strongest chloride correlation to distance-to-road, Washington 

(rho = −0.47, p<0.01) and Windham (rho = −0.46, p<0.01) had 

concentration distributions similar to the statewide chloride 

distribution (table 1).

Spearman rank correlation (Helsel and Hirsch, 2002) results 

for the entire State showed that wells closer to a paved road 

in densely populated and urban areas were significantly correlated with higher concentrations of chloride, with the strongest 

correlation related to proximity to roads (rho=−0.33; p<0.01). 

The application of road deicing salt and (or) contamination 

related to high population areas may influence the concentration 

Table 1. Summary statistics of chloride concentrations by county for 4,229 wells in Vermont from 2011 to 2018.

[Values in shaded cells represent county values greater than or equal to the State value for that column. Chloride level is designated as high, >6 shaded cells; 

moderate, 3 to 5 shaded cells; or low, less than (<) 3 shaded cells. Min, minimum; Max, maximum; %, percent; >, greater than; RL, reporting level; SMCL, 

secondary maximum contamination level; -—, not applicable]

County Number of samples Min Median 75% 90% 95% 99% Max % > RL % > SMCL Chloride level

Addison 474 <5 <5 15 51 88 225 705 42 0.8 Moderate

Bennington 141 <5 <5 12 41 77 118 189 42 0.0 Low

Caledonia 164 <5 <5 6 23 44 117 372 28 0.6 Low

Chittenden 1,057 <5 <5 16 53 116 300 3,262 45 1.6 High

Essex 34 <5 <5 <5 58 150 250 295 21 2.9 Moderate

Franklin 361 <5 <5 14 43 70 169 218 47 0.0 Low

Grand Isle 68 <5 23 65 116 223 614 885 78 4.4 High

Lamoille 242 <5 <5 15 41 60 198 270 42 0.4 Low

Orange 229 <5 <5 8 37 81 189 323 32 0.4 Low

Orleans 128 <5 <5 6 16 29 223 291 27 1.6 Low

Rutland 276 <5 <5 14 68 108 225 313 39 0.4 Moderate

Washington 497 <5 <5 11 58 97 402 1,028 35 1.8 Moderate

Windham 266 <5 <5 16 52 94 311 468 39 1.5 Moderate

Windsor 292 <5 <5 11 44 116 452 2,200 33 1.7 Moderate

Statewide 4,229 <5 <5 13 50 97 269 3,262 40 1.2 —

Photograph courtesy of the Vermont Agency of Transportation
6

FRANKLIN

ORLEANS

LAMOILLE

CHITTENDEN

WASHINGTON

CALEDONIA

ESSEX

ORANGE

WINDSOR

WINDHAM

RUTLAND

BENNINGTON

ADDISON

GRAND

ISLE

73° 72°

45°

44°

43°

Base from Vermont Department of Health digital data, 2018

Transverse Mercator projection

North American Datum of 1983

State planar feet Vermont FIPS 440

EXPLANATION

Maximum chloride concentration,

in mg/L

>250

≤250

≤50

≤10

≤5

No data

U.S. highway

State highway

ME

VT

NH

MA

RI

CT

Map area

0 15 30 KILOMETERS

0 10 20 30 MILES

Figure 3. Maximum chloride concentrations in Vermont from 2011 to 2018 by 65-square-kilometer equal-area 

grid cell. The reporting level for chloride is 5 milligrams per liter (mg/L).
7

1

10

100

1,000

10,000

Chloride, in milligrams per liter

<100 m >100 m

1,576 2,653

(689) (1,840)

RL

SMCL

EXPLANATION

1,576 Number of values

Largest value within 1.5 times

interquartile range above

75th percentile

75th percentile

50th percentile

(median)

25th percentile

Smallest value within 1.5 times

interquartile range below

25th percentile

Outlier value—Value is >1.5 and

<3 times the interquartile range

beyond either end of box

Far-outlier value—Value is ≥3 times

the interquartile range beyond

either end of box

Interquartile range

(21) Number of values <RL

Secondary maximum contaminant

level (SMCL)

Reporting level (RL)—Values <RL

are censored

Figure 4. Chloride concentrations from 4,229 domestic wells in 

Vermont sampled from 2011 to 2018, grouped by proximity (less 

than [<] or more than [>] 100 meters [m]) to nearest paved road. 

The U.S. Environmental Protection Agency’s secondary maximum 

contaminant level (SMCL) is 250 milligrams per liter (mg/L), and the 

reporting level (RL) is 5 mg/L.

Table 2. Median values of land cover variables and relation to chloride concentrations in Vermont from 2011 to 2018, by county.

[Values are medians by county. Values in red typeface denote that the correlation is significant (p<0.01) between chloride concentrations and the land use and 

demographic variable in each county. m, meter]

County

Number

of 

samples

Land cover variables

Interpretation

Median 

distance to 

nearest road 

(m)

Median

population 

density (within 

500-m radius)

Median 

percent urban 

land cover 

(within 500-m 

radius)

Median 

percent agricultural land 

cover (within 

500-m radius)

Addison 474 175 32 6 33 Agricultural, wells close to roads

Bennington 141 209 43 7 19 High population density, wells close to roads

Caledonia 164 604 24 7 12 Low population density

Chittenden 1,057 189 52 7 26 High population density, wells close to roads

Essex 34 362 25 9 3 Low population density, small sample size

Franklin 361 111 52 10 29 All variables important

Grand Isle 68 294 49 8 34 Agricultural, small sample size

Lamoille 242 322 45 9 20 All variables important

Orange 229 312 30 8 16 Low population density

Orleans 128 582 25 7 22 Low population density

Rutland 276 147 42 8 18 High population density, wells close to roads

Washington 497 413 39 9 13 All variables important

Windham 266 172 41 8 8 All variables important

Windsor 292 186 30 8 12 Wells close to roads

Statewide 4,229 220 41 8 19 All variables important
8

Photograph courtesy of the Vermont Agency of Transportation

of chloride in domestic wells. Agricultural sources may also 

contribute chloride to domestic wells, but concentrations were 

weakly correlated to agricultural land use (rho=0.15; p=<0.01). 

This analysis suggests that the proximity of wells to roads and 

high population density are indicators of elevated chloride 

concentrations at the State and county scale; however, correlations to land use and demographic variables can vary by 

county (fig. 5).

County-scale correlations between land use and demographic variables and chloride concentrations made it possible to 

determine potential sources of chloride (table 2). For most counties, concentrations of chloride correlated with distance-to-road 

and population density. Five of the 14 counties had correlations 

with agricultural land cover and only 4 counties had significant 

correlations with all the variables tested.

Three counties (Essex, Grand Isle, and Orleans), had no 

significant correlation between chloride concentrations and any 

of the variables tested. They represent counties with moderate, 

high, and low chloride levels (table 1), respectively. Essex and 

Grand Isle Counties had a relatively small sample size. Grand 

Isle had the highest chloride concentrations in the State (except 

for its maximum concentration) and also the highest median 

percent agricultural land cover. However, there was no correlation between agricultural land use and concentrations of chloride. Further investigation of Grand Isle’s water quality, geology, 

and well construction information may yield additional insight. 

Orleans County had generally low chloride concentrations 

(roughly 75 percent <RL), suggesting that high chloride concentrations in domestic wells are not widespread.

Chittenden County, which accounted for 25 percent of the 

wells used in this study, was the only county other than Grand 

Isle to have a high chloride level designation (table 1). Chittenden County also accounted for one third (17 out of 51) of the 

samples with chloride concentrations exceeding the SMCL and 

more than 40 percent (7 out of 17) of the towns that had median 

chloride concentrations greater than the statewide median. The 

percent agricultural land cover was higher and the percent urban 

land cover was lower at wells in Chittenden County than the 

statewide percentages.

Temporal

Several wells were sampled twice and used to analyze 

potential trends in chloride concentrations. Of the 90 wells sampled twice, 25 wells (28 percent) increased, 20 wells (22 percent) 

decreased, and 45 wells (50 percent) had no change. Forty-one 

of the 45 samples (91 percent) that had no change in chloride 

concentration had concentrations below the RL in both sampling 

events. The characteristics of the land use and demographic 

variables can be computed by category of change in chloride 

concentration and are shown in table 3.

Whereas the distance to the nearest road does not seem 

to greatly affect whether chloride concentrations increased or 

decreased, the wells with no change were more than three times 

farther away from a road. Most of these wells with no change 

had chloride concentrations below the RL, suggesting that wells 

far from roads are not often contaminated with chloride. Only 

22 percent of wells with no change were within the 100-m buffer zone to a paved road, but more than half the wells with an 

increase or decrease in concentration were within the 100-m 

buffer. This finding suggests that there may be a temporal or 
9

73° 72°

45°

44°

43°

EXPLANATION

Land cover

Open water

Developed urban, open space

Developed urban, low intensity

Developed urban, medium intensity

Developed urban, high intensity

Barren land (rock, sand, clay)

Deciduous forest

Evergreen forest

Mixed forest

Shrub/scrub

Grassland/herbaceous

Pasture/hay

Cultivated crops

Woody wetlands

Emergent herbaceous wetlands

Major roads

ORLEANS

GRAND

ISLE

CHITTENDEN

WINDSOR

WINDHAM

BENNINGTON

FRANKLIN

ESSEX

LAMOILLE

CALEDONIA

ORANGE

WASHINGTON

RUTLAND

ADDISON

ME

VT

NH

MA

RI

CT

Map area

Base from Vermont Department of Health digital data, 2018

Transverse Mercator projections

North American Datum of 1983

State Planar feet Vermont FIPS 440

0 15 30 KILOMETERS

0 10 20 30 MILES

Figure 5. Land cover in Vermont in 2011.
10

Photograph courtesy of www.pixabay.com

Table 3. Characteristics of variables for wells sampled in Vermont from 2011 to 2018 once before and once after September 15, 

2015, by category of change in chloride concentration.

[The September 15, 2015, date is the median sample date for all paired samples. m, meter; %, percent]

Category Number of 

samples

Percent inside 

100-m buffer to

nearest road (%)

Median distance 

to nearest paved 

road (m)

Median population 

density 

(within 500-m 

radius)

Median percent 

urban land cover 

(within 500-m radius)

Median percent agricultural land cover 

(within 500-m radius)

Increase 25 56 79 62 3.4 29

Decrease 20 60 58 47 3.1 22

No change 45 22 300 47 4.0 15

All 90 40 201 51 3.4 20

possibly seasonal component to chloride contamination in 

domestic wells near roadways. Population density also was 

higher than the overall statewide median for wells that increased 

in chloride concentration. The percent urban land cover around 

a well was the same for increases and decreases in chloride, but 

those wells with no change in chloride concentration had lower 

percent agricultural land cover than the statewide percentage. 

Further investigation of these relations may help reveal information about potential seasonality of chloride in domestic wells.

Implications

As an initial assessment, this report shows that chloride 

concentrations in domestic wells are related to urban indicators, 

such as the proximity of the wells to roadways, on a statewide 

scale (figs. 2 and 3) and that the strength of that relation varies by county (table 2). Chloride concentrations in water from 

domestic wells in Vermont are most strongly correlated to 

the proximity of the well to paved roads, urban land use, and 

population density. This finding may have implications for 

general drinking water testing recommendations for unregulated 

drinking water and provides information about possible deicing 

salt and urban runoff contamination. Determination of whether 

increasing chloride concentrations are related to increasing corrosivity in domestic wells and whether plumbing is susceptible 

to leaching of potentially harmful metals into drinking water is 

an important area of emerging public health concern.

With an estimated 258,535 households in Vermont as 

of 2017 (U.S. Census Bureau, 2017), approximately 80,000 

to 100,000 households (29 to 39 percent) or 180,000 to 

240,000 people use privately supplied well water as a main 

source of drinking water (Johnson and others, 2019; Vermont 

Department of Health, 2020). Estimates from this study suggest 

that at least 2,000 to 3,000 people are potentially using groundwater with chloride concentrations that exceed the EPA SMCL 

of 250 mg/L in Vermont (table 1). The SMCL for chloride is 

not human-health based but rather is a nonenforceable aesthetic 

level set by the EPA. However, of particular importance to 

human health, and somewhat overlooked, is the contribution of 

sodium to groundwater from salt application. For individuals 

on a salt-restricted diet, the EPA guidance level is 20 mg/L in 

drinking water (U.S. Environmental Protection Agency, 2003). 

Therefore, the effects of chloride-related corrosivity and the 
11

distribution of sodium concentrations in groundwater may warrant additional investigation.

Further analysis of land use and hydrogeology as it affects 

chloride concentrations in groundwater may be useful to better 

determine factors related to high chloride concentrations. Sampling near salt storage facilities, which play a major role in chloride contamination (Pieper and others, 2018), could be of use on 

a local scale. In addition, collecting data in unsampled areas and 

in counties with small sample sizes may strengthen the correlations between chloride concentrations and land use and demographic variables. Areas of further research might include using 

chloride to bromide mass ratios and flow studies to determine 

chloride sources to domestic wells near roadways, especially 

those with high chloride concentrations. Even though chloride 

concentrations are higher in domestic wells closer to roads 

in Vermont than in wells farther away from roads, it remains 

unclear if these concentrations have been increasing over time 

because of human effects or whether these elevated concentrations may relate to adverse human health outcomes.

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Photograph by the U.S. Geological Survey

Acknowledgments

The authors are grateful to the personnel at the Vermont 

Department of Health Laboratory and the citizens of Vermont 

whose data were used in this report. This publication was supported by cooperative agreement 5 NUE2EH001344, funded 

by the Centers for Disease Control and Prevention.

By Joseph P. Levitt1 and Sille L. Larsen2

1

U.S. Geological Survey.

2

Vermont Department of Health.

Suggested citation:

Levitt, J.P., and Larsen, S.L., 2020, Groundwater chloride concentrations

in domestic wells and proximity to roadways in Vermont, 2011–2018: 

U.S. Geological Survey Open-File Report 2019–1148, 12 p.,

https://doi.org/10.3133/ofr20191148.

For more information, contact

Director, New England Water Science Center

U.S. Geological Survey

10 Bearfoot Road

Northborough, MA 01532

dc_nweng@usgs.gov

or visit our website at

https://www.usgs.gov/centers/new-england-water

ISSN 2327-6916 (print)

ISSN 2327-6932 (online)

https://doi.org/10.3133/ofr20191148