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 granitic- and 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.
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Figure 1. Map showing
location of study area. |
Water samples were collected from 55 domestic wells
during July and August of 2003; 22 wells were completed in granitic-rock
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 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.
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Figure 2.
Diagram showing the relation of Individual Sewage Disposal System
(ISDS), domestic wells, and the water table in a fractured-rock
ground-water system. |
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.
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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 micro-grams 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 concentrations 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
granitic-rock 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 subsoilunsaturated
zone above the water table and the saturated zone below the water
table (fig. 2)that can reduce the concentrations of chemical and
biological constit-uents 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 Disposal 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 ground-water 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 Indications of Contamination from Individual
Sewage Disposal Systems
Generally, most chemicals associated with ISDS contamination were
not observed in any measurable quan-tity 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.
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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 Ground-water 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 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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