Prepared in cooperation with the New Hampshire Department of Environmental Services

Temporal and Spatial Trends of Chloride and Sodium in 

Groundwater in New Hampshire, 1960–2011

By Laura Medalie

Open-File Report 2012–1236

U.S. Department of the Interior

U.S. Geological Survey
U.S. Department of the Interior

KEN SALAZAR, Secretary

U.S. Geological Survey

Marcia K. McNutt, Director

U.S. Geological Survey, Reston, Virginia: 2012

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Suggested citation:

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., 

available at http://pubs.usgs.gov/of/2012/1236/.

Any use of trade, product, or firm names is for descriptive purposes only and does not imply 

endorsement by the U.S. Government.

Although this report is in the public domain, permission must be secured from the individual 

copyright owners to reproduce any copyrighted material contained within this report.
iii

Acknowledgments

The author is grateful to the following individuals who provided data: Patricia Bickford of the New 

Hampshire Department of Health and Human Services and Laurie Cullerot, Diana Morgan, and Gail 

Dailey of the New Hampshire Department of Environmental Services Drinking Water and Groundwater 

Bureau. Selina Makofsky and Tricia Madore of the Drinking Water and Groundwater Bureau provided 

assistance with inquiries about the data. Guidance was provided by Pierce Laskey-Rigrod, Stephen Roy, 

and Paul Susca of the Drinking Water and Groundwater Bureau. 
iv

Contents

Abstract ......................................................................................................................................................................... 1

Introduction.................................................................................................................................................................... 1

Data and Methods ......................................................................................................................................................... 3

Data Limitations and Assumptions............................................................................................................................. 3

Identification of Data Less Than the Reporting Limit .............................................................................................. 3

Extrapolation of Dataset for Values Less Than the Reporting Limit........................................................................ 4

Exclusion of Results From Treated Wells............................................................................................................... 5

Sample Representativeness................................................................................................................................... 5

Data Analysis ............................................................................................................................................................. 6

Spatial and Temporal Trends in Concentrations............................................................................................................ 7

Chloride...................................................................................................................................................................... 7

Sodium....................................................................................................................................................................... 9

Summary ..................................................................................................................................................................... 10

References Cited......................................................................................................................................................... 11

Appendix 1. Description of the Database for Salt in New Hampshire Groundwater..................................................... 13

Figures

1. Index map of New Hampshire, showing county boundaries, major roads, and regions as used in 

this report........................................................................................................................................................ 15

2. Maps showing median concentrations of chloride in water samples from wells by town in New Hampshire in 

1985, 1995, and 2005..................................................................................................................................... 16

3. Map showing differences in median concentrations of chloride at individual wells in New Hampshire for 

data before and after January 1, 1995............................................................................................................ 17

4. Boxplots of median concentrations of chloride in groundwater by source in New Hampshire, grouped by 

decade and region.......................................................................................................................................... 18

5. Maps showing median concentrations of sodium in water samples from wells by town in New Hampshire in

1985, 1995, and 2005..................................................................................................................................... 19

6. Map showing differences in median concentrations of sodium at individual wells in New Hampshire for data 

before and after January 1, 1995.................................................................................................................... 20

7. Boxplots of median concentrations of sodium in groundwater by source in New Hampshire for two 

decades by region .......................................................................................................................................... 21

Tables

1. Sources of data for concentrations of chloride and sodium in groundwater in New Hampshire...................... 22

2. Sources of data for public water system wells in the U.S. Geological Survey New Hampshire salt 

database......................................................................................................................................................... 23

3. Summary statistics for median concentrations of chloride and sodium in groundwater by source (well) in 

New Hampshire grouped by A, decade and B, region.................................................................................... 24

4. Change in median concentrations of chloride before and after January 1, 1995. ........................................... 25

5. Change in median concentrations of sodium before and after January 1, 1995. ............................................ 25
v 

Conversion Factors and Datum

Multiply By To obtain

Length

inch (in.) 2.54 centimeter (cm)

foot (ft) 0.3048 meter (m)

mile (mi) 1.609 kilometer (km)

Area

square foot (ft2) 0.09290 square meter (m2)

square mile (mi2) 2.590 square kilometer (km2) 

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

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

Concentrations of chemical constituents in water are given in milligrams per liter (mg/L).

Abbreviations

Cl chloride

DWGB Drinking Water and Groundwater Bureau

IOC inorganic contaminant

Na sodium

NHDES New Hampshire Department of Environmental Services

NHDHHS New Hampshire Department of Health and Human Services

NHDOT New Hampshire Department of Transportation

PWS public water system

RL reporting limit

SMCL secondary maximum contaminant level

SL New Hampshire Department of Health and Human Services State Laboratory

USEPA U.S. Environmental Protection Agency

USGS U.S. Geological Survey
1 

Temporal and Spatial Trends of Chloride and Sodium in 

Groundwater in New Hampshire, 1960–2011

By Laura Medalie

Abstract

Data on concentrations of chloride and sodium in groundwater in New Hampshire were 

assembled from various State and Federal agencies and organized into a database. This report provides 

documentation of many assumptions and limitations of disparate data that were collected to meet wideranging objectives and investigates temporal and spatial trends of the data. Data summaries presented in 

this report and analyses performed for this study needed to take into account the 27 percent of chloride 

and 5 percent of sodium data that were censored (less than a reporting limit) at multiple reporting limits 

that systematically decreased over time. Throughout New Hampshire, median concentrations of chloride 

were significantly greater during 2000-2011 than in every decade since the 1970s, and median 

concentrations of sodium were significantly greater during 2000-2011 than during the 1990s. Results of 

summary statistics showed that the 50th, 75th, and 90th percentiles of the median concentrations of 

chloride and sodium by source (well) from Rockingham and Strafford counties were the highest in the 

State; and the 75th and 90th percentiles from Carroll, Coos, and Grafton counties were the lowest. Large

increases in median concentrations of chloride and sodium for individual wells after 1995 compared 

with concentrations for years before were found in parts of Belknap and Rockingham counties and in 

small clusters within Carroll, Hillsborough, and Merrimack counties. 

Introduction

Several studies suggest that concentrations of chloride and sodium in ambient groundwater in 

New Hampshire have increased from the advent of the 20th century, ostensibly from road salt and other 

anthropogenic sources. Increases in concentrations of chloride in natural waters from approximately 

1945 through 1970 were documented in Hall (1975) where it was noted that increasing concentrations 

coincided with increased road deicing salt applications. Other studies in New Hampshire have shown 

that imported road salt may have long-term effects on water quality by raising baseflow concentrations 

of chloride and sodium due to storage in shallow (Harte and Trowbridge, 2010) and deep (Emery & 

Garrett Groundwater, Inc., 2007) aquifers. In a regional study of the northern United States, upward 

trends associated with use of deicing salt for roads and parking areas, number of septic systems, volume 

of wastewater discharge, and saline groundwater plumes from landfills and salt storage areas were seen 

in chloride loads (Mullaney and others, 2009). In a study of 56 well networks representing 22 principal 

aquifers nationwide, the New England coastal basins, which encompasses more than half of New 

Hampshire’s area, had the second largest median decadal increases in concentrations of chloride 

between 1988 and 2010 (Lindsey and Rupert, 2012). 

The U.S. Environmental Protection Agency (USEPA) has established a nonenforceable 

secondary maximum contaminant level (SMCL) of 250 milligrams per liter (mg/L) for chloride (for 

potential cosmetic or aesthetic effects) and a nonregulatory drinking water advisory concentration of 
2 

20 mg/L for sodium for individuals on a 500-milligram-per-day sodium-restricted diet (U.S. 

Environmental Protection Agency, 2011). The concentration of chloride, when associated with sodium, 

for the protection of freshwater aquatic organisms is 230 mg/L averaged over 4 days no more than once 

every 3 years on average (U.S. Environmental Protection Agency, 1988). In addition, sodium is 

included on the drinking water contaminant candidate list because of its possible association with 

hypertension in some individuals (U.S. Environmental Protection Agency, undated). The New 

Hampshire Department of Environmental Services (NHDES) receives water quality sample results 

submitted on behalf of public water systems (PWSs) to determine compliance with State (New 

Hampshire Department of Environmental Services, 2008) and Federal drinking water requirements. 

According to compliance water quality monitoring data reported to the NHDES between 1988 

and 2010, about 100 PWSs had at least one groundwater sample with concentrations of chloride equal to 

or exceeding 250 mg/L (Laurie Cullerot, New Hampshire Department of Environmental Services 

Drinking Water and Groundwater Bureau, written commun., July 22, 2011). Increasing concentrations 

of chloride and sodium in drinking water sources may require substantial changes to infrastructure for 

finding alternative water supply sources, diverting stormwater drainage, blending waters from wells or 

inducing artificial recharge (dilution), or abandoning public supply wells. Each of these options could 

involve significant cost and in some cases may not be viable. The need to find alternative water sources 

because of elevated concentrations of chloride and sodium due to road salt application along State 

highways has been seen occasionally in private wells. For example, the New Hampshire Department of 

Transportation (NHDOT) database on private wells that have been replaced because of concentrations 

of chloride exceeding 250 mg/L documents more than 500 such cases from 1973 through 2010 (Philip 

Huntley, New Hampshire Department of Transportation, written commun., March 11, 2011). 

The sources of groundwater for the dataset assembled for this study are primarily the fractured 

bedrock and stratified drift (overburden) aquifers of New Hampshire. Out of 5,544 untreated and 

unblended wells in the database created for this project, 61 percent are classified as bedrock, 16 percent 

are overburden, 22 percent are unclassified, and the remaining 1 percent is springs, infiltration, or 

artesian wells. From 1984 through 2007, static water levels at bedrock wells in the State decreased by 

about 13 feet, a change that is possibly related to synchronous changes in water use, well yield, well 

depth and construction, well position on the landscape, land use, or climate change (Ayotte and others, 

2010). Any of these factors could be related to changes in concentrations of chloride and sodium. 

A comprehensive assessment of chloride and sodium in groundwater in New Hampshire is 

needed for prioritizing and allocating resources to maintain high-quality groundwater from public and 

private water supply wells and for other purposes, such as supporting aquatic life in surface waters. The 

primary objective of this study is to describe spatial and temporal trends in chloride and sodium in 

groundwater used or potentially used for drinking water supplies in New Hampshire. A secondary 

objective is to assemble an integrated and thoroughly documented dataset of concentrations of chloride 

and sodium in groundwater in New Hampshire as a foundation for a comprehensive assessment. 

The purpose of this report is to integrate all data on concentrations of chloride and sodium from 

groundwater in New Hampshire available from the NHDES, the New Hampshire Department of Health 

and Human Services (NHDHHS), the NHDOT, the USEPA, and the U.S. Geological Survey (USGS) 

that meet quality assurance conditions for this study. Depending on different types of analyses and 

depictions, median concentrations are shown for individual wells, towns, decades, or regions in the State 

(fig. 1). For the purposes of this study, the State was divided into five regions, as follows: region 1, 

northern New Hampshire, included Carroll, Coos, and Grafton Counties; region 2, central New 

Hampshire, was Belknap County; region 3, south-central New Hampshire, included Hillsborough and 

Merrimack Counties; region 4, southwestern New Hampshire, included Cheshire and Sullivan Counties; 
3 

and region 5, southeastern New Hampshire, included Rockingham and Strafford Counties. Quality 

assurance conditions include having adequate documentation or assurances from data providers to be 

judged comparable and suitable for inclusion. Only data from single-source wells before water treatment 

were included in most of the summaries and analyses for this report. Data from contaminated sites were 

not used. Most of the available data were from samples collected between the late 1980s and 2011, 

although there also are some data from the 1960s and 1970s and a few samples from the 1950s. 

Data and Methods

The goal of data compilation was to collect all data on concentrations of chloride and sodium in 

untreated single sources of ambient groundwater in the State of New Hampshire into a single database 

usable for statistical summaries and analyses. The term ambient is used to describe background 

groundwater, as opposed to groundwater contaminated by a known contaminant source, such as a 

landfill or toxic spill. Because of the many differences among data provided from the various sources, 

this section and appendix 1 (which describes the database created for this project) provide a detailed 

documentation and description of data sources, assumptions, and limitations that are essential to 

understand the data and the analytical tools that were used to summarize the data. Table 1 lists the 

sources of data that were compiled for the database for salt data in New Hampshire (NH salt database).

Because several terms—detection limit, quantitation limit, reporting limit—each with a specific 

definition, were used for the lower limit of data that delineate the censoring threshold in the various 

sources of data, for uniformity in this report, the censoring threshold is referred to as a general reporting 

limit (RL). The effect for data summaries or statistics is the same regardless of how censoring is 

determined. Use of the term reporting limit also recognizes that low-concentration data provided from 

the various agencies in all cases were qualified with the <(less than) censoring symbol without a more 

specific definition.

Data Limitations and Assumptions

Data on concentrations of chloride and sodium that were provided to the USGS by the NHDES 

originally had been collected to serve compliance or storage purposes. This section of the report 

thoroughly documents the data and describes limitations and assumptions that were needed in order to 

transform data from the disparate sources into a standardized and consistent dataset suitable for analysis. 

Identification of Data Less Than the Reporting Limit 

The NHDES Drinking Water and Groundwater Bureau (DWGB) and NHDHHS State 

Laboratory (SL) databases were each developed to serve a different function—regulatory compliance 

(DWGB) or repository of results for samples analyzed at the SL. The two databases provided most of 

the data on concentrations of chloride and sodium for public water systems in the NH salt database. 

Input of data for sodium and chloride into the two original databases depended on the sample collection 

date, concentration in relation to the RL, and whether the public water system sample was analyzed at 

the SL or at a private laboratory (table 2). Beginning in 2008, chemical results for public water systems 

analyzed at the SL and initially in the SL database were transferred to the DWGB database. Also in 

2008, the NHDES developed the Data Warehouse database to receive sample results through online 

reporting. By 2011, all private laboratories were reporting sample results online directly to the Data 

Warehouse. After review by the DWGB program, results from the Data Warehouse are transferred into 

the DWGB database. 
4 

The NHDES historically entered into the DWGB database all results from inorganic contaminant 

(IOC) samples, including chloride and sodium, that were greater than the RL and that were processed at 

private laboratories. Results with values less than the RL were not available in an electronic database 

and therefore were estimated by an extrapolation method explained in the next section. Results of public 

water system samples analyzed at the SL were obtained from either the SL or DWGB databases. Before 

2000, the NHDES protocol (developed to enable close oversight by DWGB staff) for entering IOC 

results of samples processed at the SL was to enter results equal to or greater than 50 percent of the 

SMCL (125 mg/L) into the DWGB database. Because the SL was a more complete repository, including 

all results less than and greater than the RL, results before 2000 were obtained from the SL database. 

Beginning in 2001, the NHDES entered all results from SMCL samples, including chloride and sodium, 

equal to or greater than the RL into the DWGB database. Because data for the sources of the samples 

(wells) in the DWGB database are identified by a numerical field rather than a descriptive field as is the 

case in the SL database, making it easier to match to data already in the NH salt database, results for 

samples analyzed at the SL from 2001 through 2008 were obtained from the DWGB database rather 

than the SL database. As with results for public water systems that were analyzed at private laboratories, 

results for samples analyzed at the SL that were less than the RL had to be estimated and added in a 

separate step.

Extrapolation of Dataset for Values Less Than the Reporting Limit

Because of the interest in documenting changes over time in ambient conditions where the entire 

range of concentrations of chloride and sodium is important, results had to be deduced when samples 

with resulting concentrations less than the RL were collected and analyzed but were not entered into a 

database. A result of “less than the RL” is valuable information for the purpose of statistical summaries 

and trends. There is a need to supplement the datasets for sample results that were less than the RL 

(non-detects) and analyzed at private laboratories before 2010 or at the SL between 2001 and 2008. 

Non-detects were deduced by examining records of all chemical tests on samples from PWSs for 

IOCs available from the DWGB database. All groundwater sources of PWSs are required to have IOC 

tests every 3 years to comply with State regulations (New Hampshire Department of Environmental 

Services, 2011). If the values of all individual constituent results for the IOC test were less than the RL, 

then there would still be an IOC record for that water sample showing “N” in the “Detect” field. If an 

IOC record existed for a given groundwater sample with no corresponding result for chloride or sodium, 

then it was inferred that the sample was tested for chloride and sodium and had a resulting concentration 

less than the RL. In these cases, records for chloride and sodium were added to the NH salt database 

with the value of the RL in the “Concentration” field of the ChemResults (chemical results) table and 

“N” in the “Remark” field to flag the data as being at a nondetectable level. These added records were 

given the entry of “DW–IOC” in the “Data source” field.

The value of the RL varied depending on the analytical method used and on protocols defined in 

laboratory-specific quality assurance plans. Different laboratories may use different methods for the 

same constituent and their methods may change over time. In order to add records to the NH salt 

database for results for chloride or sodium that are inferred to be less than the RL, a value of the RL 

needed to be assigned. In some cases, the RL for a given constituent for a given laboratory at a 

particular period of time could be presumed by examining reported data that included that laboratory’s 

value for the RL. For other cases of missing values for RLs or where added records were from the data 

source designated “DW–IOC,” the default value of 3 was assigned as the RL for chloride and for 

sodium samples analyzed before January 1990; the default value of 1 was used for sodium samples 

analyzed beginning in January 1990. These are conservative values for the RL (many values for RL 
5 

were lower and none were higher) that were reported with results of samples analyzed at the SL for the 

time that covered the scope of this report (Patricia Bickford, New Hampshire Department of Health and 

Human Services, written commun., November, 2011). 

Exclusion of Results From Treated Wells

Because one goal of this project was to provide statistics of concentrations of chloride and 

sodium in ambient groundwater in New Hampshire, it was important to exclude sample results for 

treated water. Some PWSs treat groundwater before distribution. The DWGB and SL databases use two 

fields (“source ID” and “source/entity type”) to indicate whether a PWS sample is from a treated source. 

However, when a given PWS has records for both treated and groundwater sources, there may be cases 

where records that appear as groundwater sources show results for treated water that is not raw water. 

Without looking at paper files or being familiar with the PWS, there is no way to tell from the DWGB 

or SL databases at what point in the distribution system the water sample was collected relative to the 

addition of treatment.

A conservative approach was used to infer which samples may have been treated. The date field 

“Treatment added” was added to the ChemResults table in the NH salt database and populated with 

dates provided by NHDES (Diana Morgan, New Hampshire Department of Environmental Services, 

written commun., October 2011). Samples collected after the date that treatment was added were

disregarded for graphical depictions and statistical analyses in this report because they may have been 

treated. It is possible that valid samples were omitted because of this assumption. For many PWSs, 

treatment has been added only in recent years; thus, most systems with long-term records were not 

affected greatly by loss of data.

Sample Representativeness

The data that populate the NH salt database consist primarily of samples that were collected to 

satisfy regulatory programs. They are not a random representation of water quality of groundwater in 

New Hampshire. For example, much of the data are from public supply wells, but many households and 

commercial or industrial facilities across the State that are self-supplied do not use water from public 

supply wells. Only about one-third of statewide water withdrawals for public supply were from 

groundwater in 2000 (with the other two-thirds coming from surface water sources) according to the 

USGS National Water-Use Information Program (U.S. Geological Survey, undated). Furthermore, the 

percentage of withdrawals for public supply from groundwater ranged widely for individual counties 

from about 18 percent in Hillsborough County to as much as 80 percent in Carroll County. These 

percentages have not changed substantially over time. 

Conversely, the data for samples collected from private wells in the SL database reflect growth 

and development patterns for nonurban parts of the State or for areas not served by PWSs; thus they do 

not represent a uniform statewide coverage of information. Because of confidentially issues, locations of 

wells were not provided more specifically than by town name. As a result, it was not possible to 

determine whether each record was from a unique well. Also, it was not possible to determine whether 

chemical results from data in the SL database were affected by normal ion exchange or backwash 

generated from household water softeners.

Two additional cautions come with the nature of the data from diverse sources. The first is that 

the composition of individual wells in the decadal time bins is not constant over time. In other words, 

the set of individual wells with data values in the 1990s differed from the set with data values in the 

2000s. If the set of wells with data was constant over time, the statistical conclusions for the temporal 

comparison would be stronger. The second is that, while the method of extrapolating data for values less 
6 

than the RL was based on solid assumptions, it created a time bias in the dataset for the low values that 

it generated because the source of data that the method was based upon (chemical tests for the IOC 

group) simply was not available before 1990. As a result of extrapolating the dataset, many values were 

added during the 1990s, few values during the 2000s, and no values before 1990. 

Data Analysis

Data for chloride and sodium from all data sources listed in table 1, except the private well data 

from the SL, USEPA, and NHDOT databases, were summarized and analyzed statistically. About 

27 percent of the results for chloride and 5 percent of the results for sodium were censored (less than a 

threshold detection or RL). Further complications stemmed from data censored at multiple RLs. 

Common practice in dealing with censored values in environmental data (including water-quality data) 

is to assign the value of one-half of the RL before applying statistical tests. However, Helsel (2005) 

demonstrated that this method leads to potentially erroneous conclusions and should be avoided. 

Because censored concentrations of chloride and sodium showed systematic reductions in the RL from 

10 mg/L to 2 or 3 mg/L over time, a robust adjustment was needed (Helsel and Hirsch, 2002). The 

adjustment consisted of changing all censored and observed values less than the highest RL to that limit 

(10 mg/L) before running any of the statistical procedures including summary statistics and boxplot 

construction. Percentiles reported as “<10” incorporate some unknown combination of censored and 

observed concentrations. Although the adjustment has resulted in some loss of information for lower 

concentrations, all the information about higher concentrations is preserved. 

Preliminary exploration of data grouped by region and by decade (table 3) showed significant 

differences among the groups. Since both factors (decades and regions) defined unique populations, 

statistical comparisons of all data by decade or by region were not suitable. Data were examined in two 

ways—as summary statistics with percentiles for decades and regions compared qualitatively and by 

comparing data across decades for individual regions. The decade “2000s” included data through 2011. 

Summary statistics presented by decade and region and statistical comparisons across decades 

for regions were calculated using one value of concentration per individual source (well) for each 

category (either decade or region). If there were multiple values for a source within a decade or region, 

then the median value was used so that no single source would have excessive weight. Summary 

statistics were derived as a modification (Helsel, 2005) of the nonparametric Kaplan-Meier method 

(Kaplan and Meier, 1958), which is a standard method used to analyze right-censored data (values 

“greater than” or “more than” a number) in the survival analysis branch of statistics. Nonparametric 

tests that evaluated differences between all pairs of data across four decades (1970s through 2000s) for 

chloride and two decades (1990s and 2000s) for sodium were conducted using the Peto and Peto 

modification to the Gehan-Wilcoxon test for left-censored data (Harrington and Fleming, 1982; Helsel, 

2005). Data for chloride from the 1960s and for sodium form the 1960s through the 1980s were too 

sparse to include in the statistical comparison though they are included in the summary statistics and 

boxplots for a qualitative comparison. Methods that adapted statistical procedures to censored datasets 

were implemented using the version of the USGS library of data for TIBCO Spotfire S+ (Lorenz and 

others, 2011) or the R Project for Statistical Computing NADA package (Lee, 2012), which deals with 

values that are less than the RL in environmental data. 
7 

Spatial and Temporal Trends in Concentrations

Data are presented in several different formats. Summary statistics are presented by decade and 

region (table 3) to address the primary objective of this study. Counties were grouped into five regions 

(fig. 1) based on general similarities in concentrations of chloride and county demographics. To use data 

from the largest dataset (private well data from the SL), which were only geographically located to the 

town level, median townwide concentrations of chloride or sodium for 3 different years were compared. 

In another analysis, concentrations for individual wells were examined for changes before and after 

1995. This was done in a map-based graphic for the 900 or 800 wells for which there is at least one 

value for chloride or sodium before and after the 1995 date. Possible effects on the results of some of 

the data limitations and differences among data sources also are examined.

Chloride

A comparison of median concentrations of chloride for 1985, 1995, and 2005 by town (fig. 2) 

shows a shift towards higher concentrations in 2005. The pie charts in figure 2 show that the number of 

towns with median concentrations of chloride greater than 10 mg/L was 82 in 1985, 74 in 1995, and 96 

in 2005. Because towns indicated in the maps with white polygons have no data (rather than low 

concentrations), only towns with colored polygons should be compared across the decades. In 1985, a 

large majority of the towns had no data; consequently, interpretations based on 1985 data should be 

made cautiously. This is the only figure in the report (and the equivalent sodium figure) that includes 

data from private wells in the SL database.

Between 1985 and 1995, the number of towns with a median concentrations of chloride greater 

than 40 mg/L decreased by more than one-half, and the number of towns with a median concentration of 

chloride between 10 and 40 mg/L stayed approximately the same. The most distinctive differences are 

related to the lack of data availability in 1985 rather than actual observations, thus furthering the case 

for cautious use of 1985 data; most of the data in 1985 were based on three or fewer values (black dot in 

center of polygon in figure 2) and the number of towns with data increased more than fourfold in 1995. 

Between 1995 and 2005, the number of towns with median concentrations of chloride greater than 

20 mg/L almost doubled from 35 to 65 towns. The pattern of higher concentrations of chloride (darker 

polygons) in the southeastern part of the State that can be seen in the 1995 graph became more 

pronounced in 2005. Compared with 1995, about one-third more towns had no data in 2005. This 

probably reflects the exclusion of more of the 2005 data because of potential treatment effects (see 

“Exclusion of Results From Treated Wells” section).

A similar spatial pattern of clusters of increasing concentrations of chloride over time can be 

seen when examining median concentrations of chloride from individual wells for periods before and 

after January 1, 1995 (fig. 3; table 4). This date falls approximately midpoint in time for the entire 

dataset. Many of the data points coded as “no difference or small increase (≤ 5 mg/L)” reflect an 

assumed “0” value that was assigned to signify no change when the medians of both the before and after 

values were censored. Viewed statewide, 24 percent of the wells showed decreases in median 

concentrations of chloride, 34 percent showed no difference or small increase (less than or equal to 

5 mg/L), and 42 percent showed large increases (greater than 5 mg/L). Whereas wells that showed a 

decrease or no difference or a small increase in concentrations of chloride appear scattered uniformly 

throughout the State, clusters of wells with large increases are most evident in parts of Belknap and 

Rockingham Counties and in smaller clusters in Hillsborough and eastern Merrimack Counties. Region 

5 (southeastern) had the largest percentage (56 percent) of wells with increases in median concentrations 

of chloride greater than 5 mg/L; regions 3 (south-central, 44 percent) and 4 (southwestern, 43 percent) 
8 

were next largest; and region 1 (northern) had the smallest percentage (29 percent). Regions 1 and 2 

(central) had the largest percentages (27 percent each) of wells with decreases in median concentrations 

of chloride, and region 5 had the smallest percentage (21 percent) of wells with decreases. 

Summary statistics for median concentrations of chloride in groundwater by source (well) by 

decade and region are shown in table 3. The 10th and 25th percentile median concentrations listed in 

table 3 cannot be compared (all censored and observed median concentration values that were less than 

10 were adjusted to “<10”). Although most median concentration values at the 50th percentile were 

equal to or less than 10, median concentrations of chloride from the 2000s and from region 5 were 

higher than the RL of 10. The 75th and 90th percentiles for median concentrations of chloride in the 

2000s were higher than for all decades since the 1960s and at least double the previous two decades. 

The median concentration of chloride in the 2000s was approximately 1.5 times higher than the median 

for the 1960s through the 1980s. Data from the 1960s and 1970s should be viewed cautiously because 

there were few wells with data on concentrations of chloride. Given that qualification, the 75th and 90th 

percentiles for median concentrations of chloride from the 1960s and 1970s were higher than percentiles 

in the 1980s and 1990s, which were similar to one another. A possible explanation is that most of the 

early data were retrieved from the USGS National Water Information System database (table 1) and 

included data from a variety of projects, not all of which necessarily targeted ambient conditions (John 

Cotton, U.S. Geological Survey (retired), oral commun., February 10, 2012). 

Statewide, the percentage of raw observations (including data from private wells in the SL 

database, which were not included in the summary statistics of table 3) whose values were greater than 

the SMCL of 250 mg/L for concentrations of chloride ranged from one to seven, with the highest 

percentage for data from the 1960s. The 50th, 75th, and 90th percentiles for median concentrations of 

chloride were approximately 1.3 to 3 times higher in region 5 than in other regions. Region 1 had the 

lowest 75th and 90th percentiles compared with other regions, and regions 2, 3, and 4 were similar to 

one another. By region, the percentage of raw observations (including data from private wells in the SL 

database) whose values were greater than the SMCL of 250 mg/L for concentrations of chloride ranged 

from 2 to 5 percent, with the highest percentage for data in region 5. High concentrations of chloride in 

groundwater in region 5, which is near the seacoast, could be due to naturally higher concentrations of 

these elements in coastal precipitation from marine aerosols (Pearson and Fisher, 1971), which would 

remain constant over time, or to saltwater intrusion into aquifers, which might increase over time if 

withdrawals from groundwater increase (Masterson and Barlow, 1997). However, figure 3 shows that 

individual wells that are closest to the seacoast (easternmost part of Rockingham County) show a 

mixture of increases and decreases in concentrations of chloride, while the dense cluster of large 

increases is seen further southwest along the eastern border with Massachusetts. An alternative 

explanation for the higher concentrations and larger increases in region 5 in recent decades could be

related to greater road density in that area (fig. 1) and greater use of road salt. Further exploration of this 

hypothesis, though beyond the scope of this study, would be a worthwhile focus of future work. 

Although it was not valid to statistically compare concentration data by decade or region for the 

entire State, statistical comparisons among decades could be made for individual regions. Figure 4 

provides this detailed look at temporal changes in median concentrations of chloride for individual wells 

by region. Data from the 1960s and the 1970s, if available, are included only for a qualitative 

comparison. The minimum boundary of the plots is 10 mg/L, the highest of the multiple RLs. For each 

region except region 4, median concentrations of chloride were significantly higher in the 2000s than in 

any other decade. In region 4, median concentrations in the 1970s were not significantly different than 

those in the 2000s. For regions 1 through 4, median concentrations were not significantly different 

during the 1970s, 1980s, and 1990s. Percentiles in the rightmost boxplot in figure 4, showing all regions 
9 

together (statewide data), match values in the “All” column of table 3 (section A. By decade). The 

percentiles for median concentrations of chloride were higher in almost all measures for all decades in 

region 5 than all other regions. However, even the 90th percentile for region 5 in the 2000s, which was 

155 mg/L, was 38 percent less than the SMCL of 250 mg/L. 

Sodium

In many towns, median concentrations of sodium in groundwater samples from wells increased 

between 1995 and 2005 (fig. 5). The number of towns with median concentrations greater than 10 mg/L 

changed from 89 to 114, with increases distributed fairly evenly throughout the State. The northern part 

of the State had the lowest median concentrations in 1995 and 2005 and the southeast the highest. As 

with chloride, the number of towns without data for concentrations of sodium was greater in 2005 than 

in 1995, probably because of the effect of weeding out results due to possible treatment. The median 

concentrations of sodium from groundwater samples from the few towns that could be analyzed for 

1985 were generally larger than those in 1995. However, the distribution of data from 1985 was too 

sparse to make a statewide assessment.

A comparison of median concentrations of sodium for individual wells before and after 1995 

(fig. 6; table 5) shows clusters of large increases in regions 5 and 2 and parts of region 1, similar to the 

map for chloride concentrations (fig. 3). The distribution of the number of wells in each group 

(decrease, no difference or small change, and large increase) among regions is similar to that for the 

data on concentrations of chloride except that, for sodium, region 5 did not differ as much from the 

other regions and was very similar to region 2. 

Summary statistics for median concentrations of sodium in groundwater by source (well) 

(table 3) show that all percentiles in the 1970s and 1980s and the 10th, 25th, and 50th percentiles in the 

1990s were less than 10 mg/L. All the percentiles were greater than 10 in the 2000s. Between the 1990s 

and 2000s, all the percentiles for median concentrations of sodium increased—the 50th and the 75th 

percentiles by more than threefold, and the 90th percentile by a factor of six. The number of censored 

values for sodium in table 3 changed from 654 in the 1990s to 0 in the 2000s (the same pattern was 

seen, though less dramatically, for chloride). During the 1990s, almost all the censored values came 

from a single source of data, the DWGB database (either as reported values or as values deduced from 

IOC records (table 2)). For comparisons among decades, the percentage of raw observations (including 

data from private wells in the SL database) whose values were greater than the drinking water advisory 

of 20 mg/L for concentrations of sodium increased from 13 percent of data during the 1970s to 

30 percent of data during the 2000s. Analysis of the original data from the NHDES, which were used to 

deduce censored values from IOC records, showed that 95 percent occurrences of those censored 

sodium values were during the 1990s, and the remaining 5 percent, during the 2000s. 

Distinctions among regions are very similar to those for chloride. The only median concentration 

with a value greater than 10 mg/L was in region 5. Region 1 had the lowest 75th and 90th percentiles, 

region 5 had the highest, and regions 2, 3, and 4 were all similar with values in between the other two. 

The percentage of raw observations (including data from private wells in the SL database) with values 

for concentration of sodium higher than the drinking water advisory of 20 mg/L ranged from 19 and 

20 percent in regions 4 and 1, respectively, to 40 percent in region 5. 

Boxplots that summarize median concentrations of sodium for individual wells for each decade 

within each region are shown in figure 7. Because even the 90th percentiles for all regions were less 

than 10 mg/L during the 1970s and 1980s, data from these decades were omitted from the figure. 

Increases in all percentiles between the 1990s and 2000s seen in the summary statistics are confirmed by 

statistical tests that show significantly different medians. The pattern of significant increases for all 
10

percentiles for median concentrations of sodium between the 1990s and the 2000s is distributed 

uniformly throughout the five regions of the State. A small difference among regions is seen with 

slightly higher 50th, 75th, and 90th percentiles for regions 3 and 5 during the 2000s compared with the 

other regions. For all regions, during the 1990s, even the 90th percentile of median concentrations of 

sodium was less than the 20 mg/L drinking water advisory; but during the 2000s, median concentrations 

of sodium increased so that the 25th percentiles were higher than 20 mg/L. 

Summary 

Several studies of groundwater in New Hampshire on a statewide and regional level have 

suggested that concentrations of chloride and sodium have been increasing throughout much of the 20th 

century and into the 21st century primarily from anthropogenic sources, such as road salt. To examine 

the spatial and temporal trends of chloride and sodium on a statewide basis, the U.S. Geological Survey, 

in cooperation with the New Hampshire Department of Environmental Services, assembled all available 

data from State and Federal agencies that met specified criteria (untreated, single source, and unbiased 

sources) into a single database, performed quality assurance procedures on the compiled dataset, 

explained features of the data that may affect results, and summarized the data graphically and 

statistically.

One feature of the data was the presence of many censored values (concentrations with values 

less than a reporting limit), especially for chloride. Multiple reporting limits that changed systematically 

over time further complicated the analysis. A robust adjustment was made whereby all censored and 

observed chloride and sodium concentrations less than the highest reporting limit of 10 milligrams per 

liter (mg/L) were adjusted to 10 mg/L before compiling summary statistics or performing other 

statistical analyses. Counties in New Hampshire were grouped into five regions for summarizing data 

geographically. Summary statistics presented by decade and by region and statistical comparisons 

across decades for regions were calculated using the median concentration per individual well for each 

decade or region.

Median concentrations of chloride and sodium by town for 1985, 1995, and 2005 showed shifts 

in many towns towards higher concentrations in 2005 for both constituents, especially in the 

southeastern part of the State. Data from 1985 were too sparse statewide to make meaningful 

comparisons with more recent data. Viewed qualitatively, region 1 in northern New Hampshire had the 

lowest median concentrations of chloride and sodium, and region 5 had the highest. For each region, 

median concentrations of chloride and sodium increased significantly between earlier decades and the 

2000s. Increases in median concentrations of sodium were dramatic and relatively uniform for all 

regions—during the 1990s, all 90th percentiles were less than the 20 mg/L U.S. Environmental 

Protection Agency drinking water advisory, and during the 2000s, all 25th percentiles were higher than 

20 mg/L. 

For data grouped by decades, the percentage the values of raw observations (including data from 

private wells in the New Hampshire Department of Health and Human Services State Laboratory

database) higher than the secondary maximum contaminant level of 250 mg/L for concentrations of 

chloride ranged from 1 to 7 percent, with the highest percentage for data from the 1960s; and for 

regions, the percentage of values higher than the secondary maximum contaminant level ranged from 2 

to 5 percent, with the highest percent for data in region 5. For comparisons among decades, the 

percentage of the values of raw observations higher than the drinking water advisory of 20 mg/L for 

concentrations of sodium increased from 13 percent for data from the 1970s to 30 percent for data 

during the 2000s; and by regions, the percentage of values higher than 20 mg/L ranged from 19 and 

20 percent in regions 4 and 1, respectively, to 40 percent in region 5. 
11

References Cited

Ayotte, J.D., Kernen, B.M., Wunsch, D.R., Argue, D.M., Bennett, D.S., and Mack, T.J., 2010, 

Preliminary assessment of trends in static water levels in bedrock wells in New Hampshire, 1984 to 

2007: U.S. Geological Survey Open-File Report 2010–1189, 30 p., at 

http://pubs.usgs.gov/of/2010/1189. 

Emery & Garrett Groundwater, Inc., 2007, Implementation of a drainage study to evaluate a costeffective means of reducing sodium and chloride concentrations in the groundwater within the I–93 

Corridor, Ashland, N.H.: Meredith, New Hampshire, Emery & Garret Groundwater, Inc., 9 p. 

Hall, F.R., 1975, Chloride in natural waters of New Hampshire: Durham, N.H., University of New 

Hampshire, Agricultural Experiment Station Bulletin 504, 25 p. 

Harrington, D.P., and Fleming, T.R., 1982, A class of rank test procedures for censored survival data: 

Biometrika, v. 69, no. 3, p. 553–566. 

Harte, P.T., and Trowbridge, P.R., 2010, Mapping of road-salt-contaminated groundwater discharge and 

estimation of chloride load to a small stream in southern New Hampshire, USA: Hydrological 

Processes, v. 24, no. 17, p. 2349–2368, accessed at 

http://www3.interscience.wiley.com/journal/123338089/abstract/. 

Helsel, D.R., and Hirsch, R.M., 2002, Statistical methods in water resources: U.S. Geological Survey 

Techniques of Water-Resources Investigations, book 4, chap. A3, 510 p. (Also available at 

http://pubs.usgs.gov/twri/twri4a3/.) 

Helsel, D.R., 2005, Nondetects and data analysis—Statistics for censored environmental data: John 

Wiley & Sons, 250 p. 

Kaplan, E.L., and Meier, Paul, 1958, Nonparametric estimation from incomplete observations: Journal 

of the American Statistical Association, v. 53, no. 282, June, p. 457–481. (Also available at 

http://www.jstor.org/stable/2281868.) 

Lee, Lopaka, 2012, Package ‘NADA’—Nondetects and data analysis for environmental data (version 

1.5–3): The Comprehensive R Archive Network, accessed January 30, 2012, at http://cran.rproject.org/web/packages/NADA/NADA.pdf.

Lindsey, B.D., and Rupert, M.G., 2012, Methods for evaluating temporal groundwater quality data and 

results of decadal-scale changes in chloride, dissolved solids, and nitrate concentrations in 

groundwater in the United States, 1988–2010: U.S. Geological Survey Scientific Investigations 

Report 2012–5049, 46 p., available at http://pubs.usgs.gov/sir/2012/5046/. 

Lorenz, D.L., Ahearn, E.A., Carter, J.M., Cohn, T.A., Danchuk, W.J., Frey, J.W., Helsel, D.R., Lee, 

K.E., Leeth, D.C., Martin, J.D., McGuire, V.L., Neitzert, K.M., Robertson, D.M., Slack, J.R., Starn, 

Jeffrey, Vecchia, A.V., Wilkison, D.H., and Williamson, J.E., 2011, USGS library for S–PLUS for 

Windows—Release 4.0: U.S. Geological Survey Open-File Report 2011–1130, available at 

http://water.usgs.gov/software/S-PLUS/. 

Masterson, J.P., and Barlow, P.M., 1997, Effects of simulated ground-water pumping and recharge on 

ground-water flow in Cape Cod, Martha’s Vineyard, and Nantucket Island basins, Massachusetts: 

U.S. Geological Survey Water-Supply Paper 2447, 79 p. (Also available at 

http://pubs.usgs.gov/wsp/2447/report.pdf.) 

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. (Also available at 

http://pubs.usgs.gov/sir/2009/5086/.) 
12

New Hampshire Department of Environmental Services, 2008, Drinking water and related rules: New 

Hampshire Department of Environmental Services, accessed April 17, 2012, at 

http://des.nh.gov/organization/commissioner/legal/rules/index.htm#drinking. 

New Hampshire Department of Environmental Services, 2011, Operational responsibilities of 

community public water systems: New Hampshire Department of Environmental Services Fact Sheet 

WD–DWGB–14–1, 3 p. (Also available at 

http://des.nh.gov/organization/commissioner/pip/factsheets/dwgb/documents/dwgb-14-1.pdf.) 

Pearson, F.J., Jr., and Fisher, D.W., 1971, Geochemistry of water: chemical composition of atmospheric 

precipitation in the northeastern United States: U.S. Geological Survey Water-Supply Paper 1535–P, 

23 p. (Also available at http://pubs.usgs.gov/wsp/1535p/report.pdf.) 

U.S. Environmental Protection Agency, 1988, Ambient water quality criteria for chloride—1988: U.S. 

Environmental Protection Agency EPA 440/5–88–001, 39 p., available at 

http://nepis.epa.gov/Exe/ZyPURL.cgi?Dockey=00001N4Q.txt. 

U.S. Environmental Protection Agency, 2011, 2011 edition of the drinking water standards and health 

advisories: U.S. Environmental Protection Agency EPA 820–R–11–002, 12 p. (Also available at 

http://water.epa.gov/action/advisories/drinking/upload/dwstandards2011.pdf.) 

U.S. Environmental Protection Agency, [undated], Sodium in drinking water: U.S. Environmental 

Protection Agency Water—Contaminant Candidate List, accessed January 17, 2012, at 

http://water.epa.gov/scitech/drinkingwater/dws/ccl/sodium.cfm. 

U.S. Geological Survey, [undated], Water-use data available from USGS: U.S. Geological Survey, 

accessed February 1, 2012, at http://water.usgs.gov/watuse/wudata.html. 
13

Appendix 1. Description of the Database for Salt in New Hampshire

Groundwater

The database for salt in New Hampshire groundwater (NH salt database) is the data product that 

was compiled from several sources (table 1) and used to generate data summaries and statistics shown in 

this report. The main part of the database, which is a Microsoft Access® relational database, consists of 

three tables—Systems, Wells, and ChemResults. The PWS ID FED field of the Systems table contains 

the primary identifier of the public water system (PWS) (for New Hampshire Department of 

Environmental Services [NHDES] wells) or well identifier (for National Water Information System 

data). Other fields in the Systems table provides basic locational and descriptive information about the 

system (Name, Town, County, CPN [type of PWS system], and Population Served). The Wells table 

contains fields for the well identifier (Source) and the link to the PWS system table (PWS ID FED). 

Fields with other information (Site Description, Source Description, Lat, Long, WELL_DEPTH, 

SAFE_YIELD, and Strata) about the well are included if available. The ChemResults table contains 

links to the Systems and Wells tables (the PWS ID FED and Source fields); other fields include Date 

Sampled, DataSource, Cl, and Na (the latter two fields are chloride and sodium results expressed in 

units of milligrams per liter). Other information (laboratory identifier, sample identifier (assigned by the 

laboratory), chloride or sodium detect flag, chloride or sodium qualifier for censored values (values less 

than the detection limit), chloride or sodium laboratory method, and date that treatment was added) 

about the results also are included in the ChemResults table if available. 

Because data potentially available for the NH salt database came from many different sources, 

quality assurance of the data before entry into the database involved a thorough check of duplicate 

entries based on system identifier in combination with well identifier (together making a unique 

combination), sample collection date, and concentration value. Other quality assurance measures were 

implemented to bring data from several disparate sources into a uniform organization. For example, the 

strata and well depth fields in the well table were often blank but sometimes could be populated by 

eliciting information from the source description field. In some cases, town and county names were 

added by using a geographic information system (GIS) to overlay latitude and longitude points with 

administrative boundary maps. In other cases, a source identifier was not available, but because this 

field was necessary to link wells with their chemical results, a manufactured entry would be generated 

(using a unique prefix to distinguish it from numbers meaningful to the State databases) for use in the 

NH salt database. Ultimately, there also were cases where data could not be incorporated into the NH 

salt database because it was not possible to determine a key piece of information, such as whether the 

sample result was from a groundwater or surface-water source, whether the sample was from an 

untreated and unblended source, or how to match multiple wells from different source databases if the 

two databases did not use the same naming conventions for identifying specific wells.

Two additional tables in the NH salt database contain chloride and sodium records from private 

(domestic) well data stored in the New Hampshire Department of Health and Human Services State 

Laboratory (SL) database. These standalone tables, named Homeowners_Cl and Homeowners_Na, 

could not be integrated within the structure of the linked systems, wells, and ChemResults tables 

because individual wells were not identified, and, consequently, each record was assumed to be from a 

unique source. This assumption is not completely realistic; some percentage of the records were 

probably from nonunique wells (that is, from the same well), but this percentage is probably small. 

Because of this feature, private well data from the SL database were not used for statistical analyses.
14

Database queries were developed to filter and retrieve the data for various purposes. An initial 

query excluded information that was provided with the original data from the NHDES or NHDHHS 

databases that met the following criteria for this project: data from surface-water sources, data from 

treated or potentially treated and blended sources, and data from wells that were known to be 

contaminated. Subsequent queries created subsets of data to explore trends over time and geographic 

areas.

For more information concerning the database, contact: Director, New England Water Science 

Center, dc_nh@usgs.gov or visit http://nh.water.usgs.gov/. 
15

Figure 1. Index map of New Hampshire, showing county boundaries, major roads, and regions as used in this 

report.
16

Number of towns in each concentration category

1985 1995 2005

Three or fewer values used to calculate median

No data

Explanation

Median concentration of chloride, in milligrams per liter

Figure 2. Median concentrations of chloride in water samples from wells by town in New Hampshire in 1985, 1995, and 2005.
17

Figure 3. Differences in median concentrations of chloride at individual wells in New Hampshire for data before 

and after January 1, 1995. mg/L, milligrams per liter; ≤, less than or equal to; >greater than.
18

Figure 4. Boxplots of median concentrations of chloride in groundwater by source (well) in New Hampshire, grouped by decade (beginning with 

the 1960s and ending with the [extended] decade 2000-11) and region. The extents of the regions are shown in figure 1. Boxplots for early decades 

may be missing for some regions where data are sparse.

1 2 3 4 5 All ←Region

Median chloride concentration, in milligrams per liter

90th percentile

75th percentile

50th percentile (median)

Highest reporting limit and 

lower limit of estimated 

results

Explanation

150

100

50

10

 a a a b a a a b a a a b ba a a b ab b a c a Different letters denote 

statistically significant 

difference

 115 619 857 449 40 221 270 179 89 113 612 607 374 13 22 161 186 140 10 112 614 583 528 115 Number of values
19

1985 1995 2005

Number of towns in each concentration category

Three or fewer values used to calculate median

No data

Explanation

Median concentration of sodium, in milligrams per liter

Figure 5. Median concentrations of sodium in water samples from wells by town in New Hampshire in 1985, 1995, and 2005.
20

Figure 6. Differences in median concentrations of sodium at individual wells in New Hampshire for data before 

and after January 1, 1995. mg/L, milligrams per liter; ≤, less than or equal to; >greater than.
21

Median sodium concentration, in milligrams per liter

 698 388 295 174 703 436 171 112 674 675 698 Number of values a b a b a b a b a b a Different letters denote 

statistically significant 

difference

1 2 3 4 5 All

USEPA 

Drinking 

Water 

Advisory

 Number of values

←Region

90th percentile

75th percentile

50th percentile (median)

25th percentile

10th percentile

Highest reporting limit and 

lower limit of estimated 

results

Explanation

50

100

10

Figure 7. Boxplots of median concentrations of sodium in groundwater by source (well) in New Hampshire for two decades by region. Regions are 

defined in figure 1. Percentiles for the 1970s and 1980s are all below 10 milligrams per liter. USEPA, U.S. Environmental Protection Agency..
22

Table 1. Sources of data for concentrations of chloride and sodium in groundwater in New Hampshire.

[mg/L, milligrams per liter; na, not availale; NHDES, New Hampshire Department of Environmental Services; NHDHHS, New Hampshire Department of Health and 

Human Services; NHDOT, New Hampshire Department of Transportation; STORET, U.S. Environmental Protection Agency’s STORage and RETrieval database for 

water quality monitoring data; USEPA, U.S. Environmental Protection Agency; USGS, U. S. Geological Survey]

Dataset Name Agency Years of data

Reporting limit, in 

mg/L Number of records Number of 

wells Description

Chloride Sodium Chloride Sodium

Drinking Water and 

Groundwater Bureau 

(DWGB)1

NHDES 1988–2011 2 or 3 1 or 5 3,582 2,994 1,405 Compliance database for public water system wells, sampled once 

every 3 years.

State Laboratory (SL):

Public-water systems1 NHDHHS 1984–2000 2 or 3 1 or 3 2,497 2,090 959 Database of chemical results for water samples from public water 

systems analyzed at the NHDES SL

Private wells NHDHHS 1994 – 2011 2 or 3 1 or 2 31,949 22,486 32,082 Database of chemical results for water samples from private wells that 

have been analyzed at the NHDES SL. Town names were the only 

locational information provided.

Data Warehouse (DW) NHDES 2008–2011 1 or 3 1 64 874 19 Database developed to receive water sample results from the State and 

private laboratories through online reporting. After review, results 

are transferred to the DWGB database.

Environmental Monitoring 

(EMD)

NHDES 1972–2011 2, 3, or 10 5 or 10 1,382 551 934 Consists of NHDES data that have been stored in paper files. One part 

of this dataset consists of “old” records (before routine electronic 

filing) from small community-water systems. Another part are 

water-quality data from pump tests, including those required for 

new community-water systems and are frequently the last raw water 

samples available before the addition of treatment. Data from these 

two sources are currently (2011) being entered into the electronic 

EMD to make them available for various purposes.

National Water Information 

System (NWIS)

USGS 1952–2010 2, 3, or 10 1, 3, 5, or 

10

2,052 1,824 1,801 Database of water-related data, including chemical results, from 

various U.S. Geological Survey projects.

Legacy STORET USEPA 1971–1983 na na 50 63 17 Database of chemical results from water samples from U.S. Forest 

Service wells in Carroll, Coos, and Grafton Counties.

Salt replacement wells2 NHDOT 1973–2011 na na 426 0 426 Database of chemical results for water samples for wells possibly 

contaminated with road salt.

1

Distinctions between and apparent overlap of data between these two data sources are explained in table 2. 

2

Because data are biased towards high concentrations and do not reflect ambient conditions, they are not used in summary or statistical analyses for this study but were 

put into the New Hampshire salt database for other potential mapping applications.
23

Table 2. Sources of data for public water system wells in the U.S. Geological Survey New Hampshire salt 

database.

[RL, reporting limit1; DWGB, New Hampshire Department of Environmental Services Drinking Water and Groundwater 

Bureau; IOC, inorganic contaminants; SL, New Hampshire Department of Health and Human Services State Laboratory; 

DW, New Hampshire Department of Environmental Services Data Warehouse]

Sample collection date Source of data

Samples analyzed at private laboratories

1990 through 2010:

Samples with concentrations above RL2 DWGB database

Samples with concentrations below RL3 Deduced from IOC records of DWGB database

After 2010, all samples DW database

Samples analyzed at the State Laboratory

Before 2001, all samples4 SL database

2001 through 2008:

Samples with concentrations above RL DWGB database

Samples with concentrations below RL Deduced from IOC records of DWGB database

After 2008, all samples DW database

1

The lower limit of data that delineates the censoring threshold is called the reporting limit in this report.

2

Less than 0.1 percent of the data provided were for samples collected before 1990.

3

A few dozen results that had values below the RL were provided with the DWGB database.

4

Several hundred results that had values below the RL were provided with the DWGB database.
24

Table 3. Summary statistics for median concentrations of chloride and sodium in groundwater by source (well) in New 

Hampshire grouped by A, decade and B, region. 

[Data for concentrations of chloride and sodium reflect the median for each source (well) for each A, decade and B, region. Before 

running statistical tests, all censored (less than a detection or reporting limit) and uncensored values less than 10 milligrams per 

liter were adjusted to 10 milligrams per liter. Summary statistics were calculated using the modifed (Helsel, 2005) Kaplan-Meier 

(1958) test. mg/L, milligrams per liter; NA, not available; SMCL, secondary maximum contaminant level; USEPA, U.S. 

Environmental Protection Agency; <, less than]

A. By decade

1960s 1970s 1980s 1990s 2000s All

Chloride concentrations, in mg/L

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

25th percentile <10 <10 <10 <10 <10 <10

50th percentile 10 11 10 <10 17 <10

75th percentile 33 30 25 22 52 30

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

Percentage of observations higher than the SMCL of 250 mg/L1 7 3 1 3 3 3

Sodium concentrations, in mg/L

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

25th percentile NA <10 <10 <10 21 <10

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

75th percentile NA <10 <10 13 50 19

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

Percentage of observations higher than the USEPA drinking 

water advisory level of 20 mg/L1 NA 13 23 23 30 27

B. By region

Region 1 Region 2 Region 3 Region 4 Region 5 All

Chloride concentrations, in mg/L

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

25th percentile <10 <10 <10 <10 <10 <10

50th percentile <10 <10 <10 <10 17 <10

75th percentile 17 27 28 27 46 30

90th percentile 53 82 80 75 105 79

Number of median observations 2,043 710 1,795 522 1,847 6,917

Number censored 650 215 477 120 270 1,732

Percentage of observations higher than the SMCL of 250 mg/L1 2 2 3 2 5 3

Sodium concentrations, in mg/L

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

25th percentile <10 <10 <10 <10 <10 <10

50th percentile <10 <10 <10 <10 14 <10

75th percentile 14 19 18 17 27 19

90th percentile 29 35 39 34 55 40

Number of median observations 1,867 621 1,548 431 1,659 6,126

Number censored 263 101 189 58 200 811

Percentage of observations higher than the USEPA drinking 

water advisory level of 20 mg/L1 20 27 27 19 40 27

1

Percentages include data for private wells in the New Hampshire Department of Health and Human Services State Laboratory 

database, which are not included with results of summary statistics shown in the rest of this table.
25

Table 4. Change in median concentrations of chloride before and after January 1, 1995. 

Region Total number 

of wells

Percentage of wells

Decrease Small 

increase1

Large 

increase2

1 245 27 44 29

2 107 27 36 37

3 206 21 35 44

4 81 26 31 43

5 261 21 23 56

Total 900 24 34 42

1

Change in median concentration less than or equal to 5 milligrams per liter (mg/L).

2

Change in concentration greater than 5 mg/L.

Table 5. Change in median concentrations of sodium before and after January 1, 1995. 

Region Total number 

of wells

Percentage of wells

Decrease Small 

increase1

Large 

increase2

1 224 28 38 35

2 78 32 23 45

3 185 26 37 37

4 65 23 32 45

5 248 29 25 45

Total 800 28 32 40

1

Change in median concentration less than or equal to 5 milligrams per liter (mg/L).

2

Change in concentration greater than 5 mg/L.