U.S. Department of the Interior

U.S. Geological Survey

Scientific Investigations Report 2009–5086

National Water-Quality Assessment Program

Chloride in Groundwater and Surface Water in

Areas Underlain by the Glacial Aquifer System,

Northern United States
Cover. Rock salt being unloaded off of a ship in Oswego, New York, October, 2007 

(James Rajotte/The New York Times/Redux)
Chloride in Groundwater and Surface 

Water in Areas Underlain by the Glacial 

Aquifer System, Northern United States

By John R. Mullaney, David L. Lorenz, and Alan D. Arntson

National Water-Quality Assessment Program

Scientific Investigations Report 2009–5086

U.S. Department of the Interior

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

KEN SALAZAR, Secretary

U.S. Geological Survey

Suzette M. Kimball, Acting Director

U.S. Geological Survey, Reston, Virginia: 2009

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Suggested citation:

Mullaney, J.R., Lorenz, D.L., 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.
iii

Foreword 

The U.S. Geological Survey (USGS) is committed to providing the Nation with credible scientific information that helps to enhance and protect the overall quality of life and that facilitates effective management of water, biological, energy, and mineral resources (http://www.usgs.gov/). Information on the 

Nation’s water resources is critical to ensuring long-term availability of water that is safe for drinking and 

recreation and is suitable for industry, irrigation, and fish and wildlife. Population growth and increasing 

demands for water make the availability of that water, now measured in terms of quantity and quality, 

even more essential to the long-term sustainability of our communities and ecosystems.

The USGS implemented the National Water-Quality Assessment (NAWQA) Program in 1991 to support 

national, regional, State, and local information needs and decisions related to water-quality management 

and policy (http://water.usgs.gov/nawqa). The NAWQA Program is designed to answer: What is the condition of our Nation’s streams and groundwater? How are conditions changing over time? How do natural 

features and human activities affect the quality of streams and groundwater, and where are those effects 

most pronounced? By combining information on water chemistry, physical characteristics, stream habitat, 

and aquatic life, the NAWQA Program aims to provide science-based insights for current and emerging 

water issues and priorities. From 1991-2001, the NAWQA Program completed interdisciplinary assessments and established a baseline understanding of water-quality conditions in 51 of the Nation’s river 

basins and aquifers, referred to as Study Units (http://water.usgs.gov/nawqa/studyu.html).

In the second decade of the Program (2001–2012), a major focus is on regional assessments of waterquality conditions and trends. These regional assessments are based on major river basins and principal 

aquifers, which encompass larger regions of the country than the Study Units. Regional assessments 

extend the findings in the Study Units by filling critical gaps in characterizing the quality of surface water 

and groundwater, and by determining status and trends at sites that have been consistently monitored 

for more than a decade. In addition, the regional assessments continue to build an understanding of how 

natural features and human activities affect water quality. Many of the regional assessments employ 

modeling and other scientific tools, developed on the basis of data collected at individual sites, to help 

extend knowledge of water quality to unmonitored, yet comparable areas within the regions. The models 

thereby enhance the value of our existing data and our understanding of the hydrologic system. In addition, the models are useful in evaluating various resource-management scenarios and in predicting how 

our actions, such as reducing or managing nonpoint and point sources of contamination, land conversion, 

and altering flow and (or) pumping regimes, are likely to affect water conditions within a region.

Other activities planned during the second decade include continuing national syntheses of information on 

pesticides, volatile organic compounds (VOCs), nutrients, selected trace elements, and aquatic ecology; 

and continuing national topical studies on the fate of agricultural chemicals, effects of urbanization on 

stream ecosystems, bioaccumulation of mercury in stream ecosystems, effects of nutrient enrichment on 

stream ecosystems, and transport of contaminants to public-supply wells.

The USGS aims to disseminate credible, timely, and relevant science information to address practical and 

effective water-resource management and strategies that protect and restore water quality. We hope this 

NAWQA publication will provide you with insights and information to meet your needs, and will foster 

increased citizen awareness and involvement in the protection and restoration of our Nation’s waters. 

The USGS recognizes that a national assessment by a single program cannot address all water-resource 

issues of interest. External coordination at all levels is critical for cost-effective management, regulation, 

and conservation of our Nation’s water resources. The NAWQA Program, therefore, depends on advice 

and information from other agencies—Federal, State, regional, interstate, Tribal, and local—as well as 

nongovernmental organizations, industry, academia, and other stakeholder groups. Your assistance and 

suggestions are greatly appreciated.

Matthew C. Larsen

Associate Director for Water

v

Contents

Foreword ........................................................................................................................................................iii

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

Introduction.....................................................................................................................................................2

Purpose and Scope ..............................................................................................................................2

Sources of Salts to Water Resources ...............................................................................................2

Natural Sources...........................................................................................................................2

Atmospheric Deposition ....................................................................................................2

Weathering of Common Rocks, Minerals, and Soils.....................................................5

Salt Deposits and Brines ...................................................................................................5

Anthropogenic Sources..............................................................................................................5

Application of Deicing Salts..............................................................................................8

Landfills.................................................................................................................................8

Wastewater and Water Treatment ..................................................................................8

Agriculture .........................................................................................................................12

Methods of Data Analysis ..........................................................................................................................12

Compilation of Environmental Data .................................................................................................12

Multiple Comparison Testing ............................................................................................................12

Multiple Linear Regression Modeling ............................................................................................14

Estimation of Chloride Loads ............................................................................................................14

Chloride in Groundwater and Surface Water .........................................................................................14

Groundwater........................................................................................................................................15

Sources of Chloride in Groundwater......................................................................................16

Surface Water.....................................................................................................................................18

Relation of Maximum Chloride Concentration to Explanatory Variables .........................19

Loads and Yields of Chloride from Forested, Agricultural, and Urban Basins.................19

Relation of Chloride Yield to Explanatory Variables.............................................................23

Trends in Loads at Selected Sites and Evaluation of Sources of Chloride ......................23

Summary and Conclusions.........................................................................................................................31

References Cited..........................................................................................................................................32

Appendix 1 Surface-Water-Quality Monitoring Stations Used in Analyses for Chloride 

Concentration or Load Estimation (or Both) ..............................................................................37
vi

Figures

1. Graphs showing—

1. Salt use and consumption in the United States by (A) all end users, 1975–2003, 

and (B) consumption by use or industry, 1975–2005......................................................3

2–5. Maps showing—

2. Glacial aquifer system and National Water-Quality Assessment Program 

study basins, northern United States ...............................................................................4

3. Mean wet deposition of chloride, 1994–2003, and locations of salt mining 

in the glacial aquifer system study area, northern United States ...............................6

4. Estimated average concentration of chloride, 1994–2002, in runoff attributable 

to atmospheric (wet) deposition in the glacial aquifer system study area, 

northern United States........................................................................................................7

5. Wells and surface-water-quality monitoring stations used in the analysis of 

the glacial aquifer system study area, northern United States .................................13

6–17. Graphs showing—

6. Distribution of chloride concentrations in samples from shallow monitoring 

wells in forested, agricultural, and urban areas and in drinking-water supply 

wells in the glacial aquifer system, northern United States, 1991–2003 ..................16

7. Distribution of sodium concentrations in samples from shallow monitoring 

wells in forested, agricultural, and urban areas and in drinking-water supply 

wells in the glacial aquifer system, northern United States, 1991–2003 ..................17

8. Distribution of chloride:bromide ratios (by mass) in samples from shallow 

monitoring wells in forested, agricultural, and urban areas and in 

drinking-water supply wells in the glacial aquifer system, northern United 

States, 1991–2003...............................................................................................................18

9. (A) Binary mixing curves representing sources of chloride. The relation of 

chloride concentration to chloride:bromide ratios (by mass) for samples 

from shallow monitoring wells in (B) forested areas, (C) agricultural areas, 

and (D) urban areas...........................................................................................................20

10. Distribution of chloride:bromide ratios (by mass) in samples from drinkingwater supply wells in geographic regions of the glacial aquifer system, 

northern United States, 1991–2003 .................................................................................23

11. Distribution of maximum chloride concentrations measured at selected 

surface-water-quality monitoring stations in the glacial aquifer system, 

northern United States, 1991–2004 .................................................................................24

12. Chloride concentrations, snow depth, and discharge at station 05288705, 

Shingle Creek at Queen Avenue in Minneapolis, Minnesota, February 1997..........26

13. Chloride concentrations in base flow, plotted against maximum measured 

chloride concentrations at selected forested, agricultural, and urban basins 

in the glacial aquifer system, northern United States, 1991–2004.............................27
vii

14. Predicted natural log of maximum measured chloride concentrations from 

multiple linear regression modeling, and observed natural log of maximum 

measured chloride concentrations in selected forested, agricultural, and 

urban basins in the glacial aquifer system, northern United States, 1991–2004.....28

15. Distribution of annual mean chloride yields at selected surface-water-quality 

monitoring stations in the glacial aquifer system, northern United States, 

1991–2003 ............................................................................................................................28

16. Predicted natural log of annual chloride yield from multiple linear regression 

modeling and observed natural log of chloride yield in selected forested, 

agricultural, and urban basins in the glacial aquifer system, northern United 

States, 1991–2003...............................................................................................................29

17. Chloride loads with time at selected water-quality monitoring stations in the 

glacial aquifer system, northern United States: (A) Poplar Creek at Elgin, 

Illinois, water years 1978–2003; (B) Quinnipiac River at Wallingford, 

Connecticut, water years 1969–2003; and (C) Rahway River near Springfield, 

New Jersey, water years 1980–2003 ..............................................................................30

Tables

1. Estimated wet deposition of chloride on the glacial aquifer system, northern 

United States, 1994–2003.............................................................................................................5

2. Selected annual application rates of deicing chemicals on State and local roads 

in the glacial aquifer system, northern United States ............................................................9

3. Watershed characteristics evaluated in a regression analysis of maximum 

measured chloride concentrations .........................................................................................15

4. Chloride:bromide ratios for sources of chloride to water resources ................................22

5. Monitoring stations where surface waters had chloride concentrations greater 

than 230 milligrams per liter in the glacial aquifer system, northern United States, 

1991–2004 .....................................................................................................................................25

6. Multiple linear-regression estimates of model coefficients, standard errors, 

t-statistic and p-values for the dependent variable natural log of maximum 

measured chloride concentration in surface water for selected urban, 

agricultural, and forested basins, in the glacial aquifer system, northern 

United States, 1991–2004...........................................................................................................27

7. Multiple linear regression estimates of model coefficients and standard errors, 

t-statistics, and p-values for the dependent variable natural log of chloride yield, 

selected forested, agricultural and urban basins in the glacial aquifer system, 

northern United States, 1991–2003 ..........................................................................................29
viii

Conversion Factors

Multiply By To obtain

Length

inch (in.) 2.54 centimeter (cm)

inch (in.) 25.4 millimeter (mm)

foot (ft) 0.3048 meter (m)

mile (mi) 1.609 kilometer (km)

Area

square mile (mi2) 259.0 hectare (ha)

square mile (mi2) 2.590 square kilometer (km2) 

Volume

gallon (gal) 3.785 liter (L) 

gallon (gal) 0.003785 cubic meter (m3) 

gallon (gal) 3.785 cubic decimeter (dm3) 

million gallons (Mgal) 3,785 cubic meter (m3)

cubic foot (ft3) 28.32 cubic decimeter (dm3) 

cubic foot (ft3) 0.02832 cubic meter (m3) 

Flow rate

cubic foot per second (ft3/s) 0.02832 cubic meter per second (m3/s) 

million gallons per day (Mgal/d) 0.04381 cubic meter per second (km3/s) 

inch per year (in/yr) 25.4 millimeter per year (mm/yr)

Mass

ounce, avoirdupois (oz) 28.35 gram (g) 

pound, avoirdupois (lb) 0.4536 kilogram (kg) 

ton, short (2,000 lb) 0.9072 megagram (Mg) 

ton per year (ton/yr) 0.9072 megagram per year (Mg/yr)

ton per year (ton/yr) 0.9072 metric ton per year

tons per square mile (ton/mi2) 350.1947 kilograms per square kilometer 

(kg/km2)

pound (lb) 453,592.4 milligram (mg)

pounds per day (lb/d) 453,592.4 milligram per day (mg/d) 

pound (lb) 0.4535924 kilogram (kg)

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

Horizontal coordinate information is referenced to the North American Datum of 1983 (NAD 83).
Chloride in Groundwater and Surface Water in Areas 

Underlain by the Glacial Aquifer System, Northern 

United States

By John R. Mullaney, David L. Lorenz, and Alan D. Arntson

Abstract

A study of chloride in groundwater and surface water was 

conducted for the glacial aquifer system of the northern United 

States in forested, agricultural, and urban areas by analyzing 

data collected for the National Water-Quality Assessment 

Program from 1991 to 2004.

Groundwater-quality data from a sampling of 1,329 

wells in 19 states were analyzed. Chloride concentrations 

were greater than the secondary maximum contaminant level 

established by the U.S. Environmental Protection Agency of 

250 milligrams per liter in 2.5 percent of samples from 797 

shallow monitoring wells and in 1.7 percent of samples from 

532 drinking-water supply wells. Water samples from shallow 

monitoring wells in urban areas had the largest concentration 

of chloride, followed by water samples from agricultural and 

forested areas (medians of 46, 12, and 2.9 milligrams per liter, 

respectively).

An analysis of chloride:bromide ratios, by mass, and 

chloride concentrations compared to binary mixing curves for 

dilute groundwater, halite, sewage and animal waste, potassium chloride fertilizer, basin brines, seawater, and landfill 

leachate in samples from monitoring wells indicated multiple 

sources of chloride in samples from wells in urban areas 

and agricultural areas. Water from shallow monitoring wells 

in urban areas had the largest chloride:bromide ratio, and 

samples with chloride:bromide ratios greater than 1,000 and 

chloride concentrations greater than 100 milligrams per liter 

were dominated by halite; however, the samples commonly 

contained mixtures that indicated input from sewage or animal 

waste. Chloride:bromide ratios were significantly larger in 

samples from public-supply drinking-water wells than from 

private drinking-water wells, and ratios were significantly 

larger in all drinking-water wells in eastern and central regions 

of the glacial aquifer system than in west-central and western 

regions of the glacial aquifer system.

Surface-water-quality data collected regularly during 

varying time periods from 1991–2004 from 100 basins dominated by forested, agricultural, or urban land in 15 states were 

analyzed to determine maximum measured chloride concentrations. Samples from 15 sites in east, central, and west-central 

areas, collected primarily in winter, had chloride concentrations higher than the U.S. Environmental Protection Agency 

recommended chronic criterion concentration for aquatic life 

of 230 milligrams per liter. Concentrations of chloride in baseflow samples were predictive of maximum measured chloride 

concentrations, indicating that inputs of chloride from groundwater and (or) point-source wastewater discharges increase 

the likelihood of samples exceeding the recommended chronic 

aquatic criterion. Multiple linear regression analyses showed 

that the density of major roads, potential evapotranspiration, 

and the percentage of annual runoff from saturated overland 

flow were significant factors in describing the range of maximum measured chloride concentrations in the basins studied. 

Chloride loads and yields were determined at 95 surfacewater-monitoring stations in basins dominated by forested, 

agricultural, or urban land. Annual chloride yield was largest 

in the urban basins (median of 88 tons per square mile) and 

smallest in the forested basins (median of 6.4 tons per square 

mile). The median chloride yield in the agricultural basins was 

15.4 tons per square mile. Multiple linear regression analyses 

showed that the density of highways (roads in U.S. highway 

system), the number of major wastewater discharges in the 

basin, potential evapotranspiration, and urban minus agricultural land area were significant factors in describing the range 

of average annual chloride yields. 

Upward trends in chloride loads were apparent in several 

urban basins for which additional long-term data were available. Increases in chloride loads over time may be related 

to a variety of factors, including increases in road area and 

consequent deicing, increases in wastewater and septic-system 

discharges, recycling of chloride from drinking water, and 

leachate from landfills and salt storage areas.
2 Chloride in Groundwater and Surface Water in Areas Underlain by the Glacial Aquifer System, Northern United States

Introduction

The use of salt has increased measurably in the United 

States since 1950 (Kostick, 1993; Kostick and others, 2007) 

(fig. 1A), and the major increase in the use of salt has been for 

deicing of roads, parking lots, and other impervious surfaces 

during the winter months (fig. 1B). The application of salt for 

deicing these surfaces has raised awareness of potential 

adverse effects on water resources (Bubeck and others, 1971; 

Huling and Hollocher, 1972; Wulkowicz and Saleem, 1974).

Widespread upward trends in chloride concentrations 

in streams nationwide have been reported from 1974–81 

(Smith and others, 1987). Similar trends have been reported 

in Connecticut from the 1970s to the 1990s (Trench, 1996; 

Colombo and Trench, 2002) and in New Jersey (Hay and 

Campbell, 1990; Robinson and others, 1996). Several studies 

have shown elevated concentrations of chloride and sodium 

in glacial aquifers related to urban land use (Grady and 

Mullaney, 1998; Fong, 2000; Thomas, 2000; Savoca and 

others, 2000). 

The primary concern for water quality is the degradation 

of groundwater and surface water that may be used for 

drinking-water supply or for aquatic habitat. The U.S. 

Environmental Protection Agency (USEPA) has set a secondary maximum contaminant level (SMCL) of 250 mg/L for 

chloride in drinking water (U.S. Environmental Protection 

Agency, 1992). The SMCL for chloride is an unenforceable 

guideline that relates to the aesthetics of the water and the 

perceived salty taste of water at concentrations above 

250 mg/L. The USEPA-recommended chronic criterion for 

aquatic life is a 4-day average chloride concentration of 230 

mg/L with an occurrence interval of once every 3 years, and 

the recommended acute criterion concentration for chloride is 

860 mg/L (U.S. Environmental Protection Agency, 1988). The 

acute criterion relates to a 1-hour average concentration with 

a recurrence interval of less than once every 3 years. Other 

concerns regarding salt inputs include the effects of cationexchange reactions on the quality of water (Granato and 

others, 1995).

The U.S. Geological Survey (USGS) has been collecting 

data since 1991 on the quality of water in the Nation’s 

aquifers and streams through the National Water-Quality 

Assessment (NAWQA) Program. Data are synthesized at 

the scale of regional principal aquifers (Lapham and others, 

2005) to understand water-quality issues affecting aquifers 

used commonly for domestic and public water supply. The 

glacial aquifer system (Warner and Arnold, 2005) is the largest 

principal aquifer (fig. 2) in the United States, in terms of areal 

extent and its use as a source for drinking water. The glacial 

aquifer system is considered here to include all unconsolidated 

aquifers overlying bedrock north of the line of continental 

glaciation. Warner and Arnold (2005) divided the glaciated 

area in the United States into four major glacial regions (fig. 2) 

based on differing glacial source material: east, central, westcentral and west. 

The glacial aquifer system also is a source of surface 

water, because the aquifer and stream systems are commonly 

in hydraulic connection. Therefore, the discharge of groundwater to streams affects the quality of surface water, and 

surface water at times can become a source of groundwater.

Purpose and Scope

This report describes the concentrations of salt-related 

constituents (primarily chloride) in shallow groundwater 

and surface water in the glaciated northern United States, on 

the basis of water-quality data collected from 1991 to 2004 

for the USGS NAWQA Program at 1,329 wells in 19 states 

and 100 surface-water sites in 15 states. The report presents 

information on natural and anthropogenic sources of chloride 

in the environment, concentrations of sodium and chloride in 

groundwater, concentrations of chloride in surface water, and 

estimates of chloride loads and yields from selected basins in 

the study area. The report also provides information on the 

relation between ancillary variables (land-use, land-cover, 

hydrologic, and climatic variables) and the concentrations and 

yields of chloride in surface water. 

Groundwater data analyzed for this study generally were 

collected from coarse-grained glacial aquifers, and therefore 

the results may not translate to other areas in the glacial aquifer where coarse-grained deposits are absent, and the surficial 

material is glacial till or bedrock.

Sources of Salts to Water Resources

Natural Sources

Natural sources of salts to water resources include (1) the 

oceans; (2) the natural weathering of bedrock, surficial 

materials, and soils; (3) geologic deposits containing halite, or 

saline groundwater (brines); and (4) volcanic activity.

The oceans typically contain about 19,000 milligrams per 

liter (mg/L) of chloride (Feth, 1981). Sodium and chloride are 

the most abundant ions present in the atmosphere above the 

oceans, and atmospheric deposition of chloride and sodium 

from the oceans is highest along coastal areas. Atmospheric 

deposition may contain natural sources of chloride from the 

oceans, and dusts from surficial salt deposits such as in playa 

lakes, as well as chloride from anthropogenic sources, such as 

manufacturing and power generation. 

Atmospheric Deposition

From 1994 to 2003, wet deposition of chloride contributed an average of 0.04 to 6.2 ton/mi2

 to the glacial aquifer 

system study area (table 1, fig. 3) based on the data from the 

National Atmospheric Deposition Program (NADP) (2006). 

The largest amount of wet deposition of chloride was on the 

eastern and western coastal parts of the study area, including 

the Puget Sound area and the New England coastal basins. The 
Introduction 3

Figure 1. Salt use and consumption in the United States by (A) all end users, 1975–2003 (data from U.S. Geological Survey, 

2005b), and (B) consumption by use or industry, 1975–2005 (data from U.S. Geological Survey, 2005b; Kostick and others, 2007).

35

40

45

50

55

60

YEAR

END USE1, IN MILLIONS OF TONS PER YEAR

197519761977197819791980

1981198219831984198519861987198819891990

199119921993

199419951996

1997

199819992000

2001

20022003

65

1975197619771978197919801981198219831984198519861987198819891990199119921993199419951996199719981999200020012002200320042005

0

7.5

15

22.5

30

END USE1, IN MILLIONS OF TONS PER YEAR

YEAR

Agriculture

Alkalies and chlorine

Deicing

 Metal production

Other chemicals

Paper products

Petroleum

Textiles and dyeing

Water treatment

Other uses2

Food products

EXPLANATION

1

 End use is defined as the use of 

the mineral commodity in a particular 

industrial sector or product.

2

 The other uses category includes 

salt used in rubber, tanning and/or 

leather, industrial, institutional 

(commercial stores) and other 

wholesalers and retailers, 

U.S. Government resale, some

exports for consumption in 

overseas territories administered 

by the United States, and other 

various minor uses (U.S. Geological 

Survey, 2005b). 

SALT USE AND CONSUMPTION, ALL END USES

B CONSUMPTION BY USE OR INDUSTRY

A

Salt use and consumption
4 Chloride in Groundwater and Surface Water in Areas Underlain by the Glacial Aquifer System, Northern United States Figure 2. Glacial aquifer system and National Water-Quality Assessment Program study basins, northern United States. UMIS REDN CNBR LERI LIRB EIWA WHMI CCYK NECB WMIC ALMN CONN HDSN UIRB DELR PUGT LINJ 116° 106° 96° 86° 76° 66° 45° 40° 35°

0 200 MILES 1000 200 KILOMETERS 100

Area ofinset map

Area of map

EXPLANATION

COOK

0 100 50

50

MILES

0 100 KILOMETERS

Region

CentralEastWestWest Central

Study area basin identification

Glacial aquifer system boundary

LIRB—Lower Illinois River BasinWMIC—Western Lake Michigan DrainagesEIWA—Eastern Iowa BasinsUMIS—Upper Mississippi River BasinREDN—Red River of the North BasinCNBR—Central Nebraska BasinsCCYC—Central Columbia Plateau-Yakima River BasinPUGT—Puget Sound DrainagesCOOK—Cook Inlet Drainages

NECB—New England Coastal BasinsCONN—Connecticut, Housatonic, and Thames River BasinsHDSN—Hudson River BasinLINJ—Long Island, New Jersey Coastal DrainagesDELR—Delaware River BasinALMN—Allegheny-Monongahela River BasinsLERI—Lake Erie-Lake St. Clair WHMI—White, Great and Little Miami River UIRB—Upper Illinois River Basin

Base from U.S. Geological Survey 1:2,000,000 Digital DataAlbers Equal-Area Conic projection, standard parallels 29°30’and 45°30’, central meridian -96°, latitude of origin 23°
Introduction 5

lowest amount of wet deposition of chloride was in the central 

parts of the study area, including Iowa, Minnesota, North 

Dakota, and South Dakota. In basins in the United States with 

little or no human activity, the yield of chloride from streams 

is correlated with the amount of precipitation. The chloride 

yield from atmospheric deposition is related to precipitation 

quantity and has been reported to contribute on average 28 

to 62 percent of the chloride yield in undeveloped basins 

throughout the United States. Basins with crystalline bedrock 

were associated with the largest percentage of atmospheric 

chloride yield, followed, in order, by basins underlain by sandstone and basins underlain by limestone (Peters, 1984).

Chloride concentrations in streams and groundwater 

that originated from wet deposition of chloride are different 

from concentrations measured in precipitation because of the 

concentration of chloride by the process of evapotranspiration. 

The average concentrations of chloride from wet deposition 

in runoff (surface and groundwater) can be calculated as the 

wet deposition of chloride divided by the runoff (fig. 4). This 

calculation indicates that average concentrations in runoff 

attributable to wet deposition are typically 0.1–2.0 mg/L. 

Weathering of Common Rocks, Minerals, and Soils

In addition to the chloride deposited on a basin by precipitation, there is a natural input from the weathering of rocks 

and minerals. Chlorine is present in several minerals in common rocks, and its release to waters as chloride ions is generally slow and through processes other than dissolution (Feth, 

1981). The chloride in excess of the load from precipitation 

was attributed to the weathering of the mineral hornblende in 

glacial tills in two Adirondack Mountain basins in New York 

(Peters, 1991).

The chloride yields from 21 basins with no permanent 

residents, underlain by limestone, sandstone, and crystalline 

rocks, were analyzed by Peters (1984), who concluded, on 

the basis of a regression analysis, that 77 percent of the variation in chloride yield from these basins could be explained 

by the quantity of precipitation a basin receives. The relation 

to precipitation probably includes direct wet deposition of 

chloride, as well as chemical weathering, which increases with 

precipitation. A subset of 10 of these basins1 with very low 

population density was identified in the glacial aquifer system. 

The difference between the chloride yield attributable to direct 

input from precipitation and the amount attributable to the 

relation between precipitation and chloride yield probably 

represents an estimate of the amount derived from chemical 

weathering. This difference ranged from 0 to 72 percent of the 

chloride yield. Chloride yields from these 10 basins ranged 

from 0.7 to 68 tons/mi2, although all but one site had yields 

less than or equal to 9 tons/mi2. The estimated annual load 

from chemical weathering in these 10 basins, on the basis of 

the above percentage, ranged from 0 to 2 tons/mi2 with an 

average of 0.9 tons/mi2 (Peters, 1984).

Salt Deposits and Brines

Bedded salt deposits underlie parts of the study area, 

including parts of Michigan, Western New York, Ohio, and 

Pennsylvania (Norris, 1978). New York, Ohio, and Michigan 

produce much of the rock salt and other salts derived from 

brines in the United States. Most halite deposits occur at depth 

or at the downgradient ends of groundwater systems (Feth, 

1981) and therefore generally may not affect the shallowgroundwater quality of the glacial aquifer system. Water with 

high concentrations of dissolved solids is present in sedimentary bedrock at depth in many parts of the study area (Feth, 

1965). These halite deposits and brines may be a salinity 

source for groundwater in zones of flow convergence, where 

water of different ages and from different aquifers (surficial 

to deep bedrock) have mixed. Therefore, it is possible that 

part of the chloride budget for some basins in these areas may 

be from dissolution of halite deposits or mixing with brines. 

This component of the chloride budget is evidenced by saline 

springs in parts of the study area, such as Michigan, Illinois, 

and New York (Michigan Department of Environmental 

Quality, 1994; Panno and others, 2006; Rao and others, 2005).

Anthropogenic Sources

The use of salt in the United States has increased from 

42.9 million tons in 1975 to nearly 58.5 million tons in 2005 

(Kostick and others, 2007) (fig. 1A). The largest use of salt 

has been in the chloralkali industry that produces chlorine and 

sodium hydroxide (Kostick, 1993) (fig. 1B). In 2005, salt used 

by the chloralkali industry represented 34.7 percent of the 

end use of salt. The second largest use of salt since 1975 has 

been deicing. Deicing use has increased since then, and was 

greater than that for the chloralkali industry in 2005, when it 

represented 39.5 percent of the end use of salt in the United 

States (Kostick and others, 2007). Other uses of salt (totaling 

about 25 percent of end use in 2005) include agriculture, food 

processing, metal processing, paper production, textiles and 

dyeing, petroleum production, water treatment, and other 

1

 Basins included the following stations from Peters (1984), table 1: 

4045500, 4057004, 4126520, 4132052, 6452000, 6478500, 1066000, 

4014500, 5124480, 12447390.

Table 1. Estimated wet deposition of chloride on the glacial 

aquifer system, northern United States, 1994–2003. 

[Values are in tons per square mile per year. Data analyzed from the National 

Atmospheric Deposition Program, 2006]

Region Minimum Maximum Mean

East 0.19 4.16 0.71

Central .09 .64 .25

West central .04 .40 .13

West .06 6.22 .91
6 Chloride in Groundwater and Surface Water in Areas Underlain by the Glacial Aquifer System, Northern United States Figure 3. Mean wet deposition of chloride, 1994–2003, and locations of salt mining in the glacial aquifer system study area, northern United States. 116° 106° 96° 86° 76° 66° 45° 40° 35° Glacial aquifer system boundary Study area basin and boundary 0 to .25 greater than .25 to .5 greater than .5 to 1 greater than 1 to 1.5 greater than 1.5 to 2 greater than 2 to 2.5 greater than 2.5 to 3 greater than 3 to 3.5 greater than 3.5 to 4 greater than 4 EXPLANATION Mean wet deposition of chloride1, in tons per square mile per year Salt mining location2

0 200 100 MILES0 200 100 KILOMETERS

1. Calculated from National Atmospheric  Deposition Program, 20062. U.S. Geological Survey, 2005b 

Base from U.S. Geological Survey 1:2,000,000 Digital DataAlbers Equal-Area Conic projection, standard parallels 29°30’and 45°30’, central meridian -96°, latitude of origin 23°
Introduction 

7

Figure 4. Estimated average concentration of chloride, 1994–2002, in runoff attributable to atmospheric (wet) deposition in the glacial aquifer system study area, northern United States. Data are combined from the National Atmospheric Deposition Program (2006) and runoff data from David Wolock, U.S. Geological Survey, written commun., 2006. 

116° 106° 96° 86° 76° 66°

45°40°35°

0 to .25greater than .25 to .5greater than .5 to 1greater than 1 to 2greater than 2 to 3greater than 3 to 4greater than 4 to 5greater than 5 to 10

0 200 100 MILES0 200 100 KILOMETERS

Estimated average concentrationsof chloride in runoff attributable towet deposition, in milligrams per liter

EXPLANATION

Glacial aquifer system boundary

Study area basin and boundary

Base from U.S. Geological Survey 1:2,000,000 Digital DataAlbers Equal-Area Conic projection, standard parallels 29°30’and 45°30’, central meridian -96°, latitude of origin 23°
8 Chloride in Groundwater and Surface Water in Areas Underlain by the Glacial Aquifer System, Northern United States

manufacturing (fig. 1B). The northern states that compose 

the glacial aquifer system (fig. 2) were the end destination for 

about 76 percent of the shipments of evaporated salt and rock 

salt (excluding brine) in the United States in 2005 (Kostick 

and others, 2007).

For many uses of salt, the chloride and sodium may end 

up being discharged to groundwater and surface waters as a 

direct or indirect result of use. The common pathways include 

atmospheric deposition; the dissolution of deicing salts from 

normal use on streets, parking lots, highways, and other paved 

surfaces; storage and handling of deicing salt; release of brines 

from salt, oil, and gas production; leaching from landfills; the 

treatment of drinking water and wastewater; and discharge of 

wastewater from treatment facilities or septic systems. The 

section below describes some of the common pathways for 

salt to enter the environment from human use.

Application of Deicing Salts

The use of deicing salts has improved the safety of winter 

weather driving, reducing accident rates by a factor of 8 on 

two-lane highways and 4.5 on multi-lane highways. The use 

of salt reduces injuries, property damage, and the severity 

of accidents during winter storms, thereby reducing costs of 

accidents (Kuemmel and Hanbali, 1992).

General highway application rates for deicing salt range 

from less than 1 ton per lane mile2 per year to 20 tons per lane 

mile per year (Transportation Research Board, 1991; Jones 

and Sroka, 1997). Information on the rate of deicing-salt 

applications was acquired by compiling a list of the cities 

nearest to groundwater-sampling locations (table 2) and 

conducting an informal, qualitative survey of 20 local and 

State highway superintendents to provide information on the 

type and amount of deicing chemicals being applied to roads 

and highways in different parts of the study area. Respondents 

were asked to describe the number of lane or road miles 

serviced, and the type and amount of deicing chemicals purchased; application rates in units of tons per road or lane mile 

were calculated on the basis of that information (table 2). 

Most of the highway superintendents reported the use of 

sodium chloride rock salt. Calcium and magnesium chloride 

were used in some cases as a wetting agent for rock salt. 

Application rates per lane mile ranged from less than 

1 ton per road mile in Washington State to 74.5 tons per lane 

mile for a section of Interstate 84 in southeastern New York 

(Heisig, 2000). Most rates for deicing salt reported by highway officials ranged from 10 to 30 tons per lane mile (table 2). 

These values are similar to those reported in the literature.

In addition to the effects of application of deicing salts 

to impervious areas, salt-storage areas have been sites of 

historical contamination caused by runoff from uncovered 

salt piles, and infiltration when stored over pervious areas 

2

 The lane mile is an areal measurement of road surface. The lane mile is 

one traffic lane (12 ft wide), extending for 1 mi along a road. When applied to 

highways, this measurement unit includes paved borders and breakdown lanes 

(Church and others, 1996).

(Ostendorf and others, 2006). Many of these historical sites 

have been improved in recent years through the use of covered 

salt-storage buildings on impervious substrate. However, high 

chloride concentrations can persist in groundwater because of 

the long traveltimes from recharge areas to discharge at a well 

or surface-water body.

Highway maintenance crews in some parts of the study 

area are using techniques that enhance the effectiveness of 

deicing practices. These anti-icing techniques are designed to 

use salt more effectively by preventing a bond between ice and 

road surfaces rather than melting ice that has already bonded 

to the road. These updated practices include the use of road 

weather-information systems (RWIS) to decide which road 

deicing practices are appropriate based on weather and road 

temperature data and the use of salt that has been prewetted, or 

salt brine, on roadways prior to a predicted storm 

(Aultman-Hall and others, 2006).

Landfills

Salt from human consumption or activities is commonly 

deposited in municipal landfills. These salts typically are from 

food wastes and other products containing salt, including 

rubber, metals, and paper products. The estimated 10.3 million tons of salt or salt-bearing products deposited in landfills 

in 1990 represented 23 percent of reported consumption 

(Kostick, 1993). This estimated mass indicates that landfills 

can be source areas for salt constituents. Chloride concentrations are typically high in landfill leachate or in groundwater 

beneath or downgradient from landfills. For instance, the 

median chloride concentration in leachate from seven landfills in Illinois was 1,284 mg/L (Panno and others, 2006). 

The chloride concentration in leachate in a municipal landfill 

in Waterbury, Connecticut, was 724 mg/L in 1997 (Fuss & 

O’Neill, Inc., 1997), and 247 mg/L in shallow groundwater 

downgradient from the landfill (Mullaney and others, 1999). 

Landfills are commonly limited in aerial extent and can be 

areas with low groundwater recharge because of impervious 

capping material, which limits the release of large volumes of 

leachate to shallow aquifers or surface-water bodies. However, 

as with deicing-salt storage areas, landfills can be a longterm source of salt to aquifers and streams as a result of long 

groundwater traveltimes.

Wastewater and Water Treatment

Chloride in the environment is conservative (nonreactive), and thus there is little loss when chloride in salts 

contained in food, beverages, and household cleaning products 

is discharged to the environment through septic systems and 

wastewater-treatment facilities. In addition, water softeners 

used to treat water hardness commonly use salt brine to 

regenerate the resin in the treatment system with sodium, 

displacing calcium and magnesium. The remaining brine is 

then disposed of through the wastewater system, or to a dry 

well, where it may enter the underlying aquifer and ultimately 

discharge to a surface-water body.
Introduction 

9

Table 2. Selected annual application rates of deicing chemicals on State and local roads in the glacial aquifer system, northern United States.—Continued

State, municipality, or region State

Road 

miles 

managed

Lane 

miles 

managed

Deicing chemical

Total 

amount of 

deicing salt 

used

(tons)

Average 

use per 

lane 

mile

(tons)

Average 

use per 

road 

mile

(tons)

Source

Connecticut Department of Transportation CT 3,276 Sodium chloride 101,947 31.1 Pat Rodgers, Connecticut DOT, oral commun., 2006Manchester CT 200 Sodium chloride 2,500 12.5 Kenneth Longo, Manchester Department of 

Public Works, oral commun., 2006

Woodbury CT 86 Sodium chloride 900 10.5 Woodbury Department of Public Works, oral 

commun., 2006

Scituate Reservoir drainage basin–State 

Roads

RI 90 191 Sodium chloride 611 3.2 6.8 Nimiroski and Waldron, 2002, note that estimated sodium chloride reported based on 

calcium chloride/sodium chloride mixture

Scituate Reservoir drainage basin–Local 

Roads

RI 139 277 Sodium chloride 2,784 10.1 20.1 Nimiroski and Waldron, 2002

Iowa Department of Transportation–State 

Roads

IA Sodium chloride 3.8 Transportation Research Board, 1991

Ames IA 230 Sodium chloride 2,000 8.7 John Joiner, Director, Ames Iowa, Department of Public Works, oral commun., 2006Cedar Rapids IA 720 Sodium chloride 7,500 10.4 Department of Street Maintenance, oral commun., 2006Waverly IA 75 Sodium chloride 500 6.7 Brian Sullivan, Superintendent, Division of 

Streets, Public Works Department, Waverly, IA, oral commun., 2006

Illinois Department of Transportation IL Sodium chloride 6.6 Transportation Research Board, 1991Elgin IL 312 Sodium chloride,

some calcium chloride liquid

6,000 19.2 City of Elgin, Illinois, Web page at

http://www.cityofelgin.org/index.asp?NID=180

Gurnee IL 210 Sodium chloride 14.3 City of Gurnee, Illinois, Web page at

http://www.gurnee.il.us/public_works/about.html#streetdivision

Indiana Department of Transportation IN Sodium chloride 335,137 Indiana Department of Transporatation, 2004, 

Web page at http://www.in.gov/dot/div/communications/2004annualreport/Safety.pdf

Indiana Department of Transportation IN Sodium chloride 9.0 Transportation Research Board, 1991
10 Chloride in Groundwater and Surface Water in Areas Underlain by the Glacial Aquifer System, Northern United States Table 2. Selected annual application rates of deicing chemicals on State and local roads, in the glacial aquifer system, northern United States.—Continued State, municipality, or region State Road miles managed Lane miles managed Deicing chemical Total amount of deicing salt used (tons) Average use per lane mile (tons) Average use per road mile (tons) Source Columbus IN Sodium chloride 2,000 8.5 Steven Brown, City of Columbus, Indiana, Public Works, oral commun., 2006 Indianapolis Department of Transportation IN 7,329 4,000 Cargill ClearLane (treated sodium chloride) liquid calcium chloride, mag- nesium chloride, and brine also used 48,000 12.0 6.5 John Burkhardt, City of Indianapolis, Department of Public Works, oral com- mun., 2006 State of Michigan Department of Trans- portation MI Sodium chloride 12.9 Transportation Research Board, 1991 Brighton MI 25 50 Sodium chloride 1,505 21.0 42.0 Matthew J. Schindewolf, Director of Public Services, Brighton, Michigan, oral com- mun., 2006 Farmington Hills MI 301 Sodium chloride 6,500 21.6 Daniel Rooney, Division of Public Works, Farmington Hills, Michigan, oral com- mun., 2006 Rochester Hills MI 506 Sodium chloride 4,270 8.4 16.9 Roger Rousse, Department of Public Service, Rochester Hills, Michigan, oral commun., 2006 Wolverine Lake MI 20 Sodium chloride 750 37.5 Andy Stone, Department of Public Serivce, Wolverine Lake, Michigan, oral commun., 2006 Minnesota Department of Transportation MN 12,000 Sodium chloride 233,434 & 2.3 Mgal of brine 19.5 Minnesota Department of Transportation, 2006 Twin Cities Metro Area MN 5,900 Sodium chloride 103,000 17.5 Minnesota Department of Transportation, 2006 Brooklyn Center MN 139 Sodium chloride 27.0 Wenck Associates, Inc., 2004 Brooklyn Park MN 243 Sodium chloride 27.0 Wenck Associates, Inc., 2004 Crystal MN 112 Sodium chloride 3.0 Wenck Associates, Inc., 2004 Robbinsdale MN 88 Sodium chloride 12.0 Wenck Associates, Inc., 2004
Introduction 11

Table 2. Selected annual application rates of deicing chemicals on State and local roads, in the glacial aquifer system, northern United States.—Continued

State, municipality, or region State

Road 

miles 

managed

Lane 

miles 

managed

Deicing chemical

Total 

amount of 

deicing salt 

used

(tons)

Average 

use per 

lane 

mile

(tons)

Average 

use per 

road 

mile

(tons)

Source

New Hampshire Department of Transportation, Interstate 93 NH 111 Sodium chloride 2,762 24.9 Phil Trowbridge, New Hampshire Depart- ment of Environmental Services, written 

commun., 2006

New York, Croton Watershed–Interstate 84 NY Sodium chloride 74.5 298.0 Heisig, 2000New York, Croton Watershed–Taconic 

Parkway

NY Sodium chloride 18.8 75.0 Heisig, 2000

New York, Croton Watershed Local Roads NY Sodium chloride 9.3 37.0 Heisig, 2000Colonie Village NY 35 Sodium chloride 1,500 43.5 Carl Fleshman, Superintendent of Public 

Works, Colonie Village, NY, oral commun., 2006

Ohio Department of Transportation OH Sodium chloride,

calcium chloride also used

9.7 Kunze and Sroka, 2004 (for selected counties)

Harrison OH 100 Sodium chloride, 

calcium chloride

600, 

1,000 gal.

6.0 James Leslie, Director of Public Works, 

Harrison, OH, oral commun., 2006

West Carrollton OH 300 Sodium chloride,

calcium chloride

800 0.4

Lacey City WA 25 Sodium chloride 10 0.4 Dennis Ritter, Public Works Department, 

Lacey, WA, oral commun., 2006

Olympia WA 200 Calcium magnesium acetate, 

some sodium chloride

5 0.03 Randy Stewart, Department of Transportation, Lacey, Washington, oral commun., 

2006

Brookfield WI 482 Sodium chloride 4,400 9.1 17.2 Terry Starns, Department of Public Works, 

Brookfield, Wisconsin, oral commun., 2006

Lake Geneva WI 300 Sodium chloride 300 7.5 Lynn Allen, Department of Public Works, 

City of Lake Geneva, Wisconsin, oral commun., 2006

Milwaukee WI 7,112 Sodium chloride 50,000 7.0 Hintz and others, 2001Sussex WI 45 Sodium chloride 1,500 33.3 Jeremy Smith, Department of Public Works, 

Sussex, Wisconsin, oral commun., 2006
12 Chloride in Groundwater and Surface Water in Areas Underlain by the Glacial Aquifer System, Northern United States

A conservative estimate of the per capita salt consumption and chloride loss from wastewater can be made on the 

basis of the recommended daily adult sodium intake of 

2,300 mg/d. The salt intake associated with 2,300 mg/d 

of sodium is 5,847 mg, which includes about 3,547 mg of 

chloride. This indicates that the average person on this diet 

would consume about 2.1 kg (4.7 lbs) of salt per year, 

and release about 1.3 kg (2.9 lbs) of chloride per year to 

wastewater discharge.

Water softeners can release considerably larger amounts 

of chloride to the environment. Discharge of chloride to the 

environment from water softening is through on-site septic 

systems, dry wells, or wastewater-treatment facilities. In 2005, 

salt for water treatment represented 3.1 percent of salt use in 

the United States (Kostick and others, 2007). Salt use varies 

in a typical residential water-softener installation, however, 

because of differences in water softeners, water use, and 

water hardness. 

Water softeners likely are used extensively in the study 

area for homes with self-supplied domestic wells and in areas 

with publicly provided surface and groundwater supplies. 

Moderately hard water is found in most regions of the study 

area, although soft water predominates in the New England 

area. Hard and very hard waters are present in central and west 

central States, including Ohio, Michigan, Illinois, Wisconsin, 

Minnesota, and North Dakota (Briggs and Ficke, 1977). 

Agriculture

Agricultural use of salt was about 3.5 percent of total use 

in 2005 (Kostick and others, 2007). Salt is used as an animal 

feed additive and may be used in other agricultural products 

such as pesticides and fertilizers. Salt from animal feeds may 

discharge to surface water and groundwater from feedlots and 

manure containment areas and from the use of manure as a 

fertilizer. Panno and others (2006) reported a median chloride 

concentration of 847 mg/L from hog and horse waste, and a 

median concentration of 57 mg/L in water samples from wells 

affected by animal waste in Illinois.

Agricultural fertilizers can be a source of chloride to 

receiving waters because the potassium in most fertilizers is 

in the form of potassium chloride. Other sources of chloride 

include the concentration and dissolution of salts resulting 

from irrigation from deep groundwater sources. 

Methods of Data Analysis

NAWQA Program data from 1991 to 2004 from wells 

in the glacial aquifer system and from streams draining areas 

underlain by the glacial aquifer system were analyzed for this 

study. Ancillary information was synthesized from data compiled for each surface-water-quality station that was sampled 

in the NAWQA Program. Many surface-water monitoring stations had additional water-quality data collected by the USGS 

for State water-quality monitoring networks. 

Data analysis included graphical plotting of concentrations of water-quality constituents, multiple-comparison tests, 

and statistical analyses of the relations between ancillary variables and chloride concentrations and yields in surface water. 

A multiple linear regression model (LOADEST) (Runkel and 

others, 2004) was used to analyze chloride loads in water from 

95 surface-water monitoring stations.

Compilation of Environmental Data

Groundwater-quality and surface-water-quality data were 

synthesized from data collected from 1991 to 2004 in the 

study basins shown in figure 2. Groundwater-quality data were 

compiled from a national database established for all groundwater samples collected for the NAWQA Program in the glacial aquifer system study area. The groundwater data collected 

represent two different networks: (1) shallow monitoring wells 

in forested, agricultural, or urban land (land-use studies), 

and (2) drinking-water supply wells tapping glacial deposits 

(major aquifer studies) (Rosen and Lapham, 2008).

In land-use studies, data were collected from installed 

observation wells and selected existing observation wells to 

assess the quality of recently recharged shallow groundwater 

beneath different land-use settings (Gilliom and others, 1995). 

For this investigation, data from 797 wells from land-use studies (fig. 5) representing forested, agricultural, and urban areas 

were compiled for statistical analysis. Samples generally were 

collected only once from these wells. 

In major aquifer studies conducted by the NAWQA 

Program, data were collected from 94 public and 438 private 

drinking-water supply wells in the glacial aquifer system 

(drinking-water network, fig. 5). These wells are typically 

deeper than the wells sampled for land-use studies and may 

integrate recharge from several land uses.

Water-quality data from 100 surface-water monitoring 

stations (Appendix 1) draining basins dominated by forested, 

agricultural, or urban land were selected for analysis (fig. 5). 

The contributing areas of these basins average 286 mi2, and 

the median is 88 mi2; of these basins, at least 31 contain 

major point-source wastewater discharges listed in the Permit 

Compliance System (PCS) database (U.S. Environmental 

Protection Agency, 2007). Surface-water monitoring stations 

had at least 20 samples collected over at least a 3-year period 

from 1991 to 2004. Samples generally were collected monthly 

for 3 to 13 years. Nineteen of the stations selected have longterm-flow and water-quality records that began in the 

1960s to 1980s.

Multiple Comparison Testing

Multiple comparison tests were done by using S-PLUS 

7 software (Insightful Corporation, 2005) to determine if 

populations of log base 10 chloride and sodium concentrations 

and chloride:bromide ratios by mass (Cl:Br) in groundwater 

were significantly different by adjacent land use or well type. 
Methods of Data Analysis 13 Figure 5. Wells and surface-water-quality monitoring stations used in the analysis of the glacial aquifer system study area, northern United States.

!!!!!!! !!!!!!!!!!!! ! !! !!!! !

0 100 MILES 500 100 KILOMETERS 50

0 200 MILES 100

0 200 KILOMETERS 100

116°

45°40°35°

96° 86° 76° 66° 106°

Surface water-quality monitoring stationWell in land-use networkWell in drinking water network

EXPLANATION

RegionNational water quality assessment study units Glacial aquifer system boundary

CentralEastWestWest Central

Base from U.S. Geological Survey 1:2,000,000 Digital DataAlbers Equal-Area Conic projection, standard parallels 29°30’and 45°30’, central meridian -96°, latitude of origin 23°
14 Chloride in Groundwater and Surface Water in Areas Underlain by the Glacial Aquifer System, Northern United States

This analysis also was used with maximum measured chloride 

concentrations and yields from samples collected in streams. 

A one-way analysis of variance (ANOVA) test was used to 

determine if the null hypothesis was rejected; the null hypothesis states that the means of all groups were not significantly 

different. If the null hypothesis was rejected, a Tukey’s test for 

pairwise comparisons was done (Helsel and Hirsch, 2002). 

Multiple Linear Regression Modeling 

Multiple linear regression models were developed to 

describe maximum measured chloride concentrations 

(1991–2004) and mean chloride yield (1991–2003) for 

samples collected from surface-water monitoring stations. 

Explanatory variables that were evaluated for use in the models included land-use/land-cover characteristics, population 

density, road density, hydrologic and climatic variables, and 

wastewater discharges (table 3). Variables were selected on the 

basis of plausibility, statistical significance, and the distribution of residuals.

Data on land-use and land-cover characteristics were 

determined from LANDSAT Thematic Mapper (TM) images 

created for the National Land Cover Dataset (NLCD) 

(Vogelmann and others, 1998). The version of the NLCD 

(NLCDe) used was enhanced as described by Nakagaki and 

Wolock (2005).

Road density was computed as the length of all roads in 

a basin divided by basin area (Curtis Price, U.S. Geological 

Survey, written commun., 2006). Road density by category 

was compiled for surface-water basins from the digital 

Streetmap layer of the Tele Atlas North America (ESRI, Inc., 

2006).

Potential evapotranspiration data for each surface-water 

site were developed through the use of the Parameter-elevation 

Regressions on Independent Slopes Model (PRISM) with the 

methodology described by Daly and others (1994).

Hydrologic variables were compiled from a national 

application of the TOPMODEL rainfall-runoff model (Wolock, 

1993). Stream base-flow index was estimated by using data 

developed by Wolock (2003). 

Estimation of Chloride Loads

Chloride loads in streams were estimated for 95 basins 

(those with sufficient data for load estimation)—21, 44, and 30 

representing forested, agricultural, and urban as the dominant 

land use, respectively (Appendix 1). Loads were estimated for 

water years with available data from 1992–2003. Nineteen stations had data collected earlier than 1992, and chloride loads 

at these sites were plotted against time to identify obvious 

long-term trends.

Loads were estimated by using a multiple linear 

regression model included in the computer program 

LOADEST (Runkel and others, 2004). Given a time series 

of streamflow, additional data variables, and constituent 

concentration, LOADEST assists the user in developing 

a regression model for the estimation of constituent load. 

Explanatory variables in the regression model include various functions of streamflow, decimal time, and additional 

user-specified data variables. The formulated regression 

model then is used to estimate loads over a user-specified time 

interval. Mean load estimates, standard errors, and 95-percent confidence intervals are developed on an annual basis. 

The calibration and estimation procedures in LOADEST are 

based on three statistical estimation methods. The first two 

methods, Adjusted Maximum Likelihood Estimation (AMLE) 

and Maximum Likelihood Estimation (MLE), are appropriate 

when the calibration model errors (residuals) are normally distributed. Of the two, AMLE is the method of choice when the 

calibration data set (time series of streamflow, additional data 

variables, and concentration) contains censored data. The third 

method, Least Absolute Deviation (LAD), is an alternative to 

MLE when the residuals are not normally distributed (Runkel 

and others, 2004). Load estimates for selected urban basins 

were plotted to demonstrate visible trends with time by using a 

LOWESS smooth (Helsel and Hirsch, 2002).

Chloride in Groundwater and 

Surface Water

The concentrations of chloride and sodium in water 

from wells in the land-use studies network and major aquifer 

studies network were compared first by land use and well type 

and then to applicable drinking-water standards. Sources of 

chloride were evaluated on the basis of Cl:Br plotted against 

chloride and compared to binary mixing curves estimated for 

different sources. 

Surface-water-quality data were compared with the 

recommended criteria for chloride concentrations to protect 

aquatic life and to determine the months when the recommended criteria concentrations commonly were exceeded. A 

multiple linear regression model was developed to describe 

the variability of the natural log of the maximum measured 

chloride concentration at all surface-water monitoring stations. These maximum measured concentrations of chloride 

were compared with concentrations of chloride in base-flow 

samples from each station to determine if base-flow concentrations could be used to predict which stations would have 

chloride concentrations greater than recommended aquatic 

criteria. Lastly, the loads and yields of chloride from 95 

monitoring stations were computed, and yields were compared 

by dominant land use. A multiple linear regression model was 

used to describe the variability of the natural log of chloride 

yield. Selected stations with long-term data were used to show 

examples of apparent trends in chloride loads with time. 
Chloride in Groundwater and Surface Water 15

Groundwater

Groundwater samples collected from wells in the 

land-use studies contained different distributions of chloride 

depending on the dominant land use (fig. 6). The largest 

median concentration of chloride was in samples from urban 

land-use wells (46 mg/L) and was about 16 times larger than 

the median concentration in samples from forested land-use 

wells (2.9 mg/L). The median concentration in water samples 

from agricultural land-use wells (12 mg/L) was about four 

times larger than in samples from forested land-use wells. The 

median concentration of chloride in drinking-water supply 

wells was 26 mg/L in public-supply wells and 12 mg/L in private domestic wells (fig. 6). The results of a one-way ANOVA 

indicated that the means of the populations of log base-10 

transformed chloride data were significantly different. A 

Tukey’s test indicated that only the agricultural land-use wells 

and the private drinking-water wells were not significantly 

different (fig. 6).

Chloride concentrations in groundwater were greater 

than the SMCL of 250 mg/L for drinking water 

(U.S. Environmental Protection Agency, 1992) in samples 

from 20 of 797 land-use wells (2.5 percent); 17 were urban 

land-use wells and 3 were agricultural land-use wells. 

Chloride concentrations were greater than 250 mg/L in 10 

of 532 drinking-water wells (1.7 percent); 9 were private 

domestic wells and 1 was a public-supply well. Groundwater 

with concentrations of chloride in excess of 230 mg/L that 

discharges to surface water may cause toxic effects to aquatic 

life on the basis of the recommended criteria established by 

U.S. Environmental Protection Agency (1988).

Table 3. Watershed characteristics evaluated in a regression analysis of maximum measured chloride concentrations. 

[NLCDe, National Land Cover Dataset, Vogelmann and others, 1998, enhanced as described by Nakagaki and Wolock (2005); km, kilometer; km2, square 

kilometer; cm, centimeter; mi, mile; mi2, square mile; PRISM, Parameter-elevation Regressions on Independent Slopes Model; PCS, permit compliance 

system]

Land use, land cover (Vogelmann and others, 1998), population and road density

Percent of basin composed of low intensity residential, NLCDe

Percent basin composed of high intensity residential, NLCDe

Percent basin composed of commercial/industrial/transportation, NLCDe

Percent of basin composed of orchards/vineyards/other, NLCDe

Percent basin composed of pasture/hay, NLCDe

Percent basin composed of row crops, NLCDe

Percent basin composed of small grains, NLCDe

Percent basin composed of fallow, NLCDe

Population density 2000, in people/km2 (ESRI, 2006)

Road density in km/km2 (Curtis Price, U.S. Geological Survey, written commun., 2006)

Road density by type (ESRI, Inc., 2006)

Major highway density, in mi/mi2

Major highway connector density, in mi/mi2

Highway density, in mi/mi2

Major road density, in mi/mi2

Local road density, in mi/mi2

Hydrologic and climatic variables

Percent of basin streamflow contributed by Dunne overland flow (estimated by means of TOPMODEL (Wolock, 1993) hydrologic model)

Percent of basin streamflow contributed by Horton overland flow (estimated by means of TOPMODEL (Wolock, 1993) hydrologic model)

Mean subsurface contact time in days (estimated by means of TOPMODEL (Wolock, 1993) hydrologic model)

Base-flow index

Mean potential evapotranspiration in cm (PRISM) [Daly and others, 1994]

Wastewater discharges

Number of major discharges upstream of monitoring site, PCS database (U.S. Environmental Protection Agency, 2007)

Number of minor discharges upstream of monitoring site, PCS database (U.S. Environmental Protection Agency, 2007)
16 Chloride in Groundwater and Surface Water in Areas Underlain by the Glacial Aquifer System, Northern United States

Because sodium in drinking water is considered a human 

health issue, concentrations of sodium were also summarized 

for groundwater sites in this study. The distribution of sodium 

concentrations (in samples from 1,332 sites) was similar to 

that of chloride in the land-use wells. The largest median 

concentration of sodium was in samples from urban-land-use 

wells (26 mg/L) and was more than six times larger than the 

median concentration of sodium (4.1 mg/L) in water samples 

from forested-land-use wells. The median sodium concentration in water samples from agricultural-land-use wells (6.3 

mg/L) was larger than the median sodium concentrations in 

the forested-land-use wells (fig. 7). The median sodium concentration was 19 mg/L in public-supply wells and 12 mg/L in 

private domestic wells. The only Federal drinking-water standard for sodium is an unenforceable Drinking Water Advisory 

of 20 mg/L for individuals on a 500 milligram per day (mg/d), 

low-sodium diet (U.S. Environmental Protection Agency, 

2004). Water in the majority of urban land-use wells (57.1 

percent) had concentrations of sodium equal to or greater than 

20 mg/L, followed by agricultural land-use wells (16.7 percent) and forested land-use wells (8.0 percent). The concentration of sodium was equal to or larger than 20 mg/L in samples 

from 46.8 percent of the public-supply wells and 33.9 percent 

of the domestic wells.

Sources of Chloride in Groundwater

Chloride:bromide ratios (Cl:Br) have been used by several researchers to identify the sources of chloride in groundwater (Davis and others, 1998; Thomas, 2000; Jagucki and 

Darner, 2001; Panno and others 2006). The method is based 

on understanding the Cl:Br and chloride concentrations of 

different chloride sources, and the resulting Cl:Br and chloride 

concentrations when these sources are mixed. 

Groundwater samples collected from a network of 

forested land-use wells had the lowest median Cl:Br ratio 

(by mass) (148). Samples collected from wells in the urban 

land-use study had the highest median Cl:Br (879), followed 

by the samples from wells in the agricultural land-use well 

study (283) (fig. 8). The median Cl:Br of water samples from 

the private drinking-water wells (193) was not significantly 

different from the median for forested land-use wells, and the 

median Cl:Br of samples from the public drinking-water-supply wells (673) was not significantly different from the ratio 

for the urban land-use wells (fig. 8).

Mixing curves (fig. 9A) were developed to represent 

binary mixtures of halite, sewage or animal waste, potassium 

chloride fertilizers, landfill leachate, basin brines, and seawater sources of chloride on the basis of data and studies listed in 

table 4. Binary mixing-curve lines were determined by using 

Figure 6. Distribution of chloride concentrations in samples from shallow monitoring wells in forested, agricultural, and urban 

areas and in drinking-water supply wells in the glacial aquifer system, northern United States, 1991–2003.

50 430 317 94 438

Forest Agriculture Urban Public Private

0.1

1

10

100

1,000

10,000

CONCENTRATION OF CHLORIDE, 

IN MILLIGRAMS PER LITER

SMCL

30

90th percentile

75th percentile

median

25th percentile

10th percentile

EXPLANATION 

Number of samples

SMCL, Secondary Maximum Contaminant Level

A B C D B

CHLORIDE CONCENTRATION BY WELL TYPE, p = <0.0001

B Sample populations that share 

the same letter symbol are not significantly 

different at the 0.05 level of significance

p is the attained significance level 

for one-way analysis of variance

Land-use monitoring wells Drinking-water supply wells
Chloride in Groundwater and Surface Water 17

mixing equations described by Jagucki and Darner (2001). 

Data from the land-use wells were plotted with the binarymixing curves to evaluate if any of samples were representative of end members or simple mixtures (figs. 9B–D). 

Points representing groundwater samples that do not fall 

on any of the binary mixing lines reflect the reality that there 

may be more than two sources of chloride, and the actual mixtures in groundwater samples are not likely mixtures of fullstrength end members, but have been diluted with additional 

water. The effect of dilution of a concentrated end member 

with precipitation or dilute groundwater will be to retain the 

Cl:Br, but reduce the chloride concentration. An example of 

this would be a mixture containing chloride from sewage/

animal waste and halite. The binary mixing curves 5 and 6 on 

figures 9A–D are based on the assumption that mixtures can 

contain different proportions of the full-strength end members. 

The halite end members were assumed to have a chloride 

concentration of 10,000–20,000 mg/L; higher concentrations 

are possible based on the solubility of halite. In reality, deicing 

salt is likely to be diluted with precipitation before recharging 

the groundwater, and septic-system waste may also be diluted 

with precipitation or dilute groundwater before moving downgradient. Therefore, most mixtures are not likely equivalent to 

binary mixtures, unless samples are collected very close to the 

source of the chloride.

The samples from the forested-land-use wells generally 

had low Cl:Br and low concentrations of chloride; however, 

the positions of the plotted points for nine samples (fig. 9B) 

indicated human influence from halite or sewage and animal 

waste. Samples from the agricultural-land-use wells were not 

dominated by any one source of chloride (fig. 9C), although 

there was a large concentration of points around mixtures of 

dilute groundwater and potassium chloride or sewage and 

animal waste. Sources of chloride in agricultural areas could 

include deicing salt; salt used in animal feeds, fertilizer, and 

water softeners; salt in animal waste; and the use of deep 

groundwater (containing brines) for irrigation. Points for chloride concentrations above 100 mg/L in samples from agricultural-land-use wells plotted near the binary mixing curves 

(lines) for dilute groundwater and (1) halite, or (2) sewage 

or animal waste mixed with halite, or (3) dilute groundwater 

mixed with basin brines, or (4) landfill leachate.

Points representing samples from the urban-land-use 

wells plotted along or between the mixing lines for dilute 

groundwater/halite, or dilute groundwater/sewage or animal 

waste, and dilute groundwater and potassium chloride fertilizer (fig. 9D). Many samples had a Cl:Br of 1,000 or higher 

and chloride concentrations greater than 100 mg/L. This analysis shows that, as in the agricultural areas, there are mixed 

sources of anthropogenic inputs of chloride to groundwater. 

Samples with a Cl:Br greater than 1,000 and chloride concentrations greater than 100 mg/L are likely to be dominated by 

halite inputs from deicing or water softeners.

Points representing samples from drinking-water supply wells initially were plotted with the binary mixing curves 

that were used in figures 9A–D; however, there was a wide 

Figure 7. Distribution of sodium concentrations in samples from shallow monitoring wells in forested, agricultural, and urban 

areas and in drinking-water supply wells in the glacial aquifer system, northern United States, 1991–2003.

50 431 317 94 440

CONCENTRATION OF SODIUM, 

IN MILLIGRAMS PER LITER

A B C C D D

1

10

100

1,000

Drinking water advisory

 SODIUM CONCENTRATION BY WELL TYPE, p = <0.0001

30

90th percentile

75th percentile

median

25th percentile

10th percentile

EXPLANATION 

Number of samples

p is the attained significance level 

for one-way analysis of variance

B Sample populations that share 

the same letter symbol are not significantly 

different at the 0.05 level of significance

Forest Agriculture Urban Public Private

Land-use monitoring wells Drinking-water supply wells
18 Chloride in Groundwater and Surface Water in Areas Underlain by the Glacial Aquifer System, Northern United States

scattering of points around the binary mixing lines. There 

were, however, some statistically significant differences in 

the Cl:Br by supply-well type (fig. 8) and by the geographic 

region (fig. 10). Public drinking-water supply wells had a 

significantly higher Cl:Br than private drinking-water wells, 

indicating that water quality in these public drinking-water 

supply wells is influenced by human activities, primarily deicing, water softening, and sewage/animal waste. 

The Cl:Br in samples from the private drinking-water 

supply wells was were not significantly different from ratios 

in samples from the forested land-use wells (fig. 8); however, 

concentrations of chloride were significantly higher in the 

private drinking-water supply wells than in samples from the 

forested land-use wells (fig. 6). 

Samples from drinking-water supply wells in the east had 

significantly higher Cl:Br than samples from other regions in 

the glacial aquifer system (fig. 10). Samples from the westcentral and west parts of the glacial aquifer system had the 

lowest Cl:Br (fig. 10). The Cl:Br in 65 percent of the samples 

from the east and 43 percent of the samples from the central 

parts of the glacial aquifer system were greater than 460 (table 

4, minimum value for sewage and animal waste), indicating 

the influence of chloride from deicing salt, water-softening 

salt, and sewage and animal waste (fig. 10).

Surface Water

Surface-water-quality data from monitoring sites on 

100 streams or rivers that were sampled 10 or more times for 

chloride from 1991 to 2004 were analyzed to make comparisons of chloride concentration to basinwide land use. Samples 

generally were collected monthly for 3 to 13 years. Some sites 

that were part of State networks had many more years of chloride data. Multiple linear regression models were developed 

to describe the distribution of maximum measured chloride 

concentrations and chloride yields for samples collected 

from 1991 to 2004 in forested, agricultural, and urban basins. 

Chloride yields were compared among forested, agricultural, 

and urban basins.

To evaluate the potential for chloride concentrations to 

be greater than the USEPA recommended aquatic life criteria 

described below for chloride (U.S. Environmental Protection 

Agency, 1988), the maximum measured concentrations 

(samples collected from 1991 to 2004) were compared by land 

use (fig. 11). The recommended chronic criterion for aquatic 

life is a 4-day average of 230 mg/L with a recurrence interval 

of once every 3 years (U.S. Environmental Protection Agency, 

1988). Samples from 13 sites with urban land use and 2 sites 

with agricultural land use had chloride values that were greater 

Figure 8. Distribution of chloride:bromide ratios (by mass) in samples from shallow monitoring wells in forested, agricultural, 

and urban areas and in drinking-water supply wells in the glacial aquifer system, northern United States, 1991–2003.

0.1

1

10

100

1,000

10,000

CHLORIDE:BROMIDE RATIO BY MASS

A B C C A

CHLORIDE:BROMIDE RATIO BY WELL TYPE, p = <0.0001

30

35 373 294 86 378

90th percentile

75th percentile

median

25th percentile

10th percentile

EXPLANATION 

Number of samples

p is the attained significance level 

for one-way analysis of variance

B Sample populations that share 

the same letter symbol are not significantly 

different at the 0.05 level of significance

Forest Agriculture Urban Public Private

Land-use monitoring wells Drinking-water supply wells
Chloride in Groundwater and Surface Water 19

than the chronic concentration value at least once (table 5). 

Sampling frequency was insufficient to determine whether the 

4-day average would have been greater than 230 mg/L with a 

recurrence interval of less than once every 3 years. Six of the 

sites had chloride concentrations greater than 230 mg/L in 

10 percent or more of the samples collected. Concentrations in 

samples from three sites were greater than the recommended 

acute aquatic criterion concentration for chloride of 860 mg/L 

(U.S. Environmental Protection Agency, 1988). This criterion 

relates to a 1-hour average concentration with a recurrence 

interval of less than once every 3 years. 

Samples with concentrations of chloride greater than 

230 mg/L generally were collected during the winter and 

spring months, primarily November–April, indicating a 

possible relation with winter deicing activity (table 5). 

Concentrations above the recommended criteria may occur 

during a rain or freezing rain event following the application 

of deicing chemicals, or when daytime temperatures rise high 

enough to melt roadside and parking-lot snow and ice (fig. 12). 

Samples with chloride concentrations above the recommended 

aquatic criteria in late spring and summer may be explained by 

the discharge of groundwater containing high concentrations 

of chloride (for example, Shingle Creek, station 05288705 and 

Lincoln Creek, station 040869415; table 5) or could be related 

to wastewater discharges with high chloride during a low-flow 

period, as in the case of two monitoring stations in Illinois 

(05531500, 05532000) identified in table 5 (CH2M HILL Inc., 

2003).

Concentrations of chloride considered to represent 

base-flow samples collected from 1991–2004 were selected 

from the data by using the sample with the lowest river 

discharge at each site. Samples collected during base-flow 

periods are presumed to represent the quality of groundwater, 

except for basins with major wastewater discharges. The 

median concentration of chloride was 81 mg/L in base-flow 

samples from urban basins, 21 mg/L from agricultural basins, 

and 3.5 mg/L from forested basins. Chloride concentrations 

for the selected base-flow samples were plotted against the 

maximum measured chloride concentrations to determine 

whether there was a relation (fig. 13). Maximum measured 

chloride concentrations were greater than 230 mg/L in samples 

from sites at which base-flow chloride concentrations were 

greater than about 75–90 mg/L. This result suggests that baseflow concentrations can be used to predict which streams may 

have concentrations larger than the USEPA recommended 

chronic criteria for chloride. Streams with elevated concentrations of chloride in base flow may be more likely to exceed 

the recommended criteria during the winter months, because 

the chloride concentrations are high at the beginning of 

snowmelt events that contribute stormwater discharge containing concentrated chloride. Therefore, the loads in base flow 

from groundwater discharge and wastewater discharge are an 

important consideration in basins with chloride concentrations 

larger than 230 mg/L.

Relation of Maximum Chloride Concentration to 

Explanatory Variables

A multiple linear regression model was developed 

to describe the variability of the natural log of maximum 

measured chloride concentrations. The regression analysis 

was performed on chloride data from 83 of the 100 stations 

previously described. This subset was used because data for 

some of the explanatory variables (table 3) were unavailable 

for some stations. The data set included 15 forested basins, 

41 agricultural basins, and 27 urban basins. Three significant 

variables—major road density, potential evapotranspiration, 

and the percentage of the annual streamflow from saturated 

overland flow (Dunne overland flow), (table 6)—explained 

66 percent of the variability in the natural log of the maximum measured chloride concentration. Model residuals were 

approximately normally distributed with a constant variance 

(fig. 14).

Basins with greater density of major roads had higher 

maximum chloride concentrations. This variable indicates a 

likely relation with areas receiving deicing salts, but road 

densities also are usually correlated with overall urbanization 

and population density, indicating the possibility for multiple 

sources of chloride, as described in earlier sections of this 

report. 

Basins with higher average potential evapotranspiration 

generally had higher maximum concentrations of chloride. 

The significance of this variable may be explained as follows: 

sites with low evapotranspiration are more likely to have more 

groundwater recharge (Granato and others, 1995), and areas 

with high potential evapotranspiration may be more likely to 

concentrate salts in basin recharge and runoff.

The variable “Dunne overland flow” was significant and 

had a negative sign. This variable is also known as “saturation 

overland flow” and was described by Dunne and Black (1970). 

The negative sign indicates that basins with the most annual 

saturated overland flow had lower maximum chloride concentrations, because with more overland runoff, there is a greater 

potential for dilution of salts. 

Loads and Yields of Chloride from Forested, 

Agricultural, and Urban Basins

Chloride loads were estimated for 20 forested, 

44 agricultural, and 31 urban basins from 1991 to 2003. The 

average load was determined from the LOADEST results for 

the number of years of available data within the study time 

period. Loads were normalized by drainage area and 

converted to yields in tons of chloride per square mile. The 

median yield was 6.4 tons/mi2 from the forested basins, 15.4 

tons/mi2 from the agricultural basins, and 88 tons/mi2 from the 

urban basins (fig. 15). 
20 Chloride in Groundwater and Surface Water in Areas Underlain by the Glacial Aquifer System, Northern United States

Figure 9. (A) Binary mixing curves representing sources of chloride. The relation of chloride 

concentration to chloride:bromide ratios (by mass) for samples from shallow monitoring wells in 

(B) forested areas, (C) agricultural areas, and (D) urban areas.

10 100 1,000 10,000

1

10

100

1,000

10,000

0.1 1

1. 2.

3.

4.

5. 6.

7. 8.

9.

10.

Range for sewage and 

animal waste end member

10 100 1,000 10,000

1

10

100

1,000

10,000

0.1 1

Approximate range 

for dilute

groundwater

1. 2.

3.

4.

5. 6.

7. 8.

9.

10.

CONCENTRATION OF CHLORIDE, IN MILLIGRAMS PER LITER

CHLORIDE:BROMIDE RATIO BY MASS

Approximate range 

for dilute

groundwater

Binary mixing curves

1. Dilute groundwater/halite used for deicing, high range

2. Dilute groundwater/halite used for deicing, low range

3. Dilute groundwater/sewage or animal waste high range

4. Dilute groundwater/sewage or animal waste low range

5. Halite used for deicing/sewage or animal waste high range

6. Halite used for deicing/sewage or animal waste low range

7. Dilute groundwater/potassium chloride fertilizer

8. Dilute groundwater/basin brines

9. Dilute groundwater/seawater

10. Dilute groundwater/landfill leachate

A

B
Chloride in Groundwater and Surface Water 21

Figure 9. (A) Binary mixing curves representing sources of chloride. The relation of chloride 

concentration to chloride:bromide ratios (by mass) for samples from shallow monitoring wells in 

(B) forested areas, (C) agricultural areas, and (D) urban areas.—Continued

Approximate range 

for dilute

groundwater

Approximate range 

for dilute

groundwater

10 100 1,000 10,000

1

10

100

1,000

10,000

0.1 1

1. 2.

3.

4.

5. 6.

7. 8.

9.

10.

C

10 100 1,000 10,000

1

10

100

1,000

10,000

0.1 1

1. 2.

5. 6.

8.

9.

10.

3.

4.

7.

CONCENTRATION OF CHLORIDE, IN MILLIGRAMS PER LITER

CHLORIDE:BROMIDE RATIO BY MASS

D

Binary mixing curves

1. Dilute groundwater/halite used for deicing, high range

2. Dilute groundwater/halite used for deicing, low range

3. Dilute groundwater/sewage or animal waste high range

4. Dilute groundwater/sewage or animal waste low range

5. Halite used for deicing/sewage or animal waste high range

6. Halite used for deicing/sewage or animal waste low range

7. Dilute groundwater/potassium chloride fertilizer

8. Dilute groundwater/basin brines

9. Dilute groundwater/seawater

10. Dilute groundwater/landfill leachate
22 Chloride in Groundwater and Surface Water in Areas Underlain by the Glacial Aquifer System, Northern United States

Table 4. Chloride:bromide ratios for sources of chloride to water resources.

[mg/L, milligrams per liter; Cl:Br, chloride to bromide ratio by mass; shaded areas represent end members used to draw binary mixing curves]

Chloride source and reference Chloride 

(mg/L)

Bromide 

(mg/L) Cl:Br

Dilute groundwater

Forested land-use wells, glacial aquifer system (median of 35 samples) 3 0.02 148

End member used for dilute groundwater, from forested land-use well network .33 .015 22

Sewage and animal waste

Panno and others, 2006, median of samples from 6 private septic systems 91 .09 769

1

1,011

Davis and others, 1998 (assembled for U.S., England) -- -- 300–600

Thomas, 2000; Jagucki and Darner, 2001 69 -- 460

Peavy, 1978 37–101 -- --

Panno and others, 2006, median of 4 samples of hog and horse waste 847 .570 1,395

Halite

Granato, 1996; analysis of deicing salt -- -- 5,000

End member range used in binary mixing curves for deicing salt 210,000–20,000

Knuth and others, 1990, analysis of deicing salt -- -- 8,400

Davis and others, 1998 -- -- 1,000–10,000

Panno and others, 2006, analysis of deicing salt -- -- 13,497

Panno and others, 2006, analysis of water softening salt -- -- 5,139

Panno and others, 2006, analysis of water softening salt -- -- 3,438

Seawater

Feth, 1981 19,300 67 288

Landfill leachate

Panno and others, 2006, median of 10 leachate samples from Illinois 1,284 7.8 193

3

164

Mullaney and others, 1999, landfill leachate affected groundwater sample 247 1.4 173

Fertilizer

Panno and others, 2006, potassium chloride fertilizer sample analysis 440 -- 510

Basin brines

Panno and others, 2006, Illinois Basin brines summarized from other studies 64,600 -- 406

1

 Cl:Br ratio based on dividing Cl and Br values reported by Panno and others (2006).

2

 Assumed range in concentrated deicing salt runoff in binary mixing curves used.

3

 Cl:Br ratio based on dividing Cl and Br values reported by Panno and others (2006).

4

 Estimated upper limit based on analysis in Panno and others (2006).
Chloride in Groundwater and Surface Water 23

Relation of Chloride Yield to Explanatory 

Variables

A multiple linear regression model was developed 

to describe the natural log of chloride yield. The data set 

included 15 forested basins, 41 agricultural basins, and 27 

urban basins. Four variables (table 7) explained 69 percent of 

the variability in the natural log of chloride yield. Significant 

variables selected or modified from table 3 and shown in table 

7 were Highway density and the Number of major discharges 

upstream of monitoring site in USEPA PCS database + (plus) 

1, potential evapotranspiration, and the difference between the 

percent of urban and agricultural land. Model residuals were 

approximately normally distributed, with a constant 

variance (fig. 16).

Major dischargers include municipal wastewater-treatment facilities with discharges greater than 1 million gallons 

per day (Mgal/d) and other facilities with discharges that have 

been rated by USEPA as major, based on volume and type of 

pollutants and type of receiving waters (Steven Winnett, U.S. 

Environmental Protection Agency, written commun., 2007). 

The variable “number of major discharges upstream of the 

monitoring site in the USEPA Permit Compliance System 

(PCS) database,” if used alone in a simple linear regression 

model, explains about 17 percent of the variability in the chloride yield, indicating that wastewater discharges are a likely 

component of chloride loads in some streams sampled. 

The significance of the variable Highway density 

(table 3) is probably related to areas that receive deicing salts 

in winter. The density of different road categories is generally correlated with urban land use and population density, 

and therefore other sources of chloride from urban settings 

cannot be excluded from this analysis. The difference between 

percent urban and percent agricultural land was only weakly 

correlated (correlation coefficient 0.44) with highway density, 

and explained an additional 14 percent of the variability in 

chloride yield.

Trends in Loads at Selected Sites and Evaluation 

of Sources of Chloride

Long-term data were available for 19 of the monitoring sites for which loads were estimated. Load estimates for 

three selected urban basins with long-term data were plotted 

to demonstrate visible trends with time by using a LOWESS 

smooth (Helsel and Hirsch, 2002) (figs. 17A–C). The apparent trends are similar to those reported by Kaushal and others 

(2005). Increases in chloride load over time generally can be 

Figure 10. Distribution of chloride:bromide ratios (by mass) in samples from drinking-water supply wells in geographic 

regions of the glacial aquifer system, northern United States, 1991–2003.

30

90th percentile

75th percentile

median

25th percentile

10th percentile

EXPLANATION 

Number of samples

B Sample populations that share 

the same letter symbol are not significantly 

different at the 0.05 level of significance

p is the attained significance level 

for one-way analysis of variance

103 185 108 68

0.1

1

10

100

1,000

10,000

CHLORIDE:BROMIDE RATIO BY MASS

EAST CENTRAL WESTCENTRALWEST

CHLORIDE:BROMIDE IN DRINKING-WATER SUPPLY WELLS

BY GEOGRAPHIC REGION, p = <0.0001

A B C C
24 Chloride in Groundwater and Surface Water in Areas Underlain by the Glacial Aquifer System, Northern United States

Figure 11. Distribution of maximum chloride concentrations measured at selected surface-water-quality monitoring stations in 

the glacial aquifer system, northern United States, 1991–2004.

20 49 31

FORESTED URBAN

USEPA-recommended chronic

criterion for aquatic life

1

10

100

1,000

10,000

0.1

 MAXIUMUM CONCENTRATION OF CHLORIDE, 

IN MILLIGRAMS PER LITER

AGRICULTURAL

A B C

MAXIMUM MEASURED CHLORIDE IN SURFACE WATER, p = <0.0001

30

90th percentile

75th percentile

median

25th percentile

10th percentile

EXPLANATION 

Number of samples

B Sample populations that share 

the same letter symbol are not significantly 

different at the 0.05 level of significance

p is the attained significance level 

for one-way analysis of variance

attributed to changes in the application of deicing salt, the 

expansion of road networks and impervious areas that require 

deicing, increases in the number of septic systems, increases in 

the volume of wastewater discharge, and the arrival of saline 

groundwater plumes from landfills and salt-storage areas over 

time. These trends suggest that chloride concentrations in 

some urban basins may be greater than USEPA-recommended 

criteria in the coming years and decades.

Of the examples shown in figure 17, only Poplar Creek 

at Elgin, Illinois, receives no major wastewater discharges 

(Sullivan, 2000). The Rahway River near Springfield, New 

Jersey, may have some combined sewer overflows 

(J.G. Kennen, U.S. Geological Survey, written commun., 

2006) and has 29 minor discharges listed in the PCS database. 

The Quinnipiac River Basin in Connecticut was studied 

in greater detail to evaluate changes that have taken place during the period of record to determine potential causes for the 

increase in chloride load and the relative input from different 

sources. The chloride load at this station increased from less 

than 6,000 tons in water year 1982 to more than 9,000 tons 

in water year 2003 (fig. 17B). The average estimated chloride load from atmospheric deposition was 66 tons based on 

information used to create figure 3. The Quinnipiac River at 

Wallingford, Connecticut, had about 17 Mgal/d of wastewater 

discharge from three municipal wastewater-treatment facilities 

in 1998 (Mullaney and others, 2002); this represents about 

12 percent of the average discharge for water years 1992 to 

2003. In addition, the PCS database lists eight other major 

wastewater discharges, and four minor discharges.

The wastewater discharge is likely responsible for some 

of the chloride load. At a typical concentration of 91 mg/L for 

chloride in wastewater from sewage and animal waste 

(table 4), the load of chloride would be equivalent to about 

2,360 tons per year, representing at least 20 percent of the load 

in 1998. An increase in wastewater discharge of 

2.5 Mgal/d to the Quinnipiac River Basin from 1985 to 2001 

was reported by Ahearn (2002). This may explain some of the 

increase (approximately 346 tons) in load that is apparent in 

figure 17B. Some of the chloride load in wastewater discharge is derived from the chloride initially in the drinkingwater supply. For instance, one upstream utility in Meriden, 

Connecticut, reported concentrations of chloride in drinking 

water as large as 250 mg/L (City of Meriden, Connecticut, 

2005); therefore, some chloride that initially may be derived 

from anthropogenic sources may be recycled. 

The load of chloride from residential septic systems in 

the Quinnipiac River Basin (not considering systems with 

water softeners) was estimated by determining the number 

of septic systems in the basin in 1990 and then applying 

block-group data from the 1990 Census on the method of 
Chloride in Groundwater and Surface Water 25

Table 5. Monitoring stations where surface waters had chloride concentrations greater than 230 milligrams per liter in the glacial 

aquifer system, northern United States, 1991–2004.—Continued 

[mg, milligram; L, liter; U, urban land use; A, agricultural land use]

Station 

identifier Station name

Dominant

land

use

Number 

of 

chloride 

samples

Number of 

samples 

greater 

than 230 

mg/L

Percent

Maximum 

chloride 

concentration

(mg/L)

Months with concentration greater than 230 mg/L

J F M A M J J A S O N D

01102500

Aberjona River 

at Winchester, 

Massachusetts

U 127 12 9.4 673 X X X X X

01356190

Lisha Kill northwest of 

Niskayuna, 

New York

U 90 7 7.8 353 X X X

01391500

Saddle River at 

Lodi, New 

Jersey

U 59 1 1.7 408 X

01394500

Rahway River 

near 

Springfield, 

New Jersey

U 59 4 6.8 1,320 X X X

03353637

Little Buck 

Creek near 

Indianapolis, 

Indiana

U 205 4 2.0 470 X X X

04072050

Duck Creek at 

Seminary 

Road near 

Oneida, 

Wisconsin

A 93 2 2.2 425 X X

040869415

Lincoln Creek at 

47th Street at 

Milwaukee, 

Wisconsin

U 184 42 22.8 4,330 X X X X X X X X X X X

04161820

Clinton River 

at Sterling 

Heights, 

Michigan

U 43 3 7.0 300 X X X

04186500

Auglaize River 

near Fort 

Jennings, Ohio

A 80 1 1.3 251 X

05288705

Shingle Creek at 

Queen Avenue 

in Minneapolis, Minnesota

U 115 25 21.7 2,020 X X X X X

05330902

Nine Mile Creek 

near James 

Circle at 

Bloomington, 

Minnesota

U 56 7 12.5 522 X X X
26 Chloride in Groundwater and Surface Water in Areas Underlain by the Glacial Aquifer System, Northern United States

Figure 12. Chloride concentrations, snow depth, and discharge at station 05288705, Shingle Creek at Queen 

Avenue in Minneapolis, Minnesota, February 1997. (Data from James Fallon, U.S. Geological Survey, written 

commun., 2006.)

2/7/1997 2/14/1997 2/21/1997 2/28/1997

10

100

1,000

5

10

15

20

25

Chloride

Snow Depth

Discharge

CHLORIDE CONCENTRATION, IN MILLIGRAMS PER LITER

DATE

SNOW DEPTH, IN INCHES

30

2/1/1997

DISCHARGE, IN CUBIC FEET PER SECOND

Table 5. Monitoring stations where surface waters had chloride concentrations greater than 230 milligrams per liter in the glacial 

aquifer system, northern United States, 1991–2004.—Continued 

[mg, milligram; L, liter; U, urban land use; A, agricultural land use]

Station 

identifier Station name

Dominant

land

use

Number 

of 

chloride 

samples

Number of 

samples 

greater 

than 230 

mg/L

Percent

Maximum 

chloride 

concentration

(mg/L)

Months with concentration greater than 230 mg/L

J F M A M J J A S O N D

05531500

Salt Creek at 

Western 

Springs, 

Illinois

U 137 40 29.2 635 X X X X X

05527800

Des Plaines 

River at 

Russell, 

Illinois

U 39 1 2.6 252 X

05532000

Addison Creek 

at Bellwood, 

Illinois

U 51 16 31.4 829 X X X X X X X

05550500 Poplar Creek at 

Elgin, Illinois

U 45 12 26.7 432 X
Chloride in Groundwater and Surface Water 27

Figure 13. Chloride concentrations in base flow, plotted against maximum measured chloride 

concentrations at selected forested, agricultural, and urban basins in the glacial aquifer system, northern 

United States, 1991–2004.

Table 6. Multiple linear-regression estimates of model coefficients, standard errors, t-statistic and p-values for the dependent 

variable natural log of maximum measured chloride concentration in surface water for selected urban, agricultural, and forested 

basins, in the glacial aquifer system, northern United States, 1991–2004. 

[mi, mile; mi2, square mile; mm, millimeter; yr, year; <, less than; -, minus]

Variable Units Parameter estimate Standard error t-statistic p-value

Intercept Dimensionless -8.3728 1.6394 -5.1071 <0.0001

Major road density mi/mi2 1.1229 .1498 7.4941 <.0001

Potential evapotranspiration 1961–90 

(effect applied only up to 625 mm/

year)

mm/yr .0212 .0028 7.4909 <.0001

Percent Dunne overland flow

(saturated overland flow)

Percent of annual streamflow -.4547 .1061 -4.2838 .0001

Agriculture

Forested

Urban

0.1

1

10

100

1,000

10,000

BASE-FLOW CHLORIDE CONCENTRATION, IN MILLIGRAMS PER LITER

MAXIMUM MEASURED CHLORIDE CONCENTRATION, 

IN MILLIGRAMS PER LITER

0.1 1 10 100 1,000

log10(Y) = 1.025(log10(X)) + 0.2875

R2 = 0.84

USEPA-recommended chronic

criterion for aquatic life
28 Chloride in Groundwater and Surface Water in Areas Underlain by the Glacial Aquifer System, Northern United States

Figure 15. Distribution of annual mean chloride yields at selected surface-water-quality monitoring stations in the glacial 

aquifer system, northern United States, 1991–2003.

FORESTED AGRICULTURAL URBAN

1

10

100

1,000

CHLORIDE YIELD, IN TONS PER SQUARE MILE PER YEAR

CHLORIDE YIELD FROM STREAMS, p = <0.0001

30

90th percentile

75th percentile

median

25th percentile

10th percentile

EXPLANATION 

Number of samples

B Sample populations that share 

the same letter symbol are not significantly 

different at the 0.05 level of significance

p is the attained significance level 

for one-way analysis of variance

20 44 31

A A B

Figure 14. Predicted natural log of maximum measured chloride concentrations from multiple linear 

regression modeling, and observed natural log of maximum measured chloride concentrations in selected 

forested, agricultural, and urban basins in the glacial aquifer system, northern United States, 1991–2004.

PREDICTED NATURAL LOG OF MAXIMUM MEASURED CHLORIDE CONCENTRATION

OBSERVED NATURAL LOG OF MAXIMUM 

MEASURED CHLORIDE CONCENTRATION

1:1

line

least squares 

fitted line

0

2

4

6

8

1 2 3 4 5 6 7
Chloride in Groundwater and Surface Water 29

Figure 16. Predicted natural log of annual chloride yield from multiple linear regression modeling and 

observed natural log of chloride yield in selected forested, agricultural, and urban basins in the glacial aquifer 

system, northern United States, 1991–2003.

Table 7. Multiple linear regression estimates of model coefficients and standard errors, t-statistics, and p-values for the dependent 

variable natural log of chloride yield in selected forested, agricultural and urban basins in the glacial aquifer system, northern United 

States, 1991–2003. 

[mi, mile; mm, millimeter; yr, year; USEPA PCS, U.S. Environmental Protection Agency Permit Compliance System; NLCDe, National Landcover Dataset 

(Vogelmann and others (1998), enhanced by Nakagaki and Wolock (2005)); +, plus; -, minus; <, less than]

Variable Units Parameter estimate Standard error t-statistic p-value

Intercept Dimensionless -5.0085 1.0433 -4.8005 <0.0001

Highway density (U.S. numbered 

routes; does not include limited 

access highways)

 mi/mi2 2.3846 .8936 2.6687 .0093

natural log of (number of major 

discharges upstream of monitoring 

site in USEPA PCS database+1)

 Dimensionless .4149 .1809 2.2934 .0245

Potential evapotranspiration 1961–90 

(effect applied only up to 700 mm/

year)

 mm/yr .0123 .0017 7.2562 <.0001

Percent of urban-agricultural land 

(NLCDe) Percent of basin area .0109 .0018 5.9849 <.0001

PREDICTED NATURAL LOG OF ANNUAL CHLORIDE YIELD

OBSERVED NATURAL LOG OF ANNUAL CHLORIDE YIELD

1:1

line

least squares 

fitted line

0

1

2

3

4

5

1 2 3 4 5 6
30 Chloride in Groundwater and Surface Water in Areas Underlain by the Glacial Aquifer System, Northern United States

Figure 17. Chloride loads with time at selected waterquality monitoring stations in the glacial aquifer system, 

northern United States: (A) Poplar Creek at Elgin, 

Illinois, water years 1978–2003; (B) Quinnipiac River at 

Wallingford, Connecticut, water years 1969–2003; and (C)

Rahway River near Springfield, New Jersey, water years 

1980–2003.

 1978 1979 1980 1981

 1982 1983 1984

 1985 1986 1987 1988 1989 1990 1991

 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003

0

4,000

8,000

12,000

16,000

A 05550500 Poplar Creek at Elgin, Illinois

Fitted line is a LOWESS smooth

196919711973

19751977

19791981

1983

1985

198719891991

199319951997

199920012003

2,000

4,000

6,000

8,000

10,000

12,000

0

CHLORIDE LOAD, IN TONS PER YEAR

B 01196500 Quinnipiac River at Wallingford, Connecticut 

Fitted line is a LOWESS smooth

1980198119821983198419851986198719881989199019911992199319941995199619971998199920002001

20022003

WATER YEAR

0

1,000

2,000

3,000

4,000

5,000

6,000

C 01394500 Rahway River near Springfield, New Jersey 

Fitted line is a LOWESS smooth
Summary and Conclusions 31

sewage disposal (U.S. Department of Commerce, 1991). The 

estimated number of septic systems in the basin upstream from 

the monitoring station was 10,400. By using the estimated 

chloride concentration of 91 mg/L for wastewater (table 4) and 

assuming 300 gal/d of water use per household, the estimated 

load of chloride from septic systems was 430 tons in 1990. 

The number of septic systems likely has increased since 

1990, and the presence of water softeners could increase this 

load substantially.

The other likely source of the increase in chloride load 

in this basin is the increase in the impervious area, including 

roads, during the period of record, and subsequent increases 

in deicing these areas. The impervious area of the basin 

above the Quinnipiac monitoring station was analyzed for 

1985 and 2002 using the Impervious Surface Analysis Tool 

(ISAT) described by Chabaeva and others (2004). This method 

was used with impervious surface coefficients developed 

for Connecticut and population-density classes (Prisloe and 

others, 2003). The University of Connecticut has developed 

consistently interpreted satellite imagery for Connecticut land 

use and land cover for 1985, 1990, 1995, and 2002 (Civco and 

others, 1998). The ISAT method estimated that the impervious 

cover for the basin upstream from the monitoring station was 

13.3 percent in 1985, but increased to 14.7 percent in 2002—

an increase of 1.4 percent, or about 890 acres. In addition to 

these increases, Interstate 691 was completed in 1988, adding 

about 5.5 miles of four-lane road, and an estimated 85 tons of 

deicing salt (or about 52 tons of chloride) (Patrick Rodgers, 

Connecticut Department of Transportation, oral commun., 

2004). Additional potential sources of chloride include 

9 salt-storage areas and 20 landfills (Connecticut Department 

of Environmental Protection, 1995). This analysis indicates 

that although deicing salts may be the dominant input of chloride in many of the urban basins studied, other sources should 

be considered in the establishment of Total Maximum Daily 

Loads (TMDLs).

Summary and Conclusions

The use of salt in the United States has increased since 

the 1950s. Salt use has increased from 42.9 million tons in 

1975 to nearly 58.5 million tons in 2005. However, the largest 

use of salt for many years has been the chloralkali industry. In 

2005 the largest use was for deicing (39.5 percent), followed 

by use in the chloralkali industry (34.7 percent). The remaining uses total about 25 percent. The Northern States that 

compose the glacial aquifer system were the end destination 

for about 76 percent of the shipments of evaporated salt and 

rock salt. 

Chloride concentrations were greater than the U.S. 

Environmental Protection Agency secondary maximum contaminant level (SMCL) of 250 milligrams per liter in 2.5 percent of samples from 797 shallow monitoring wells sampled 

(land-use wells), and in 1.7 percent of 532 drinking-water 

supply wells sampled from 1991 to 2003. Chloride concentrations were largest in samples from shallow monitoring wells in 

urban areas (median concentration of 46 milligrams per liter), 

followed by the concentrations in samples from agricultural 

areas (median, 12 milligrams per liter), and forested areas 

(median, 2.9 milligrams per liter).

Chloride:bromide ratios, by mass, in groundwater 

samples were plotted against chloride concentrations along 

with binary mixing curves for different sources of chloride. 

Samples from shallow monitoring wells in the urban areas had 

the largest ratios of chloride to bromide, and generally plotted 

on or between binary mixing curves for dilute groundwater, 

and (1) halite, (2) sewage and animal waste, and (3) potassium 

chloride fertilizer; these results indicate mixed sources of 

chloride. Samples that had a chloride:bromide ratio greater 

than 460 are dominated by anthropogenic sources of chloride. 

Samples that had a chloride:bromide ratio greater than 1,000 

and chloride concentrations greater than 100 milligrams per 

liter are likely dominated by halite by mass, but may contain 

lesser proportions of other sources.

Samples from shallow monitoring wells in agricultural 

areas were not dominated by any one source of chloride, but 

there was a cluster of points representing samples that plotted 

on or near the binary mixing lines for dilute groundwater and 

potassium chloride or dilute groundwater and sewage and 

animal waste. Sources of chloride in agricultural areas include 

deicing salt, salt used in animal feeds, water-softening salts, 

salts in animal waste and fertilizer, and the use of irrigation 

water containing brines.

Ratios for samples from drinking-water wells indicated 

mixed sources of chloride, but two general patterns emerged 

when chloride:bromide ratios were compared by well type 

(private domestic drinking-water wells, and public drinkingwater wells) and by region in the glacial aquifer system. The 

chloride:bromide ratios of the samples from the public drinking-water wells were significantly larger than for the private 

domestic wells, indicating anthropogenic sources of chloride. 

This may be a result of the integration of water of different 

depths and ages into public drinking-water wells due to pumping. Drinking-water-well samples from the east region of the 

glacial aquifer system had the highest chloride:bromide ratios 

followed by the central region. Samples from the west-central 

and west areas had low chloride:bromide ratios, indicating less 

human influence on water quality.

Data collected from 1991 to 2004 from 100 surface-water 

monitoring sites in the glacial aquifer system draining land 

dominated by forested land cover or urban, and agricultural 

land use were analyzed to compare chloride concentrations 

to USEPA-recommended chronic aquatic criteria, to relate 

maximum measured chloride concentrations to ancillary 

factors, and to compute annual loads so that chloride yields 

could be compared. Fifteen (12 urban, 3 agricultural) of the 

100 sites had samples with concentrations greater than the 

USEPA-recommended aquatic criteria concentration for 

chloride of 230 milligrams per liter. Six of the 15 sites had 

10 percent or more of their values greater than 230 milligrams 
32 Chloride in Groundwater and Surface Water in Areas Underlain by the Glacial Aquifer System, Northern United States

per liter, indicating frequent occurrence of high concentrations. Concentrations of chloride greater than 230 milligrams 

per liter occurred most frequently during the months from 

November to April, indicating a likely relation with winter 

deicing activities. 

Sites with maximum concentrations of chloride greater 

than 230 milligrams per liter had base-flow concentrations 

of chloride greater than 75 to 90 milligrams per liter. This 

result indicates that basins with high chloride concentrations 

in groundwater or wastewater discharge were more likely to 

exceed the recommended chronic criteria. 

A multiple linear regression model was used to describe 

the natural log of the maximum measured chloride at 

84 surface-water-monitoring stations that had ancillary data. 

Significant variables were the density of major roads (roads 

in the U.S. highway system), potential annual evapotranspiration, and the percentage of the annual streamflow derived from 

saturated overland flow (runoff). 

The load of chloride was determined for each of 

95 surface-water monitoring stations for the years with data 

collected from 1991 to 2003. The loads were normalized 

by drainage area to calculate yields of chloride. The median 

chloride yield was 88 tons per square mile from the basins 

dominated by urban land use, 15.4 tons per square mile from 

agricultural basins, and 6.4 tons per square mile from forested 

basins. The significant variables to describe chloride yields 

were highway density, number of major wastewater discharges, potential evapotranspiration, and percentage of urban 

minus agricultural land in each watershed.

Long-term historical data and the literature on this subject 

suggest that chloride concentrations in streams are currently 

increasing in urbanized and urbanizing areas, potentially 

exceeding recommended criteria for aquatic life now or in the 

coming decades. Increases in chloride load over time generally can be attributed to changes in the application of deicing 

salt, the expansion of road networks and impervious areas that 

require deicing, increases in the number of septic systems, 

increases in the volume of wastewater discharge, and the 

arrival of saline groundwater plumes from landfills and saltstorage areas over time.

Data from the Quinnipiac River in Connecticut were 

analyzed to explore the role of different sources of chloride in 

the overall chloride load. The analysis indicated that in basins 

with a high volume of wastewater discharge and a moderate 

to high density of septic systems, at least 31 percent of the 

chloride load can be derived from these sources. The wet 

deposition of chloride from precipitation represents less than 

one percent of the chloride load. The source of the chloride in 

wastewater and septic-system leachate can include chloride 

recycled from anthropogenic sources in the public and private 

water supplies; landfills and storage sites for deicing chemicals 

represent additional anthropogenic sources. A large part of the 

remaining load can be attributed to the use of deicing salt on 

impervious areas.

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Appendix 1 37

Appendix 1
38 Chloride in Groundwater and Surface Water in Areas Underlain by the Glacial Aquifer System, Northern United States Appendix 1. Surface-water-quality monitoring stations used in analyses for chloride concentration or load estimation (or both).—Continued [NF, North Fork; SF, South Fork; --, data not available] USGS identifier Station name Hydrologic unit code Contrib- uting drain- age area (square miles) Dominant land use Number of chloride samples 1992– 2004 Long Term (pre- 1991) data avail- able Maximum measured chloride concen- tration (milli- grams per liter) Mean chloride yield for period of selected data (tons per square mile) Number of major permit- com- pliance system dis- charg- ers 01095220 Stillwater River near Sterling, MA 01070004 31.6 Forested 69 68.4 29.1 0 01101500 Ipswich River at South Middleton, MA 01090001 44.5 Urban 46 98.7 84.8 1 01102345 Saugus River at Saugus Iron Works at Saugus, MA 01090001 20.8 Urban 44 197 109.8 0 01102500 Aberjona River at Winchester, MA 01090001 24.7 Urban 127 673 182.6 1 01105000 Neponset River at Norwood, MA 01090001 34.7 Urban 32 149 88.0 0 01109000 Wading River near Norton, MA 01090004 43.3 Urban 34 77.6 64.6 2 01135300 Sleepers River (site W-5) near St. Johnsbury, VT 01080102 42.9 Agricultural 34 9.7 6.3 0 01137500 Ammonoosuc River at Bethlehem Junction, NH 01080101 87.6 Forested 34 8 5.3 0 01170100 Green River near Colrain, MA 01080203 41.4 Forested 78 15 7.3 0 01184490 Broad Brook at Broad Brook, CT 01080205 15.5 Agricultural 101 36.9 37.0 0 01184500 Scantic River at Broad Brook, CT 01080205 98.2 Agricultural 21 19 -- -- 01188000 Bunnell (Burlington) Brook near Burlington, CT 01080207 4.1 Forested 52 Yes 27 25.0 0 01189000 Pequabuck River at Forestville, CT 01080207 45.8 Urban 40 76 48.1 3 01192500 Hockanum River near East Hartford, CT 01080205 73.4 Urban 129 124 60.3 2 01193500 Salmon River near East Hampton, CT 01080205 100 Forested 74 Yes 27.4 22.8 0 01196500 Quinnipiac River at Wallingford, CT 01100004 115 Urban 108 Yes 82.3 67.1 11 01199900 Tenmile River at South Dover near Wingdale, NY 01100006 192.5 Agricultural 54 35 23.6 0 01208873 Rooster River at Fairfield, CT 01100006 10.6 Urban 35 140 50.4 0 01208990 Saugatuck River near Redding, CT 01100006 21 Forested 75 Yes 38 36.2 0 01209710 Norwalk River at Winnipauk, CT 01100006 33 Urban 209 Yes 83.1 58.0 1 01349150 Canajoharie Creek near Canajoharie, NY 02020004 59.7 Agricultural 136 118 28.4 0 01356190 Lisha Kill northwest of Niskayuna, NY 02020004 15.6 Urban 90 353 103.8 0 01361200 Claverack Creek at Claverack, NY 02020006 60.6 Agricultural 24 30 -- -- 01362200 Esopus Creek at Allaben, NY 02020006 63.7 Forested 55 14.8 10.5 0 01372051 Fall Kill at Poughkeepsie, NY 02020008 18.8 Urban 24 110 78.8 0

01379500 Passaic River near Chatham, NJ 02030103 100 Urban 31 Yes 210 94.7 8
Appendix 1 39

Appendix 1. Surface-water-quality monitoring stations used in analyses for chloride concentration or load estimation (or both).—Continued[NF, North Fork; SF, South Fork; --, data not available]

USGS

identifier Station name Hydrologic unit code

Contributing

drainage 

area 

(square 

miles)

Dominant 

land use

Number 

of

chloride 

samples 

1992–

2004

Long 

Term 

(pre1991) 

data 

available 

Maximum 

measured

chloride 

concentration 

(milligrams per 

liter)

Mean 

chloride 

yield for

period of 

selected 

data 

(tons per 

square 

mile)

Number 

of major

permitcompliance 

system

dischargers

01380500 Rockaway River above reservoir at Boonton, NJ 02030103 116 Forested 31 Yes 120 60.7 201381500 Whippany River at Morristown, NJ 02030103 29.4 Urban 31 Yes 190 106.4 101381800 Whippany River near Pine Brook, NJ 02030103 68.5 Urban 59 165 168.6 701390500 Saddle River at Ridgewood, NJ 02030103 21.6 Urban 31 197 130.4 001391500 Saddle River at Lodi, NJ 02030103 54.6 Urban 59 Yes 408 184.1 401394500 Rahway River near Springfield, NJ 02030104 25.5 Urban 59 Yes 1,320 141.6 101435000 Neversink River near Claryville, NY 02040104 66.6 Forested 375 17.6 6.0 001440000 Flat Brook near Flatbrookville, NJ 02040104 64 Forested 49 21.7 20.6 001443500 Paulins Kill at Blairstown, NJ 02040105 126 Forested 59 Yes 73.9 61.2 203267900 Mad River at St. Paris Pike at Eagle City, OH 05080001 310 Agricultural 97 30.5 19.8 103353637 Little Buck Creek near Indianapolis, IN 05120201 17 Urban 205 470 66.9 003360895 Kessinger Ditch near Monroe City, IN 05120202 56.2 Agricultural 55 35 21.8 003366500 Muscatatuck River near Deputy, IN 05120207 293 Agricultural 28 15 9.9 004062085 Peshekee River near Martins Landing, MI 04030107 43.9 Forested 10 1 0.5 004063700 Popple River near Fence, WI 04030108 139 Forested 124 3.6 1.0 004071795 Pensaukee River near Krakow, WI 04030103 35.8 Agricultural 34 61 7.8 004072050 Duck Creek at Seminary Road near Oneida, WI 04030103 95.5 Agricultural 93 425 24.9 004080798 Tomorrow River near Nelsonville, WI 04030202 44 Agricultural 32 6.6 3.5 --04085109 East River at Midway Road near de Pere, WI 04030204 47 Agricultural 26 86 -- --040863075 North Branch Milwaukee River near Random Lake, WI 04040003 51.4 Agricultural 53 40.7 16.5 --04086600 Milwaukee River near Cedarburg, WI 04040003 607 Agricultural 80 104 24.2 --040869415 Lincoln Creek at 47th Street at Milwaukee, WI 04040003 9.6 Urban 184 4,330 162.9 004159492 Black River near Jeddo, MI 04090001 464 Agricultural 29 69.2 -- --04161820 Clinton River at Sterling Heights, MI 04090003 309 Urban 43 300 112.1 304175600 River Raisin near Manchester, MI 04100002 132 Agricultural 30 25.6 17.8 004186500 Auglaize River near Fort Jennings, OH 04100007 332 Agricultural 80 251 26.4 3
40 Chloride in Groundwater and Surface Water in Areas Underlain by the Glacial Aquifer System, Northern United States Appendix 1. Surface-water-quality monitoring stations used in analyses for chloride concentration or load estimation (or both).—Continued [NF, North Fork; SF, South Fork; --, data not available] USGS identifier Station name Hydrologic unit code Contrib- uting drain- age area (square miles) Dominant land use Number of chloride samples 1992– 2004 Long Term (pre- 1991) data avail- able Maximum measured chloride concen- tration (milli- grams per liter) Mean chloride yield for period of selected data (tons per square mile) Number of major permit- com- pliance system dis- chargers 05030150 Otter Tail River near Perham, MN 09020103 335 Forested 23 5 1.8 0 05051300 Bois de Sioux River near Doran, MN 09020101 1,880 Agricultural 26 47 3.4 0 05053000 Wild Rice River near Abercrombie, ND 09020105 1,490 Agricultural 29 Yes 62.4 2.4 0 05056000 Sheyenne River near Warwick, ND 09020203 760 Agricultural 35 Yes 26 1.9 0 05058700 Sheyenne River at Lisbon, ND 09020204 2,490 Agricultural 45 Yes 48 2.8 0 05062500 Wild Rice River at Twin Valley, MN 09020108 934 Agricultural 45 15 2.2 0 05066500 Goose River at Hillsboro, ND 09020109 1,093 Agricultural 30 Yes 53.2 3.7 0 05082625 Turtle River at Turtle River State Park near Arvilla, ND 09020307 311 Agricultural 91 160 3.6 0 05085900 Snake River above Alvarado, MN 09020309 218 Agricultural 26 38 2.6 0 05112000 Roseau River below State Ditch 51 near Caribou, MN 09020314 1,420 Agricultural 29 Yes 38 1.6 0 05276005 North Fork Crow River above Paynesville, MN 07010204 232 Agricultural 35 19.9 5.5 0 05287890 Elm Creek near Champlin, MN 07010206 86 Urban 103 109 23.2 0 05288705 Shingle Creek at Queen Avenue in Minneapolis, MN 07010206 28.2 Urban 115 2,020 151.5 0 05320270 Little Cobb River near Beauford, MN 07020011 130 Agricultural 95 50.3 7.6 0 05330902 Nine Mile Creek near James Circle at Bloomington, MN 07020012 44.6 Urban 56 522 52.6 0 05331833 Namekagon River at Leonards, WI 07030002 126 Forested 32 16.4 3.3 0 05420680 Wapsipinicon River near Tripoli, IA 07080102 346 Agricultural 74 36.6 15.8 1 05451210 South Fork Iowa River northeast of New Providence, IA 07080207 224 Agricultural 100 41.6 13.3 0 05461390 Flood Creek near Powersville, IA 07080202 124 Agricultural 34 20.8 14.0 0 05464220 Wolf Creek near Dysart, IA 07080205 299 Agricultural 56 25.4 16.0 0 05520500 Kankakee River at Momence, IL 07120001 2,294 Agricultural 72 Yes 84 27.0 8 05525500 Sugar Creek at Milford, IL 07120002 446 Agricultural 75 149 15.0 0 05526000 Iroquois River near Chebanse, IL 07120002 2,091 Agricultural 33 50.3 12.9 3 05527800 Des Plaines River at Russell, IL 07120004 123 Agricultural 39 252 61.1 1 05531500 Salt Creek at Western Springs, IL 07120004 115 Urban 137 635 304.6 9

05532000 Addison Creek at Bellwood, IL 07120004 17.9 Urban 51 Yes 829 229.5 1
Appendix 1 41

Appendix 1. Surface-water-quality monitoring stations used in analyses for chloride concentration or load estimation (or both).—Continued[NF, North Fork; SF, South Fork; --, data not available]

USGS 

identifier Station name Hydrologic unit code

Contributing

drainage 

area 

(square 

miles)

Dominant 

land use

Number 

of

chloride 

samples 

1992–

2004

Long 

Term 

(pre1991) 

data 

available 

Maximum 

measured

chloride 

concentration 

(milligrams per 

liter)

Mean 

chloride 

yield for

period of 

selected 

data 

(tons per 

square 

mile)

Number 

of major

permitcompliance 

system

dischargers

05548105 Nippersink Creek above Wonder Lake, IL 07120006 84.5 Agricultural 38 176 51.7 105550500 Poplar Creek at Elgin, IL 07120006 35.2 Urban 45 Yes 432 257.1 005567000 Panther Creek near El Paso, IL 07130004 93.9 Agricultural 29 150 23.4 005568800 Indian Creek near Wyoming, IL 07130005 62.7 Agricultural 29 51 20.8 005572000 Sangamon River at Monticello, IL 07130006 550 Agricultural 105 95 20.3 005584500 La Moine River at Colmar, IL 07130010 655 Agricultural 73 60.1 14.3 106795500 Shell Creek near Columbus, NE 10200201 294 Agricultural 24 9.7 1.6 006800000 Maple Creek near Nickerson, NE 10220003 368 Agricultural 200 58.3 2.9 012056500 NF Skokomish River below Staircase Rapids near Hoodsport, WA 17110017 57.2 Forested 63 1.4 6.8 012103380 Green River above Twin Camp Creek near Lester, WA 17110013 16.5 Forested 24 1.2 3.6 012112600 Big Soos Creek above Hatchery near Auburn, WA 17110013 66.7 Urban 36 4.9 7.7 012113375 Springbrook Creek at Tukwila, WA 17110013 19 Urban 37 53.9 22.5 012128000 Thornton Creek near Seattle, WA 17110012 12.1 Urban 98 27 5.1 015241600 Ninilchik River at Ninilchik, AK 19020301 135 Forested 36 4.3 1.9 --15266110 Kenai River below Skilak Lake Outlet near Sterling AK 19020302 1,206 Forested 36 0.1 3.0 --15274000 SF Campbell Creek near Anchorage, AK 19020401 29.2 Forested 38 0.7 0.5 --15275100 Chester Creek at Arctic Boulevard at Anchorage, AK 19020401 27.4 Urban 42 94.2 15.1 --391732085414401 Clifty Creek at County Road 1150 E near Hartsville, IN 05120206 87.9 Agricultural 33 30 23.4 0393306086585201 Big Walnut Creek at County Road 700 W at Reelsville, IN 05120203 318 Agricultural 31 40 22.0 1394340085524601 Sugar Creek at County Road 400 S at New Palestine, IN 05120204 92.6 Agricultural 246 100 26.2 0395355084173600 Stillwater River on Martindale Road near Union, OH 05080001 646 Agricultural 30 66.2 20.6 1395457084095100 Great Miami River near Vandalia, OH 05080001 1,142 Agricultural 30 84.1 -- --

Prepared by the Pembroke Publishing Service Center.

For more information concerning this report, contact:

Director

U.S. Geological Survey

Connecticut Water Science Center

101 Pitkin Street

East Hartford, CT 06108

dc_ct@usgs.gov

or visit our Web site at:

http://ct.water.usgs.gov
Mullaney and others—Chloride in Groundwater and Surface Water in Areas Underlain by the Glacial Aquifer System, Northern United States—Scientific Investigations Report 2009–5086

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EXPLANATION

Glacial aquifer system

Science Center Director

USGS Connecticut Water Science Center

101 Pitkin Street

East Hartford, CT 06108

http://ct.water.usgs.gov