U.S. Department of the Interior

U.S. Geological Survey

Fact Sheet 2004–3066

July 2004

Ground-Water Quality of Granitic- and Volcanic-Rock 

Aquifers in Southeastern Park County, Colorado, 

July-August 2003

Printed on recycled paper

Introduction

Park County is one of the fastest growing counties in 

Colorado. Located southwest of Denver (fig. 1), the predominantly rural county has experienced a substantial increase in 

development as commuter communities and vacation homes 

continue to be built in the county. Health department officials, 

planners, and County Commissioners in Park County are 

interested in obtaining information regarding water quality in 

aquifers that serve the residents of the county. Of particular 

interest is the potential degradation of ground-water quality 

due to the increasing number and density of individual sewage 

disposal systems (ISDS). 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. This report summarizes the ground-water quality of samples collected in July 

and August 2003 from domestic wells completed in the graniticand volcanic-rock aquifers in southeastern Park County, Colo. 

(fig. 1). Additionally, this report provides an initial assessment 

of the potential effects of ISDSs on ground-water quality in 

granitic- and sedimentary-rock aquifers in southeastern Park 

County, Colo.

Water samples were collected from 55 domestic wells during 

July and August of 2003; 22 wells were completed in graniticrock aquifers, and 33 wells were completed in volcanic-rock 

aquifers. Measurements of pH, specific conductance, and 

bacteria were conducted in the field and sample collection 

procedures were followed as described in USGS National Field 

Manual (Wilde and others, 1998). Water samples were analyzed 

for various chemical groups including major ions, nitrogen 

species, phosphorus species, selected trace metals, and radiochemical constituents (Fishman, 1993). At selected wells, water 

samples were analyzed for an extensive list of organic chemicals that are indicative of contamination from wastewater effluent (Zaugg and others, 2002). Analyses were done at the USGS 

National Water Quality Laboratory in Denver, Colo. Water 

samples from selected wells also were analyzed for tritium 

and analyzed at the USGS Chlorofluorocarbon Laboratory in 

Reston, Virginia. Quality-control samples collected in the field 

included two blank and two replicate samples. Results indicate 

that field procedures did not contaminate the environmental 

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

at URL http://waterdata.usgs.gov/co/nwis/qwdata (search on 

Park County, data type ground water, and the date range of 

July 1 through August 31, 2003). 

Granitic rocks of Precambrian age are more common in the 

vicinity of Lake George, whereas volcanic rocks of Tertiary 

age are more common in the vicinity of Guffey (fig. 1). The 

granitic and volcanic rocks in the study area compose aquifers 

that contain ground water in fractures (Tweto, 1979). Well 

yields for sampled wells ranged from about 0.25 to 30 gallons 

per minute (gal/min). The variability in well yields depends on 

many things, including the number of fractures intercepted by a 

well, the degree of openness of those fractures, and the length of 

the open interval of the well (fig. 2). Depths for wells chosen for 

sampling ranged from 62 to 600 feet (ft). The well yields and 

By Roderick F. Ortiz

Figure 1. Map showing location of study area.

Elevenmile Canyon 

Reservoir

Park

County

Lake George

Study area

COLORADO

DENVER

Guffey

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depths for wells sampled are representative of most of the wells 

drilled in the various aquifers in the vicinity of Guffey and Lake 

George, Colo. (Colorado Division of Water Resources, 2000). 

There was no correlation between the concentration of the various chemicals sampled as part of this study and the well depths 

or well yields.

The amount of time it takes for water to recharge the aquifer 

and reach the wells varies with the openness and connectedness 

of the fractures, the distance from the recharge point to the open 

interval of a well, and the rate of water flow in the fractures. 

Because the rate of recharge and flow in the vicinity of each 

well can vary, it is not known whether ISDS effluent can reach 

the ground water before chemical and biological contaminants 

are reduced in concentration or removed from the effluent by 

geochemical and physical processes. 

This report provides a general assessment of ground-water 

quality and an initial assessment of whether contamination of 

ground water has occurred. The closeness of neighboring wells 

and ISDS’s differs depending on the size of the lots in each 

development. Each residence has its own ISDS. House densities 

range from several houses per acre to a single house on many 

acres. 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.

General Assessment of Ground-Water Quality

Water from granitic- and volcanic-rock aquifers sampled 

during this study were similar in water type. The predominant 

cation and anion were calcium and bicarbonate; however, the 

chemical concentrations of many of the constituents were dissimilar (table 1). Wilcoxson rank-sum tests (Helsel and Hirsch, 

1992) were used to determine if a statistical difference existed 

between constituents. Differences between data sets were determined to be significant when the p-value was 0.05 or smaller. A 

p-value of 0.05 means that there is a 95-percent confidence that 

two data sets being compared are different. Significant differences in concentration in samples collected from the two aquifer 

types were identified for pH, specific conductance, magnesium, 

potassium, sodium, fluoride, sulfate, alkalinity, bicarbonate, 

phosphorus, orthophosphate, uranium, and radon. 

The median pH values were 7.2 and 7.6 standard units for the 

granitic- and volcanic-rock aquifers, respectively. The median 

dissolved-solids concentration was 155 and 193 milligrams 

per liter (mg/L) for the granitic- and volcanic-rock aquifers, 

respectively. Median hardness concentrations for both aquifer 

types was 140 mg/L, which would be considered “hard” water 

(Hem, 1985). The maximum hardness concentration measured 

in samples collected from wells completed in the volcanic-rock 

aquifers was 400 mg/L. The maximum hardness concentration 

measured from samples collected from wells completed in the 

granitic-rock aquifers was 250 mg/L. The median alkalinity 

concentration in samples collected from both aquifers was 

130 mg/L; alkalinity is a measure of the ability of the water to 

neutralize acids. The median boron concentration for both aquifer types was reported as 20 micrograms per liter (μg/L). About 

15 percent of the samples collected from wells completed in 

the volcanic-rock aquifers had concentrations of boron greater 

than the maximum boron concentration of samples collected 

from wells completed in the granitic-rock aquifers. The median 

dissolved-radon concentration for samples collected from the 

granitic-rock aquifers was 6,300 picocuries per liter (pCi/L), 

whereas, the median concentration of samples collected from 

the volcanic-rock aquifers was 795 pCi/L. The largest radon 

concentration reported in water from samples collected from the 

volcanic-rock aquifers was only slightly larger than the minimum concentration reported from samples collected from the 

granitic-rock aquifers.

Dissolved organic carbon concentrations from either aquifer 

type generally were about 1 mg/L. Samples collected from all 

the wells showed total-coliform bacteria was detected in about 

18 percent of the samples; about 80 percent of those samples 

were at or near the reporting limit of 1 colony per 100 milliliters 

of sample. No Escherichia coli (E. coli) colonies were detected 

in any sample. Total coliform and E. coli can originate from 

humans and other warm-blooded animals. Phenol concentrations in 7 of the 15 samples collected were reported as being 

at or above the analytical reporting limit of 0.5 μg/L. Of these 

seven samples, six were collected from wells completed in the 

volcanic-rock aquifers. Phenols are used in the formation of 

phenolic resins and in the manufacture of nylon and other synthetic fibers. Phenols also are used in slimicides (chemicals that 

kill bacteria and fungi in slimes), disinfectants, antiseptics, and 

medicinal preparations such as mouthwash and throat lozenges.

Data collected as part of this study were compared to 

primary drinking-water standards (U.S. Environmental 

Protection Agency, 2002) to assess the general quality of the 

water in the study area for domestic use. Although the primary 

drinking-water regulations only apply to public water systems, 

these standards are used here as a basis for comparing how 

water-quality results from well samples compare to standards. 

Nearly all nitrite concentrations were reported as less than the 

reporting limit of 0.008 mg/L, which is below the primary standard of 1 mg/L reported as nitrogen. Because nitrite concentraFigure 2. Diagram showing the relation of Individual Sewage Disposal System (ISDS), domestic wells, and the water 

table in a fractured-rock ground-water system.

WELL WITH

LOW YIELD

-OPEN TO FEW

FRACTURES

WELL WITH

HIGH YIELD

-OPEN TO MANY

FRACTURES

SOIL/WEATHERED

ROCK

LEACH FIELDS

UNSATURATED

ZONE

SATURATED

ZONE

OPEN

INTERVAL

WATER

TABLE

OPEN

INTERVAL

FRACTURES

FILLED WITH

WATER

ISDS

EFFLUENT

ISDS

ISDS
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�������� Summary of water-quality data collected from domestic wells installed in granitic- or volcanic-rock aquifers in

southeastern Park County, Colorado, July–August 2003.

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tions were small, comparisons to the nitrate standard were done 

using nitrite plus nitrate data. The maximum nitrite plus nitrate 

concentration was 6.6 mg/L, which was less than the 10 mg/L 

standard for nitrate (reported as nitrogen) set by the U.S. 

Environmental Protection Agency. The standard for fluoride 

(4 mg/L) was equalled in one sample collected from a well 

completed in the granitic-rock aquifer; fluoride concentrations 

in all other samples did not exceed the standard.

Primary drinking-water standards have been established for 

some radionuclides (U.S. Environmental Protection Agency, 

2002). Radioactive elements sampled as part of this study 

included uranium and radon. Drinking-water standards have 

been established for uranium because of increased risk of cancer 

and kidney toxicity. The current (2004) standard of 30 μg/L 

was exceeded in one sample collected from a well completed 

in the granitic-rock aquifer. Radon in ground water that is used 

for domestic purposes is a concern because of increased risk 

of lung cancer (off-gassing) and stomach cancer (ingestion). 

Currently (2004), there is no federally enforced drinking-water 

standard for radon in community water-supply systems, but proposed regulations suggest concentrations of 300 or 4,000 pCi/L 

contingent on other mitigating remedial activities (U.S. 

Environmental Protection Agency, 1999). The proposed 

standards do not pertain to private wells. The median radon 

concentration was 795 pCi/L for 14 samples collected from 

wells completed in the volcanic-rock aquifers and 6,300 pCi/L 

for 6 samples collected from wells completed in the graniticrock aquifers; all but one of the 6 samples exceeded the higher 

proposed standard of 4,000 pCi/L.

Secondary drinking-water standards also have been defined 

for common chemicals that can change the aesthetic characteristics of water such as taste, odor, or color (U.S. Environmental 

Protection Agency, 2002). The secondary standard for chloride 

(250 mg/L) was not exceeded in any sample. Only one sample 

exceeded the secondary standard for sulfate (250 mg/L); the 

sample was collected from a well completed in a volcanic-rock 

aquifer.

Eight tritium samples were collected to attempt to determine an approximate age, or time since recharge, of the ground 

water pumped by wells: six samples were collected from wells 

completed in the volcanic-rock aquifers, and two samples were 

collected from wells completed in the granitic-rock aquifers. 

Tritium is used as an age-dating tracer because it was produced 

in relatively large concentrations as a result of atmospheric 

nuclear bomb testing beginning in 1954 (Kendall and 

McDonnell, 1998). Concentrations of tritium do not definitively 

yield the age of the water in a sample but must be used with 

other age-dating chemicals to refine the age estimate. Ground 

water having a certain tritium concentration is likely to contain 

a mixture of waters of different ages that exhibit a composite 

age based on the proportions of different-age waters contributing to a sample. Concentrations greater than 10 pCi/L indicate 

that recharge to the ground-water system occurred after 1954 

(Kendall and McDonnell, 1998). Tritium concentrations in 

wells completed in the volcanic-rock aquifers ranged from less 

than 2 to 19 pCi/L with three of the six samples reported as less 

than 10 pCi/L. Tritium concentrations in the two wells completed in the granitic-rock aquifers were 35 and 41 pCi/L. Wells 

completed in the volcanic-rock aquifers tended to have smaller 

tritium concentrations than those completed in the granitic-rock 

aquifers. Some of water collected from wells completed in 

the volcanic-rock aquifers may predate 1954. Additional data 

analysis of the age-dating chemicals is needed to evaluate the 

age of ground water and the vulnerability of the ground water to 

contamination.

Potential Effects of Individual Sewage Disposal 

System Effluent on Ground-Water Quality

Geochemical and physical processes occur in the subsoil—

unsaturated zone above the water table and the saturated zone 

below the water table (fig. 2)—that can reduce the concentrations of chemical and biological constituents in ISDS effluent. 

For 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 granitic and volcanic 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 (nitrite plus nitrate, 

as nitrogen). 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 

gasses. 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. However, organic chemicals can persist in ground water 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 saturated and unsaturated zones below a leach field. 

For example, E. coli can potentially survive for several weeks in 

the subsurface if conditions are favorable (Matthess and

Pekdeger, 1981). It is not known whether E. coli can survive 

long enough in a fractured-rock setting to be transported to the 

water table and eventually to wells. 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). However, if the water table lies close to the 

land surface, the unsaturated zone is thin and more of the 

bacteria in the effluent can potentially reach the ground water 

(Canter and Knox, 1985).

Indicators of Ground-Water Contamination from 

Individual Sewage Dispoal Systems

Samples collected from wells were analyzed for selected 

chemicals and bacteria that can originate from ISDS’s. Many 

of these chemicals and bacteria also can enter the ground water 

from natural sources. Chemicals and bacteria originating from 

natural sources usually do not occur at elevated concentrations 
because they come from dispersed sources such as waste from 

warm-blooded animals, decomposition of forest material, deposition from the atmosphere, or from decomposition of rocks. 

Whether from ISDS’s or natural sources, elevated concentrations of chemicals or bacteria can indicate degraded groundwater quality. An ISDS can provide a focused source of these 

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 containing boron, dietary salt containing chloride, 

caffeinated beverages, pesticides, perfumes, or human waste 

containing nitrite, nitrate, and ammonia. Persistent detections or 

elevated concentrations of bacteria also may indicate contamination from an ISDS, but bacteria such as total coliform and 

E. coli can originate from other warm-blooded animals as well 

as humans. 

Potential Indicators of Contamination from Individual 

Sewage Disposal Systems

Generally, most chemicals associated with ISDS contamination were not observed in any measurable quantity nor at 

elevated concentrations in the water samples collected during 

this study. Bacteria and some chemicals associated with ISDS 

effluent were reported, however, which could indicate potential 

contamination from ISDS. Although there were no reportable 

counts of E. coli bacteria in any sample, total coliform was 

detected in about 18 percent of the samples, of which nearly all 

were collected from wells completed in the volcanic-rock aquifers. The largest concentration of total coliform was 57 colonies 

per 100 milliliters. Boron was detected in all of the samples 

collected from wells completed in the two aquifer types. The 

largest boron concentration was 142 μg/L, but the median 

values for the two aquifer types were similar. Of the 15 samples 

analyzed for selected organic chemicals associated with contamination from waste water, 7 samples contained phenol at 

concentrations above the reporting limit and 5 samples contained trace concentrations of phenol. Trace concentrations are 

quantifiable amounts of a compound detected at concentrations 

below the reporting limit of the analytical method. All 10 of the 

samples collected in the vicinity of Guffey, Colo., were reported 

as containing some detectable concentration of phenol. Wells 

in the Guffey area are completed in volcanic-rock aquifers but 

there is no indication that detections of organic chemicals are 

associated with volcanic rocks. Rather, the high incidence of 

detections may be due to the large number of wells and ISDS’s 

in the Guffey area. Trace concentrations of at least one other 

organic compound were detected in about 75 percent of the 

samples. Organic compounds identified at trace concentrations 

in at least two samples included 4-nonylphenol and 

N,N-diethyl-m-toluamide (commonly known as DEET).

Sample Plan to Compare Individual Sewage Disposal 

System Densities

The overall sampling plan was designed, in part, 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. Private wells were used as 

a surrogate for ISDS’s because those lots that have a well and a 

house also have an ISDS, and records for wells are more easily 

accessible than records of ISDS’s. Thus, well density based on 

information from Colorado Division of Water Resources (2000) 

was assumed to be equivalent to ISDS density for the purposes 

of this analysis. The 22 wells completed in granitic-rock aquifers and the 33 wells completed in volcanic-rock aquifers were 

each divided into 4 density categories based on the number of 

wells per acre (table 2). The high-density category consisted of 

more than one well per acre. The medium-density category consisted of one well in 3 acres. The low-density category consisted 

of one well in 5 or more acres. Finally, the background wells 

were not expected to be influenced by other wells and ISDSs. 

No comparisons were done using data from the medium-density 

category because of the small number of wells in this category.

Potential Effects of Individual Sewage Disposal 

System Density on Ground-Water Quality

The data were grouped by aquifer type and ISDS density 

category. Wilcoxson rank-sum tests (Helsel and Hirsch, 1992) 

were run to determine whether a statistical difference exists 

between constituents for any combination of two density categories within an aquifer type. Differences between data sets 

were determined to be significant when the p-value was 0.05 or 

smaller, as discussed in the “General Assessment of Groundwater Quality” section of this report.

Comparisons using Wilcoxon rank-sum tests for the wells 

completed in the granitic-rock aquifers did not identify any 

significant differences between ISDS density categories for any 

constituent. Wells completed in granitic rock generally were 

located in the vicinity of Lake George, Colo., in the northeastern part of the study area.

For samples collected from the wells completed in the volcanic-rock aquifers, the concentrations of boron and chloride 

in the high-density category wells were significantly higher 

High density

 (1 well per acre)

11 9

1 0

6 14

4 10

Medium density

 (1 well per 3 acres)

Low density

 (1 well per 5 acres 

 or more)

Background

Individual sewage

disposal sytem

(ISDS) density 

category

Graniticrock

aquifer

Volcanicrock

aquifer

Table 2. Number of wells in each ISDS density 

category in the two aquifer types sampled 

in July–August 2003.
than concentrations in both the low-density category and the 

background wells. Boron and chloride are often associated with 

ISDS contamination. Similarly, significant differences were 

observed for radon, uranium, and many of the major ions. These 

differences, however, were likely due to the varied water chemistry within the different volcanic-rock aquifers in the study 

area. Significant differences in phosphorus and orthophosphorus 

concentrations also were observed, but concentrations in the 

low-density and background wells were significantly larger than 

concentrations in samples collected from the high-density wells. 

Wells completed in volcanic-rock aquifers, as part of this study, 

generally are located in the Thirtynine Mile Volcanic Field or 

the Guffey Volcanic Center in southeastern Park County (Epis 

and others, 1979). All nine of the high-density wells installed in 

volcanic-rock aquifers are located in or near the small town of 

Guffey. Overall, these data indicated that there is some relation 

between ISDS density and ground-water quality as evidenced 

by the boron and chloride concentration data; however, it also 

appears that significant differences in concentration are associated with natural geochemical processes as evidenced by the 

radon, uranium, and major ion data. 

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