|
8 Water-Quality Characteristics and Ground-Water Quantity of the Fraser River Watershed, Grand County, Colorado, 1998-2001
<br />false negative is less than or equal to 1 percent. Concentrations
<br />detected between the LT-MDL and LRL are reported as esti-
<br />mated (E) concentrations. Concentrations detected at values
<br />less than the MRL, LT-MDL, or undetected are reported with a
<br />"less than" (<) symbol at the MRL or the LRL (Childress and
<br />others, 1999). The LT-MDL and LRL for a constituent may
<br />change annually on a water-year basis.
<br />Statistical Methods
<br />Water-quality data were analyzed using nonparametric
<br />statistical methods. Nonparametric methods were used because
<br />they do not require data sets to be normally distributed, are
<br />resistant to outliers, and are appropriate for use with small data
<br />sets (Helsel and Hirsch, 1992). The two-tailed Wilcoxon rank-
<br />sum test was used to determine if the distribution of the ground-
<br />water-quality data was significantly different among aquifer
<br />types (alluvial, Troublesome Formation), land use (urban, non-
<br />urban), and ISDS and non-ISDS areas. Results of the test are
<br />expressed in terms of p-values-the probability of rejecting or
<br />not rejecting the hypothesis that the data sets of the comparison
<br />categories are similar. The test does not prove that the data sets
<br />are similar, only that similarity in data sets can be rejected or not
<br />rejected. Test results were considered to be statistically signifi-
<br />cant when the p-value of the respective test was less than or
<br />equal to 0.05, indicating that there was at least a 95-percent
<br />probability that there were differences in water-quality data and
<br />related median values between the comparison categories. The
<br />case against similarity in data increased as p-values got smaller
<br />and smaller. For results with p-values greater than 0.05, the
<br />hypothesis of similarity in data sets could not be rejected. Data
<br />for the Precambrian granite aquifer were excluded from the
<br />comparison of aquifer types because of the small size of the data
<br />set (seven samples). Water temperature and radon-222 data
<br />were excluded from comparisons for land use and ISDS/non-
<br />ISDS areas because processes on or near the land surface should
<br />have no effect on water temperature and radon-222 concentra-
<br />tions in ground water. Data that were reported with a less-than
<br />(<) symbol were given a value equal to one-half the respective
<br />MRL or LRL for comparison testing. Results of the two-tailed
<br />Wilcoxon rank-sum tests for some constituents were excluded
<br />from discussion in this report because elevated concentrations
<br />in one or more wells biased the results. Additional information
<br />on the Wilcoxon rank-sum test is found in Helsel and Hirsch
<br />(1992) and Ott (1993).
<br />With statistical testing such as the Wilcoxon rank-sum test,
<br />differences in water-quality data between categories can be
<br />statistically significant but not environmentally significant. For
<br />example, statistically significant differences in nitrate concen-
<br />trations between two land uses would not be environmentally
<br />important if nitrate concentrations were centered around
<br />1.50 mg/L for one land use and 0.090 mg/L for the other land
<br />use. Nitrate concentrations in the water for both land uses
<br />would be considered low and would not be of concern
<br />environmentally.
<br />Changes in ground-water quality with time were qualita-
<br />tively examined by graphing individual constituents for a site
<br />against time, fitting a LOWESS smooth curve (Helsel and
<br />Hirsch, 1992) to the data, and visually examining the graphs for
<br />changes. Quantitative analyses of changes with time or trends
<br />were not used because the time periods of the data sets were of
<br />insufficient length (maximum of 4 years), and the data sets were
<br />of insufficient size (maximum of seven samples) to perform sta-
<br />tistical tests for trend analysis, such as the Mann-Kendall test or
<br />the seasonal Kendall test. Also, data collection only occurred in
<br />2 months of each year. Graphs of changes with time were devel-
<br />oped for most water-quality constituents, including water tem-
<br />perature, specific conductance, pH, turbidity, dissolved-oxygen
<br />concentration, acid-neutralizing capacity, dissolved solids,
<br />major ions (calcium, magnesium, sodium, potassium, chloride,
<br />and sulfate), silica, nutrients (nitrate and orthophosphate), and
<br />dissolved organic carbon. Manganese was included when most
<br />or all manganese concentrations for a given well were greater
<br />than the reporting level. Graphs were not developed for bro-
<br />mide, fluoride, ammonia, ammonia plus organic nitrogen,
<br />nitrite, iron, total coliform bacteria, and MBAS because most
<br />concentrations for these constituents were less than the respec-
<br />tive laboratory reporting levels. Phosphorus was excluded
<br />because of its similarity in concentration to orthophosphate.
<br />Radon-222 and data for wells 1 and 3 also were excluded
<br />because only four or fewer samples were collected at each site
<br />or from the two wells.
<br />Water-Quality Standards
<br />Ground-water data were compared to U.S. Environmental
<br />Protection Agency (USEPA) primary, secondary, proposed,
<br />and alternative drinking-water standards and treatment tech-
<br />niques (U.S. Environmental Protection Agency, 2002b) and
<br />State of Colorado water-quality standards, where applicable
<br />(Colorado Department of Public Health and Environmental,
<br />2001). Primary drinking-water standards consist of maximum
<br />contaminant levels (MCL's), which are the highest values or
<br />concentrations of contaminants allowed in public water-supply
<br />systems to protect human health. MCL's are legally enforceable
<br />standards. Secondary drinking-water regulations (SDWR) are
<br />related to the esthetic and cosmetic qualities of the water, such
<br />as taste and odor or tooth and skin discoloration, and are not
<br />legally enforceable. The USEPA-proposed MCL and proposed
<br />alternative MCL (AMCL) that are applicable to this report are
<br />under review and are not currently (2003) enforceable. USEPA
<br />treatment techniques are required processes that aim to lower
<br />the levels of contaminants in drinking water.
<br />Instream water-quality standards have been established for
<br />selected field properties and water-quality constituents for the
<br />Fraser River and its tributaries by the Colorado Department of
<br />Public Health and Environment (Colorado Department of Pub-
<br />lic Health and Environment, 2002). The water-quality standards
<br />are numeric values or table value standards (TVS). The Fraser
|