WSP09923

• File Number: 8102
• Description: Arkansas River Basin Basic Hydrology
• State: CO
• Basin: Arkansas
• Water Division: 2
• Date: 1/1/1997
• Author: USGS
• Title: St Charles Mesa and La Junta Publications
• Water Supply Pro - Doc Type: Publication

001979

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COMPARISON OF MEASURED AND
EXPECTED BACKGROUND CONCENTRA-
TIONS OF CHEMICAL AND BIOLOGICAL
CONSTITUENTS

The 25 water samples collected in July and
August 1997 were analyzed for chemical and bacterial
indicators to determine whether Mesa ground water has
been affected by the byproducts of human activities,
including septic-system effluent. If contaminants from
septic-system effluent are reaching the ground water, a
long, nan.ow contaminant plume can be expected to form
in the direction of ground-water flow, with little dispersion
of the contaminants vertically or horizontally (fig. 2)
(Robertson and others, 1989). Thus, because the concen-
tration of chemical and biological constituents may vary
with depth in the aquifer and the wells that were sampled
are open to most of the saturated thickness of the aquifer
(fig. 2), the water samples represent a composite of the
ground-water quality in the aquifer.
The ground water of the Mesa is recharged by
precipitation and water originating in Pueblo Reservoir,
which is delivered to the Mesa and to the St. Charles Mesa
Water District through Bessemer Ditch. Chemical anal-
yses of Bessemer Ditch water and precipitation were not
. available for comparison with ground-water-sample anal-
yses. Data from analyses of water collected from Pueblo
Reservoir were assumed to be representative of water
reaching the Mesa through the Bessemer Ditch (table 1).
Additionally, it was assumed that water that was conveyed
from Pueblo Reservoir to the Mesa through Bessemer
Ditch did not become enriched in any of the chemical or
biological constituents considered in this study.
To determine whether the ground water of the Mesa
has been affected by the byproducts of human practices,
each chemical or biological constituent was compared to
the expected maximum background concentration
(concentrations from Pueblo Reservoir) or to the appli-
cable U.S. Environmental Protection Agency (USEPA)
maximum contaminant level (MCL) or maximum contam-
inant level goal (MCLG) (U.S. Environmental Protection
Agency, 1995). The MCL and MCLG values are listed in
table I. If the concentration of a chemical or biological
constituent in a sample exceeded the expected maximum
background concentration or the USEPA MCL or MCLG,
that sample was considered to have been affected by the
byproducts of human practices. It was not possible to
differentiate the particular practice that may have caused
. the concentration of a constituent to be higher than the
expected background concentration, except for caffeine or

MBAS, both of which are strong indicators that the ground
water has been affected by septic-system effluent.
Additionally, an assessment was made to determine
whether there was evidence to indicate widespread degrada-
tion of the ground water that could be attributed to septic-
system effluent. For this assessment, an approach was used
that considered multiple lines of evidence. For a particular
sample to be considered affected by septic-system effluent,
concentrations of several of the chemical and biological
constituents had to be higher than the expected maximum
background concentrations or USEPA MCL or MCLG, and
there needed to be constituents present, such as caffeine or
MBAS, that most likely originated from septic systems.
This multiple-lines-of-evidence approach was used because
all of the chemical and biological constituents, except
caffeine and probably MBAS, can originate from multiple
sources.
Concentrations of nitrate in all the wells sampled were
below the USEPA maximum contaminant level of 10 milli-
grams per liter (mg/L) (U.S. Environmental Protection
Agency, 1995); concentrations ranged from 0.07 to 9.2 mg/L
(table 2), with a median concentration of 4.2 mg/L. Three of
the four samples in which the nitrate concentration was
higher than 7 mg/L were outside the area identified as
having a high water table (depth to water less than 10 feet
from the land surface) (fig. 3). The observation that
80 percent of the ground-water samples exceeded the
expected maximum background concentration for nitrate of
less than I mgIL (table I) indicates that human practices
probably have contributed to these concentrations being
higher than the expected background concentration.
Ammonia was detected in 18 samples. The range of
ammonia concentrations was from <0.01 to 0.08 mg/L
(table 2), with a median concentration of 0.03 mg/L. There
is no USEPA maximum contaminant level for ammonia.
The expected maximum background concentration of
ammonia, based on samples obtained from Pueblo Reser-
voir, was 0.17 mg/L (table 1). All the detections of
ammonia were below the expected maximum background
concentration. The concentrations of ammonia do not indi-
cate that human practices have contributed ammonia to the
ground water.
Dissolved organic carbon was detected in 24 samples,
ranging from 0.8 to 9.5 mg/L (table 2), with a median
concentration of 1.8 mg/L. There is no USEPA maximum
contaminant level for DOC. The concentration of DOC was
below the expected maximum background concentration of
3.2 mg/L in 23 samples (table 1). Only one ground-water
sample (9.5 mg/L) exceeded the expected maximum back-
ground concentration. The concentrations of DOC in 23 of
the samples, which were below the expected maximum

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