Prepared in cooperation with Park County, Colorado

Ground-Water Quality and Potential Effects of Individual 

Sewage Disposal System Effluent on Ground-Water Quality 

in Park County, Colorado, 2001–2004

Scientific Investigations Report 2007–5220

U.S. Department of the Interior

U.S. Geological Survey
Cover. Photograph by Gary E. Nichols, Park County, Colorado.
Ground-Water Quality and Potential 

Effects of Individual Sewage Disposal 

System Effluent on Ground-Water Quality 

in Park County, Colorado, 2001–2004

By Lisa D. Miller and Roderick F. Ortiz

Prepared in cooperation with Park County, Colorado

Scientific Investigations Report 2007–5220

U.S. Department of the Interior

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

DIRK KEMPTHORNE, Secretary

U.S. Geological Survey

Mark D. Myers, Director

U.S. Geological Survey, Reston, Virginia: 2007

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

reproduce any copyrighted materials contained within this report.

Suggested citation:

Miller, L.D., and Ortiz, R.F., 2007, Ground-water quality and potential effects of individual sewage disposal system 

effluent on ground-water quality in Park County, Colorado, 2001–2004: U.S. Geological Survey Scientific Investigations 

Report 2007–5220, 48 p.
iii

Contents

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

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

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

Description of the Study Area ............................................................................................................4

Previous Studies ...................................................................................................................................4

Acknowledgments................................................................................................................................6

Study Approach..............................................................................................................................................6

Sample Collection and Processing....................................................................................................6

Analytical Methods.............................................................................................................................13

Quality Control.....................................................................................................................................13

Data Analysis Methods......................................................................................................................13

Quality of Ground Water.............................................................................................................................15

Age of Ground Water .........................................................................................................................15

Natural Factors Affecting Drinking-Water Quality in Park County.............................................18

pH.................................................................................................................................................18

Fluoride........................................................................................................................................18

Sulfate .........................................................................................................................................18

Dissolved Solids.........................................................................................................................20

Radionuclides.............................................................................................................................22

Human Factors Affecting Drinking-Water Quality in Park County..............................................24

Chloride........................................................................................................................................26

Nitrate..........................................................................................................................................26

Bacteria.......................................................................................................................................28

Summary of Drinking-Water Quality in Domestic Wells in Park County ...................................28

Potential Effects of Individual Sewage Disposal System Effluent on Ground-Water Quality.........30

Occurrence of Constituents Associated with Individual Sewage Disposal System 

Effluent in Ground-Water Samples.....................................................................................34

Potential Effects of Individual Sewage Disposal System Density and Aquifer Type 

on Ground-Water Quality .....................................................................................................37

Summary........................................................................................................................................................42

References Cited..........................................................................................................................................45

Figures

1. Locations of the study area, domestic wells sampled by aquifer type, and all 

domestic wells listed in the Colorado Division of Water Resources Well Permit 

database (2004) in Park County, Colorado................................................................................3

2. Surficial geology of Park County, Colorado..............................................................................5

3. (A) Estimated concentrations of tritium in precipitation near Park County, 

Colorado, and (B) atmospheric concentrations of CFC-11, CFC-12, and CFC-113 

in air in North America...............................................................................................................14

4. Explanation of a boxplot ............................................................................................................15

5. Distribution of (A) tritium concentrations and (B) approximate recharge dates 

based on chlorofluorocarbon data for alluvial, crystalline-rock, sedimentaryrock, and volcanic-rock aquifers throughout Park County, Colorado, 2001–2004 ...........16

6. Approximate ground-water recharge date based on CFC concentrations in 

selected wells in Park County, Colorado, 2001–2004............................................................17
iv

7. (A) Distribution of pH by aquifer type and (B) spatial distribution of pH in samples

collected from selected domestic wells in Park County, Colorado, 2001–2004 ...............20

8. (A) Distribution of fluoride by aquifer type and (B) spatial distribution of fluoride 

in samples collected from selected domestic wells in Park County, Colorado, 

2001–2004 .....................................................................................................................................22

9. (A) Distribution of sulfate by aquifer type and (B) spatial distribution of sulfate 

in samples collected from selected domestic wells in Park County, Colorado, 

2001–2004 .....................................................................................................................................24

10. (A) Distribution of dissolved solids by aquifer type and (B) spatial distribution 

of dissolved solids in samples collected from selected domestic wells in Park 

County, Colorado, 2001–2004.....................................................................................................26

11. (A) Distribution of radon by aquifer type and (B) spatial distribution of radon 

in samples collected from selected domestic wells in Park County, Colorado, 

2000–2004 .....................................................................................................................................28

12. (A) Distribution of uranium by aquifer type and (B) spatial distribution of uranium 

in samples collected from selected domestic wells in Park County, Colorado, 

2000–2004 .....................................................................................................................................30

13. Spatial distribution of nitrite-plus-nitrate in samples collected from selected 

domestic wells in Park County, Colorado, 2001–2004...........................................................32

14. Spatial distribution of total coliform detections in samples collected from 

selected domestic wells in Park County, Colorado, 2001–2004...........................................33

15. Number of wastewater compounds detected in samples collected from 

selected domestic wells in Park County, Colorado, 2001–2004...........................................36

16. Distribution of nitrite-plus-nitrate concentrations in samples collected from 

selected domestic wells in the Bailey area in 1974 and 2001 .............................................38

17. Distribution of chloride concentrations in samples collected from selected 

domestic wells in the Bailey area in 1974 and 2001..............................................................38

18. Distribution of (A) potassium, (B) chloride, (C) nitrite-plus-nitrate, and 

(D) boron concentrations in samples collected from domestic wells in Park 

County, Colorado, 2001–2004, in relation to average subdivision lot size..........................39

19. Distribution of (A) potassium, (B) chloride, (C) nitrite-plus-nitrate, and (D) boron 

concentrations in samples collected from wells completed in the crystalline-rock

aquifers in Park County, Colorado, 2001–2004, in relation to average subdivision

lot size...........................................................................................................................................40

20. Distribution of (A) potassium, (B) chloride, (C) nitrite-plus-nitrate, and (D) boron 

concentrations in samples collected from wells completed in the volcanic-rock 

aquifers in Park County, Colorado, 2001–2004, in relation to average subdivision 

lot size...........................................................................................................................................41

21. Relation of nitrite-plus-nitrate concentrations in samples collected from selected 

domestic wells to year of individual sewage disposal system installation, Park 

County, Colorado.........................................................................................................................42

Tables

1. Ground-water-quality sites and summary of ground-water-quality data for 

Park County, Colorado, 2001–2004 .............................................................................................7

2. Summary of selected ground-water-quality data for Park County, Colorado, 

2000–2004 .....................................................................................................................................19

3. List of organic compounds identified in samples collected from domestic wells 

in Park County, Colorado, 2001–2004 .......................................................................................35
v

Conversion Factors, Definitions, and Abbreviations

Inch/Pound to SI

Multiply By To obtain

Length

inch (in.) 2.54 centimeter (cm)

foot (ft) 0.3048 meter (m)

Area

acre 4,047 square meter (m2

)

square mile (mi2

) 2.590 square kilometer (km2) 

Flow rate

gallon per minute (gal/min) 0.06309 liter per second (L/s)

Radioactivity

picocurie per liter (pCi/L) 0.037 becquerel per liter (Bq/L) 

Mass

ounce, avoirdupois (oz) 28.35 gram (g)

pound, avoirdupois (lb) 0.4536 kilogram (kg)

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

°F=(1.8×°C)+32

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

°C=(°F-32)/1.8

Vertical coordinate information is referenced to the North American Vertical Datum of 1988 

(NAVD 88).

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

Altitude, as used in this report, refers to distance above the vertical datum.

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

25 °C).

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

micrograms per liter (μg/L), picocuries per liter (pCi/L), or picograms per kilogram (pg/kg).

Additional Abbreviations Used in This Report

BPA Bisphenol A

CFC Chlorofluorocarbon

CWCB Colorado Water Conservation Board

DEET N,N-diethyl-meta-toluamide

DS Dissolved solids

E. coli Escherichia coli

ISDS Individual sewage disposal system

IQR Interquartile range

MCL Maximum contaminant level

4-NP 4-nonylphenol

NPDWR National Primary Drinking Water Regulation

NSDWR National Secondary Drinking Water Regulation

NWIS National Water Information System

PPACG Pikes Peak Area Council of Governments

SMCL Secondary maximum contaminant level

USEPA U.S. Environmental Protection Agency

USGS U.S. Geological Survey

Abstract

In 2000, the U.S. Geological Survey, in cooperation 

with Park County, Colorado, began a study to evaluate groundwater quality in the various aquifers in Park County that 

supply water to domestic wells. The focus of this study was to 

identify and describe the principal natural and human factors 

that affect ground-water quality. In addition, the potential 

effects of individual sewage disposal system (ISDS) effluent 

on ground-water quality were evaluated.

Ground-water samples were collected from domestic 

water-supply wells from July 2001 through October 2004 in 

the alluvial, crystalline-rock, sedimentary-rock, and volcanicrock aquifers to assess general ground-water quality and 

effects of ISDS’s on ground-water quality throughout Park 

County. Samples were analyzed for physical properties, major 

ions, nutrients, bacteria, and boron; and selected samples also 

were analyzed for dissolved organic carbon, human-related 

(wastewater) compounds, trace elements, radionuclides, and 

age-dating constituents (tritium and chlorofluorocarbons).

Drinking-water quality is adequate for domestic 

use throughout Park County with a few exceptions. Only 

about 3 percent of wells had concentrations of fluoride, 

nitrate, and (or) uranium that exceeded U.S. Environmental 

Protection Agency national, primary drinking-water standards. 

These primary drinking-water standards were exceeded only 

in wells completed in the crystalline-rock aquifers in eastern 

Park County. Escherichia coli bacteria were detected in one 

well near Guffey, and total coliform bacteria were detected in 

about 11 percent of wells sampled throughout the county. The 

highest total coliform concentrations were measured southeast 

of the city of Jefferson and west of Tarryall Reservoir. 

Secondary drinking-water standards were exceeded more 

frequently. About 19 percent of wells had concentrations of 

one or more constituents (pH, chloride, fluoride, sulfate, and 

dissolved solids) that exceeded secondary drinking-water 

standards. Currently (2004), there is no federally enforced 

drinking-water standard for radon in public water-supply 

systems, but proposed regulations suggest a maximum 

contaminant level of 300 picocuries per liter (pCi/L) and 

an alternative maximum contaminant level of 4,000 pCi/L 

contingent on other mitigating remedial activities to reduce 

radon levels in indoor air. Radon concentrations in about 

91 percent of ground-water samples were greater than or equal 

to 300 pCi/L, and about 25 percent had radon concentrations 

greater than or equal to 4,000 pCi/L. Generally, the highest 

radon concentrations were measured in samples collected from 

wells completed in the crystalline-rock aquifers.

Analyses of ground-water-quality data indicate that 

recharge from ISDS effluent has affected some local groundwater systems in Park County. Because roughly 90 percent of 

domestic water used is assumed to be recharged by ISDS’s, 

detections of human-related (wastewater) compounds in 

ground water in Park County are not surprising; however, 

concentrations of constituents associated with ISDS effluent 

generally are low (concentrations near the laboratory 

reporting levels). Thirty-eight different organic wastewater 

compounds were detected in 46 percent of ground-water 

samples, and the number of compounds detected per sample 

ranged from 1 to 17 compounds. Samples collected from 

wells with detections of wastewater compounds also had 

significantly higher (p-value < 0.05) chloride and boron 

concentrations than samples from wells with no detections 

of wastewater compounds.

ISDS density (average subdivision lot size used to 

estimate ISDS density) was related to ground-water quality 

in Park County. Chloride and boron concentrations were 

significantly higher in ground-water samples collected from 

wells located in areas that had average subdivision lot sizes 

of less than 1 acre than in areas that had average subdivision 

lot sizes greater than or equal to 1 acre. For wells completed 

in the crystalline-rock aquifers, chloride concentrations were 

significantly (p-value < 0.05) higher in wells located in areas 

with average subdivision lot sizes less than 1 acre than in 

areas with average subdivision lot sizes greater than 5 acres, 

and boron concentrations were significantly higher in wells 

located in areas with average subdivision lot sizes of less than 

1 acre than in areas with average subdivision lot sizes greater 

than 3 acres. In the volcanic-rock aquifers, wells were sampled 

only in areas that had average subdivision lot sizes less than 

1 acre or greater than 5 acres. Potassium, chloride, and boron 

concentrations were significantly higher in wells located in 

areas that had average subdivision lot sizes of less than 1 

acre than in areas with average subdivision lot sizes greater 

Ground-Water Quality and Potential Effects of Individual 

Sewage Disposal System Effluent on Ground-Water 

Quality in Park County, Colorado, 2001–2004 

By Lisa D. Miller and Roderick F. Ortiz
2 Potential Effects of Individual Sewage Disposal System Effluent on Ground-Water Quality in Park County, Colorado

than 5 acres in the volcanic-rock aquifers. No significant 

increases in constituent concentrations were observed in wells 

completed in the sedimentary-rock aquifers for any lot-size 

category, and too few samples were collected from wells 

completed in the alluvial aquifers to do statistical tests.

The year of ISDS installation also was related to 

ground-water quality in Park County. For example, significantly higher nitrite-plus-nitrate concentrations were measured 

between wells with ISDS’s installed in the 1970’s and those 

installed in the 1980’s. Significantly higher nitrite-plus-nitrate 

concentrations were not measured between wells with ISDS’s 

installed in the 1980’s and those installed in the 1990’s. 

However, significantly higher nitrite-plus-nitrate concentrations were measured between wells with ISDS’s installed in 

the 1990’s and those installed after 1999. The lowest overall 

nitrite-plus-nitrate concentrations were measured in wells that 

had ISDS’s installed after 1999, and the highest concentrations 

were measured in wells with ISDS’s installed before 1980. 

Nitrate concentrations may be less in samples collected from 

wells with ISDS’s installed after 1980 because effluent has 

not had enough time to move through the unsaturated zone to 

the ground-water table in sufficient quantities to significantly 

affect ground-water quality.

Concentrations of nitrite-plus-nitrate and chloride were 

significantly higher (p-value < 0.05) in ground-water samples 

collected during 2001 than in samples collected during 1974 

in the Bailey area. Increases in nitrite-plus-nitrate and chloride 

concentrations in ground water in the Bailey area over time 

probably are related to a larger part of the overall recharge 

being from ISDS effluent either as a result of an increase in 

ISDS density or more time for recharge from ISDS effluent 

to reach the ground-water table.

Many of the wells sampled in Park County probably 

have a mixture of water from several parts of the aquifer 

system that may have different recharge areas, flow paths, 

and ground-water recharge dates. Because most of the development in Park County has occurred since 1990 and only 

about 8 percent of wells sampled for chlorofluorocarbons had 

ground-water recharge dates since 1990, there may not have 

been enough elapsed time for effluent water from ISDS’s 

to noticeably affect sampled ground-water quality. As time 

passes, recharge from ISDS’s may become a larger part of the 

ground-water resource, as a result, concentrations of chloride, 

boron, and other constituents associated with ISDS effluent 

may increase in ground water if not effectively removed by 

treatment processes.

Introduction

Park County, in central Colorado (fig. 1), has been one 

of the fastest growing counties in Colorado based on the 

percentage of population growth. Since 1970, the population 

has increased nearly eightfold (U.S. Bureau of the Census, 

1982, 2004). Specifically, the population of Park County 

was 2,185 in 1970 and 16,949 in 2005 (U.S. Bureau of the 

Census, 1982, 2006). The largest decadal population increase 

(7,349) occurred during the 1990’s. Continued development 

of commuter communities and vacation homes has increased 

the demand on water resources and, as such, has increased 

the potential for contamination of ground water. Although the 

towns of Alma, Bailey, and Fairplay have public water-supply 

systems, most county residences receive their domestic water 

supply from individual wells. In 2000, about 89 percent of the 

total population of Park County received their water from individual wells (Tamara Ivahnenko, U.S. Geological Survey, oral 

commun., June 21, 2006). Most of these households, served by 

individual wells, also have individual sewage disposal systems 

(ISDS). Health department officials, planners, and County 

Commissioners in Park County are interested in obtaining 

current information regarding water quality in the aquifers that 

serve the residents of the county and in assessing the potential 

degradation of ground-water quality due to the increased number and density of ISDS’s installed throughout the county.

Park County land-use regulations were put in place to 

“ensure that development in all areas of Park County provides for a water supply that is sufficient in terms of quantity, long-term dependability of supply, and quality without 

adversely affecting water-supply systems of neighboring 

uses” (Park County, 2005). To meet this goal, local entities 

need current information regarding the quality of ground 

water in Park County. Additional ground-water-quality data 

are needed to evaluate the effects of changing land-use and 

water-management practices on water resources, and to 

identify, describe, and explain, where possible, the major 

natural and human factors that affect observed water-quality 

conditions. In 2000, the U.S. Geological Survey (USGS), 

in cooperation with Park County, began a study to evaluate 

ground-water quality in the various aquifers in Park County 

that supply water to domestic wells. As part of this study, 

ground-water-quality samples were collected from domestic wells throughout Park County (fig.1). Water-quality data 

were compiled and analyzed to provide a general assessment 

of present (2001–2004) ground-water quality in the various 

aquifers that supply water to residents of the county.

Purpose and Scope

The purpose of this report is to provide relevant data 

and interpretations needed by water managers and planners 

to assess ground-water quality in Park County. Specifically, 

the report provides (1) a general assessment of the quality of 

ground water by locale and aquifer type, and (2) an assessment 

of the potential effects from ISDS’s on ground-water quality 

in the county. The report uses radionuclide data collected by 

the USGS in September 2000 in the Bailey area and data collected from July 2001 through October 2004, in cooperation 

with Park County, Colorado, to evaluate ground-water quality 

in the major aquifers that serve the residents of the county. In 

addition, selected data collected in 1974 by Klein and others 

(1978) also are used to determine if changes in constituent 

concentrations have occurred over time.
Introduction 3

Figure 1. Locations of the study area, domestic wells sampled by aquifer type, and all domestic wells listed in the Colorado 

Division of Water Resources Well Permit database (2004) in Park County, Colorado.

0 2.5 5 KILOMETERS

0 4 8 MILES 2

South Platte River Basin

Arkansas River Basin

EXPLANATION

Antero

Reservoir

Aquifer Type

Domestic well

Unknown

Alluvial

Crystalline

Sedimentary

Volcanic

Base map from U.S. Geological Survey digital data, 1:100,000

Universal Transverse Mercator projection,

Zone 13

Continental Divide

Hartsel Spinney Mountain

Reservoir

Elevenmile Canyon

Reservoir

Guffey

Lake George

Fairplay

Alma

Tarryall

Reservoir

Tarryall

Jefferson

Bailey

106°00' 105°30'

PARK COUNTY

C O L O R A D O

39°30'

39°00'

Denver

Colorado

Springs
4 Potential Effects of Individual Sewage Disposal System Effluent on Ground-Water Quality in Park County, Colorado

Description of the Study Area

Park County is located on the eastern side of the 

Continental Divide and encompasses about 2,210 square 

miles (mi2

) in central Colorado (fig. 1). Most of Park County is 

situated in the South Platte River Basin whereas a smaller area 

to the south is situated in the Arkansas River Basin. The terrain 

in Park County varies from a grass-covered plateau, encompassing much of the interior part of the county, to tree-covered 

mountains along the northern and western edges of the county. 

Land-surface altitude ranges from about 7,200 feet (ft) on the 

eastern edge of the county to more than 14,000 ft in the northwestern part of the county. Average annual precipitation ranges 

from about 10 to 40 inches and varies with altitude (Daly and 

Taylor, 1998). The lowest annual precipitation amounts occur 

in the vicinity of Hartsel (fig. 1), and the highest amounts 

occur along the Continental Divide in the western part of the 

county. Much of the precipitation in the higher attitudes is in 

the form of snow. On average, snow accumulates to more than 

300 inches per year in the mountains north and west of Alma 

(Kimbrough, 2001).

Land cover in Park County is 53 percent natural herbaceous plants, grasses, and sedges (herbaceous rangeland at 

lower altitudes, tundra at higher altitudes); 39 percent forest (deciduous and evergreen); 5 percent alpine (mixture of 

tundra, snow, ice, and barren rock); 2 percent pasture or hay; 

and 1 percent open water (Kimbrough, 2001). Trees and 

shrubs also exist with the herbaceous land cover but generally 

account for less than 25 percent of the cover.

The estimated population of Park County in 2005 was 

16,949 residents (http://www.census.gov/popest/counties/

tables/CO-EST2005-01-08.csv, accessed May 30, 2007). This 

number is about eight times the number of people identified in 

1970 (U.S. Bureau of the Census, 1982). Much of the population of Park County is located within subdivisions near the 

towns of Alma, Bailey, Fairplay, Guffey, Hartsel, and Lake 

George (fig. 1). Typically, house densities range from several 

houses per acre to a single house per many acres. Although 

the towns of Alma, Bailey, and Fairplay have public watersupply systems, these systems only serve a small segment of 

the population. In 2000, about 89 percent of the total population of Park County received their water from individual wells 

(Tamara Ivahnenko, U.S. Geological Survey, oral commun., 

June 21, 2006). The largest clusters of domestic supply wells 

in the South Platte Basin are located in subdivisions northeast 

of Bailey, near Alma and Fairplay, south of Jefferson, and 

southwest of Hartsel near the county line (fig. 1). Wells in the 

Arkansas River Basin are located primarily in the southeastern corner of the county from Guffey east to the county line 

(Kimbrough, 2001).

The surficial geology of Park County (fig. 2) ranges from 

unconsolidated alluvial deposits of Quaternary age to crystalline rocks of Precambrian age (Tweto, 1979; Klein and others, 

1978). It consists of intrusive and extrusive igneous rocks and 

depositions of marine and terrestrial sediments. Periods of 

folding and faulting and erosion by glaciation and rivers collectively produced a very complex geologic setting. Detailed 

descriptions of the geology of Park County are provided by 

Tweto (1979) and Klein and others (1978). The four primary aquifer types in Park County are alluvial deposits of 

Quaternary age; crystalline-rock (metamorphic and granitic) 

of Precambrian age; sedimentary rock of Tertiary, Cretaceous, 

Jurassic, and Paleozoic age; and volcanic rock of Tertiary age 

(Klein and others, 1978).

Reported well yields from domestic wells are variable 

in Park County ranging from about 0.01 to about 140 gallons 

per minute (gal/min) with an average reported well yield of 

about 7.5 gal/min (Colorado Division of Water Resources, 

2004). The variability in reported well yields depends on 

factors such as the aquifer properties and well construction. 

Reported well yields for wells sampled as part of this study 

ranged from about 0.3 to 96 gal/min with an average reported 

well yield of about 8.0 gal/min (Colorado Division of Water 

Resources, 2004).

Previous Studies

In the mid-1970’s, the USGS completed a reconnaissance water-resources appraisal (surface water, ground water, 

and water quality) of Park County (Klein and others, 1978). 

This work was done in cooperation with the Colorado Water 

Conservation Board (CWCB) and the Pikes Peak Area 

Council of Governments (PPACG). Klein and others (1978) 

determined that, in general, ground water in the unconsolidated alluvial deposits, consolidated sedimentary rocks, and 

Precambrian crystalline rocks was of suitable quality for 

domestic supplies with several exceptions. Specifically, dissolved fluoride concentrations in several areas exceeded the 

Colorado Department of Health’s (1971) recommended limits 

for drinking water. In particular, Klein and others (1978) 

indicated that water from wells completed in the Pikes Peak 

Granite north of Bailey tended to have a greater percentage 

of exceedances of Colorado Department of Health’s recommended fluoride limits for drinking water than water from 

wells completed in other geologic formations. In addition, 

chemical analysis of water from the Precambrian crystalline 

rocks in northeast Park County indicated that some wells had 

elevated concentrations of nitrate.

In 2001, the USGS completed a retrospective assessment of water resources for Park County. As part of this effort, 

surface- and ground-water resource data from 1962–1998 

were compiled and analyzed by Kimbrough (2001). This 

report indicated that (1) ground-water contamination may 

be occurring in some aquifers and may be associated with 

population growth and the resulting increase in ISDS’s, and 

(2) broad-scale geographic and seasonal distribution of current ground-water-quality conditions in Park County were not 

available. In 2004, the ground-water quality in the alluvial, 

crystalline-rock, sedimentary-rock, and volcanic-rock aquifers in Park County was assessed in three separate USGS fact 

sheets (Brendle, 2004; Ortiz, 2004a, 2004b). The fact sheets 

provided a preliminary assessment of the potential effects of 

ISDS density on ground-water quality. Brendle (2004) and 
Introduction 5

Figure 2. Surficial geology of Park County, Colorado.

EXPLANATION

Lakes

Towns

0 5

Base map from U.S. Geological Survey digital data, 1:100,000 0 5

Universal Transverse Mercator projection,

Zone 13

10 MILES

10 15 KILOMETERS

Alluvial deposits of Quaternary age

Crystalline rocks of Precambrian age

Sedimentary rocks of Tertiary, 

 Cretaceous, Jurassic, and 

 Paleozoic age

Intrusive and volcanic rocks of 

 Tertiary and Cretaceous age

Jefferson

Alma

Fairplay

Bailey

Tarryall

Hartsel

Lake George

Guffey

Antero

Reservoir

Elevenmile Canyon

Reservoir

Spinney Mountain

Reservoir

Tarryall

Reservoir

106°00' 105°30'

39°30'

39°00'

Geology modified from Green (1992)
6 Potential Effects of Individual Sewage Disposal System Effluent on Ground-Water Quality in Park County, Colorado

Ortiz (2004a, 2004b) used private well density as a surrogate 

for ISDS density, because (1) lots that have a house and a 

well also have an ISDS, and (2) records for wells were more 

easily accessible. Brendle (2004) determined that, for wells 

completed in the crystalline-rock aquifers in the vicinity of 

Bailey, concentrations of nitrate, chloride, and boron tended 

to be higher in areas with higher ISDS densities than in areas 

with lower ISDS densities. Ortiz (2004a) determined that for 

wells completed in the volcanic-rock aquifers in the vicinity 

of Guffey, concentrations of chloride and boron tended to be 

higher in areas with higher ISDS densities than in areas with 

lower ISDS densities.

Acknowledgments

The authors thank the many residents of Park County 

who permitted access to and sampling from their wells. The 

authors also thank Tom Eisenman, Director of Park County 

Environmental Health Department, for his support and assistance in study design and selection of areas to be sampled; 

and Brenda Green, Park County, for compiling ISDS data for 

the Park County sample sites. These data greatly enhanced the 

ability to evaluate the potential effects of ISDS’s on groundwater quality.

Study Approach

The general study approach was to identify and prioritize 

specific sampling areas within the alluvial, crystalline-rock, 

sedimentary-rock, and volcanic-rock aquifers in Park County 

and to collect water samples from domestic wells within these 

areas to characterize the spatial distribution of the ground-water 

quality in the four major aquifers. The focus of the study was 

to identify and describe the principal natural and human factors 

that affect ground-water quality and evaluate the relation of 

selected ground-water-quality contaminants with ISDS density, 

aquifer rock type, geography, and other hydrologic factors.

Well completion data were available for 223 of the 

224 wells sampled (Colorado Division of Water Resources, 

2004). For the purpose of this study, a well was identified 

by the aquifer-rock type within the open interval of the well. 

Nineteen wells were completed in the alluvial aquifers, 99 

in the crystalline-rock aquifers, 70 in the sedimentary-rock 

aquifers, and 35 in the volcanic-rock aquifers. The surficial 

geology as shown in figure 2 does not necessarily correspond 

to the aquifer-rock type of all wells sampled; some wells 

penetrate rock types other than those identified as surficial 

based on location and (or) well depth. For example, several 

wells west of Jefferson (fig. 1) were completed in the crystalline rock even though the surficial geology (fig. 2) shows only 

sedimentary and alluvial rocks in this area.

The overall sampling plan, in part, was designed to 

allow an evaluation of whether the density of development 

(proximity of wells and ISDS’s) was a significant factor in 

potential degradation of ground-water quality. The closeness 

of neighboring wells and ISDS’s varies depending on the 

size of the lots in each development. Hypothetically, there is 

a greater potential for degradation of ground-water quality 

as houses are built closer to each other and as the density of 

ISDS’s increases.

Ground water from domestic wells (that is general 

residential with lawn/garden irrigation and domestic animal/

livestock watering wells and residential household-use only 

wells) installed in the major aquifers in Park County was 

sampled from July 2001 through October 2004 as part of this 

study. During 2001 and 2002, the wells were sampled once 

each in the summer and fall to determine if there were detectable variations in water quality with time. Statistical tests 

indicated no significant differences between these samples, 

so only the fall samples were used in the data analysis in this 

report (Brendle, 2004). Radionuclide data collected by the 

USGS in the Bailey area also were included in this analysis, 

because radionuclide data were not collected in the Bailey 

area in 2001 (during this study).

Almost all wells were sampled for physical properties 

(water temperature, pH, and specific conductance), selected 

major ions, nutrients, bacteria, and boron (table 1). Selected 

wells also were sampled for dissolved organic carbon, specific 

human-related (wastewater) compounds, trace elements, 

radionuclides, and age-dating constituents (tritium and chlorofluorocarbons). The samples analyzed for human-related 

(wastewater) compounds were collected from most wells 

sampled in 2001 and from selected wells in 2002, 2003, and 

2004. The presence of specific human-related compounds such 

as fecal indicators, caffeine, detergent metabolites, cholesterol, 

and pharmaceuticals or elevated concentrations of bacteria or 

chemicals in ground waters might indicate contamination of 

ground water from ISDS’s.

All data are stored in the USGS National Water 

Information System (NWIS) database to aid in assessing 

current water-quality conditions and future trend and change 

analysis. Data used in this analysis can be obtained on the 

Web at URL http://nwis.waterdata.usgs.gov/co/nwis/qwdata

(search on Park County, data type ground water, and the date 

range of September 1, 2000, through October 10, 2004).

Sample Collection and Processing

Ground-water samples were collected for analysis of 

physical properties, major ions, nutrients, dissolved organic 

carbon, and bacteria following procedures described in the 

USGS National Field Manual (Wilde and others, 1998). 

Wastewater compounds were collected using methods 

described in Brown and others (1999). Samples for radon 

were collected and processed using the standard method for 

radon in drinking water (ASTM International, 2006). Uranium 

samples were collected and processed following methods 

described by Faires (1993). Procedures used to collect and 

analyze samples for chlorofluorocarbons (CFC) are given in 

Busenberg and Plummer (1992) and Plummer and Busenberg 

(1999), and those for determination of tritium are given in 

Ekwurzel and others (1994) and Ludin and others (1998).
Table 1. Ground-water-quality sites and summary of ground-water-quality data for Park County, Colorado, 2001–2004.—Continued[USGS, U.S. Geological Survey; PP, physical properties; MI, major ions; N, nutrients; Bact, bacteria; B, boron; DOC, dissolved organic carbon; WW, wastewater organics; TE, seleU, Uranium; AD, age dating; --, data not available; ALLV, alluvial; CRYS, crystalline; SEDT, sedimentary; VOLC, volcanic]

USGS 

site number

Sampling

date

(mm/dd/yy)

Well depth

(feet below

land surface)

Apparent

aquifer

type

PP MI N Bact B DOC WW TE 

384411105345301 8/7/2003 150 CRYS X X X X X X

384451105312101 8/2/2003 205 VOLC X X X X X X X 

384456105311601 8/11/2003 300 VOLC X X X X X X X 

384457105311901 8/11/2003 130 VOLC X X X X X X X 

384458105311601 7/31/2003 180 VOLC X X X X X X X

384500105352301 8/8/2003 250 VOLC X X X X X X

384502105312301 8/9/2003 200 VOLC X X X X X X

384503105311901 7/30/2003 140 VOLC X X X X X X X 

384504105290701 8/1/2003 460 VOLC X X X X X

384504105353401 8/8/2003 150 VOLC X X X X X

384512105311001 7/30/2003 140 VOLC X X X X X 

384513105311301 7/31/2003 75 VOLC X X X X X 

384513105311701 8/11/2003 120 VOLC X X X X X X X 

384523105374801 8/7/2003 260 VOLC X X X X X

384528105290801 8/1/2003 240 VOLC X X X X X X 

384538105284501 8/4/2003 180 VOLC X X X X X 

384547105334601 10/6/2004 150 SEDT X X X X X X X 

384601105354701 8/9/2003 180 VOLC X X X X X

384602105345701 8/11/2003 160 CRYS X X X X X X 

384615105262201 8/6/2003 -- VOLC X X X X X 

384629105281101 7/30/2003 320 VOLC X X X X X X

384637105260501 8/2/2003 203 VOLC X X X X X X X 

384639105262501 8/5/2003 128 VOLC X X X X X 

384644105282401 8/9/2003 400 VOLC X X X X X

384703105262201 8/1/2003 62 VOLC X X X X X X X

384707105285701 7/30/2003 450 VOLC X X X X X X X 

384711105290301 7/31/2003 325 VOLC X X X X X

384729105270501 8/4/2003 300 VOLC X X X X X 

384734105271901 8/4/2003 320 VOLC X X X X X 

384738105290501 8/7/2003 420 VOLC X X X X X

384739105283901 8/9/2003 250 VOLC X X X X X

384743105210901 8/1/2003 600 CRYS X X X X X

384752105273401 8/4/2003 340 VOLC X X X X X 

384839105364701 10/6/2004 20 VOLC X X X X X X X 

384841105283001 8/6/2003 250 VOLC X X X X X 

384855105281801 8/5/2003 280 VOLC X X X X X 

384920105290401 8/5/2003 150 VOLC X X X X X 

385001105531301 9/27/2004 220 SEDT X X X X X

385140105225001 7/22/2003 380 CRYS X X X X X
Table 1. Ground-water-quality sites and summary of ground-water-quality data for Park County, Colorado, 2001–2004.—Continued [USGS, U.S. Geological Survey; PP, physical properties; MI, major ions; N, nutrients; Bact, bacteria; B, boron; DOC, dissolved organic carbon; WW, wastewater organics; TE, seleU, Uranium; AD, age dating; --, data not available; ALLV, alluvial; CRYS, crystalline; SEDT, sedimentary; VOLC, volcanic] USGS site number Sampling date (mm/dd/yy) Well depth (feet below land surface) Apparent aquifer type PP MI N Bact B DOC WW TE 

385203105230301 7/22/2003 260 CRYS X X X X X 385210105281501 8/5/2003 155 VOLC X X X X X 

385250105242401 7/19/2003 240 CRYS X X X X X 385254105542201 9/21/2004 478 SEDT X X X X X X 

385258105554301 9/24/2004 240 SEDT X X X X X 385303105243701 7/21/2003 -- CRYS X X X X X X 385305105555901 9/27/2004 100 SEDT X X X X X X 

385310105241001 7/21/2003 230 CRYS X X X X X X 385310105555901 9/23/2004 202 SEDT X X X X X 385318105540501 9/21/2004 107 CRYS X X X X X X X 

385319105432601 10/6/2004 220 CRYS X X X X X X 

385325105231101 7/22/2003 500 CRYS X X X X X 385332105572301 9/20/2004 80 SEDT X X X X X 385351105552501 9/23/2004 140 SEDT X X X X X X 385352105411501 10/1/2004 150 VOLC X X X X X 385358105553601 9/21/2004 145 ALLV X X X X X 385406105544201 9/22/2004 88 SEDT X X X X X X 385411105552301 9/27/2004 120 SEDT X X X X X X 385411105553001 9/22/2004 180 SEDT X X X X X X X 

385411105560501 9/20/2004 260 SEDT X X X X X 385412105572501 9/22/2004 340 SEDT X X X X X X 

385413105233501 7/21/2003 150 CRYS X X X X X 

385415105413701 10/1/2004 100 SEDT X X X X X 

385416105545901 9/22/2004 120 SEDT X X X X X 385418105554701 9/20/2004 120 SEDT X X X X X 385427105240901 7/21/2003 320 CRYS X X X X X X X 

385429105575201 9/23/2004 405 SEDT X X X X X X 

385549105544701 9/23/2004 260 SEDT X X X X X 385607105293001 7/14/2003 180 CRYS X X X X X X X 

385616105293301 7/16/2003 160 CRYS X X X X X X 385623105292301 7/15/2003 200 CRYS X X X X X 

385631105291001 7/18/2003 150 CRYS X X X X X X X 

385651105283301 7/17/2003 100 CRYS X X X X X 

385704105282901 7/17/2003 175 CRYS X X X X X 385727105283701 7/16/2003 150 CRYS X X X X X X X 385808105303501 7/17/2003 168 CRYS X X X X X X X 385854105211701 7/18/2003 180 CRYS1 X X X X X X

385856105212301 7/18/2003 149 CRYS1 X X X X X X

385857105212601 7/22/2003 190 CRYS1 X X X X X 
Table 1. Ground-water-quality sites and summary of ground-water-quality data for Park County, Colorado, 2001–2004.—Continued[USGS, U.S. Geological Survey; PP, physical properties; MI, major ions; N, nutrients; Bact, bacteria; B, boron; DOC, dissolved organic carbon; WW, wastewater organics; TE, seleU, Uranium; AD, age dating; --, data not available; ALLV, alluvial; CRYS, crystalline; SEDT, sedimentary; VOLC, volcanic]

USGS 

site number

Sampling

date

(mm/dd/yy)

Well depth

(feet below

land surface)

Apparent

aquifer

type

PP MI N Bact B DOC WW TE 

390322105582901 9/28/2004 40 ALLV X X X X X X X 

390451105565401 10/1/2004 92 SEDT X X X X X

390526105533801 9/28/2004 180 SEDT X X X X X X 

390533106054901 9/24/2004 205 SEDT X X X X X

390610105574501 8/8/2002 4 -- X X X X

390625105531601 9/28/2004 160 SEDT X X X X X X 

390719105593201 10/4/2002 197 SEDT X X X X X 

390756105592301 9/29/2004 128 SEDT X X X X X X

390813105593101 9/26/2002 160 SEDT X X X X X X

390817106011401 10/8/2002 120 SEDT X X X X X 

390819105584301 9/18/2002 135 SEDT X X X X X X

390953106025801 9/26/2002 -- SEDT X X X X X X

391022105571701 10/4/2004 60 ALLV X X X X X

391107105455501 9/16/2004 280 CRYS X X X X X 

391144106013201 9/19/2002 150 ALLV X X X X X

391144106014801 9/25/2002 140 ALLV X X X X X 

391147106002901 9/24/2002 200 SEDT X X X X X 

391207106014501 9/23/2002 165 ALLV X X X X X 

391215106020001 9/24/2002 160 SEDT X X X X X 

391218106020801 9/19/2002 140 SEDT X X X X X

391229105471901 9/17/2004 380 CRYS X X X X X

391233106022101 9/18/2002 160 SEDT X X X X X X

391250106003701 9/25/2002 155 SEDT X X X X X X X

391306106013101 9/24/2002 130 ALLV X X X X X X X 

391320106015201 9/28/2002 198 SEDT X X X X X 

391331106050001 9/23/2002 123 SEDT X X X X X 

391334106025901 7/10/2002 183 SEDT X X X X X

391338105443301 10/5/2004 350 CRYS X X X X X

391338106010701 10/1/2002 -- ALLV X X X X X 

391340106033101 9/23/2002 220 SEDT X X X X X X 

391344106024701 9/19/2002 160 SEDT X X X X X X

391346106023101 9/23/2002 80 ALLV X X X X X X X

391351105460301 9/15/2004 240 CRYS X X X X X X X 

391354105451001 9/13/2004 280 CRYS X X X X X

391356106054701 9/20/2002 102 SEDT X X X X X

391400106030401 9/29/2004 98 ALLV X X X X X

391405105431201 9/16/2004 600 CRYS X X X X X

391405106034101 10/8/2002 160 ALLV X X X X X 

391407106062701 9/20/2002 110 CRYS X X X X X X X X
Table 1. Ground-water-quality sites and summary of ground-water-quality data for Park County, Colorado, 2001–2004.—Continued [USGS, U.S. Geological Survey; PP, physical properties; MI, major ions; N, nutrients; Bact, bacteria; B, boron; DOC, dissolved organic carbon; WW, wastewater organics; TE, seleU, Uranium; AD, age dating; --, data not available; ALLV, alluvial; CRYS, crystalline; SEDT, sedimentary; VOLC, volcanic] USGS site number Sampling date (mm/dd/yy) Well depth (feet below land surface) Apparent aquifer type PP MI N Bact B DOC WW TE 

391423106024501 10/10/2002 115 SEDT X X X X X X 391425105465901 9/16/2004 255 CRYS X X X X X X 391428105470701 9/15/2004 200 CRYS X X X X X X 

391434106023701 9/25/2002 115 ALLV X X X X X 

391437106025701 10/7/2002 75 ALLV X X X X X 

391440106044901 10/10/2002 150 SEDT X X X X X 

391445106041001 9/30/2002 130 SEDT X X X X X X 391450105443701 9/15/2004 302 CRYS X X X X X 391455106032801 10/8/2002 140 SEDT X X X X X 391458106035701 10/9/2002 100 SEDT X X X X X X 

391500106023301 10/14/2002 195 SEDT X X X X X 391503105445001 9/17/2004 420 CRYS X X X X X 391504106013801 9/27/2002 220 SEDT X X X X X X 391504106033201 10/14/2002 180 SEDT X X X X X 

391509106030701 10/5/2002 108 CRYS X X X X X X 

391513105464401 9/14/2004 485 CRYS X X X X X X 

391525106014001 10/5/2002 252 SEDT X X X X X 

391526106051301 10/7/2002 140 SEDT X X X X X X 

391527106021301 7/25/2002 125 SEDT X X X X X 391530106021301 10/14/2002 175 SEDT X X X X X X 

391531106003901 10/4/2004 126 CRYS X X X X X 391535105444101 9/14/2004 550 CRYS X X X X X X 

391536106024201 10/5/2002 175 SEDT X X X X X 391550106014501 9/28/2002 305 SEDT X X X X X X X 391629105433301 9/14/2004 100 CRYS X X X X X X X 

391629106043801 10/4/2002 130 SEDT X X X X X X 391629106044401 9/27/2002 120 SEDT X X X X X X X X 391629106051601 10/11/2002 110 SEDT X X X X X X X 391632106044301 9/28/2004 100 SEDT X X X X X X 391632106054201 10/10/2002 -- ALLV X X X X X 391638106042001 7/23/2002 105 SEDT X X X X X 391641106052901 10/3/2002 80 SEDT X X X X X X 

391717105431501 9/16/2004 300 CRYS X X X X X X 391742106042301 10/3/2002 50 CRYS X X X X X X 

391743106040101 10/9/2002 70 ALLV X X X X X X 391747106041101 10/1/2002 140 SEDT X X X X X X 

391820106040201 10/2/2002 -- SEDT X X X X X 

391831106035301 9/28/2002 155 SEDT X X X X X X

391852105533401 9/30/2004 100 SEDT X X X X X X
Table 1. Ground-water-quality sites and summary of ground-water-quality data for Park County, Colorado, 2001–2004.—Continued[USGS, U.S. Geological Survey; PP, physical properties; MI, major ions; N, nutrients; Bact, bacteria; B, boron; DOC, dissolved organic carbon; WW, wastewater organics; TE, seleU, Uranium; AD, age dating; --, data not available; ALLV, alluvial; CRYS, crystalline; SEDT, sedimentary; VOLC, volcanic]

USGS 

site number

Sampling

date

(mm/dd/yy)

Well depth

(feet below

land surface)

Apparent

aquifer

type

PP MI N Bact B DOC WW TE 

391902105534101 9/29/2004 100 SEDT X X X X X X X 

391903106034901 10/7/2002 165 SEDT X X X X X

391911105534501 9/29/2004 80 SEDT X X X X X X

391940106033401 9/26/2002 110 SEDT X X X X X X

391947106033701 10/2/2002 195 SEDT X X X X X X X

392006106034101 9/27/2002 151 SEDT X X X X X X

392031106033101 9/30/2002 50 ALLV X X X X X X 

392035106033301 10/2/2002 -- ALLV X X X X X X X

392050105504301 9/30/2004 18 ALLV X X X X X X 

392222105482701 9/30/2004 38 ALLV X X X X X 

392237105480601 9/30/2004 140 SEDT X X X X X X

392250105492301 9/13/2004 125 CRYS X X X X X

392255105503801 9/13/2004 155 CRYS X X X X X

392427105252501 9/11/2001 221 CRYS X X X X X

392435105280901 9/13/2001 400 CRYS X X X X X X

392437105244401 *9/10/2001 603 CRYS X X X X X X

392439105251901 9/11/2001 452 CRYS X X X X X

392447105253501 9/11/2001 400 CRYS X X X X X X

392448105253201 9/11/2001 275 CRYS X X X X X X

392452105271401 9/12/2001 340 CRYS X X X X X X

392453105245501 9/11/2001 575 CRYS X X X X X X

392504105255401 9/12/2001 752 CRYS X X X X X X

392517105281801 9/14/2001 295 CRYS X X X X X X

392526105281101 9/13/2001 600 CRYS X X X X X X

392527105243701 9/10/2001 183 CRYS X X X X X X

392532105241801 9/10/2001 203 CRYS X X X X X X

392533105272601 9/13/2001 220 CRYS X X X X X X

392536105241701 9/18/2001 405 CRYS X X X X X X

392536105273201 9/13/2001 200 CRYS X X X X X X

392542105260501 9/12/2001 702 CRYS X X X X X X

392613105245401 9/10/2001 250 CRYS X X X X X X

392618105265801 9/12/2001 400 CRYS X X X X X X

392633105252301 9/10/2001 220 CRYS X X X X X X

392652105275601 9/13/2001 100 CRYS X X X X X X

392659105255201 9/25/2001 100 CRYS1 X X X X X X

392712105284301 9/17/2001 270 CRYS X X X X X X

392714105253001 9/12/2001 103 CRYS X X X X X X

392830105273501 9/19/2001 700 CRYS1 X X X X X X

392832105275001 9/19/2001 400 CRYS1 X X X X X X
Table 1. Ground-water-quality sites and summary of ground-water-quality data for Park County, Colorado, 2001–2004.—Continued [USGS, U.S. Geological Survey; PP, physical properties; MI, major ions; N, nutrients; Bact, bacteria; B, boron; DOC, dissolved organic carbon; WW, wastewater organics; TE, seleU, Uranium; AD, age dating; --, data not available; ALLV, alluvial; CRYS, crystalline; SEDT, sedimentary; VOLC, volcanic] USGS site number Sampling date (mm/dd/yy) Well depth (feet below land surface) Apparent aquifer type PP MI N Bact B DOC WW TE 

392833105281301 9/21/2001 700 CRYS1 X X X X X X 392834105245801 9/14/2001 275 CRYS1 X X X X X X 392840105241501 9/14/2001 503 CRYS1 X X X X X X 392843105283601 9/18/2001 680 CRYS1 X X X X X X 392855105280901 7/23/2001 130 CRYS1 X X X X 392857105241201 9/25/2001 125 CRYS1 X X X X X X 392857105283301 9/18/2001 283 CRYS1 X X X X X X 392905105275301 9/20/2001 124 CRYS1 X X X X X X 392916105290501 9/17/2001 265 CRYS1 X X X X X X 392923105273901 9/20/2001 203 CRYS1 X X X X X X 392923105313501 9/20/2001 250 CRYS1 X X X X X X 392925105313301 9/21/2001 200 CRYS1 X X X X X X 392926105284801 9/17/2001 200 CRYS1 X X X X X X 392933105282901 9/17/2001 550 CRYS1 X X X X X X 392939105314101 9/25/2001 400 CRYS1 X X X X X X 393010105242401 9/18/2001 324 CRYS1 X X X X X X 393011105311501 9/17/2001 85 CRYS1 X X X X X X 393014105314201 9/19/2001 500 CRYS1 X X X X X X 393017105313601 9/17/2001 305 CRYS1 X X X X X X 393019105262601 9/14/2001 340 CRYS1 X X X X X 393030105324701 9/20/2001 320 CRYS1 X X X X X X 393037105305201 9/18/2001 480 CRYS1 X X X X X X 393039105311501 9/18/2001 400 CRYS1 X X X X X X 393040105310201 9/20/2001 310 CRYS1 X X X X X X 393041105300501 9/21/2001 175 CRYS1 X X X X X X 393046105310701 9/19/2001 385 CRYS1 X X X X X X 393047105295201 9/19/2001 180 CRYS1 X X X X X X 393054105263801 9/15/2001 240 CRYS1 X X X X X X 393054105314101 9/24/2001 450 CRYS1 X X X X X X *392527105243701 10/12/2002 183 CRYS 

*392533105272601 10/15/2002 220 CRYS 

*392830105273501 10/15/2002 700 CRYS1 

*392939105314101 10/11/2002 400 CRYS1 *393030105324701 10/14/2002 320 CRYS1 

*Well resampled for age-dating constituents.1Well completed in the Pikes Peak Granite.
Study Approach 13

Analytical Methods

Water samples were analyzed for various chemical 

groups including major ions, nitrogen species, phosphorus 

species, and radiochemical constituents following procedures 

outlined by Fishman (1993) and Faires (1993). Additionally, 

water samples at selected wells were analyzed for an extensive 

list of organic chemicals that are assumed to be indicative of 

contamination from ISDS effluent using methods described 

by Zaugg and others (2002). Analyses of the aforementioned 

constituents were done at the USGS National Water Quality 

Laboratory in Denver, Colorado. Tritium samples were 

analyzed at the USGS Isotope Research Laboratory in Menlo 

Park, California. CFC samples were analyzed at the USGS 

Chlorofluorocarbon Laboratory in Reston, Virginia.

Quality Control

Quality-control samples collected in the field included 

18 blank and 19 replicate samples. About 14 percent (37 of 

261) of the water-quality samples collected during this study 

(2001–2004) were quality-control samples. Field blanks were 

collected and analyzed to test for sample bias from contamination. Field blanks are discussed in detail in the following 

paragraphs. Replicate samples were collected to estimate variability in the environmental data. Replicate samples showed 

little variability in environmental data. Spike samples were not 

collected during this study. However, mean percent recoveries for laboratory spikes from 2001–2004 for the wastewater 

compounds ranged from 4 to 106 percent with a mean percent 

recovery of 81 percent (Quality control data provided by the 

National Water Quality Laboratory, U.S. Geological Survey, 

written commun., 2007).

Calcium, magnesium, sulfate, nitrite-plus-nitrate, 

dissolved organic carbon, and boron were detected in one or 

more field blanks usually at or below the laboratory reporting 

level. Concentrations of calcium, magnesium, and sulfate in 

the environmental samples were at least one order of magnitude higher than concentrations in blank samples; therefore, 

concentrations of these constituents in field blanks were 

small relative to the concentrations in environmental samples 

and did not affect reported environmental concentrations. 

Concentrations of nitrite-plus-nitrate and dissolved organic 

carbon in field blanks were reported below the laboratory 

reporting levels. Nitrite-plus-nitrate was detected only in 1 

of the 11 field blanks analyzed for nitrite-plus-nitrate, whereas 

organic carbon was detected in 2 of the 4 field blanks analyzed for organic carbon. Because nitrite-plus-nitrate was 

detected in only one sample (collected during 2001), reported 

environmental concentrations of nitrite-plus-nitrate probably 

are not substantially affected by contamination. Environmental samples with reported concentrations of dissolved 

organic carbon near the laboratory reporting level may be 

periodically affected by trace amounts of sample contamination. Trace quantities of organic carbon in field blanks did 

not substantially affect the assessment of environmental data 

collected, because multiple lines of evidence were used to 

determine if human activities have an effect on ground-water 

quality in Park County. Potential sources of organic carbon in 

field blanks may include plasticizers in tubing or bottles and 

(or) blank water used to collect samples.

Wastewater compounds, including 4-nonylphenol (4-NP), 

acetophenone, benzophenone, DEET, diethoxynonylphenol, 

isophorone, methyl salicylate, phenol, and tris(2-butoxyethyl) 

phosphate, were detected in one or more field blanks usually at or below the laboratory reporting level. However, 

acetophenone and methyl salicylate were not detected in 

environmental samples. Potential sources of wastewater 

compounds in blanks could be field personnel (fragrances, 

DEET), the field environment (dust, fumes), sample bottles, 

sample transport, or the laboratory environment, or the blank 

water used for field blanks may have contained the compounds. DEET, diethoxynonylphenol, methyl salicylate, 

and tris(2-butoxyethyl) phosphate only were detected once 

(one of eight) in field blanks, and 4-NP, benzophenone, and 

isophorone only were detected twice (two of eight) in field 

blanks. Trace quantities of these compounds in field blanks did 

not substantially affect the assessment of environmental data 

collected. The most frequently detected wastewater compound 

in field blanks that also was detected in environmental samples 

was phenol. Phenol was detected in three out of eight field 

blanks. Zaugg and Leiker (2006) suggest that phenol sample 

data should be interpreted cautiously because of the frequent 

detections in field and laboratory blank samples at concentrations that are important relative to reported environmental 

concentrations. In total, phenol was detected in 26 environmental samples. In 20 of the 26 samples, 1 or more additional 

wastewater compounds also were detected. This indicates that 

the presence of phenol in these samples is probably not the 

result of field or laboratory contamination but that phenol may 

be present in the ground water in some areas of Park County. 

Consequently, the trace quantities of phenol that were sporadically detected in blank samples were determined not to affect 

the assessment of environmental data collected.

Data Analysis Methods

To determine the approximate time elapsed since 

recharge (apparent recharge date of the ground water) and 

thus the potential for a given sample to be affected by development, ground-water samples were collected from selected 

wells in the alluvial, crystalline-rock, sedimentary-rock, and 

volcanic-rock aquifers throughout Park County and analyzed for tritium and CFC’s. Tritium is used as an age-dating 

tracer because atmospheric nuclear bomb testing beginning 

in 1954 produced relatively large concentrations of tritium in 

the atmosphere (Kendall and McDonnell, 1998). Concentrations of tritium greater than 10 picocuries per liter (pCi/L), 

or about 30 tritium units (TU), indicate that at least a part of 

the recharge to the ground-water system occurred after 1954 
14 Potential Effects of Individual Sewage Disposal System Effluent on Ground-Water Quality in Park County, Colorado

(Kendall and McDonnell, 1998). Concentrations of tritium 

do not definitively yield the apparent recharge date of the 

water in a sample but must be used with other age-dating 

chemicals such as CFC’s to refine the recharge date estimate. 

Commercial production of CFC’s began in 1930. CFC’s 

gradually replaced older refrigerants in cooling devices. After 

the mid-1940’s, CFC’s became the preferred aerosol propellants and were widely used as solvents, degreasers, and as 

blowing agents for plastic foam (Plummer and others, 2006). 

Concentrations of tritium and CFC’s in the atmosphere over 

time are shown in figures 3A and 3B. Concentrations of CFC’s 

in the atmosphere have increased steadily from the 1940’s 

through the early 1990’s (fig. 3B). To determine the apparent 

ground-water recharge date based on CFC concentrations, 

ground water is assumed to be in solubility equilibrium with 

the air in the unsaturated zone (the zone between the land 

surface and the water table) at the time of recharge (Plummer 

and others, 2006). The apparent ground-water recharge date 

can be estimated by comparing the calculated partial pressures of CFC’s in solubility equilibrium with the water sample 

with the historical CFC concentrations in local air (Plummer 

and others, 2006). Ground-water samples were analyzed for 

CFC-11, CFC-12, and CFC-113. Multiple CFC compounds 

(CFC-11, CFC-12, and CFC-113) are analyzed to aid in determining the apparent ground-water recharge date of a sample 

because each CFC compound has a unique concentration over 

time (fig. 3B).

CFC-based recharge dates were determined assuming no 

dilution by mixing or delay by interaction with aquifer materials, which are common processes for constituents moving with 

ground water. Because water that is pumped from a well is a 

mixture of the waters from all the flow lines reaching the discharge point, water samples from wells with relatively narrow 

open (screened) intervals will tend to have apparent recharge 

dates that are less affected by mixing than those samples 

from wells with large open intervals. Open intervals in wells 

sampled in Park County ranged from about 10 to about 300 ft 

based on available well-log data. Because of the large open 

intervals in many of the wells sampled, apparent ground-water 

recharge dates probably have been affected by mixing of 

water of different ages. Apparent ground-water recharge dates, 

based on CFC concentrations, for Park County samples were 

determined by the USGS Chlorofluorocarbon Laboratory in 

Reston, Virginia.

The maximum, minimum, median, and 10th, 25th, 75th, 

and 90th percentile values were computed for each constituent analyzed with the exception of the wastewater compounds, 

because of the small number of detections per constituent. 

These basic descriptive statistics were done for radionuclide 

data using data collected from September 2000 through 

October 2004, and basic descriptive statistics for the remaining 

constituents were done using data collected from July 2001 

through October 2004.

Boxplots were generated to show simple graphical summaries of selected data sets (that is constituents) and to compare data sets based on ISDS density, aquifer type, and year of 

ISDS installation. Boxplots are useful because the variability 

0

1,000

2,000

3,000

4,000

5,000

6,000

TRITIUM CONCENTRATION, 

IN TRITIUM UNITS 

1945 1955 1965 1975 1985

YEAR

1995 2005

0

100

200

300

400

500

600

CONCENTRATION, 

IN PARTS PER TRILLION, BY VOLUME

1945 1955 1965 1975

YEAR

1985 1995 2005

CFC-11

CFC-12

CFC-113

A

B

Figure 3. (A) Estimated concentrations of tritium in precipitation 

near Park County, Colorado (International Atomic Energy Agency, 

2004), and (B) atmospheric concentrations of CFC-11, CFC-12, 

and CFC-113 in air in North America (http://water.usgs.gov/lab/

software/air_curve/, accessed February 22, 2007).

between data sets, unusual values, and selected summary statistics are easily observed. The horizontal line within the box 

represents the median value (50 percent of the data are larger 

than this value and 50 percent of the data are less than this 

value). The lower horizontal line of the box is the 25th percentile or lower quartile (25 percent of the data are less than this 

value). The upper horizontal line of the box is the 75th percentile or upper quartile (75 percent of the data are less than 

this value). The interquartile range (IQR) contains the values 

between the 25th and 75th percentiles and is the difference 

between the 25th and 75th percentiles. The bottom of the vertical line on the boxplot is the smallest value within 1.5 times 

the IQR of the box. The top of the vertical line on the boxplot is the largest value within 1.5 times the IQR of the box. 

Outside values are greater than 1.5 times the IQR from the box 

and outlier values are greater than 3 times the IQR from the 

box. An example of a boxplot is shown in figure 4.
Quality of Ground Water 15

Data, collected as part of this study, were compared to 

U.S. Environmental Protection Agency (USEPA) primary 

and secondary drinking-water standards (U.S. Environmental 

Protection Agency, 1999b, 2002a) to assess the general quality 

of the ground water in the study area for domestic use. However, the primary and secondary drinking-water standards only 

apply to public water systems that provide water to at least 

15 connections or 25 persons at least 60 days out of the year 

(most cities and towns, schools, businesses, campgrounds, and 

shopping malls are served by public water systems).

National Primary Drinking Water Regulations (NPDWR’s 

or primary standards) are legally enforceable standards that 

apply to public water systems. Primary standards protect 

public health by limiting the levels of contaminants in drinking water. National Secondary Drinking Water Regulations 

(NSDWR’s or secondary standards) are nonenforceable 

guidelines regulating contaminants that may cause cosmetic 

effects (such as skin or tooth discoloration) or aesthetic effects 

(such as taste, odor, or color) in drinking water. The USEPA 

recommends secondary standards to water systems but does 

not require systems to comply (http://www.epa.gov/safewater/

contaminants/index.html#listsec, accessed June 27, 2006).

To explore the potential human effects, particularly 

effects of ISDS’s, on ground-water quality the occurrence/

co-occurrence of selected constituents (that is nitrate, ammonia, chloride, total coliform, E. coli, boron, and wastewater 

compounds) associated (in part) with ISDS effluent were 

examined. The relations between these constituents and ISDS 

density, average lot size in surrounding area, aquifer type, 

year of ISDS installation, and other data were evaluated using 

the nonparametric Wilcoxon Mann-Whitney test (SYSTAT 

Software, Inc., 2004). Nonparametric analysis allows the user 

to analyze data without assuming an underlying distribution. 

The Wilcoxon Mann-Whitney test is a nonparametric test 

used to compare two populations (data sets). Specifically, it is 

used to test the null hypothesis that two data sets have identical distribution functions against the alternative hypothesis 

that the two distribution functions differ only with respect to 

location (median), if at all. For the purpose of this study, the 

null hypothesis is the statistical hypothesis that there is no 

difference in constituent concentrations between data sets. 

The p-value is the probability of wrongly rejecting the null 

hypothesis (no difference between data sets) if it is in fact true. 

Small p-values indicate that the null hypothesis is unlikely to 

be true. In other words, small p-values indicate that differences 

between the two data sets likely exist. Differences between 

data sets were determined to be significant when the p-value 

was 0.05 or smaller.

Quality of Ground Water

A focus of this study was to identify and describe the 

principal natural and human factors that affect ground-water 

quality throughout Park County. Knowledge of the approximate time elapsed since recharge (age of ground water) 

helps to understand the processes that affect ground-water 

quality and the potential for ground water to be affected by 

human activities such as wastewater discharge (ISDS effluent) and runoff from urban areas. The longer the water is in 

contact with soils and aquifer material, the more time there 

is for dissolution of minerals and time for sorption and ion 

exchange processes. In addition, if samples represent recharge 

to the aquifer that occurred prior to development, effects of 

ISDS’s will not be detected in ground-water-quality samples. 

Therefore, it may not be possible to determine if ISDS’s are 

affecting or will affect ground-water quality in the future. If 

samples represent recharge that has occurred since development, however, the effects of ISDS’s on ground-water quality 

may be observed.

Age of Ground Water

Samples from selected wells throughout Park County 

were analyzed for tritium and CFC’s to determine the approximate time elapsed since recharge. Tritium concentrations 

greater than 10 pCi/L in a sample indicate that at least some 

part of the recharge to the ground-water system occurred after 

1954 (Kendall and McDonnell, 1998). Tritium concentrations 

were greater than 10 pCi/L in about 75 percent of the samples 

collected from wells in Park County. The highest tritium concentrations in precipitation were measured during the early to 

mid-1960’s (fig. 3A). Tritium concentrations for Park County 

(fig. 3A) were estimated using data collected throughout 

North America (International Atomic Energy Agency, 2004). 

Ground-water recharge was assumed to be mostly from snowmelt; as a result, only tritium concentrations in precipitation Figure 4. Explanation of a boxplot.

* Outlier value

 (greater than 1.5 times the IQR)

Outlier value

 (greater than 3 times the IQR)

Upper tail

 (less than or equal to 

 1.5 times the IQR)

75th percentile

50th percentile

 (median or middle value)

25th percentile

Lower tail

 (less than or equal to 

 1.5 times the IQR)

IQR

(interquartile 

range)
16 Potential Effects of Individual Sewage Disposal System Effluent on Ground-Water Quality in Park County, Colorado

from October through March (winter months) were used to 

estimate annual tritium values for Park County. Current (2004) 

concentrations of tritium in precipitation in Park County 

probably are between 6 and 10 pCi/L (International Atomic 

Energy Agency, 2004). Consequently, tritium concentrations 

between 6 and 10 pCi/L may indicate ground-water recharge 

dates prior to about 1960 or more recent recharge since about 

the mid-1990’s. Only one sample, collected from a well in the 

volcanic-rock aquifer, had a tritium concentration between 6 

and 10 pCi/L. Overall the highest tritium concentrations were 

measured in wells completed in the alluvial and crystallinerock aquifers, and the lowest concentrations were measured in 

wells completed in the sedimentary and volcanic-rock aquifers 

(fig. 5A).

Based on CFC data, ground-water recharge dates in Park 

County range from the mid-1940’s to modern water. In other 

words, the ground-water recharge age (based on CFC data) is 

between zero and about 60 years old (fig. 5B and fig. 6). Based 

on CFC and tritium data, wells completed in the sedimentaryrock aquifers tended to have older waters than wells completed 

Figure 5. Distribution of (A) tritium concentrations and (B) approximate recharge dates based on chlorofluorocarbon data for alluvial, 

crystalline-rock, sedimentary-rock, and volcanic-rock aquifers throughout Park County, Colorado, 2001–2004.

in the other aquifer-rock types (alluvial, crystalline, and volcanic) (fig. 5B) indicating that one or more of the following conditions occur: (1) infiltration of water through the sedimentary 

rocks is slower than other formations, (2) the ground-water 

flow paths are longer, or (3) that ground-water flow velocities 

are slower.

Many of the wells sampled in Park County probably have 

a mixture of water from several parts of the aquifer system that 

may have different recharge areas, flow paths, and ground-water 

recharge dates. Given that most of the development in Park 

County has occurred since 1990 and only about 8 percent of 

wells (3 of 38) sampled for CFC’s had ground-water recharge 

dates since 1990 (fig. 6), there may not have been enough time 

elapsed for effluent water from ISDS’s to noticeably affect 

sampled ground-water quality. About 47 percent of wells 

sampled for age-dating constituents and wastewater compounds 

had detectable quantities of wastewater compounds. However, 

concentrations of wastewater compounds were generally low 

(near the laboratory reporting level). Detections of wastewater 

compounds in ground water indicate that these compounds are 

TRITIUM, IN PICOCURIES PER LITER

n = 12 n = 24

n = 11

n = 8

AQUIFER TYPE

0

10

20

30

40

50

60

70

A B

AQUIFER TYPE

1940

1950

1960

1970

1980

1990

2000

2010

APPARENT RECHARGE DATE

n = 11

n = 12

n = 8

n = 7

Outlier value

Outside value

75th percentile

Median

25th percentile

Smallest value within 1.5 times the IQR

Largest value within 1.5 times the IQR

Interquartile range (IQR)

n = 10 Number of samples

EXPLANATION

Alluvial Crystalline

rock

Sedimentary

rock

Volcanic

rock

Alluvial Crystalline

rock

Sedimentary

rock

Volcanic

rock
Quality of Ground Water 17

Figure 6. Approximate ground-water recharge date based on CFC concentrations in selected wells in Park County, 

Colorado, 2001–2004.

Volcanic

Mean ground-water

 recharge date

9

1945–1970

1971–1989

1990–2003

EXPLANATION

24

285

9

285

Sedimentary

Alma

Fairplay

Hartsel Spinney Mountain

Reservoir

Elevenmile Canyon

Reservoir

Lake George

Tarryall

Tarryall

Reservoir

Jefferson Bailey

Antero

Reservoir

Guffey

0 3

0 5 10 KILOMETERS

1.5 6 MILES

39°00'

39°30'

106°00' 105°30'

Note: Wells may be completed in rock type other than the surficial geology shown on this figure.

Crystalline

(Igneous and Metamorphic)

Base map from U.S. Geological Survey digital data, 1:100,000

Universal Transverse Mercator projection,

Zone 13

Alluvial 
18 Potential Effects of Individual Sewage Disposal System Effluent on Ground-Water Quality in Park County, Colorado

not being completely removed by ISDS’s or the unsaturated

zone. Because roughly 90 percent of domestic water used is 

assumed to be recharged by ISDS’s (Van Slyke and Simpson, 

1974), detections of wastewater compounds in ground water in 

Park County are not surprising. As time passes, it is likely that 

recharge from ISDS’s may become a larger part of the groundwater resource.

Natural Factors Affecting Drinking-Water 

Quality in Park County

Ground-water quality in Park County is affected by natural 

factors resulting from the interaction of water with the soil and 

mineralogy of the various geologic formations as it infiltrates 

to the water table in addition to human factors. Ground-water 

samples were analyzed for many constituents as part of this 

study (2001–2004); however, the following discussion is limited to those constituents that exceeded national primary and 

(or) secondary drinking-water standards (U.S. Environmental 

Protection Agency, 1999b, 2002a) most likely as a result of 

natural geologic factors. Secondary drinking-water standards 

for pH, fluoride, sulfate, and dissolved solids (DS) and primary 

drinking-water standards for fluoride and uranium were equaled 

or exceeded in one or more samples collected from domestic 

wells in Park County (table 2). Concentrations of radon-222 

(referred to as radon hereafter) in some wells exceeded the 

proposed drinking-water regulations (U.S. Environmental 

Protection Agency, 1999a). The occurrence and sources of 

pH, fluoride, sulfate, DS, uranium, and radon in ground-water 

samples are discussed in the following paragraphs.

pH

Only about 2 percent (5 of 224) of pH values measured 

were outside of the national secondary drinking-water regulation’s secondary maximum contaminant level (SMCL) range 

for pH of 6.5 to 8.5 established by the U.S. Environmental 

Protection Agency (2006, 1999b). Values of pH less than 

6.5 standard units were measured only in wells in the Pikes 

Peak Granite (crystalline-rock aquifer) in the vicinity of Bailey 

(fig. 7B). However, not all samples from wells in the Pikes 

Peak Granite were less than 6.5: pH values ranged from 5.9 

to 8.4 standard units in samples from these wells (fig. 7A, B). 

Low pH values in some wells may indicate that ground-water 

flow (or perhaps recharge) is more rapid and has less contact 

time with geologic units, in certain areas of the Pikes Peak 

Granite. This may occur because of thinner soils and (or) a 

higher percentage of fractures to transport water. In these 

areas, the pH of the ground water may be closer to the pH of 

the precipitation that recharges the aquifer. The pH in precipitation in Colorado typically ranges from about 4.7 to 5.9 standard units (National Atmospheric Deposition Program, 2007). 

A pH of 8.7 was measured in one well in the volcanic-rock 

aquifer near Guffey (fig. 7B). High pH in certain types of volcanic rocks has been well documented in parts of the Western 

United States and Argentina (Welch, 2001; Spencer, 2002).

When pH decreases to less than 6.5, water becomes more 

corrosive and has a bitter metallic taste. Corrosion of pipes, 

distribution lines, and well casings becomes more substantial at 

a pH of less than 6.5. Values of pH greater than 8.5 can cause 

aesthetic problems such as a soda taste, slippery feel, buildup of 

scale on pipes and fixtures that can lead to lower water pressure, 

and deposits on dishes, utensils and laundry basins.

Fluoride

The USEPA has set primary and secondary drinkingwater standards for fluoride of 4.0 and 2.0 mg/L, respectively. 

Only one well, completed in the Pikes Peak Granite, near 

Lake George, exceeded the primary drinking-water standard 

for fluoride of 4.0 milligrams per liter (mg/L). Concentrations 

of fluoride in about 12 percent (27 of 224) of the samples 

exceeded the secondary drinking-water standard of 2.0 mg/L 

(fig. 8A, B). Long-term exposure to drinking-water concentrations of fluoride greater than 4.0 mg/L may result in serious bone disorders (U.S. Environmental Protection Agency, 

2002a). Excess fluoride exposures at concentrations greater 

than 2.0 mg/L during the formative period prior to tooth eruption may cause tooth discoloration and (or) pitting. 

Fluoride (or fluorine) occurs in igneous and sedimentary 

rock. Concentrations of fluoride measured in wells in Park 

County ranged from less than 0.2 to 4.0 mg/L (fig. 8A, B and 

table 2). Generally the highest fluoride concentrations were 

measured in samples collected from wells completed in the 

crystalline-rock aquifers, and the lowest concentrations were 

measured in samples collected from wells completed in the 

alluvial aquifers. Ground water containing fluoride concentrations exceeding 1 mg/L occurs in many places in the United 

States, in a wide variety of geologic settings. Specifically, 

Hawley and Wobus (1977) determined fluorine was abundant 

in minerals in the Pikes Peak Granite, located primarily in 

eastern Park County. Fluoride concentrations were greater 

than 1 mg/L in about 97 percent (34 of 35) of samples collected (during this study) from wells completed in the Pikes 

Peak Granite, and about 71 percent (25 of 35) were equal to 

or greater than 2.0 mg/L (fig. 8). All samples with fluoride 

concentrations equal to or greater than 2.0 mg/L were collected from wells completed in the crystalline-rock aquifers 

near Bailey and Lake George (fig. 8B); 93 percent of these 

samples were collected from wells completed in the Pikes 

Peak Granite. Concentrations of fluoride in wells completed 

in the alluvial, sedimentary-rock, and volcanic-rock aquifers 

generally were less than 0.5 mg/L.

Sulfate

Sulfates are a combination of sulfur and oxygen and are 

part of naturally occurring minerals in some soil and rock formations that contain ground water. Sulfur is widely distributed 

in reduced form in igneous and sedimentary rocks as metallic 

sulfides. When sulfide minerals undergo weathering in contact 

with aerated water, the sulfur is oxidized to sulfate ions that go 
Table 2. Summary of selected ground-water-quality data for Park County, Colorado, 2000–2004.[CFC, chlorofluorocarbon; <, less than; oC, degrees Celsius; μS/cm, microsiemens per centimeter; cols/100 mL, colonies per 100 milliliters; mg/L, milligrams per liter; pCi/L, pgrams per liter; gal/min, gallons per minute; picograms per kilogram, pg/kg; CaCO3, calcium carbonate; E, estimated value; M, constituent measured but not quantified; --, not a

Constituent or property Number of analyses Minimum Concentration or value at indicated percentile Maximum 

10% 25% 50% 75% 90%

Physical properties

pH, standard units 224 5.9 6.8 7.2 7.5 7.8 8.0 8.7 

Specific conductance, μS/cm 224 86 185 233 340 483 731 3,070 

Water temperature, oC 224 4.5 5.7 7.0 8.7 10.0 11.0 14.8 

Dissolved oxygen, mg/L 196 0 0.3 0.8 3.9 6.2 7.8 9.7 

Major ions and dissolved solids, in mg/L

Bicarbonate, dissolved 215 4 75 127 184 254 319 623 

Calcium, dissolved 165 6.17 26.1 33.6 44.8 60.3 87.9 684 

Chloride, dissolved 224 0.28 1.01 1.80 3.59 6.84 12.8 873 

Fluoride, dissolved 224 <0.2 <0.2 0.2 0.3 0.8 2.2 4.0 

Magnesium, dissolved 165 0.56 4.43 6.52 15.5 27.0 48.0 195 

Potassium, dissolved 165 <.16 0.62 0.87 1.24 2.01 3.92 8.05 

Sodium, dissolved 165 0.73 2.49 4.05 9.74 17.9 39.5 219 

Sulfate, dissolved 220 2.4 5.8 10.2 18.0 36.3 105 1,760 

Dissolved solids 158 49 124 158 202 299 496 2,810 

Nutrients and organic carbon, in mg/L

Ammonia 224 <.04 <.04 <.04 <.04 <.04 E 0.02 2.04 

Nitrate 6 0.21 0.21 0.24 0.53 0.79 1.40 1.47 

Nitrite plus nitrate 224 <0.06 <0.06 E 0.04 0.34 1.18 3.54 25.7 

Nitrite 224 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 0.28 

Orthophosphate 224 <0.02 <0.02 <0.02 <0.02 E 0.01 0.06 0.34 

Phosphorus 223 <.006 <.006 <.006 E 0.004 0.017 0.062 0.37 Organic carbon 70 0.2 0.3 0.5 0.7 1.2 1.7 7.5 

Bacteria, in colonies/100 mL

Escherichia coli 220 <1 <1 <1 <1 <1 <1 1 

Total coliform 220 <1 <1 <1 <1 <1 1 E 340 

Trace elements, in μg/L

Aluminum, dissolved 9 <20 <20 <20 <20 <20 <20 <20 

Arsenic, dissolved 9 <2 <2 <2 <2 <2 2.6 3 

Boron, dissolved 223 <13 6.1 11 16 26 61 574 

Cadmium, dissolved 9 <8 <8 <8 <8 <8 <8 <8 

Copper, dissolved 9 <6 <6 <6 <6 5.2 11 13 

Iron, dissolved 9 <10 <10 <10 75 334 648 810 

Manganese, dissolved 9 <2 <2 <2 7.8 16 26 30.8 

Zinc, dissolved 9 <24 <24 <24 <24 <24 197 317 

Other constituents or properties

Alkalinity, mg/L as CaCO3 215 3 61 104 152 209 262 511 

Hardness, mg/L as CaCO3 165 19 87 120 180 252 400 2,500 Radon, pCi/L 477 40 314 805 1,670 3,952 6,854 19,200 

Uranium, μg/L 480 <1 <1 1.85 2.93 6.12 11.2 48 

Well depth, feet below land surface 217 4 100 130 180 296 444 752 

Well yield, gal/min 207 0.3 2 4 7 10 15 96 

1U.S. Environmental Protection Agency drinking water standards.2Secondary drinking water standard (U.S. Environmental Protection Agency, 1999b).3Primary drinking water standard (U.S. Environmental Protection Agency, 2002a).4Samples collected in the Bailey area in 2000 were included in the analysis.
20 Potential Effects of Individual Sewage Disposal System Effluent on Ground-Water Quality in Park County, Colorado

AQUIFER TYPE

5

6

7

8

9

n = 19

n = 99

n = 70 n = 35

A

pH, IN STANDARD UNITS

Outlier value

Outside value

75th percentile

Median

25th percentile

Smallest value within 1.5 times the IQR

Largest value within 1.5 times the IQR

Interquartile range (IQR)

n = 10 Number of samples—Total does not equal number 

 in table 2; aquifer type unknown for one well

EXPLANATION

Alluvial Crystalline

rock

Sedimentary

rock

Volcanic

rock

Figure 7 (above and facing page). (A) Distribution of pH by aquifer type and (B) spatial distribution of pH in samples 

collected from selected domestic wells in Park County, Colorado, 2001–2004.

into solution in the water (Hem, 1992). In addition, the combustion of coal and petroleum and other industrial processes 

such as smelting of sulfide ores can release sulfur into the 

atmosphere (Hem, 1992). Sulfate in drinking water currently 

(2004) has a SMCL of 250 mg/L based on aesthetic effects 

(that is taste and odor).

Sulfate minerals can cause scale buildup in water pipes 

similar to other minerals and may be associated with a bitter taste in water. Sulfate can make cleaning clothes difficult. 

Using chlorine bleach in sulfur water may reduce the cleaning power of detergents. Health concerns regarding sulfate in 

drinking water have been raised because of reports of diarrhea 

associated with the ingestion of water containing high levels 

of sulfate. Available data indicate that people may acclimate 

rapidly to the presence of sulfates in their drinking water. 

However, groups within the general population, such as infants, 

may be at greater risk from the laxative effects of sulfate when 

they experience an abrupt change from drinking water with 

low sulfate concentrations to drinking water with high sulfate 

concentrations (U.S. Environmental Protection Agency, 1999c). 

According to panelists at the Health Effects from Exposure to 

Sulfate Workshop sponsored by the Centers for Disease Control 

and Prevention on September 28, 1998 (U.S. Environmental 

Protection Agency, 1999d), there is not enough scientific 

evidence on which to base a regulation for sulfate in drinking 

water, but panelists favored a health advisory in places where 

drinking water has sulfate levels greater than 500 mg/L.

Sulfate concentrations ranged from 2.4 to 1,760 mg/L, 

and the median concentration was 18 mg/L (table 2). The 

highest concentrations of sulfate generally were measured 

in samples collected from wells in the sedimentary-rock 

aquifers near the Park County line southwest of Hartsel and 

between Alma and Fairplay (figs. 9A, B). The lowest sulfate 

concentrations were measured in samples collected from wells 

completed in the crystalline-rock aquifers (fig. 9A, B). Sulfate 

concentrations measured in samples from wells completed in 

the crystalline-rock aquifers were generally less than 50 mg/L. 

Most of the samples that exceeded the SMCL of 250 mg/L for 

sulfate also exceeded the SMCL of 500 mg/L for DS. Concentrations in about 5 percent of the samples (12 of 220) exceeded 

the SMCL of 250 mg/L, and concentrations in about 4 percent 

of the samples (8 of 220) were greater than 500 mg/L, more 

than twice the SMCL (figs. 9A, B).

Dissolved Solids

DS consist of inorganic salts (principally calcium, 

magnesium, potassium, sodium, bicarbonates, chlorides, 

and sulfates) and some small amounts of organic matter 

that are dissolved in water. High concentrations of DS can 

give water a murky appearance and detract from the quality of taste of the water. In drinking water, DS originate from 

natural sources, sewage, urban runoff, and chemicals used 
Quality of Ground Water 21

0 3 6 MILES 1.5

0 5 10 KILOMETERS 2.5

Note: Wells may be completed in rock type other than the surficial geology shown on this figure.

Base map from U.S. Geological Survey digital data, 1:100,000

Universal Transverse Mercator projection,

Zone 13

Sedimentary

Volcanic

Crystalline

(Igneous and Metamorphic)

Alluvial 

EXPLANATION

pH less than 6.5 (U.S. Environmental 

 Protection Agency Secondary Maximum 

 Contaminant Level less than 6.5)

pH 6.5 to 8.5

pH greater than 8.5 (U.S. Environmental 

 Protection Agency Secondary Maximum 

 Contaminant Level greater than 8.5)

285

24

9

9

285

B

Pikes Peak

Granite

Tarryall

Reservoir

Alma

Hartsel

Tarryall

Antero

Reservoir

Elevenmile Canyon

Reservoir

Spinney Mountain

Reservoir

Lake George

Bailey

Fairplay

Jefferson

106°00' 105°30'

39°30'

39°00'

Guffey
22 Potential Effects of Individual Sewage Disposal System Effluent on Ground-Water Quality in Park County, Colorado

in the water-treatment process, and the nature of the piping 

or hardware used to convey the water. Estimates of DS were 

made for samples collected from the Bailey area in 2001 when 

DS concentrations were not available using a linear regression relation developed between DS and specific conductance 

data from 43 wells sampled during 2000 in the Bailey area 

(DS = [0.55*specific conductance] + 12.41, coefficient of 

determination (r2

) = 0.99, standard error = 6.8). The 55 estimated values were not used to compute the summary statistics 

presented in table 2 but are shown in figure 10 and included in 

the following discussion.

Elevated concentrations of DS in Park County groundwater samples probably are mostly from natural sources. 

However, some elevated concentrations of DS in the Bailey 

and Guffey areas may result from contamination by ISDS’s, 

because chloride and nitrate concentrations in these groundwater samples were considerably higher than concentrations 

in samples collected from surrounding wells. Concentrations 

of DS measured in samples ranged from 49 to 2,810 mg/L, 

and the median DS concentration was 202 mg/L (fig. 10A and 

table 2). In general, DS concentrations were lowest in samples 

collected from wells completed in the crystalline-rock aquifers and highest in wells completed in the sedimentary-rock 

aquifers (figs. 10A, B). The SMCL of 500 mg/L for DS was 

exceeded in about 7 percent of samples (16 of 214), mostly 

in samples collected from wells completed in the sedimentaryrock aquifer (fig. 10). DS concentrations in about 4 percent 

of the samples (9 of 214) were greater than 1,000 mg/L (twice 

the SMCL).

Radionuclides

Radionuclides are radioactive elements such as radium 

and uranium. Radioactive elements sampled as part of this 

study included radon (radon-222) and uranium. Radon-222 

(produced by the decay of radium-226) is the main radon 

isotope of environmental importance because it has a longer 

half-life (3.8 days) than other radon isotopes (Hem, 1992). 

Radon-222 is commonly called radon when referring to radon 

in homes. Radon is an invisible and odorless gas that forms 

from the radioactive decay of uranium and thorium naturally 

present in rocks and soils. Because radon is a gas, it can easily 

move through soil and cracks in building slabs or basement 

walls and concentrate in a building’s indoor air. Although soil 

is the primary source, the use of household water that contains 

radon contributes to elevated indoor radon levels. Most of the 

cancer risk from radon in drinking water arises from the transfer of radon into indoor air, and exposure through inhalation, 

although some risk exists from ingesting water that contains 

radon. The National Academy of Sciences (1999) recommends that the USEPA continues to use 10,000 pCi/L in water 

to 1 pCi/L in air as the best estimate of the transfer of radon 

in drinking water to radon in indoor air (through showering, 

cooking, and other household water uses). The recommended 

maximum radon concentration for indoor air is 4 pCi/L 

(U.S. Environmental Protection Agency, 2005).

Currently (2004), there is no federally enforced drinkingwater standard for radon in public water-supply systems, but 

proposed regulations indicate a maximum contaminant level 

Figure 8 (above and facing page). (A) Distribution of fluoride by aquifer type and (B) spatial distribution of fluoride in 

samples collected from selected domestic wells in Park County, Colorado, 2001–2004.

0

1

2

3

4

FLUORIDE, IN MILLIGRAMS PER LITER

AQUIFER TYPE

A

n = 19

n = 99

n = 70

n = 35

Outlier value

Outside value

75th percentile

Median

25th percentile

Smallest value within 1.5 times the IQR

Largest value within 1.5 times the IQR

Interquartile range (IQR)

n = 10

EXPLANATION

Alluvial Crystalline

rock

Sedimentary

rock

Number of samples—Total does not equal number

 in table 2; aquifer type unknown for one well

Volcanic

rock
Quality of Ground Water 23

Crystalline

(Igneous and Metamorphic)

Volcanic

Sedimentary

Alluvial

1.0 to 1.9

Less than 1.0

Fluoride concentration, 

 in milligrams per liter

2.0 to 3.9 (U.S. Environmental Protection Agency

 Secondary Maximum Contaminant Level,

 2.0 milligrams per liter)

Equal to or greater than 4.0 (U.S. Environmental

 Protection Agency Maximum Contaminant

 Level, 4.0 milligrams per liter)

Note: Wells may be completed in rock type other than the surficial geology shown on this figure.

EXPLANATION

Base map from U.S. Geological Survey digital data, 1:100,000

Universal Transverse Mercator projection,

Zone 13

24

285

285

9

9

B

Pikes Peak

Granite

106°00' 105°30'

39°30'

39°00'

Jefferson

Hartsel

Guffey

Elevenmile Canyon

Reservoir

Spinney Mountain

Reservoir

Tarryall

Antero

Reservoir

Fairplay

Alma

Tarryall

Reservoir

Bailey

Lake George

0 11⁄2 3 6 MILES

0 2.5 5 10 KILOMETERS
24 Potential Effects of Individual Sewage Disposal System Effluent on Ground-Water Quality in Park County, Colorado

(MCL) for radon (radon-222) of 300 pCi/L and an alternative 

MCL of 4,000 pCi/L contingent upon other mitigating remedial activities (U.S. Environmental Protection Agency, 1999a). 

Under the proposed regulations, the USEPA would encourage 

States and (or) managers of public water systems, to develop 

programs to address the health risks from radon in indoor air 

and require public water systems to reduce radon levels in 

drinking water to 4,000 pCi/L or less. If States and (or) public 

water systems do not develop programs to aid in reducing 

radon in indoor air, public water systems would be required to 

have radon levels in drinking water of 300 pCi/L or less. The 

proposed standards will apply only to public water systems 

that regularly serve 25 or more people and that use ground 

water or mixed ground and surface water. These standards will 

not apply to systems that rely on surface water where radon 

levels in the water are very low. They also will not apply to 

private wells, because the USEPA does not regulate them 

(U.S. Environmental Protection Agency, 1999a).

Radon concentrations in ground-water samples in Park 

County ranged from 40 to 19,200 pCi/L (table 2 and fig. 11A, 

B). Radon concentrations in about 91 percent of ground-water 

samples (70 of 77) collected from domestic wells throughout Park County were greater than or equal to 300 pCi/L, 

and about 25 percent (19 of 77) were greater than or equal 

to 4,000 pCi/L. Generally, the highest radon concentrations 

were measured in samples collected from wells completed in 

the crystalline-rock aquifers, and the lowest concentrations 

were measured in samples collected from wells completed in 

the volcanic-rock aquifers (fig. 11A). The spatial distribution 

of radon concentrations in ground-water samples collected 

throughout Park County is shown in figure 11B.

Primary drinking-water standards have been established 

for uranium because of increased risk of cancer and kidney 

toxicity (U.S. Environmental Protection Agency, 2006). The 

current (2004) MCL for uranium (30 micrograms per liter 

[μg/L]) in drinking water was exceeded in two samples collected from wells completed in the crystalline-rock aquifer. 

Uranium concentrations in ground-water samples ranged from 

less than 1.0 to 48 μg/L (table 2 and fig. 12A, B). Similar to 

radon, the highest uranium concentrations generally occurred 

in samples collected from wells completed in the crystallinerock aquifers, and the lowest concentrations generally 

occurred in samples collected from wells completed in the 

volcanic-rock aquifers (fig. 12A, B). The spatial distribution 

of uranium concentrations in ground-water samples collected 

throughout Park County is shown in figure 12B.

Human Factors Affecting Drinking-Water 

Quality in Park County

Effluent from ISDS’s, runoff from parking lots and 

roadways, and use of chemicals (solvents for cleaning, motor 

oil, and paint) around the home or in industrial processes can 

affect ground-water quality. The following discussion is limited 

to those constituents that exceeded national primary and (or) 

secondary drinking-water standards (U.S. Environmental 

Figure 9 (above and facing page). (A) Distribution of sulfate by aquifer type and (B) spatial distribution of sulfate in 

samples collected from selected domestic wells in Park County, Colorado, 2001–2004.

Outlier value

Outside value

75th percentile

Median

25th percentile

Smallest value within 1.5 times the IQR

Largest value within 1.5 times the IQR

Interquartile Range (IQR)

n = 10 Number of samples—Total does not equal number 

 in table 2; aquifer type unknown for one well 

10

100

1,000

SULFATE, IN MILLIGRAMS PER LITER

n = 19 n = 99 n = 67 n = 35

A

AQUIFER TYPE

EXPLANATION

Alluvial Crystalline

rock

Sedimentary

rock

Volcanic

rock
Quality of Ground Water 25

Crystalline

(Igneous and Metamorphic)

Volcanic

Sedimentary

Alluvial 

Base map from U.S. Geological Survey digital data, 1:100,000

Universal Transverse Mercator projection,

Zone 13

Note: Wells may be completed in rock type other than the surficial geology shown on this figure.

B

24

285

285

9

9

Less than 125

125 to 249

250 to 499 (U.S. Environmental Protection

 Agency Secondary Maximum Contaminant

 Level, 250 milligrams per liter)

Equal to or greater than 500

Sulfate concentrations, 

 in milligrams per liter

EXPLANATION

Alma

Fairplay

Antero

Reservoir

Jefferson

Hartsell Spinney Mountain

Reservoir

Elevenmile Canyon

Reservoir

Tarryall

Reservoir

Bailey

Tarryall

Guffey

Lake George

106°00' 105°30'

39°30'

39°00'

0 3 6 MILES

0 5 10 KILOMETERS
26 Potential Effects of Individual Sewage Disposal System Effluent on Ground-Water Quality in Park County, Colorado

Figure 10 (above and facing page). (A) Distribution of dissolved solids by aquifer type and (B) spatial distribution of 

dissolved solids in samples collected from selected domestic wells in Park County, Colorado, 2001–2004.

Protection Agency, 2002a) in Park County ground-water 

samples most likely as a result of human factors. The USEPA 

secondary drinking-water standards for chloride and primary 

drinking-water standards for nitrate (shown as nitrite-plusnitrate in table 2), total coliform, and E. coli were exceeded 

in one or more samples collected from domestic wells in Park 

County (table 2). 

Chloride

High levels of chloride tend to indicate contamination by 

human activities including use of road salts, discharges from 

water softeners, human-waste disposal, and other activities. 

Chloride may be naturally present in various rock types (Hem, 

1992). However, higher concentrations of chloride generally 

exist in ISDS effluent than in natural ground water. Chloride is a conservative chemical species that generally does 

not degrade in natural ground water, is not readily removed 

through natural soil treatment, and travels faster than other 

less conservative chemicals in unsaturated zone and aquifer 

systems (Heatwole and others, 2005).

Concentrations of chloride in ground-water samples 

ranged from 0.28 to 873 mg/L; 75 percent of measured 

concentrations were less than about 7 mg/L and 90 percent 

were less than about 13 mg/L (table 2). The highest overall 

chloride concentrations were measured in wells completed 

in the crystalline-rock aquifers (median chloride concentration in crystalline-rock aquifer wells was 5.16 mg/L), and 

the lowest concentrations in wells completed in the alluvial 

aquifers (median chloride concentration in alluvial aquifer 

wells was 1.66 mg/L). The SMCL of 250 mg/L for chloride 

was exceeded in only one sample collected from a well in the 

crystalline-rock aquifer near Bailey. (This well also had an elevated nitrite-plus-nitrate concentration of 11.1 mg/L that also 

exceeded the 10 mg/L MCL for nitrate.) All other measured 

chloride concentrations were at least an order of magnitude 

less than the SMCL.

Nitrate

Nitrate concentrations at or above the primary drinkingwater standard of 10 mg/L generally are attributed to human 

sources. In Park County, elevated nitrate concentrations most 

likely are caused by wastewater effluent from ISDS’s. Most 

ground-water samples were not analyzed for nitrate independently but for nitrite and nitrite-plus-nitrate. Nearly all nitrite 

concentrations were reported as less than the reporting level of 

0.008 mg/L, which is below the primary standard of 1 mg/L 

reported as nitrogen. Because nitrite concentrations were small, 

comparisons to the nitrate standard were done using nitriteplus-nitrate data. The maximum nitrite-plus-nitrate concentration was 25.7 mg/L (table 2), which exceeds the MCL of 

10 mg/L for nitrate (reported as nitrogen). Concentrations of 

nitrite-plus-nitrate in about 2 percent (4 of 224) of samples 

equaled or exceeded the MCL for nitrate (fig. 13). All four of 

these wells were in the crystalline-rock aquifer near Bailey 

(fig. 13). The highest nitrite-plus-nitrate concentrations were 

measured in ground-water samples collected from wells near 

DISSOLVED SOLIDS, IN MILLIGRAMS PER LITER 1,000

100

AQUIFER TYPE

Outlier value

Outside value

75th percentile

Median

25th percentile

Smallest value within 1.5 times the IQR

Largest value within 1.5 times the IQR

Interquartile range (IQR)

n = 10 Number of samples—Total does not equal number 

 in table 2; aquifer type unknown for some wells

n = 18

n = 43

n = 64 n = 33

A

EXPLANATION

Alluvial Crystalline

rock

Sedimentary

rock

Volcanic

rock
Quality of Ground Water 27

Note: Wells may be completed in rock type other than the surficial geology shown on this figure.

Base map from U.S. Geological Survey digital data, 1:100,000

Universal Transverse Mercator projection,

Zone 13

Sedimentary

Volcanic

Crystalline

(Igneous and Metamorphic)

Alluvial 

B

EXPLANATION

Less than 250

250 to 499

Equal to or greater than 1,000

Reservoir

500 to 999 (U.S. Environmental

 Protection Agency Secondary

 Maximum Contaminant Level, 

 500 milligrams per liter)

285

285

24

9

9

Dissolved solids concentration,

 in milligrams per liter

Bailey

Jefferson

Tarryall

Reservoir

Alma

Fairplay

Antero

Reservoir

Tarryall

Lake George

Elevenmile Canyon

Reservoir

Guffey

Hartsel Spinney Mountain

Reservoir

0 11⁄2 3 6 MILES

0 2.5 5 10 KILOMETERS

39°30'

39°00'

106°00' 105°30'
28 Potential Effects of Individual Sewage Disposal System Effluent on Ground-Water Quality in Park County, Colorado

Bailey and Guffey (fig. 13). The median nitrite-plus-nitrate concentration for samples collected throughout Park County during 

this study was 0.34 mg/L, which is well below the drinkingwater standard for nitrate (table 2). Nitrate concentrations in 

drinking water are important because ingestion of nitrate by 

infants can cause low oxygen levels in the blood, a potentially 

fatal condition (U.S. Environmental Protection Agency, 2006).

Bacteria

Total coliform and E. coli bacteria live in the intestines 

of warm-blooded animals and in waste material or feces 

excreted from the intestinal tract. The presence of total 

coliform and E. coli in water samples is used to indicate 

recent sewage or animal-waste contamination. Although not 

typically disease causing, their presence is correlated with 

the presence of several waterborne disease-causing organisms. A few strains of E. coli are pathogenic, such as E. coli

O157:H7, but most strains are not (Wilde and others, 1998). 

For public water-supply systems, the USEPA requires that 

no more than 5.0 percent of samples collected in a month 

have detections of total coliform; and for water systems that 

collect fewer than 40 routine samples per month, no more 

than 1 sample can test positive for total coliform per month 

(U.S. Environmental Protection Agency, 2006). Because only 

one sample was collected and analyzed for total coliform and 

E. coli at each well during this study, a well was considered to 

exceed the USEPA primary drinking-water standards if bacteria were detected.

E. coli only were detected in one sample from a well 

near Guffey, whereas, total coliform were detected in about 

11 percent (24 of 220) of wells sampled. Concentrations of 

total coliform in samples ranged from less than 1 colony 

per 100 milliliters (col/100 mL) of water to an estimated 

340 col/100 mL (table 2). The highest total-coliform concentrations were measured in samples collected from wells southeast of Jefferson and west of Tarryall Reservoir (fig. 14).

In 1974, ground-water-quality samples were collected 

by the USGS from 80 wells throughout Park County. Total 

coliform concentrations in the crystalline-rock aquifer wells in 

the Bailey area were higher in samples collected during 1974 

(Kimbrough, 2001) than in samples collected during this study 

(the Bailey area was sampled during 2001). Total coliform 

were detected in about 20 percent of the crystalline-rock aquifer wells sampled in the Bailey area in 1974 compared to 5 percent in 2001. Concentrations of total coliform in the Bailey 

area ranged from less than 1 to 2,000 col/100 mL in 1974 and 

from less than 1 to an estimated 2 col/100 mL in 2001.

Summary of Drinking-Water Quality in Domestic 

Wells in Park County

Overall, ground water throughout Park County is suitable for domestic use. Only about 3 percent of ground-water 

samples had concentrations of fluoride, nitrate, and (or) 

uranium that exceeded a primary drinking-water standard. 

These primary drinking-water standards only were exceeded 

Outlier value

Outside value

75th percentile

Median

25th percentile

Smallest value within 1.5 times the IQR

Largest value within 1.5 times the IQR

Interquartile range (IQR)

n = 10 Number of samples—Total does not equal number 

 in table 2; aquifer type unknown for some wells

AQUIFER TYPE

0

5,000

10,000

RADON, IN PICOCURIES PER LITER

n = 11

n = 27

n = 22

n = 15

15,000

20,000

A

EXPLANATION

Alluvial Crystalline

rock

Sedimentary

rock

Volcanic

rock

Figure 11 (above and facing page). (A) Distribution of radon by aquifer type and (B) spatial distribution of radon in samples 

collected from selected domestic wells in Park County, Colorado, 2000–2004.
Quality of Ground Water 29

Note: Wells may be completed in rock type other than the surficial geology shown on this figure.

Base map from U.S. Geological Survey digital data, 1:100,000

Universal Transverse Mercator projection,

Zone 13

Sedimentary

Volcanic

Crystalline

(Igneous and Metamorphic)

Alluvial 

285

285

24

9

9

B

Less than 300

300 to 3,999 (U.S. Environmental Protection 

 Agency #1 proposed Maximum Contaminant 

 Level, 300 picocuries per liter)

4,000 to 7,999 (U.S. Environmental Protection 

 Agency #2 proposed Maximum Contaminant 

 Level, 4,000 picocuries per liter)

Equal to or greater than 8,000 

Lakes

EXPLANATION

Radon concentration, in picocuries per liter

Jefferson

Alma

Antero

Reservoir

Spinney Mountain

Reservoir

Elevenmile Canyon

Reservoir

Guffey

Lake George

Bailey

Tarryall

Reservoir

Tarryall

39°30'

39°00'

106°00' 105°30'

Hartsel

Fairplay

0 11⁄2 3 6 MILES

0 2.5 5 10 KILOMETERS
30 Potential Effects of Individual Sewage Disposal System Effluent on Ground-Water Quality in Park County, Colorado

in samples collected from wells completed in the crystallinerock aquifers. Radon concentrations in about 25 percent of the 

samples were greater than or equal to 4,000 pCi/L, the highest 

proposed maximum contaminant level for radon in drinking 

water. Most radon concentrations greater than 4,000 pCi/L 

were measured in samples collected from wells completed in 

the crystalline-rock aquifers; however, some high concentrations of radon (greater than 4,000 pCi/L) were measured in 

samples collected from wells completed in the alluvial and 

sedimentary-rock aquifers. Total coliform were detected in 

about 11 percent of samples throughout Park County. These 

samples were collected from wells completed in each aquiferrock type (alluvial, crystalline, sedimentary, and volcanic). 

Secondary standards were exceeded more frequently than 

primary standards. Concentrations of one or more constituents 

exceeded secondary drinking-water standards in about 19 percent of samples.

Based on age-dating data, only about 8 percent of wells 

sampled for CFC’s had ground-water recharge dates after 

1990. Because most of the development in Park County has 

occurred since 1990, there may not have been enough time 

elapsed for human activities to considerably affect sampled 

ground-water quality. As time passes, it is possible that 

recharge affected by human activities may become a larger 

part of the ground-water resource. As a result, concentrations 

and (or) frequency of detection of constituents associated with 

human activities may increase.

The USEPA recommends that well owners have their 

wells tested for pH, DS, nitrates, and total-coliform bacteria 

levels annually (U.S. Environmental Protection Agency, 

2002b). One way to reduce the risk of contaminating nearby 

wells is to properly maintain the onsite ISDS. Many useful 

bulletins describing proper use and maintenance of septic tanks 

(ISDS’s) can be found on the World Wide Web at sites such as: 

http://www.agnr.umd.edu/users/wye/personel/Miller/septic.html, 

http://www.soil.ncsu.edu/publications/Soilfacts/AG-439-13/, and 

http://www.montana.edu/wwwpb/pubs/mt9401.html.

Potential Effects of Individual Sewage 

Disposal System Effluent on GroundWater Quality

An ISDS can provide a source of chemicals and bacteria 

if the leach-field pipe is too close to the water table or if the 

ground-water-flow velocity is too rapid to allow for proper 

geochemical or physical treatment of the ISDS effluent. 

Chemicals from products that are used in households can enter 

the ground-water system as a more concentrated effluent from 

an ISDS (Kolpin and others, 2002) than from natural sources. 

Examples of products containing these chemicals include 

soaps that contain boron, dietary salt that contains chloride, 

Outlier value

Outside value

75th percentile

Median

25th percentile

Smallest value within 1.5 times the IQR

Largest value within 1.5 times the IQR

Interquartile range (IQR)

n = 10 Number of samples—Total does not equal number

 in table 2; aquifer type unknown for some wells

AQUIFER TYPE

URANIUM, IN MILLIGRAMS PER LITER

0

10

20

30

40

50

n = 12

n = 29

n = 22

n = 15

A

EXPLANATION

Alluvial Crystalline

rock

Sedimentary

rock

Volcanic

rock

Figure 12 (above and facing page). (A) Distribution of uranium by aquifer type and (B) spatial distribution of uranium in 

samples collected from selected domestic wells in Park County, Colorado, 2000–2004.
Potential Effects of Individual Sewage Disposal System Effluent on Ground-Water Quality 31

Crystalline

(Igneous and Metamorphic)

Volcanic

Sedimentary

Alluvial 

Less than15

15 to 30

Equal to or greater than 30 (U.S. Environmental 

 Protection Agency Maximum Contaminant 

 Level, 30 micrograms per liter)

EXPLANATION

Note: Wells may be completed in rock type other than the surficial geology shown on this figure.

Base map from U.S. Geological Survey digital data, 1:100,000

Universal Transverse Mercator projection,

Zone 13

B

285

285

9

9

24

Uranium concentration, in micrograms 

 per liter

39°00'

39°30'

106°00' 105°30'

Jefferson Bailey

Tarryall

Reservoir

Tarryall

Fairplay

Alma

Hartsel

Lake George

Spinney Mountain

Reservoir

Elevenmile Canyon

Reservoir

Antero

Reservoir

0 11⁄2 3 6 MILES

0 2.5 5 10 KILOMETERS

Guffey
32 Potential Effects of Individual Sewage Disposal System Effluent on Ground-Water Quality in Park County, Colorado

Figure 13. Spatial distribution of nitrite-plus-nitrate in samples collected from selected domestic wells in Park 

County, Colorado, 2001–2004.

Lake George

Note: Wells may be completed in rock type other than the surficial geology shown on this figure.

Base map from U.S. Geological Survey digital data, 1:100,000

Universal Transverse Mercator projection,

Zone 13

Sedimentary

Volcanic

Crystalline

(Igneous and Metamorphic)

Alluvial 

285

24

9

9

285

Less than 1.0

1.0 to 4.9

5.0 to 9.9

Equal to or greater than 10 

(U.S. Environmental Protection 

Agency Maximum Contaminant 

Level, 10 milligrams per liter)

EXPLANATION

Nitrite-plus nitrate concentrations,

in milligrams per liter

106°00' 105°30'

39°30'

39°00'

0 11⁄2 3 6 MILES

0 2.5 5 10 KILOMETERS

Bailey

Tarryall

Guffey

Elevenmile Canyon

Reservoir

Spinney Mountain

Reservoir

Antero

Reservoir

Tarryall

Reservoir

Jefferson

Alma

Fairplay

Hartsel
Potential Effects of Individual Sewage Disposal System Effluent on Ground-Water Quality 33

Figure 14. Spatial distribution of total coliform detections in samples collected from selected domestic wells in 

Park County, Colorado, 2001–2004.

Note: Wells may be completed in rock type other than the surficial geology shown on this figure.

Base map from U.S. Geological Survey digital data, 1:100,000

Universal Transverse Mercator projection,

Zone 13

Sedimentary

Volcanic

Crystalline

(Igneous and Metamorphic)

Alluvial 

285

24

9

9

285

EXPLANATION

Not detected

Detected

106°00' 105°30'

39°30'

39°00'

Alma

Fairplay

Antero

Reservoir

Guffey

Elevenmile Canyon

Reservoir

Spinney Mountain

Reservoir

Lake George

Hartsel

Jefferson

Bailey

Tarryall

Reservoir

Tarryall

0 21⁄2 5 10 MILES

0 2.5 5 10 KILOMETERS
34 Potential Effects of Individual Sewage Disposal System Effluent on Ground-Water Quality in Park County, Colorado

caffeinated beverages, pesticides, perfumes, or human waste 

that contains nitrite, nitrate, and ammonia. Persistent detections or elevated concentrations of bacteria also may indicate 

contamination from an ISDS, especially if detections of 

bacteria also are accompanied by detections of wastewater 

compounds or elevated concentrations of other chemicals.

Geochemical and physical processes occur in the 

subsoil—the unsaturated zone above the water table and 

the saturated zone below the water table that can reduce the 

concentrations of chemical and biological constituents in ISDS 

effluent. In a properly functioning ISDS, most of the potential 

contaminants in effluent are removed by filtration or oxidation in the unsaturated zone below the leach field and above 

the water table (Wilhelm and others, 1994). When effluent 

reaches the unsaturated zone above the water table, it flows 

through the pores between the particles, such as sand and 

gravel from the weathered sedimentary rock, that make up 

the subsoil. Large particles and bacteria in the effluent can be 

filtered by the subsoil, leaving mostly dissolved compounds 

in the effluent. As the effluent flows through the subsoil and 

is exposed to oxygen, ammonia is oxidized to form nitrate. 

When nitrate reaches the water table, and if dissolved organic 

carbon is present and dissolved oxygen is absent, the nitrate 

and dissolved organic carbon may be consumed by denitrifying bacteria to produce nitrogen and carbon dioxide gases. 

Thus, the concentration of nitrate increases beyond the leach 

field but then can decrease as it travels through the saturated 

zone (Robertson and others, 1989).

Caffeine and other organic chemicals can be degraded 

to other compounds by bacteria in the saturated zone in the 

vicinity of the leach field from which the chemicals originated. 

Organic chemicals can persist in ground water, however, if 

degrading bacteria are not present. Biological constituents in 

ISDS effluent that can cause disease (pathogenic organisms) 

include bacteria and viruses. These microorganisms have different survival rates and transport properties in the unsaturated 

and saturated zones below a leach field. For example, E. coli

potentially can survive for several weeks in the subsurface if 

conditions are favorable (Matthess and Pekdeger, 1981). Total 

coliform and E. coli bacteria can be removed from ISDS effluent by filtration as the effluent flows through the unsaturated 

zone (Viraraghavan and Warnock, 1976). If the water table 

is close to the land surface, the unsaturated zone is thin, and 

more of the bacteria in the effluent potentially can reach the 

water table (Canter and Knox, 1985).

Occurrence of Constituents Associated with 

Individual Sewage Disposal System Effluent 

in Ground-Water Samples

Wastewater compounds were detected in 46 percent (43 

of 93) of samples collected from wells in Park County. Thirtyeight different organic wastewater compounds were detected 

in ground-water samples (table 3); most were at low concentrations at or near the laboratory reporting levels. Only 1 

(tetrachloroethylene [PCE]) of the 38 wastewater compounds 

detected in ground-water samples is regulated in drinking 

water by the USEPA. PCE was detected in about 6 percent 

of samples (6 of 93); however, PCE concentrations were low. 

The maximum PCE concentration measured was an estimated 

0.3 μg/L, which is an order of magnitude lower than the MCL 

for PCE of 5 μg/L (0.005 mg/L). The number of wastewater 

compounds detected per sample ranged from 1 to 17 compounds. The spatial distribution of wastewater compound 

detections and number of wastewater compounds detected 

at each site is shown in figure 15. Detections of wastewater 

compounds in ground water indicate that these compounds are 

not being completely removed by ISDS’s or the unsaturated 

zone. Because ISDS’s typically are not designed to remove all 

wastewater compounds, and roughly 90 percent of domestic 

water used is assumed to be recharged by ISDS’s (Van Slyke 

and Simpson, 1974), detections of wastewater compounds in 

ground water in Park County are not surprising.

The most frequently detected wastewater compounds were 

phenol, bisphenol A (BPA), isophorone, 4-nonylphenol (4-NP), 

and tris (2-chloroethyl) phosphate. Wastewater compounds 

detected in ground-water samples, frequency of detection, and 

possible use or source of the compound are listed in table 3. 

Phenol was detected in 28 percent of the samples. Phenols are 

used in the formation of phenolic resins and in the manufacture 

of nylon and other synthetic fibers (http://www.atsdr.cdc.gov/

tfacts115.html#bookmark02, September 17, 2007). Phenols 

also are used in chemicals that kill bacteria and fungi in slimes, 

disinfectants, antiseptics, and medicinal preparations such as 

mouthwash and throat lozenges. BPA was detected in 24 percent 

of ground-water samples analyzed for wastewater constituents. 

BPA is used as a plasticizer in the manufacture of polycarbonate plastic and epoxy resins that are used in baby bottles, as 

protective coatings on food containers, and as composites and 

sealants in dentistry (Calafat and others, 2005). Isophorone was 

detected in 9 percent of the samples. Isophorone is used as a 

solvent in some printing inks, paints, lacquers, and adhesives 

(http://www.eco-usa.net/toxics, June 13, 2006). In 8 percent of 

wells sampled, 4-NP was detected. 4-nonylphenoNPl is used to 

make nonylphenol ethoxylates, or nonionic detergent metabolites, that are in numerous industrial, agricultural, and domestic 

products such as detergents and pesticide formulations (http://

www.pesticideinfo.org/Docs/ref_general2.html#ChemUseType, 

September 17, 2007). Tris(2-chloroethyl) phosphate was 

detected in about 8 percent of the samples. Tris(2-chloroethyl) 

phosphate is used as a flame retardant in plastics, especially in 

flexible foams used in automobiles and furniture, and in rigid 

foams used for building insulation (http://www.inchem.org/

documents/iarc/vol48/48-06.html, July 3, 2006).

The co-occurrence of multiple constituents possibly 

associated with ISDS effluent (such as bacteria, wastewater 

compounds, and elevated concentrations of nitrate, chloride, 

or boron) in a single sample can provide strong evidence of 

contamination by an ISDS. For example, samples collected 

from one well in the volcanic-rock aquifer near Guffey had 

detectable concentrations of E. coli bacteria, total coliform 

bacteria, six wastewater compounds, and a nitrite-plus-nitrate 
Table 3. List of organic compounds identified in samples collected from domestic wells in Park County, Colorado, 2001–2004.[Possible use or source: Listings not exhaustive. Pcode, U.S. Gelogical Survey National Water Information System (NWIS) database parameter code; CAS number; Chemical ANumber; mg/L, milligram per liter; μg/L, microgram per liter; --, unknown; NA; not applicable]

Pcode Compound name CAS number Number of samples Number of detects Frequency of detection (percent) Reporting level (μg/L) Possible 

34466 Phenol 108-95-2 93 26 28 0.50 disinfectants, antiseptic

synthetic fibers

62069 Bisphenol A (BPA) 80-05-7 93 22 24 1.00 plasticizer

34409 Isophorone 78-59-1 93 8 9 0.50 solvent

62085 4-Nonylphenol (total) 84852-15-3 93 7 8 5.00 nonionic detergent meta

62087 Tris(2-chloroethyl) phosphate 115-96-8 93 7 8 0.50 flame retardant

38260 Methylene blue active substances, unfiltered NA 58 4 7 *0.1 surfactants or detergent

62075 Hexahydrohexamethyl cyclopentabenzopyran (HHCB) 1222-05-5 93 6 6 0.50 fragrance

62082 N,N-diethyl-meta-toluamide (DEET) 134-62-3 93 6 6 0.50 insect repellant

34476 Tetrachloroethylene (PCE) 127-18-4 93 6 6 0.50 paint and spot removers

degreasers, glues

62088 Tris(dichloroisopropyl) phosphate 13674-87-8 93 6 6 0.50 flame retardant

34572 1,4-Dichlorobenzene 106-46-7 93 5 5 0.50 deodorant, moth repelle

62065 Acetyl hexamethyl tetrahydro naphthalene (AHTN) 21145-77-7 93 5 5 0.50 fragrance

62084 p-Cresol 106-44-5 93 5 5 1.00 wood preservative

34443 Naphthalene 91-20-30 93 4 4 0.50 fumigant, dyes, insectic

62092 Triphenyl phosphate 115-86-6 93 4 4 0.50 flame retardant, plastici

62067 Benzophenone 119-61-9 93 3 3 0.50 fixative for perfumes an

62072 Cholesterol 57-88-5 93 3 3 2.00 plant and animal steroid

62093 Tris(2-butoxyethyl) phosphate 78-51-3 93 3 3 0.50 flame retardant, plastici

62057 3-beta-Coprostanol 360-68-9 93 2 2 2.00 animal fecal steroid

62062 4-tert-Octylphenol 140-66-9 93 2 2 1.00 nonionic detergent meta

62070 Camphor 76-22-2 93 2 2 0.50 flavor, ointments, fragra

62076 Indole 120-72-9 93 2 2 0.50 fragrance, human feces

62078 Isopropylbenzene (cumene) 120-72-9 93 2 2 0.50 solvent, petroleum prod

34288 Tribromomethane (Bromoform) 75-25-2 93 2 2 0.50 chlorination by-product

62090 Triclosan 3380-34-5 93 2 2 1.00 disinfectants in detergen

toothpastes

62054 1-Methylnaphthalene 90-12-0 93 1 1 0.50 gasoline, diesel fuel, cru

62055 2,6-Dimethylnaphthalene 581-42-0 93 1 1 0.50 diesel fuel, kerosene

62056 2-Methylnaphthalene 91-57-6 93 1 1 0.50 gasoline, diesel fuel, cru

62058 3-Methyl-1H-indole 83-34-1 93 1 1 1.00 fragrance

62063 5-Methyl-1(H)-benzotriazole 136-85-6 93 1 1 2.00 antioxidant in antifreeze 

62066 Anthraquinone 84-65-1 93 1 1 0.50 dye, insecticide, bird re

50305 Caffeine 58-08-2 93 1 1 0.50 stimulant, beverages, di

82680 Carbaryl 63-25-2 93 1 1 1.00 insecticide

62083 Diethoxynonylphenol -- 93 1 1 5.00 nonionic detergent meta

61705 Diethoxyoctylphenol -- 93 1 1 1.00 nonionic detergent meta

62073 d-Limonene 5989-27-5 93 1 1 0.50 cleaning products, aero

61706 Ethoxyoctylphenol -- 93 1 1 1.00

62091 Triethyl citrate 77-93-0 93 1 1 0.50 food additive, cosmetic

*Concentration of constituent in milligrams per liter.
36 Potential Effects of Individual Sewage Disposal System Effluent on Ground-Water Quality in Park County, Colorado

Figure 15. Number of wastewater compounds detected in samples collected from selected domestic wells in Park 

County, Colorado, 2001–2004.

Hartsel

0 11⁄2 3 6 MILES

0 2.5 5 10 KILOMETERS

Crystalline

(Igneous and Metamorphic)

Volcanic

Sedimentary

Not detected

1 to 3

4 to 6

7 to 9

Equal to or greater 

 than 10

EXPLANATION

Base map from U.S. Geological Survey digital data, 1:100,000

Universal Transverse Mercator projection,

Zone 13

285

285

9

9

24

Alluvial 

Note: Wells may be completed in rock type other than the surficial geology shown on this figure.

Alma

Fairplay

Antero

Reservoir

Bailey

Tarryall

Reservoir

Spinney Mountain

Reservoir

Elevenmile Canyon

Reservoir

Lake George

Tarryall

Guffey

Jefferson

39°00'

39°30'

106°00' 105°30'
Potential Effects of Individual Sewage Disposal System Effluent on Ground-Water Quality 37

concentration of 1.43 mg/L, which was more than double the 

nitrite-plus-nitrate concentrations in nearby wells. The cooccurrence of these multiple constituents in one well probably 

indicates contamination by sewage effluent.

Chloride and boron concentrations were significantly 

higher (p-value < 0.05) in samples collected from wells with 

detections of wastewater compounds than in wells with no 

detections of wastewater compounds. However, nitrite-plusnitrate concentrations were not significantly different between 

wells with detections of wastewater compounds and those with 

no detections. Heatwole and others (2005) suggested that chloride measurements (1) can serve as a precursor to contamination from other ISDS constituents (including nitrogen), (2) may 

help determine if contaminants in monitoring wells originate 

from ISDS’s or other sources, and (3) can be used to estimate 

the relative ratio of ISDS water and aquifer water (mixing factors). Similarly, boron isotopes have been useful in determining 

human influences on ground water. Babcock and others (2006) 

developed a method using boron isotopes to track the transport 

and fate of recycled water following recharge into a shallow 

brackish aquifer. Barth and others (1996) used boron isotopes 

to identify contamination of ground water from a solid-waste 

disposal site. Higher concentrations of chloride and boron in 

samples from wells that have detections of wastewater compounds than in samples from wells that do not have detections 

of wastewater compounds indicate that recharge from ISDS 

effluent is affecting ground-water quality in Park County. If 

the overall recharge from ISDS’s increases, concentrations of 

chloride, boron, and other constituents associated with ISDS 

effluent also are likely to increase.

Concentrations of nitrite-plus-nitrate (p-value = 0.0002) 

and chloride (p-value = 0.0006) were significantly higher 

in ground-water samples collected from the crystalline-rock 

aquifers in the Bailey area during 2001 than in 1974 (figs. 16

and 17). The median nitrite-plus-nitrate concentration measured during 1974 was 0.82 mg/L compared to 2.58 mg/L 

for samples collected during this study (2001) (fig. 16), and 

the median chloride concentration measured during 1974 

was 3.4 mg/L compared to 7.75 mg/L measured during 2001 

(fig. 17). Changes in nitrite-plus-nitrate and chloride concentrations in the Bailey area over time likely are related to a larger 

part of the overall recharge being from ISDS effluent either 

as a result of an increase in ISDS density or more time for 

recharge from ISDS effluent to reach the ground-water table.

Potential Effects of Individual Sewage Disposal 

System Density and Aquifer Type on GroundWater Quality

Ground-water-quality data were grouped by average 

subdivision lot size, aquifer type, and year of ISDS installation to investigate the effects of ISDS density and aquifer type 

on ground-water quality. The average subdivision lot size and 

year of ISDS installation were provided by Park County for 

selected wells (Brenda Green, Park County, written commun., 

2007). The nonparametric Wilcoxon Mann-Whitney test was 

used to determine if concentrations of selected constituents, 

associated with ISDS effluent, were significantly different in 

ground-water samples based on average subdivision lot size, 

aquifer type, and (or) year of ISDS installation.

Throughout Park County the average subdivision lot size 

was used as a surrogate for ISDS density. Wells were placed 

into four categories based on average subdivision lot size 

(ISDS density): (1) average lot size less than 1 acre, (2) average lot size from 1 to 3 acres, (3) average lot size from 3 to 

5 acres, and (4) average lot size greater than 5 acres. Chloride 

and boron concentrations were significantly higher in groundwater samples collected from wells located in areas that had 

average subdivision lot sizes less than 1 acre than in areas 

that had average subdivision lot sizes equal to or greater than 

1 acre (fig. 18). Median potassium concentrations increased as 

average subdivision lot size decreased (fig. 18). Significantly 

higher (p-value < 0.05) potassium concentrations were measured only in wells located in areas that have average lot sizes 

less than or equal to 5 acres than in wells located in areas that 

have average lot sizes greater than 5 acres (fig. 18).

Wells were grouped by aquifer rock type (alluvial, 

crystalline-rock, sedimentary-rock, and volcanic-rock) and 

then by average subdivision lot size. Too few samples were 

collected from alluvial aquifer wells to determine if differences 

in constituent concentrations were present between lot-size 

categories. For wells completed in the crystalline-rock aquifers, 

median chloride and boron concentrations increased as average 

subdivision lot size decreased (fig. 19). However, increases in 

chloride concentrations were significant (p-value < 0.05) only 

between wells located in areas that had average subdivision 

lot sizes less than 1 acre and wells located in areas that had 

average subdivision lot sizes greater than 5 acres. Increases in 

boron concentrations were significant (p-value < 0.05) only 

between wells located in areas that had average subdivision lot 

sizes less than 1 acre and wells located in areas that had average subdivision lot sizes greater than 3 acres. Increases in constituent concentrations were not significant in wells completed 

in the sedimentary-rock aquifers for any lot-size category. One 

reason for this may be that (based on CFC and tritium data) 

ground water in the sedimentary-rock aquifers tended to be 

older than ground water in the other aquifer-rock types. There 

may not have been enough time for ISDS effluent to reach the 

ground-water table in sufficient quantities to noticeably affect 

ground-water quality. For wells completed in the volcanic-rock 

aquifers, potassium, chloride, and boron concentrations were 

significantly higher in wells located in areas that had average 

subdivision lot sizes of less than 1 acre than in areas that had 

average subdivision lot sizes greater than 5 acres (fig. 20). 

Significant increases in constituent concentrations may occur 

between other lot size categories in the volcanic-rock aquifers, 

but samples were collected only from wells located in areas 

that had average subdivision lot sizes less than 1 acre and 

greater than 5 acres.

Nitrate concentrations were significantly higher in 

wells with ISDS’s installed in the 1970’s than in wells with 

ISDS’s installed in the 1980’s (fig. 21). Significantly higher 
38 Potential Effects of Individual Sewage Disposal System Effluent on Ground-Water Quality in Park County, Colorado

Figure 16. Distribution of nitrite-plus-nitrate concentrations in samples collected from selected domestic wells 

in the Bailey area in 1974 and 2001.

Figure 17. Distribution of chloride concentrations in samples collected from selected domestic wells in the Bailey 

area in 1974 and 2001.

0.01

0.10

1

10

NITRITE-PLUS-NITRATE, IN MILLIGRAMS PER LITER

1974 2001

U.S. Environmental Protection Agency 

 maximum contaminant level equal to 

 10 milligrams per liter

n = 55 n = 55

Outlier value

Outside value

75th percentile

Median

25th percentile

Smallest value within 1.5 times the IQR

Largest value within 1.5 times the IQR

Interquartile range (IQR)

n = 10 Number of samples

EXPLANATION

CHLORIDE, IN MILLIGRAMS PER LITER

0.01

0.1

1

10

100

Outlier value

Outside value

75th percentile

Median

25th percentile

Smallest value within 1.5 times the IQR

Largest value within 1.5 times the IQR

Interquartile range (IQR)

n = 10 Number of samples

EXPLANATION

1974 2001

n = 55

n = 55
Potential Effects of Individual Sewage Disposal System Effluent on Ground-Water Quality 39

Figure 18. Distribution of (A) potassium, (B) chloride, (C) nitrite-plus-nitrate, and (D) boron concentrations in samples 

collected from domestic wells in Park County, Colorado, 2001–2004, in relation to average subdivision lot size.

n = 59

n = 34 n = 63 n = 34

n = 59

n = 34 n = 63

n = 34

n = 58 n = 34

n = 63 n = 34

1

5

9

1

10

100

0.1

1

10

10

100

POTASSIUM,

IN MILLIGRAMS PER LITER

NITRITE-PLUS-NITRATE, 

IN MILLIGRAMS PER LITER

BORON,

IN MICROGRAMS PER LITER

CHLORIDE,

IN MILLIGRAMS PER LITER

n = 44

n = 27

n = 38 n = 26

AVERAGE SUBDIVISION LOT SIZE,

IN ACRES

A

B

C

D

Outlier value

Outside value

75th percentile

Median

25th percentile

Smallest value within 1.5 times the IQR

Largest value within 1.5 times the IQR

Interquartile Range (IQR)

EXPLANATION

n = 10 Number of samples—Total number may not equal number 

 of samples; average subdivision lot size unknown for 

 some wells

>5 3−5 1−3 <1

AVERAGE SUBDIVISION LOT SIZE,

IN ACRES

>5 3−5 1−3 <1

AVERAGE SUBDIVISION LOT SIZE,

IN ACRES

>5 3−5 1−3 <1

AVERAGE SUBDIVISION LOT SIZE,

IN ACRES

>5 3−5 1−3 <1

<

>

Less than

Greater than
40 Potential Effects of Individual Sewage Disposal System Effluent on Ground-Water Quality in Park County, Colorado

Figure 19. Distribution of (A) potassium, (B) chloride, (C) nitrite-plus-nitrate, and (D) boron concentrations in 

samples collected from wells completed in the crystalline-rock aquifers in Park County, Colorado, 2001–2004, 

in relation to average subdivision lot size.

1

2

3

4

5

6

1.0

10

100

0.1

1

10

40

80

120

160

>5 3–5 1–3 <1

AVERAGE SUBDIVISION LOT SIZE,

IN ACRES

>5 3–5 1–3 <1

AVERAGE SUBDIVISION LOT SIZE,

IN ACRES

>5 3–5 1–3 <1

AVERAGE SUBDIVISION LOT SIZE,

IN ACRES

>5 3–5 1–3 <1

AVERAGE SUBDIVISION LOT SIZE,

IN ACRES

POTASSIUM,

IN MILLIGRAMS PER LITER

CHLORIDE,

IN MILLIGRAMS PER LITER

NITRITE-PLUS-NITRATE,

 IN MILLIGRAMS PER LITER

BORON,

IN MICROGRAMS PER LITER

Outlier value

Outside value

75th percentile

Median

25th percentile

Smallest value within 1.5 times the IQR

Largest value within 1.5 times the IQR

Interquartile Range (IQR)

n = 10

EXPLANATION

A

B

C

D

n = 6

n = 11

n = 12

n = 11 n = 21

n = 18

n = 35 n = 18 n = 18

n = 18

n = 18

n = 18

n = 35

n = 35

n = 21

n = 21

Number of samples—Total number may not equal number 

 of samples; average subdivision lot size unknown for 

 some wells

Laboratory reporting limit

<

>

Less than

Greater than
Potential Effects of Individual Sewage Disposal System Effluent on Ground-Water Quality 41

Figure 20. Distribution of (A) potassium, (B) chloride, (C) nitrite-plus-nitrate, and (D) boron concentrations in 

samples collected from wells completed in the volcanic-rock aquifers in Park County, Colorado, 2001–2004, in 

relation to average subdivision lot size.

Laboratory reporting limit

1

2

3

4

5

10

15

20

25

30

0.1

1

20

40

60

100

160

5

n = 9

n = 9

n = 9

n = 9

n = 20

n = 20

n = 20

n = 20

POTASSIUM,

IN MILLIGRAMS PER LITER

NITRITE-PLUS-NITRATE,

IN MILLIGRAMS PER LITER

BORON,

IN MICROGRAMS PER LITER

CHLORIDE,

IN MILLIGRAMS PER LITER

AVERAGE SUBDIVISION LOT SIZE

IN ACRES

AVERAGE SUBDIVISION LOT SIZE

IN ACRES

AVERAGE SUBDIVISION LOT SIZE, 

IN ACRES

AVERAGE SUBDIVISION LOT SIZE,

IN ACRES

Outlier value

Outside value

75th percentile

Median

25th percentile

Smallest value within 1.5 times the IQR

Largest value within 1.5 times the IQR

Interquartile Range (IQR)

EXPLANATION

A

B

C

D

>5 <1

>5 <1 >5 <1

>5 <1

n = 10 Number of samples—Total number may not equal number 

 of samples; average subdivision lot size unknown for 

 some wells

Less than

Greater than

<

>
42 Potential Effects of Individual Sewage Disposal System Effluent on Ground-Water Quality in Park County, Colorado

nitrite-plus-nitrate concentrations were not measured between 

wells with ISDS’s installed in the 1980’s compared to those 

with ISDS’s installed in the 1990’s (fig. 21). However, nitriteplus-nitrate concentrations were significantly higher in wells 

with ISDS’s installed in the 1990’s than in wells with ISDS’s 

installed from 2000 to 2006 (fig. 21). The highest nitrite-plusnitrate concentrations were measured in wells with ISDS’s 

installed before 1980. Nitrate concentrations may be higher in 

wells where ISDS’s were installed before 1980 because effluent has had enough time to move through the unsaturated zone 

to the ground-water table in sufficient quantities to affect local 

ground-water quality. Other possibilities may be (1) ISDS’s 

installed before 1980 may not be as effective at removing contaminants as those installed after 1980, or (2) ISDS’s installed 

before 1980 are more likely to need major service or repairs 

than newer systems.

Ground-water-quality data collected from wells throughout Park County indicate that recharge from ISDS effluent has 

affected local ground-water systems; however, concentrations 

of constituents associated with ISDS effluent are low, and 

drinking-water quality is good throughout Park County with 

some exceptions. Data also indicate that ISDS density and 

year of ISDS installation are related to ground-water quality. 

In general, ground-water quality was least affected by ISDS’s 

in areas that had average subdivision lot sizes greater than 

5 acres, and most affected in areas that had average subdivision lot sizes less than 1 acre. Ground-water quality in wells 

with ISDS’s installed before 1980 was most affected by ISDS 

effluent; and wells with ISDS’s installed after 1999 were least 

affected by ISDS effluent.

Summary

In 2000, the U.S. Geological Survey, in cooperation with 

Park County, began a study to evaluate ground-water quality 

in the various aquifers in Park County that supply water to 

domestic wells. The focus of this study was to identify and 

describe the principal natural and human factors that affect 

ground-water quality. Of particular interest to health department officials, planners, and County Commissioners in Park 

County is the potential degradation of ground-water quality 

due to the increased number and density of individual sewage disposal systems (ISDS) installed throughout the county. 

As a result, in addition to providing a general assessment of 

ground-water quality in Park County, the potential effects of 

ISDS’s on ground-water quality were evaluated.

As part of this study, ground-water-quality samples were 

collected from 224 existing domestic water-supply wells 

sampled from July 2001 through October 2004 in the alluvial, 

crystalline-rock, sedimentary-rock, and volcanic-rock aquifers 

in Park County. Almost all wells were sampled for physical 

properties (water temperature, pH, specific conductance, and 

dissolved oxygen), selected major ions, nutrients, and bacteria. Selected wells also were sampled for dissolved organic 

carbon, human-related (wastewater) compounds, radionuclides, and age-dating constituents (tritium and chlorofluorocarbons). The presence of specific human-related compounds 

such as fecal indicators, caffeine, detergent metabolites, 

cholesterol, and pharmaceuticals or elevated concentrations of 

bacteria or chemicals in ground waters were used to indicate 

Figure 21. Relation of nitrite-plus-nitrate concentrations in samples collected from selected domestic wells to year of individual 

sewage disposal system installation, Park County, Colorado.

INDIVIDUAL SEWAGE DISPOSAL SYSTEM YEAR GROUP

2000–2006 1990–1999 1980–1989 1970–1979

Laboratory reporting limit 

 (0.06 milligram per liter)

0.1

1

10

NITRITE-PLUS-NITRATE, IN MILLIGRAMS PER LITER

n = 19 n = 93

n = 29

n = 26

30

Outlier value

Outside value

75th percentile

Median

25th percentile

Smallest value within 1.5 times the IQR

Largest value within 1.5 times the IQR

Interquartile Range (IQR)

EXPLANATION

n = 10 Number of samples—Total may not equal number 

 of samples; year individual sewage disposal 

 system installed unknown for some wells
Summary 43

contamination of ground water from ISDS’s. The overall 

sampling plan, in part, was designed to allow an evaluation of 

whether the density of development (proximity of wells and 

ISDS’s) was a significant factor in potential degradation of 

ground-water quality.

The natural processes that affect ground-water quality 

and the potential for ground water to be affected by human 

activities such as wastewater discharge (ISDS effluent) are 

related to the age of the ground water. To determine the 

approximate time elapsed since recharge (apparent recharge 

date of the ground water) and thus the potential for a given 

sample to be affected by development, ground-water samples 

were collected from selected wells in the alluvial, crystallinerock, sedimentary-rock, and volcanic-rock aquifers throughout 

Park County and analyzed for tritium and chlorofluorocarbons. Tritium concentrations greater than 10 picocuries per 

liter (pCi/L) in a sample indicate that at least some part of 

the recharge to the ground-water system occurred after 1954. 

About 75 percent of samples collected had tritium concentrations greater than 10 pCi/L. Generally, the highest tritium concentrations were measured in wells completed in the alluvial 

and crystalline-rock aquifers, and the lowest concentrations 

were measured in wells completed in the sedimentary- and 

volcanic-rock aquifers. Based on chlorofluorocarbon data, 

ground-water recharge dates in Park County range from the 

mid-1940’s to modern water (2004). Chlorofluorocarbon and 

tritium data indicate that wells completed in the sedimentaryrock aquifers tended to have older waters than wells completed in the other aquifer-rock types (alluvial, crystalline, 

and volcanic) indicating that (1) infiltration of water through 

the sedimentary rocks is slower than other formations, (2) the 

ground-water flow paths are longer, or (3) that ground-water 

flow velocities are slower.

Many of the wells sampled in Park County probably have 

a mixture of water from several parts of the aquifer system that 

may have different recharge areas, flow paths, and groundwater recharge dates. Because most of the development in 

Park County has occurred since 1990 and only about 8 percent 

of wells sampled for chlorofluorocarbons had ground-water 

recharge dates after 1990, there may not have been enough 

elapsed time for effluent water from ISDS’s to noticably affect 

sampled ground-water quality. About 47 percent of wells 

sampled for age-dating constituents and wastewater compounds had detectable quantities of wastewater compounds. 

Detections of wastewater compounds in ground water indicate 

that these compounds are not being completely removed by 

ISDS’s or the unsaturated zone. Because roughly 90 percent 

of domestic water used is assumed to be recharged by ISDS’s, 

detections of wastewater compounds in ground water in Park 

County are not surprising.

Data collected as part of this study were compared to the 

U.S. Environmental Protection Agency’s primary and secondary drinking-water standards to assess the general quality 

of the ground water in the study area for domestic use even 

though the drinking-water standards only apply to public water 

systems. Primary drinking-water standards are legally enforceable standards that are put in place to protect public health by 

limiting the levels of contaminants in drinking water. Secondary drinking-water regulations are nonenforceable guidelines 

regulating contaminants that may cause cosmetic effects (such 

as skin or tooth discoloration) or aesthetic effects (such as 

taste, odor, or color) in drinking water.

Ground-water quality is related to natural factors (resulting from the interaction of water with the soil and mineralogy 

of the various geologic formations as it infiltrates to the water 

table) and to human factors. Because elevated concentrations 

of some constituents are more related to natural factors and 

others are more related to human factors, constituents that 

exceeded drinking-water standards were grouped accordingly. 

Secondary drinking-water standards for pH, fluoride, sulfate, 

and dissolved solids, primary drinking-water standards for 

fluoride and uranium, and proposed drinking-water standards 

for radon were exceeded in one or more samples collected 

from domestic wells in Park County most likely because of 

natural factors.

Only about 2 percent of measured pH values were outside 

of the national secondary drinking-water regulation’s secondary maximum contaminant level range for pH of 6.5 to 8.5. 

Values of pH less than 6.5 standard units were measured only 

in wells in the Pikes Peak Granite (crystalline-rock aquifer) in the vicinity of Bailey. However, not all pH values in 

samples from wells in the Pikes Peak Granite were low. Low 

pH values in some wells may indicate that ground-water flow 

(or perhaps recharge) is more rapid and has less contact time 

with geologic units, in certain areas of the Pikes Peak Granite. 

This lesser contact time may occur because of thinner soils 

and (or) a higher percentage of fractures to transport water. 

In these areas, the pH of the ground water may be closer to 

the pH of the precipitation that recharges the aquifer.

Only one sample collected from a well completed in the 

Pikes Peak Granite near Lake George exceeded the primary 

drinking-water standard for fluoride of 4.0 milligrams per liter 

(mg/L); however, fluoride concentrations in about 12 percent 

of samples throughout Park County exceeded the secondary drinking-water standard of 2.0 mg/L. All samples with 

fluoride concentrations equal to or greater than 2.0 mg/L 

were collected from wells completed in the crystalline-rock 

aquifers near Bailey and Lake George; 93 percent of these 

samples (with fluoride concentrations greater than or equal 

to 2.0 mg/L) were collected from wells completed in the Pikes 

Peak Granite.

Sulfate concentrations ranged from 2.4 to 1,760 mg/L, 

and the median concentration was 18 mg/L. The highest concentrations of sulfate generally were measured in wells in the 

sedimentary-rock aquifers near the Park County county line 

southwest of Hartsel and between Alma and Fairplay. Most of 

the wells that exceeded the secondary maximum contaminant 

level of 250 mg/L for sulfate also exceeded the secondary 

maximum contaminant level of 500 mg/L for dissolved solids. 
44 Potential Effects of Individual Sewage Disposal System Effluent on Ground-Water Quality in Park County, Colorado

About 5 percent of the samples had concentrations of sulfate 

that exceeded the secondary maximum contaminant level of 

250 mg/L, and about 4 percent of these samples had sulfate 

concentrations greater than 500 mg/L, more than twice the 

secondary maximum contaminant level level.

Elevated concentrations of dissolved solids in Park 

County wells probably are mostly from natural sources. 

However, some elevated concentrations of dissolved solids in 

the Bailey and Guffey areas may result from contamination 

by ISDS’s, because chloride and nitrite-plus-nitrate concentrations in these ground-water samples were considerably higher 

than concentrations in samples collected from surrounding 

wells. Concentrations of dissolved solids measured in samples 

ranged from 49 to 2,810 mg/L. The secondary maximum 

contaminant level for dissolved solids (500 mg/L) was 

exceeded in about 7 percent of samples, mostly in samples 

collected from wells completed in the sedimentary-rock aquifer. Dissolved-solids concentrations in about 4 percent of the 

samples were greater than 1,000 mg/L (twice the secondary 

maximum contaminant level).

Currently (2004), there is no federally enforced drinkingwater standard for radon in public water-supply systems, but 

proposed regulations indicate a maximum contaminant level 

of 300 pCi/L and an alternative maximum contaminant level 

of 4,000 pCi/L (contingent upon other mitigating remedial 

activities to reduce concentrations of radon in indoor air). 

Radon concentrations in ground-water samples in Park County 

ranged from 40 to 19,200 pCi/L. Radon concentrations in 

about 91 percent of ground-water samples collected from 

domestic wells throughout Park County were greater than or 

equal to 300 pCi/L, and about 25 percent had radon concentrations greater than or equal to 4,000 pCi/L. Generally, the highest radon concentrations were measured in samples collected 

from wells completed in the crystalline-rock aquifers, and the 

lowest concentrations were measured in samples collected 

from wells completed in the volcanic-rock aquifers.

The current (2004) MCL for uranium of 30 micrograms 

per liter (μg/L) in drinking water was exceeded in two samples 

collected from wells completed in the crystalline-rock aquifer. 

Uranium concentrations in samples ranged from less than 1.0 

to 48 μg/L. The highest uranium concentrations generally were 

measured in wells completed in the crystalline-rock aquifers, 

and the lowest concentrations generally were measured in 

wells completed in the volcanic-rock aquifers.

National secondary drinking-water standards for chloride 

and primary drinking-water standards for nitrate, total coliform bacteria, and E. coli bacteria were exceeded in one or 

more samples collected from domestic wells in Park County. 

Chloride concentrations in ground-water samples ranged 

from 0.28 to 873 mg/L; 75 percent of measured concentrations 

were less than 7 mg/L, and 90 percent were less than 13 mg/L. 

The secondary maximum contaminant level of 250 mg/L for 

chloride was exceeded in only one sample collected from a 

well completed in the crystalline-rock aquifer near Bailey. 

All other measured chloride concentrations were at least 

an order of magnitude less than the secondary maximum 

contaminant level.

Concentrations of nitrite-plus-nitrate in about 2 percent 

of samples equaled or exceeded the maximum contaminant 

level of 10 mg/L for nitrate. All of these samples were collected from wells in the crystalline-rock aquifer near Bailey. 

Generally, the highest nitrite-plus-nitrate concentrations in the 

county were measured in ground-water samples collected from 

wells near the towns of Bailey and Guffey. The median nitriteplus-nitrate concentration for samples collected throughout 

Park County during this study was 0.34 mg/L.

E. coli bacteria were detected in one well near Guffey; 

whereas, total-coliform bacteria were detected in about 11 percent of wells sampled throughout Park County. Concentrations 

of total coliform in samples ranged from near the reporting 

level of 1 colony per 100 milliliters of water to an estimated 

340 colonies per 100 milliliters of sample. The highest totalcoliform concentrations were measured southeast of Jefferson 

and west of Tarryall Reservoir. Total-coliform concentrations 

and the frequency of detections in the crystalline-rock aquifer wells in the Bailey area were higher in samples collected 

during 1974 than in samples collected during this study (the 

Bailey area was sampled during 2001).

Overall, ground water throughout Park County is suitable 

for domestic drinking water. Only about 3 percent of wells 

had concentrations of fluoride, nitrate, and (or) uranium that 

exceeded a primary drinking-water standard. These primary 

drinking-water standards were exceeded only in wells completed in the crystalline-rock aquifers. Secondary standards 

were exceeded more frequently. About 19 percent of wells 

had concentrations of one or more constituents that exceeded 

secondary drinking-water standards.

Chemicals from products that are used in households can 

enter the ground-water system as a more concentrated effluent 

from an ISDS than from natural sources. Persistent detections 

or elevated concentrations of bacteria also may indicate contamination from an ISDS, especially if detections of bacteria 

also are accompanied by detections of wastewater compounds 

or elevated concentrations of other chemicals.

Wastewater compounds were detected in 46 percent of 

samples collected from wells in Park County. Thirty-eight 

different organic wastewater compounds were detected in 

ground-water samples, and most at low concentrations at or 

near the laboratory reporting levels. The number of wastewater 

compounds detected per sample ranged from 1 to 17 compounds. The most frequently detected wastewater compounds 

were phenol, bisphenol A, isophorone, 4-nonylphenol, and 

tris(2-chloroethyl) phosphate. Phenol was detected in 28 percent of the samples. Bisphenol A was detected in 24 percent of 

ground-water samples. Isophorone was detected in 9 percent 

of the samples, and 4-nonylphenol and tris(2-chloroethyl) 

phosphate were detected in 8 percent of samples. 

The co-occurrence of multiple constituents possibly 

associated with ISDS effluent (such as bacteria, wastewater 

compounds, and elevated concentrations of nitrate, chloride, 

or boron) in a single sample can provide strong evidence of 

contamination by an ISDS. Chloride and boron concentrations 

were significantly higher (p-value < 0.05) in samples collected 

from wells with detections of wastewater compounds than in 
References Cited 45

wells with no detections of wastewater compounds. However, 

nitrite-plus-nitrate concentrations were not significantly different between wells with detections of wastewater compounds 

and those with no detections.

Changes in nitrite-plus-nitrate and chloride concentrations in the Bailey area over time probably are related to a 

larger part of the overall recharge being from ISDS effluent 

either as a result of an increase in ISDS density or more time 

for recharge from ISDS effluent to reach the ground-water 

table. Concentrations of nitrite-plus-nitrate (p-value = 0.0002) 

and chloride (p-value = 0.0006) were significantly higher in 

ground-water samples collected during 2001 than in 1974. 

The median nitrite-plus-nitrate concentration measured during 1974 was 0.82 mg/L compared to 2.58 mg/L for samples 

collected during this study (2001), and the median chloride 

concentration measured during 1974 was 3.4 mg/L compared 

to 7.75 mg/L measured during 2001.

Throughout Park County the average subdivision lot size 

was used as a surrogate for ISDS density. Chloride and boron 

concentrations were significantly higher in ground-water 

samples collected from wells located in areas that had average 

subdivision lot sizes less than 1 acre than in wells located in 

areas that had average subdivision lot sizes equal to or greater 

than 1 acre. Median potassium concentrations increased as 

average subdivision lot size decreased; however, statistically 

significant (p-value < 0.05) increases in potassium concentrations only occurred between wells located in areas that had 

average lot sizes less than or equal to 5 acres and wells located 

in areas that had average lot sizes greater than 5 acres.

Wells were grouped by aquifer rock type (alluvial, 

crystalline-rock, sedimentary-rock, and volcanic-rock) and 

then by average subdivision lot size. Too few samples were 

collected from alluvial aquifer wells to determine if differences 

in constituent concentrations were present between lot-size 

categories. For wells completed in the crystalline-rock aquifers, 

median boron and chloride concentrations increased as average 

subdivision lot size decreased. However, statistically significant (p-value < 0.05) increases in concentrations of boron were 

detected only between wells located in areas that had average 

subdivision lot sizes less than 1 acre and wells located in areas 

that had average subdivision lot sizes greater than 3 acres. Significant increases in chloride concentrations were detected only 

between wells located in areas that had average subdivision lot 

sizes less than 1 acre and wells located in areas that had average subdivision lot sizes greater than 5 acres. No significant 

increases in constituent concentrations were observed in wells 

completed in the sedimentary-rock aquifers for any lot-size category. In the volcanic-rock aquifers, wells were sampled only 

in areas that had average subdivision lot sizes less than 1 acre 

or greater than 5 acres. Potassium, chloride, and boron concentrations were significantly higher in wells located in areas 

that had average subdivision lot sizes of less than 1 acre than in 

areas with average subdivision lot sizes greater than 5 acres in 

the volcanic-rock aquifers.

Significant increases in nitrite-plus-nitrate concentrations were observed between wells with ISDS’s installed 

in the 1970’s and wells with ISDS’s installed in the 1980’s. 

Significantly higher nitrite-plus-nitrate concentrations were 

not observed between wells with ISDS’s installed in the 

1980’s and wells with ISDS’s installed in the 1990’s. The lowest overall nitrite-plus-nitrate concentrations were measured in 

wells that had ISDS’s installed after 1999. Nitrite-plus-nitrate 

concentrations may be higher in samples collected from wells 

with ISDS’s installed before 1980 because effluent has had 

enough time to move through the unsaturated zone to the 

ground-water table in sufficient quantities to affect groundwater quality. Other possibilities may be (1) ISDS’s installed 

before 1980 may not be as effective at removing contaminants 

as those installed after 1980, or (2) ISDS’s installed before 

1980 are more likely to need major service or repairs.

Ground-water-quality data collected from wells throughout Park County indicate that recharge from ISDS effluent has 

affected local ground-water systems; however, concentrations 

of constituents associated with ISDS effluent are low, and 

drinking-water quality is good throughout Park County with 

some exceptions. Data also indicate that ISDS density and 

year of ISDS installation are related to ground-water quality. 

In general, ground-water quality was least affected by ISDS’s 

in areas that had average subdivision lot sizes greater than 

5 acres, and most affected in areas that had average subdivision lot sizes less than 1 acre. Ground-water quality in wells 

with ISDS’s installed before 1980 was most affected by ISDS 

effluent, and wells with ISDS’s installed after 1999 was least 

affected. As time passes, recharge from ISDS’s may become 

a larger part of the ground-water resource; and, as a result, 

concentrations of nitrate, chloride, boron, and other constituents associated with ISDS effluent also are likely to increase 

in ground water.

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