Abstract

Elevated selenium concentrations in streams are a water-quality concern in western Colorado. Sweitzer Lake was placed on the State 303(d) list as impaired with respect to dissolved selenium. In Colorado, the Water Quality Control Division (Division) of the Colorado Department of Public Health and Environment is required to develop total maximum daily loads of selenium for the 303(d) list segments. The U.S. Geological Survey, in cooperation with Colorado Department of Public Health and Environment, summarized selenium loading from surface water and groundwater to Sweitzer Lake to support the total maximum daily loads development process. Surface-water and groundwater data were collected to quantify selenium concentrations and loads to Sweitzer Lake from October 2006 to October 2007. These data were used to determine the amount of the selenium load (pounds annually) that would need to be reduced for the contributing sources (surface water and groundwater) to meet the State chronic aquatic-life standard of 4.6 micrograms per liter for dissolved selenium, herein referred to as “a load reduction.” Selenium concentration data were also compared to the State acute aquatic-life standard of 18.4 micrograms per liter for dissolved selenium. Both surface-water and groundwater-quality samples collected during this study were found to exceed the chronic standard. Surface-water quantity and quality data were collected at Garnet Canal Diversion and Diversion Drain. Groundwater flux data were collected at 10 seepage-meter sites in Sweitzer Lake, and groundwater-quality data were collected at a groundwater seep (seep-1) and inferred from a January 2007 sample collected at Garnet Canal Diversion. Selenium concentrations and loads were greater at Garnet Canal Diversion than those observed at Diversion Drain. Approximately one-third of Garnet Canal Diversion’s streamflow originates from Loutzenhizer Arroyo. Selenium concentrations observed during previous studies at Loutzenhizer Arroyo indicate high selenium concentrations and high selenium loads. All selenium concentrations in samples from Garnet Canal Diversion were greater than the chronic standard and were less than the acute standard (18.4 μg/L) during the irrigation season. Seventy-three percent (or 35.9 pounds) of the annual selenium load at Garnet Canal Diversion would need to be reduced in order to meet the chronic standard. All daily mean selenium concentrations and selenium-concentration samples were greater than the chronic standard at Diversion Drain, but were less than the acute standard during the irrigation season. Forty percent (or 7.32 pounds) of the mean annual selenium load at Diversion Drain would need to be reduced in order to meet the chronic standard. Estimated groundwater selenium loads and reductions of selenium loads to Sweitzer Lake were estimated using ranges of lake-bottom areas with positive groundwater flux and groundwater selenium concentrations. Estimated annual groundwater selenium load reductions ranged from 0.900 pound of the 1.17 pounds of annual load to 86.3 pounds of the 88.3 pounds of annual load for the selenium concentration scenarios over the range of lake-bottom areas. Groundwater selenium load and load reductions determined from this study identify the probable minimum and maximum values for these parameters. Further data collection and analysis could refine the range of groundwater selenium loads and load reductions.

First posted January 22, 2010