Water quality and trend analysis of Colorado-Big Thompson system reservoirs and related conveyances, 1969 through 2000

Abstract

The U.S. Geological Survey, in an ongoing cooperative monitoring program with the Northern Colorado Water Conservancy District, Bureau of Reclamation, and City of Fort Collins, has collected water-quality data in north-central Colorado since 1969 in reservoirs and conveyances, such as canals and tunnels, related to the Colorado–Big Thompson Project, a water-storage, collection, and distribution system. Ongoing changes in water use among agricultural and municipal users on the eastern slope of the Rocky Mountains in Colorado, changing land use in reservoir watersheds, and other water-quality issues among Northern Colorado Water Conservancy District customers necessitated a reexamination of water-quality trends in the Colorado–Big Thompson system reservoirs and related conveyances. The sampling sites are on reservoirs, canals, and tunnels in the headwaters of the Colorado River (on the western side of the transcontinental diversion operations) and the headwaters of the Big Thompson River (on the eastern side of the transcontinental diversion operations). Carter Lake Reservoir and Horsetooth Reservoir are offchannel water-storage facilities, located in the foothills of the northern Colorado Front Range, for water supplied from the Colorado–Big Thompson Project. The length of waterquality record ranges from approximately 3 to 30 years depending on the site and the type of measurement or constituent. Changes in sampling frequency, analytical methods, and minimum reporting limits have occurred repeatedly over the period of record. The objective of this report was to complete a retrospective water-quality and trend analysis of reservoir profiles, nutrients, major ions, selected trace elements, chlorophyll-a, and hypolimnetic oxygen data from 1969 through 2000 in Lake Granby, Shadow Mountain Lake, and the Granby Pump Canal in Grand County, Colorado, and Horsetooth Reservoir, Carter Lake, Lake Estes, Alva B. Adams Tunnel, and Olympus Tunnel in Larimer County, Colorado. This report summarizes and assesses: 1. Water-quality and field-measurement profile data collected by the U.S. Geological Survey and stored in the U.S. Geological Survey National Water Information System, 2. Time-series trends of chemical constituents and physical properties, 3. Trends in oxygen deficits in the hypolimnion of the reservoirs in the late summer season by the seasonal Kendall trend test method, 4. Nutrient limitation and trophic status indicators, and 5. Water-quality data in terms of Colorado water-quality standards. Water quality was generally acceptable for primary uses throughout the Colorado–Big Thompson system over the site periods of record, which are all within the span of 1969 to 2000. Dissolved solids and nutrient concentrations were low and typical of a forested/mountainous/crystalline bedrock hydrologic setting. Most of the more toxic trace elements were rarely detected or were found in low concentrations, due at least in part to a relative lack of ore-mineral deposits within the drainage areas of the Colorado–Big Thompson Project. Constituent concentrations consistently met waterquality standard thresholds set by the State of Colorado. Trophic-State Index Values indicated mesotrophic conditions generally prevailed at reservoirs, based on available Secchi depth, total phosphorus concentrations, and chlorophyll-a concentrations. Based on plots of time-series values and concentrations and seasonal Kendall nonparametric trends testing, dissolved solids and most major ions are decreasing at most sites. Many of the nutrient data did not meet the minimum criteria for time-series testing; but for those that did, nutrient concentrations were generally stable (no statistical trend) or decreasing (ammonia plus organic nitrogen and total phosphorus). Iron and manganese concentrations were stable or decreasing at most sites that met testing criteria. Chlorophyll-a data were only collected for 11 years but generally indicated quasi-stable or downward temporal trends. Water Quality and Trend Analysis of Colorado– Big Thompson System Reservoirs and Related Conveyances, 1969 Through 2000 By Michael R. Stevens 2 Water Quality and Trend Analysis of Colorado–Big Thompson System Reservoirs and Related Conveyances Introduction The U.S. Geological Survey (USGS), in an ongoing cooperative monitoring program with the Northern Colorado Water Conservancy District (NCWCD), Bureau of Reclamation (BOR), and City of Fort Collins, has collected water-quality data in north-central Colorado since 1969 in reservoirs and conveyances such as canals and tunnels related to the Colorado–Big Thompson (CBT) Project, a waterstorage, collection, and distribution system. The sampling sites are on reservoirs, canals, and tunnels in the headwaters of the Colorado River (on the western side of the transcontinental diversion operations) and the headwaters of the Big Thompson River (on the eastern side of the transcontinental diversion operations). Carter Lake and Horsetooth Reservoir are off-channel water-storage facilities, located in the foothills of the northern Colorado Front Range, for water supplied from the CBT Project (table 1, fig. 1). The length of water-quality record ranges from approximately 3 to 30 years depending on the site and the type of measurement or constituent. Sampling frequency, analytical methods, and minimum reporting limits have changed repeatedly over the period of record. The CBT Project was authorized by Congress in 1937 and completed in 1956, resulting in the construction of several reservoirs, more than 120 miles of tunnels and canals, and numerous power-generation facilities. The USGS became involved in monitoring in the CBT Project area as part of a cooperative water-quality evaluation of the BOR Pick-Sloan Missouri Basin Program. When operation of the CBT Project was assumed by NCWCD, collection of baseline waterquality information continued. A report by Mueller (1990) summarized water-quality data available at the time, analyzed time-series trends, and evaluated the sampling network design for most of the sampling sites considered in this report and other sites on the eastern plains where the water is used for agricultural purposes. Purpose and Scope Part of the value of such a long-term monitoring record is the ability to test for temporal trends in water quality. Temporal trends in nutrients, major ions, trophic variables, and oxygen can indicate changes in water quality related to eutrophication. Ongoing changes in water use among agricultural and municipal users on the eastern slope of Colorado, changing land uses in reservoir watersheds, and other water-quality issues among NCWCD customers necessitated a reexamination of water-quality trends in CBT system reservoirs and related conveyances. The overall objective of the project is to complete a retrospective analysis of reservoir profiles, nutrients, major ions, selected trace elements, chlorophyll-a, and hypolimnetic oxygen data from 1969 through 2000 in Lake Granby, Shadow Mountain Lake, and the Granby Pump Canal in Grand County, Colo., and Horsetooth Reservoir, Carter Lake, Lake Estes, east portal of the Alva B. Adams Tunnel (hereinafter “Adams Tunnel”), and Olympus Tunnel in Larimer County, Colo. Sampling sites in this report are in the headwater areas of the CBT Project where diversion and storage facilities are located. Areas downstream from Lake Granby on the western slope and water delivery areas downstream from Carter Lake and Horsetooth Reservoir are beyond the scope of this report. Table 1. Water-quality sampling sites. [15-digit U.S. Geological Survey (USGS) identification numbers are latitude and longitude of site location with a 2-digit code at the end; 8-digit identification numbers are USGS downstream order numbering] Site number (fig. 1) U.S. Geological Survey identification number Site name Number of samples Period of record 1 09018500 Lake Granby near Granby, Colo. (near spillway) 154 1973–75, 1979–present 2 400844105530800 Lake Granby near Granby, Colo. (near dam in Rainbow Bay) 123 1989–present 3 09018300 Granby Pump Canal near Grand Lake, Colo. 179 1970–present 4 09014500 Shadow Mountain Lake near Grand Lake, Colo. 125 1989–present 5 09013000 Alva B. Adams Tunnel east portal, near Estes Park, Colo. 283 1970–present 6 402231105291900 Lake Estes near dam near Estes Park, Colo. 16 1998–present 7 06734900 Olympus Tunnel at Lake Estes, Colo. 247 1970–present 8 06742500 Carter Lake near Berthoud, Colo. 309 1970–present 9 06737500 Horsetooth Reservoir near Fort Collins, Colo. (Soldier Canyon Dam) 518 1969–present 10 403147105083800 Horsetooth Reservoir near Fort Collins, Colo. (Spring Canyon Dam) 10