Potential Effects of Individual Sewage Disposal System
Density on Ground-Water Quality in the Fractured-Rock Aquifer in the Vicinity
of Bailey, Park County, Colorado, 2001-2002
Fact Sheet 20043009
By Daniel L. Brendle
Available from the U.S. Geological Survey,
Branch of Information Services, Box 25286,
Denver Federal Center, Denver, CO 80225, USGS Fact Sheet 2004-3009.
This document also is available in pdf format:
FS-2004-3009.pdf (242 KB)
(Requires Adobe
Acrobat Reader)
Introduction
Park County, about 30 miles southwest of Denver (fig. 1), is one
of the fastest growing counties in Colorado. With the increasing
population and development in rural areas comes an increase in demand
for water resources, the number of individual sewage disposal systems
(ISDS), and the potential to affect the ground-water quality. 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. In 2000, the
U.S. Geological Survey (USGS), in cooperation with Park County,
began a study to determine the effects of residential development
and the concurrent increase in ISDSs and the density of ISDSs on
ground-water quality in the various aquifers in Park County that
supply water to domestic wells. This report provides a preliminary
assessment of water-quality data collected in 2001 from domestic
wells completed in the fractured-rock aquifer in the vicinity of
Bailey in northeastern Park County (fig. 1). Water samples were
collected from 57 domestic wells during 2001, once in July and once
in September. Samples were analyzed for chemicals and bacteria that
might indicate whether ISDS effluent has caused degradation of ground-water
quality. This report also describes the preliminary results of five
water samples collected in October 2002 for tritium analysis.
The granitic and metamorphic (crystalline) rocks (Tweto,
1979) in the Bailey area contain ground water in fractures and form
the principal aquifer supplying domestic wells. Reported well yields
for wells drilled into the fractured crystalline rock range from
less than 1 gallon per minute (gal/min) to 60 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). Well yields
for sampled wells ranged from about 0.5 to 25.5 gal/min. Depths
for wells chosen for sampling ranged from 85 to 752 feet. The well
yields and depths for wells sampled are representative of most of
the wells drilled in the crystalline-rock aquifer in the vicinity
of Bailey (Colorado Division of Water Resources, 2000). |
Figure 1.
Location of study area.
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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 removed from the effluent or reduced
in concentration. This study was designed to assess whether contamination
of ground water has occurred and to obtain information that might
help determine the length of time it takes for ISDS effluent to
recharge the aquifer. The size of the lots in each development,
and thus the closeness of neighboring wells and ISDSs, varies. Each
residence has its own ISDS. House densities range from several houses
per acre to a single house on many acres. As houses are built closer
to each other and as ISDS density increases, there could be a greater
potential for degradation of ground-water quality.
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 the 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 crystalline 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 (nitrate plus nitrite, as nitrogen).
When nitrate reaches the water table, and if dissolved organic carbon
(DOC) is present and dissolved oxygen is absent, the nitrate and
DOC 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 decreases 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
compounds 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, Escherichia
coli (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 closer to the land surface, the
unsaturated zone is thinner and more of the bacteria in the effluent
can potentially reach the ground water (Canter and Knox, 1985). |
Figure 2. Diagram showing the
relation of Individual Sewage Disposal System, domestic wells, and
the water table in a fractured-rock ground-water system.
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Indicators of Contamination
to the Ground Water
Samples collected from wells were analyzed
for chemicals and bacteria that can originate from an ISDS. Many
of these chemicals and bacteria can enter the ground water through
natural processes, but if they are observed in elevated concentrations
it can indicate degraded ground-water quality. Chloride, nitrate,
nitrite, ammonia, and boron can all occur naturally in ground water.
These chemicals usually do not occur naturally at elevated concentrations
because these chemicals come from dispersed sources, such as waste
from wild animals, decomposition of forest material, deposition
from the atmosphere, or from weathering of rocks. An ISDS can provide
a focused source of these chemicals if its leach field pipe is too
close to the water table or if the infiltration rate and ground-water
flow velocity are too rapid to allow for proper geochemical or physical
treatment of the ISDS effluent. Products that are used in households,
such as soaps containing boron, dietary salt containing chloride,
caffeine, pesticides, perfumes, or human waste containing nitrate,
nitrite, and ammonia, can enter the ground-water system as a more
concentrated effluent from an ISDS. Total coliform and E. coli
bacteria can originate from humans and other warm-blooded animals.
Sampling Plan Design and Methods
The plan for collecting ground-water samples was
designed to allow an evaluation of whether the density of development
and thus the proximity of wells and ISDSs (ISDS density) is a factor
in potential degradation of ground-water quality. Private wells
were used as a surrogate for locating ISDSs because those lots having
a well and a house also have an ISDS, and records for wells are
more easily accessible than records of ISDSs. Thus, well density
is equivalent to ISDS density for the purposes of this analysis.
Wells listed in Colorado public well records (Colorado Division
of Water Resources, 2000) were divided into four categories based
on the number of wells per acre: more than one well per acre (high-density);
one well in 3 acres (medium-density); one well in 5 or more acres
(low-density); and background wells (wells that were not expected
to be influenced by other wells and ISDSs) (background) (table 1).
Candidate wells were classified into one of the three
density categories by plotting their locations on a map and overlaying
a grid on the map in GIS. Each of the overlay grids was made of
cells that represent areas of 1 acre, 3 acres, or 5 acres. Well
density was confirmed by observations made in the field. When the
list of candidate wells was exhausted, additional wells needed to
complete the sample size for each density category were chosen based
on field observations. Background wells were chosen using the criterion
that the wells were located such that the water pumped by the wells
was not expected to be influenced by ISDSs or other human activities.
Wells were sampled for chemicals and bacteria
that can originate from septic systems and can be used as indicators
of ground-water-quality degradation, including boron, chloride,
fluoride, sulfate, nitrate, ammonia, phosphorus, total coliform
and E. coli bacteria, and 67 organic chemicals that can
only originate from households, such as caffeine, perfumes, pharmaceuticals,
and the metabolites of organic chemicals (Kolpin and others, 2002).
Samples were collected from a faucet in the plumbing system of each
house and the water was pumped by the existing pump in each of the
wells. Standard USGS protocols for the collection of water-quality
samples were followed (Wilde and others, 1998). Measurements of
the physical characteristics of water, such as pH, dissolved oxygen,
and specific conductance also were made. Additionally, most wells
were sampled twice, once each in July and September, to determine
whether there were detectable variations in water quality with time
(table 1). Quality-control samples collected in the field included
eight 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 this URL: http://waterdata.usgs.gov/co/nwis/qwdata
(search for Park County and the date range when the samples were
collected). Water samples were analyzed at the USGS National Water
Quality Laboratory in Denver, Colorado, using methods described
in Fishman (1993) for inorganic chemicals and in Zaugg and others
(2002) for organic chemicals.
Methods used to compare data
The results of analysis of the samples were
grouped by the density categories and the month the well was sampled.
The concentrations of chemicals and bacteria in the samples were then
compared to assess whether differences existed between the density
categories and the month the sample was collected. Sample results
were compared by using boxplots and testing for differences between
the groupings of data by using the Wilcoxon rank-sum test (Helsel
and Hirsch, 1992).
Boxplots provide a visual comparison of the variability between
data from different density categories and from different months.
An example of a boxplot is shown below:
The Wilcoxon rank-sum test is used to determine whether a statistical
difference exists between two data sets. All possible combinations
of two data sets for a particular chemical for all density categories
and months were evaluated using the rank-sum test. Differences between
data sets were determined to be significant when the significance
level (p-value) was 0.05 or smaller. A p-value
of 0.05 means that it can be said with 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.
Potential Effects of Individual Sewage Disposal System
Density on Ground-Water Quality
Four of the samples collected in July and four collected in September
contained bacteria; only one well had detections of bacteria in
both months. Detections of bacteria indicate contamination of the
ground water, but not necessarily from an ISDS. Bacteria were present
in samples from wells in the low-, medium-, and high-density categories.
Detections of bacteria did not appear to be correlated with ISDS
density. Additionally, concentrations of the other chemicals associated
with ISDSs in these samples were not above expected background concentrations.
Samples from four wells in the low-density and background categories
contained organic chemicals that can originate only from an ISDS.
One of the 67 organic chemicals was detected in each of 3 wells,
and 2 of the chemicals were detected in 1 well. The detections of
organic chemicals might be due to ISDS contamination, but concentrations
of the other chemicals in the samples were not elevated above expected
background concentrations.
A comparison was made to determine whether there was a correlation
between concentrations of the various chemicals and the depths or
yields of wells. This comparison indicated that the concentrations
of several chemicals were inversely related to the depths or yields
of wells (as yield or depth increases, concentrations decrease),
but the correlations were not very strong.
Data from all wells were plotted on boxplots, and the Wilcoxon
rank-sum test was used to identify statistically significant differences
between months for a selected density category and chemical, or
between density categories for a particular month and chemical.
Most of the tests indicated no significant differences. None of
the tests indicated a significant difference between July and September
for a particular density category and chemical. The boxplots are
shown only for those chemicals that had at least one comparison
that was statistically significant: nitrate, chloride, and boron.
The boxplots indicated the lowest concentration for a particular
chemical was nearly the same for all density categories. These plots
also show that the difference between the highest and lowest concentrations
for a particular chemical was greatest for the high-density category
and smallest for the background category.
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Significant differences
as determined by the Wilcoxon rank-sum test for the nitrate data
were found (fig. 3): between the high- and low-density categories
for July data; and between the high- and low-density categories
and the high-density and background categories for September data.
Figure 3 indicates that the median nitrate concentration in the
high-density category was about 35 percent greater than the median
for the medium-density category, about 75 percent greater than the
median for the low-density category, and about 64 percent greater
than the median for the background category. These comparisons and
the boxplots indicate nitrate concentrations tended to be higher
in the high- and medium-density categories than in the low-density
or background categories. The comparisons also indicate a higher
probability of transport of nitrate to the ground water in areas
with a higher density of houses and their associated ISDSs. However,
in the high-density category only 7 percent (two samples) of the
samples had nitrate concentrations greater than the U.S. Environmental
Protection Agency (USEPA) primary drinking-water standard of 10
milligrams per liter (mg/L), and 17 percent (five samples) had nitrate
concentrations from 9 to 10 mg/L. The maximum nitrate concentration
was 25.7 mg/L from a well in the high-density category. None of
the nitrate samples from the low-density category exceeded the USEPA
standard for nitrate; the maximum nitrate concentration in this
density category was 9.2 mg/L. These data indicate a propensity
for elevated nitrate concentrations in areas where ISDS densities
are more than one ISDS per 5 acres, but the data also indicate that
elevated nitrate also occurs in the low-density and background categories.
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Figure 3. Boxplot of nitrate data for all density categories and both months when samples were collected.
See table 1 for the number of samples in each category.
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Significant differences
as determined by the Wilcoxon rank-sum test for the chloride data
were found (fig. 4): between the high- and low-density categories
and the high-density and background categories for July data; and
between the high- and medium-density categories, the high- and low-density
categories, and the high-density and background categories for September
data. Figure 4 indicates that the median chloride concentration in
the high-density category was about 26 percent greater than the median
for the medium-density category, about 65 percent greater than the
median for the low-density category, and about 69 percent greater
than the median for the background category. These comparisons and
the boxplots indicate chloride concentrations tended to be higher
in the high- and medium-density categories than in the low-density
or background categories. The comparisons also indicate that there
may be a higher probability of transport of chloride to the ground
water in areas with higher density of houses and their associated
ISDSs. However, in the high-density category only 7 percent (two samples)
of the samples had chloride concentrations greater than the USEPA
secondary drinking-water standard of 250 mg/L. None of the chloride
samples from the low-density category exceeded the USEPA standard
for chloride; the maximum chloride concentration in this density category
was 51.3 mg/L. The maximum chloride concentration was 886 mg/L for
a sample from the high-density category. |
Figure 4. Boxplot
of chloride data for all density categories and both months when samples
were collected. See table 1 for the number of samples in each category.
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Significant
differences as determined by the Wilcoxon rank-sum test for the
boron data were found (fig. 5) only between the high- and low-density
categories for September 2001 data. Figure 5 indicates that the
median boron concentration in the high-density category was about
24 percent greater than the median for the medium-density category
and about 39 percent greater than the medians for the low-density
and background categories. The maximum boron concentration was 144
micrograms per liter (µg/L) for a sample from the high-density
category. |
Five tritium samples
were collected between October 11–15, 2002, to attempt to
determine an approximate age, or time since recharge, of the ground
water pumped by wells. 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
others, 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
of tritium in the samples ranged from 15 to 38 picocuries per liter
(tritium data can be obtained on the Web at URL http://waterdata.usgs.gov/co/nwis/qwdata
(search for Park County and date range October 11-15, 2002)). Concentrations
greater than 10 picocuries/liter indicate that recharge to the ground-water
system occurred after 1954 (Kendall and others, 1998). 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. |
Figure 5. Boxplot of
boron data for all density categories for samples collected in September.
See table 1 for the number of samples in each category.
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References Cited
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