Scientific Investigations Report
2007-5220 PDF (3 MB)

Abstract

In 2000, the U.S. Geological Survey, in cooperation with Park County, Colorado, began a study to evaluate ground-water quality in the various aquifers in Park County that supply water to domestic wells. The focus of this study was to identify and describe the principal natural and human factors that affect ground-water quality. In addition, the potential effects of individual sewage disposal system (ISDS) effluent on ground-water quality were evaluated.

Ground-water samples were collected from domestic water-supply wells from July 2001 through October 2004 in the alluvial, crystalline-rock, sedimentary-rock, and volcanic-rock aquifers to assess general ground-water quality and effects of ISDS’s on ground-water quality throughout Park County. Samples were analyzed for physical properties, major ions, nutrients, bacteria, and boron; and selected samples also were analyzed for dissolved organic carbon, human-related (wastewater) compounds, trace elements, radionuclides, and age-dating constituents (tritium and chlorofluorocarbons).

Drinking-water quality is adequate for domestic use throughout Park County with a few exceptions. Only about 3 percent of wells had concentrations of fluoride, nitrate, and (or) uranium that exceeded U.S. Environmental Protection Agency national, primary drinking-water standards. These primary drinking-water standards were exceeded only in wells completed in the crystalline-rock aquifers in eastern Park County. Escherichia coli bacteria were detected in one well near Guffey, and total coliform bacteria were detected in about 11 percent of wells sampled throughout the county. The highest total coliform concentrations were measured southeast of the city of Jefferson and west of Tarryall Reservoir. Secondary drinking-water standards were exceeded more frequently. About 19 percent of wells had concentrations of one or more constituents (pH, chloride, fluoride, sulfate, and dissolved solids) that exceeded secondary drinking-water standards. Currently (2004), there is no federally enforced drinking-water standard for radon in public water-supply systems, but proposed regulations suggest a maximum contaminant level of 300 picocuries per liter (pCi/L) and an alternative maximum contaminant level of 4,000 pCi/L contingent on other mitigating remedial activities to reduce radon levels in indoor air. Radon concentrations in about 91 percent of ground-water samples were greater than or equal to 300 pCi/L, and about 25 percent had radon concentrations greater than or equal to 4,000 pCi/L. Generally, the highest radon concentrations were measured in samples collected from wells completed in the crystalline-rock aquifers.

Analyses of ground-water-quality data indicate that recharge from ISDS effluent has affected some local ground-water systems in Park County. Because roughly 90 percent of domestic water used is assumed to be recharged by ISDS’s, detections of human-related (wastewater) compounds in ground water in Park County are not surprising; however, concentrations of constituents associated with ISDS effluent generally are low (concentrations near the laboratory reporting levels). Thirty-eight different organic wastewater compounds were detected in 46 percent of ground-water samples, and the number of compounds detected per sample ranged from 1 to 17 compounds. Samples collected from wells with detections of wastewater compounds also had significantly higher (p-value < 0.05) chloride and boron concentrations than samples from wells with no detections of wastewater compounds.

ISDS density (average subdivision lot size used to estimate ISDS density) was related to ground-water quality in Park County. Chloride and boron concentrations were significantly higher in ground-water samples collected from wells located in areas that had average subdivision lot sizes of less than 1 acre than in areas that had average subdivision lot sizes greater than or equal to 1 acre. For wells completed in the crystalline-rock aquifers, chloride concentrations were significantly (p-value < 0.05) higher in wells located in areas with average subdivision lot sizes less than 1 acre than in areas with average subdivision lot sizes greater than 5 acres, and boron concentrations were significantly higher in wells located in areas with average subdivision lot sizes of less than 1 acre than in areas with average subdivision lot sizes greater than 3 acres. In the volcanic-rock aquifers, wells were sampled only in areas that had average subdivision lot sizes less than 1 acre or greater than 5 acres. Potassium, chloride, and boron concentrations were significantly higher in wells located in areas that had average subdivision lot sizes of less than 1 acre than in areas with average subdivision lot sizes greater than 5 acres in the volcanic-rock aquifers. No significant increases in constituent concentrations were observed in wells completed in the sedimentary-rock aquifers for any lot-size category, and too few samples were collected from wells completed in the alluvial aquifers to do statistical tests.

The year of ISDS installation also was related to ground-water quality in Park County. For example, significantly higher nitrite-plus-nitrate concentrations were measured between wells with ISDS’s installed in the 1970’s and those installed in the 1980’s. Significantly higher nitrite-plus-nitrate concentrations were not measured between wells with ISDS’s installed in the 1980’s and those installed in the 1990’s. However, significantly higher nitrite-plus-nitrate concentrations were measured between wells with ISDS’s installed in the 1990’s and those installed after 1999. The lowest overall nitrite-plus-nitrate concentrations were measured in wells that had ISDS’s installed after 1999, and the highest concentrations were measured in wells with ISDS’s installed before 1980. Nitrate concentrations may be less in samples collected from wells with ISDS’s installed after 1980 because effluent has not had enough time to move through the unsaturated zone to the ground-water table in sufficient quantities to significantly affect ground-water quality.

Concentrations of nitrite-plus-nitrate and chloride were significantly higher (p-value < 0.05) in ground-water samples collected during 2001 than in samples collected during 1974 in the Bailey area. Increases in nitrite-plus-nitrate and chloride concentrations in ground water in the Bailey area over time probably are related to a larger part of the overall recharge being from ISDS effluent either as a result of an increase in ISDS density or more time for recharge from ISDS effluent to reach the ground-water table.

Many of the wells sampled in Park County probably have a mixture of water from several parts of the aquifer system that may have different recharge areas, flow paths, and ground-water recharge dates. Because most of the development in Park County has occurred since 1990 and only about 8 percent of wells sampled for chlorofluorocarbons had ground-water recharge dates since 1990, there may not have been enough elapsed time for effluent water from ISDS’s to noticeably affect sampled ground-water quality. As time passes, recharge from ISDS’s may become a larger part of the ground-water resource, as a result, concentrations of chloride, boron, and other constituents associated with ISDS effluent may increase in ground water if not effectively removed by treatment processes.

Version 1.0

Posted December 2007

• Report PDF (3 MB)