Introduction
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Figure 1.
Map showing location of study area. |
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 September or October 2002 from domestic
wells completed in alluvial and sedimentary-rock aquifers in the
vicinity of Fairplay and Alma, Colo. (fig. 1). Additionally, this
report provides an initial assessment of the potential effects of
ISDSs on ground-water quality in sedimentary-rock aquifers in the
vicinity of Fairplay and Alma, Colorado.
Water samples were collected from 53 domestic wells during September
and October of 2002; 13 of the wells were completed in alluvial aquifers,
and 40 were completed in sedimentary-rock aquifers. Measurements of
pH, specific conductance, and bacteria were conducted in the field,
and water samples were collected following procedures 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). Additionally, water samples at selected
wells were analyzed for an extensive list of organic chemicals that
are indicative of contamination from ISDS effluent (Zaugg and others,
2002). Analyses of these samples 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 five 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 September 1 through
October 31, 2002). The geology of the study area
consists of unconsolidated alluvial deposits of Quaternary age which
compose alluvial aquifers that contain water in the void spaces
between the grains of sand and gravel that make up the deposits,
and sedimentary rocks of Tertiary and Paleozoic age which compose
sedimentary-rock aquifers that contain water in the pore spaces
of the various sedimentary layers and in the fractures that cut
across the sedimentary-rock layers (Tweto, 1979). Reported well
yields for wells in the vicinity of Fairplay, Colo., range from
less than 1 gallon per minute (gal/min) to more than 200 gal/min.
The variability in well yields depends on many things, including
the type of geologic material the well is completed in (alluvial
or sedimentary), the number of fractures intercepted by a well,
the degree of openness of those fractures, the percentage of pore
spaces (porosity), the interconnectedness of pores containing water
(permeability), and the length of the open interval of the well
(fig. 2). Well yields for sampled wells in the alluvial aquifers
ranged from 5 to 15 gal/min, and well yields for sampled wells in
the sedimentary-rock aquifers ranged from 2 to 15 gal/min.
Depths for wells chosen for sampling ranged from 50 to 305 feet
(ft). The well yields and depths for the wells sampled are representative
of most of the wells drilled in the various aquifers in the vicinity
of Fairplay, 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 yields.
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Figure 2.
Diagram showing the relation of Individual Sewage Disposal Systems
(ISDS), domestic wells, and the water table in an alluvial and
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 fractures and pore spaces, the distance from the recharge point
to the open interval of a well, and the velocity of water flow in
the fractures or pore spaces. Because the rate of recharge and the
flow velocity 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
are 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 ISDSs
varies 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 ISDSs
increases.
General Assessment of Ground-Water Quality
Generally, the water quality was similar in samples
collected from the alluvial and sedimentary-rock aquifers (table
1). The median pH value and dissolved-solids concentration for both
types of aquifers were 7.7 standard units and 219 milligrams per
liter (mg/L), respectively. Median hardness concentrations for
both aquifer types were 210 mg/L, which would be considered "very
hard" water (Hem, 1985). The maximum hardness concentration of 1,600
mg/L was measured from a sample collected from a well completed
in the sedimentary-rock aquifer. The maximum hardness concentration
measured from samples collected from wells completed in the alluvial
aquifer was 350 mg/L. The median alkalinity concentration in samples
collected from both aquifers was 180 mg/L. Alkalinity is a measure
of the ability of the water to neutralize acids. Trace-metal concentrations
generally were small and similar to concentrations in samples collected
from 1962 to 1998 in Park County (Kimbrough, 2001). The median boron
concentration for both aquifer types was reported as less than the
reporting limit of 13 micrograms per liter (µg/L). The median
dissolved-radon concentration for samples collected from both aquifer
types was 1,600 picocuries per liter (pCi/L); however, the range
in concentrations for the alluvial wells was 990 to 2,400 pCi/L,
whereas the range in concentrations for the wells completed in the
sedimentary-rock aquifers was 50 to 6,000 pCi/L.
Dissolved organic carbon concentrations from either
aquifer type generally were less than 1 mg/L. Total coliform bacteria
were detected in only one well and there were no detections of Escherichia
coli (E. coli) bacteria; total coliform and E. coli
can originate from humans and other warm-blooded animals. Isophorone
was the only organic chemical reported at a concentration above
the reporting limit for a specific compound. The concentration of
isophorone in the sample from one well completed in the sedimentary-rock
aquifer was 3.3 µg/L. Isophorone is an industrial chemical
used as a solvent in some printing inks, paints, lacquers, and adhesives.
It also is used as an intermediate in the production of certain
chemicals, and it occurs naturally in cranberries (Agency for Toxic
Substances and Disease Registry, 1989).
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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. Nitrite concentrations in
all samples 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 low, comparisons to
the nitrate standard were done using nitrite plus nitrate data.
The maximum nitrite plus nitrate concentration was 4.5 mg/L, which
was much less than the 10 mg/L standard for nitrate (reported as
nitrogen) set by the U.S. Environmental Protection Agency. Additionally,
the standard for fluoride (4 mg/L) was not exceeded in any sample.
Proposed standards for arsenic (10 µg/L) and current standards
for copper (1,300 mg/L) were not exceeded. All cadmium concentrations
were reported as less than 8 mg/L; however, a direct comparison
to the standard could not be made because the current standard for
cadmium is 5 µg/L.
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 standard
(2004) of 30 µg/L was not exceeded in any sample analyzed
for uranium. 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 levels of 300 or 4,000
pCi/L (U.S. Environmental Protection Agency, 1999). The varying
levels are dependent on other mitigating remedial activities. The
proposed standards do not pertain to private wells. Radon may be
of concern because the median radon concentration was 1,600 pCi/L
for the 19 wells sampled.
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. About 6 percent of the sulfate concentrations exceeded
the secondary standard of 250 mg/L; the maximum sulfate concentration
was 1,380 mg/L. All sulfate concentrations greater than 250 mg/L
were associated with wells completed in the sedimentary-rock aquifers.
Aluminum concentrations were below the reporting limit of 15 µg/L
and the secondary standard of 50 µg/L. Zinc concentrations
generally were below the reporting limit of 24 mg/L and the secondary
standard of 5,000 µg/L. Iron and manganese concentrations
were more often reported above the reporting limit. No manganese
sample exceeded the secondary standard of 50 µg/L. Three of
six samples collected in the sedimentary-rock aquifers exceeded
the secondary standard of 300 µg/L for iron.
Nine tritium samples were collected to attempt to determine an
approximate age, or time since recharge, of the ground water pumped
by wells: four samples were collected from wells completed in the
alluvial aquifers, and five samples were collected from wells completed
in the sedimentary-rock aquifers. Tritium is used as an age-dating
tracer because it was produced in relatively high 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 ranged from 19 to 49 picocuries per liter in seven
of the nine samples, which included samples collected from both
aquifer types. One sample from each aquifer type had tritium concentrations
at or near the reporting limit of 2.5 pCi/L, which indicated that
water collected from these wells predates 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 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 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 (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 unsaturated
and saturated 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).
If the water table lies close to the land surface, however, 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 ISDSs, however, 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 ISDSs 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 nitrate, nitrite, 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 detected in the water samples collected during this study. However,
quantification of even small concentrations of bacteria and chemicals
associated with ISDS effluent can indicate a potential for contamination.
Only one sample had detectable concentrations of total coliform
bacteria, and none of the 43 ground-water samples analyzed had detectable
concentrations of E. coli. Boron was detected in 23 percent
of the samples collected from wells completed in the alluvial aquifer
and in 27 percent of the samples collected from wells completed
in the sedimentary-rock aquifer. Only one of the seven samples analyzed
for selected organic chemicals associated with contamination from
human activities had detectable concentrations of an organic chemical.
Isophorone was reported at 3.3 µg/L in a sample collected
from well completed in a sedimentary-rock aquifer. D-limonene, methylene-blue
active substances, 4-tert-octylphenol, and bisphenol-A were detected
in one sample each at trace concentrations. Para-nonylphenol and
phenol were detected at trace concentrations in two samples each.
D-limonene is a citrus degreaser and methylene-blue active substances
are used in surfactants or detergents. Phenols are used primarily
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. Although the presence of these organic chemicals could
indicate contamination from ISDSs, the extent of the contamination
was limited to mostly trace-level detections.
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 ISDSs)
was a significant factor in potential degradation of ground-water
quality. The density of private wells was used as a surrogate for
the density of ISDSs 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 ISDSs. Thus, well density based on information obtained
from Colorado Division of Water Resources (2000) was assumed to
be equivalent to ISDS density for the purposes of this analysis.
Only wells installed in sedimentary-rock aquifers were evaluated
for differences in density categories because the number of samples
collected in each ISDS density category associated with the alluvial
aquifers was small. The 40 wells installed in sedimentary-rock aquifers
wells were divided into 4 density categories based on the number
of wells per acre. The high-density category (10 wells) consisted
of more than 1 well per acre. The medium-density category (12 wells)
consisted of 1 well in 3 acres. The low-density category (14 wells)
consisted of 1 well in 5 or more acres. Finally, the four background
wells were not expected to be influenced by other wells and ISDSs.
Potential Effects of Individual Sewage Disposal System
Density on Ground-Water Quality
The data were grouped by 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. 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. As the p-value increases,
the level of confidence that two data sets are different decreases.
Comparisons using Wilcoxon rank-sum tests did not identify significant
differences between ISDS density categories for any constituent
with the exception of phosphorus. Significant differences for phosphorus
were observed between the high-density category and both the low-density
category and the background wells. Medians for the low-density category
and the background wells were higher than the median for samples
from the high-density category. Phosphorus concentrations in samples
collected from the high-density category wells were all reported
as less than the reporting limit. Overall, the data did not indicate
major effects of ISDS on ground-water quality.
References
Agency for Toxic Substances and Disease Registry, 1989, Public
health statement for Isophorone, accessed March 17, 2004, at http://www.atsdr.cdc.gov/toxprofiles/phs138.html.
Canter, L.W., and Knox, R.C., 1985, Septic tank system effects
on ground water quality: Chelsea, Mich., Lewis Publishers, 336 p.
Colorado Division of Water Resources, 2000, Public well records
available from the Division of Water Resources Records Section,
Denver, Colo.
Fishman, M.J., ed., 1993, Methods of analysis by the U.S. Geological
Survey National Water Quality LaboratoryDetermination of inorganic
and organic constituents in water and fluvial sediments: U.S. Geological
Survey Open-File Report 93125, 217 p.
Helsel, D.R., and Hirsch, R.M., 1992, Statistical methods in water
resources: N.Y., Elsevier Science Publishing Company, Inc., 522
p., 1 diskette.
Hem, J.D., 1985, Study and interpretation of the chemical characteristics
of natural water: U.S. Geological Survey Water-Supply Paper 2254,
263 p.
Kendall, C., and McDonnell, J.J., eds., 1998, Isotope tracers
in catchment hydrology: N.Y., Elsevier Science Publishing Company,
Inc., 839 p.
Kimbrough, R.A., 2001, Review and analysis of available streamflow
and water-quality data for Park County, Colorado, 1962-98: U.S.
Geological Survey Water-Resources Investigations Report 01–4034,
66 p.
Kolpin, D.W., Furlong, E.T., Meyer, M.T., Thurman, E.M., Zaugg,
S.D., Barber, L.B., and Buxton, H.T., 2002, Pharmaceuticals,
hormones, and other organic wastewater compounds in U.S. streams,
1999-2000- A national reconnaissance: Environmental Science
and Technology, v. 36, n. 6, p. 1202–1211.
Matthess, G., and Pekdeger, A., 1981, Survival and transport of
pathogenic bacteria and viruses in ground water, in Proceedings,
First International Conference on Ground-Water-Quality Research,
Houston, Texas, John Wiley and Sons, N.Y., p. 472–482.
Robertson, W.D., Sudicky, E.A., Cherry, J.A., Rappaport, R.A.,
and Shimp, R.J., 1989, in Kobus, H.E., and Kinzelbach, W., eds.,
Impact of a domestic septic system on an unconfined sand aquifer:
Proceedings of the international symposium on contaminant transport
in ground water, Stuttgart, Federal Republic of Germany, April 4-6,
1989, v. 3, p. 105–112.
Tweto, Ogden, comp., 1979, Geologic map of Colorado: U.S. Geological
Survey State Geologic Map, scale 1:500,000 (reprinted).
U.S. Environmental Protection Agency, 1999, National primary drinking
water regulations- Radon-222, proposed rule: Code of Federal Regulations,
v. 64, Title 40, chap. 1, part 141 and part 142, p. 59246-59344.
U.S. Environmental Protection Agency, 2002, National primary drinking
water regulations: Code of Federal Regulations, v. 64, Title 40,
chap. 1, part 141 and part 143.2.
Viraraghavan, T., and Warnock, R.G., 1976, Groundwater quality
adjacent to a septic tank system: Journal of the American Water
Works Association, v. 68, no. 11, part 1, p. 611–614.
Wilde, F.D., Radke, D.B., Gibs, J., and Iwatsubo, R.T., 1998,
National field manual for the collection of water-quality data:
U.S. Geological Survey Techniques of Water-Resources Investigations,
book 9, chap. A1–A9.
Wilhelm, S.R., Schiff, S.L., and Cherry, J.A., 1994, Biogeochemical
evolution of domestic waste water in septic systems, 1. Conceptual
model: Ground Water, v. 32, no. 6, p. 905–916.
Zaugg, S.D., Smith, S.G., Schroeder, M.P., Barber, L.B., and Burkhardt,
M.R., 2002, Methods of analysis by the U.S. Geological Survey National
Water Quality Laboratory—Determination of wastewater compounds
by polystyrene-divinylbenzene solid-phase extraction and capillary-column
gas chromatography/mass spectrometry: U.S. Geological Survey Water-Resources
Investigations Report 01–4186, 37 p. |