Prepared in cooperation with the New Hampshire Department of Environmental Services

Concentrations of Chloride and Sodium in Groundwater in 

New Hampshire From 1960 Through 2011

Median chloride concentrations were at least 11⁄2 times higher and sodium concentrations

at least 3 times higher between 2000–11 than in all previous decades.

Introduction

Several studies from the 1970s 

and more recently (for example, Hall 

(1975), Daley and others (2009), and 

Mullaney (2009)) have found that 

concentrations of chloride and sodium 

in groundwater in New Hampshire 

have increased during the past 50 years.

Increases likely are related to road salt 

and other anthropogenic sources, such 

as septic systems, wastewater, and 

contamination from landfills and saltstorage areas. According to water-quality data reported to the New Hampshire 

Department of Environmental Services 

(NHDES), about 100 public water 

systems (5 percent) in 2010 had at least 

one groundwater sample with chloride concentrations that were equal to 

or exceeded the U.S. Environmental 

Protection Agency (USEPA) secondary 

maximum contaminant level (SMCL) 

of 250 milligrams per liter (mg/L) 

before the water was treated for public 

consumption. The SMCL for chloride is 

a measurement of potential cosmetic or aesthetic effects of chloride 

in water. High concentrations of chloride and sodium in drinkingwater sources can be costly to remove.

A new cooperative study between the U.S. Geological Survey 

(USGS) and the NHDES (Medalie, 2012) assessed chloride and 

sodium levels in groundwater in New Hampshire from the 1960s 

through 2011. The purpose of the study was to integrate all data 

on concentrations of chloride and sodium from groundwater in 

New Hampshire available from various Federal and State sources, 

including from the NHDES, the New Hamsphire Department of 

Health and Human Services, the USGS, and the U.S. Environmental Protection Agency (USEPA), for public and private (domestic) 

wells and to organize the data into a database. Medalie (2012) 

explained the many assumptions and limitations of disparate data 

The amount of a substance present in a given volume of water is often 

expressed as a concentration in mg/L.

The U.S. Environmental Protection Agency drinking water 

regulations specify the following:

• Chloride: SMCL of 250 mg/L

• Sodium: DWA for individuals

on a sodium-restricted diet of

20 mg/L

that were collected to meet wide-ranging 

objectives. This fact sheet summarizes 

the most important findings of the data.

Overview of Significant 

Findings

Individual wells that had concentrations reported in both 1995 and 2005 

were used to subset the data to show 

meaningful changes over time. The 

comparison of median concentrations 

of chloride (fig. 1) and sodium (fig. 2) 

by town for 1995 and 2005 shows a 

shift towards increased concentrations 

in 2005. The median is the middle 

value in the dataset after the data have 

been ordered by magnitude. Between 

1995 and 2005, the number of towns 

with median concentrations of chloride 

greater than 20 mg/L almost doubled 

from 35 to 65, and the number of towns 

with concentrations of chloride greater 

than 40 mg/L more than tripled from 

10 to 34. Over the same time span, 

the number of towns with concentrations of sodium greater than 

10 mg/L increased from 89 to 114, and the number of towns with 

concentrations greater than 20 mg/L increased from 26 to 43. The 

numbers themselves are not as important as their utility in describing increases over time.

Table 1 shows that the 17 mg/L median concentration of 

chloride in the 2000s was at least one and a half times higher than 

the median for all previous decades. The 90th percentile (highest 

10 percent of values) of concentration for 2000–11 was higher than 

for all decades and double all the values for the 1980s and 1990s. 

Low concentrations among decades, shown by the 10th percentile 

(lowest 10 percent of values), cannot be compared because many 

of the reported values were below a reporting threshold (indicated 

with “<”). To compare data with different reporting thresholds, 

all values below the reporting threshold (censored values) were 

adjusted to the common value of 10 mg/L, which was the highest 

censored value among the data.

Most of the percentiles of concentrations of sodium for 

data through the 1990s were less than 10 mg/L; however, all the 

percentiles were greater than 10 mg/L in the 2000s. Between 

the 1990s and 2000s, all percentiles of concentrations of sodium 

increased—the median by more than a factor of three, and the 

90th percentile by a factor of six. These data provide substantial 

evidence that sodium concentrations in New Hampshire groundwater for 2000–11 increased considerably from previous decades. 

5

EXPLANATION

Median chloride

concentration,

in milligrams per liter

No data

0 - 10

> 10 - 20

> 20 - 40

> 40

Three or fewer

values used to

calculate median

Chloride

1995 200

Figure 1. Median concentrations of chloride in groundwater 

in New Hampshire, using well data reported in 1995 and 2005. 

Differences between the maps illustrate changes in chloride 

concentrations by town. Towns with three or fewer values 

are flagged so the reader can get a sense of the distribution 

of data abundance. For this comparison, 687 records were 

used out of the 41,500 chloride records assembled for 

this project. 

U.S. Department of the Interior

U.S. Geological Survey

Photograph by Steven Corsi,

U.S. Geological Survey

Fact Sheet 2013–3011

February 2013
Table 1. Summary statistics for chloride and sodium in groundwater in New Hampshire.

[Data for concentrations of chloride and sodium reflect the median (50th percentile) for each source (well) for each decade. mg/L, milligrams per liter; SMCL, secondary maximum contaminant level; USEPA, U.S. Environmental Protection Agency; DWA, drinking water advisory; <, less than]

Statistic1 1960s 1970s 1980s 1990s 2000s2 All

Chloride

Concentrations, in mg/L:

10th percentile <10 <10 <10 <10 <10 <10

50th percentile 10 11 10 <10 17 <10

90th percentile 88 77 60 61 122 79

Number of median observations 114 402 2,227 2,503 1,671 6,917

Number censored 0 159 640 831 102 1,732

Observations with concentrations higher than the 250 mg/L SMCL3

, in percent 7 3 1 3 3 3

Sodium

Concentrations, in mg/L:

10th percentile NA <10 <10 <10 19 <10

50th percentile NA <10 <10 <10 30 <10

90th percentile NA <10 <10 15 89 40

Number of median observations NA 183 1,616 2,542 1,785 6,126

Number censored NA 0 157 654 0 811

Observations with concentrations higher than the 20 mg/L USEPA DWA3

, in percent NA 13 23 23 30 27 1

The 10th, 50th, and 90th percententile are defined as the values below which 10, 50, or 90 percent, respectively, of the observations fall. 2

Includes 2011.

3

Observations include data for private wells in the New Hampshire Department of Health and Human Services State Laboratory database, which are not included with the summary statistics.

The percentage of observations with 

concentrations higher than the 20-mg/L 

USEPA drinking water advisory (DWA) 

for individuals on a sodium-restricted 

diet for sodium increased from 13 

percent during the 1970s to 30 percent 

during the 2000s. Higher concentrations 

of chloride and sodium in 2005 were 

especially pronounced in the southeastern part of the State (figs. 1 and 2).

Selected References

Daley, M.L., Potter, J.D., and 

McDowell, W.H., 2009, Salinization of urbanizing New Hampshire 

streams and groundwater—Effects 

of road salt and hydrologic variability: Journal of North American 

Benthological Society, v. 28, no. 4, p. 

929–940.

Hall, F.R, 1975, Chloride in natural 

waters of New Hampshire, station 

bulletin 504, New Hampshire agricultural experiment station: Durham, New Hampshire, University 

of New Hampshire, 25 p.

Medalie, Laura, 2012, Temporal and spatial trends of chloride and 

sodium in groundwater in New Hampshire, 1960–2011: U.S. 

Geological Survey Open-File Report 2012–1236, 25 p., accessed 

January 31, 2013, at http://pubs.usgs.gov/of/2012/1236/.

Mullaney, J.R., Lorenz, D.L., and Arntson, A.D., 2009, Chloride in 

groundwater and surface water in areas underlain by the glacial 

aquifer system, northern United States: U.S. Geological Survey 

Scientific Investigations Report 2009–5086, 41 p., accessed 

January 31, 2013, at http://pubs.usgs.gov/of/2009/5086.

New Hampshire Department of 

Environmental Services, 2008, New 

Hampshire road salt reduction initiative: New Hampshire Department of 

Environmental Services, accessed 

January 31, 2013, at http://des.

nh.gov/organization/divisions/water/

wmb/was/salt-reduction-initiative/

index.htm.

New Hampshire Department of Environmental Services, 2010, Sodium 

and chloride in drinking water: New 

Hampshire Department of Environmental Services Environmental 

Fact Sheet WD–DWGB–3–17, 3 p., 

accessed January 31, 2013, at http://

des.nh.gov/organization/commissioner/pip/factsheets/dwgb/documents/dwgb-3-17.pdf.

By Laura Medalie

0 - 5

> 5 - 10

> 10 - 20

> 20

EXPLANATION

Median sodium

concentration,

in milligrams per liter

Three or fewer

values used to

calculate median

No data

Sodium

1995 2005

Figure 2. Median concentrations of sodium in groundwater 

in New Hampshire, using well data reported in 1995 and 

2005. Differences between the maps illustrate changes 

in sodium concentrations by town. Towns with three or 

fewer values are flagged so the reader can get a sense of 

the distribution of data abundance. For this comparison, 

648 records were used out of the 30,800 sodium records 

assembled for this project.

Photo by Dennis Adams,

courtesy of National Scenic Byways Online (www.byways.org).

For More Information, Contact:

U.S. Geological Survey

New England Water Science Center, New Hampshire-Vermont Office

331 Commerce Way, Suite 2, Pembroke, NH 03275

http://nh.water.usgs.gov Photograph by Steven Corsi,

U.S. Geological Survey