Water-Quality Monitoring of the Vallecito Reservoir Watershed

Science Center Objects

The water quality of the Vallecito Reservoir watershed in southwestern Colorado was initially studied by the U.S. Geological Survey (USGS) during July 1996 to July 1997, in cooperation with the Pine River Irrigation District, to assess the suitability of the reservoir as a potential source of drinking water (Ranalli and Evans, 1999). As a result of this study, in December 1997 a group of residents in and around Bayfield, Colo., created the Pine River Watershed Stakeholders Group.

One water-quality issue identified by the stakeholder group is to increase the understanding of the current water quality of Vallecito Reservoir, its two major inflows (Vallecito Creek and Los Piños River), and its outflow (Los Piños River) downstream from the reservoir. 


    The U.S. Geological Survey (USGS), in cooperation with volunteers from the Pine River Watershed Stakeholders Group and the U.S. Environmental Protection Agency (USEPA), U.S. Bureau of Reclamation (BOR), Colorado Department of Public Health and Environment (CDPHE), Pine River Irrigation District, Southern Ute Tribe, San Juan Basin Health Department, and San Juan Resource Conservation and Development, collected water-quality samples from Vallecito Reservoir, its two major inflows, and its outflow between August 1999 and November 2002 at about monthly intervals from April through November. The water-quality samples were analyzed for total and dissolved metals (aluminum, arsenic, cadmium, copper, chromium, iron, lead, manganese, mercury, nickel, silver, and zinc), dissolved major ions (calcium, magnesium, sodium, potassium, chloride, bicarbonate, and sulfate), dissolved silica, dissolved organic carbon (DOC), ultraviolet (UV) absorbance at 254 and 280 nanometers, nutrients (total organic nitrogen, dissolved organic nitrogen, dissolved ammonia, dissolved nitrate, total phosphorus, dissolved phosphorus, and orthophosphate), chlorophyll-a (reservoir only), and suspended sediment (inlets to the reservoir only). Measurements of field properties (pH, specific conductance, water temperature, and dissolved oxygen) were also made at each sampling site each time a water-quality sample was collected.

    The overall objectives of water-quality monitoring were to:

    1. characterize the current water quality of the study area
    2. periodically monitor the water quality of the study area over time to document trends in water quality and the causes for the trends if they occur.