Arkansas River Basin, Colorado

Pueblo Reservoir is a 357,000 acre-foot reservoir, located about six miles west of Pueblo, Colorado on the main stem of the Arkansas River. The reservoir is owned and managed by the US Bureau of Reclamation. Water from Pueblo Reservoir is used as a water supply for three drinking water treatment facilities, including the Pueblo Board of Waterworks. Treated water is then supplied to six different service areas. The reservoir is designated as a Direct Use Water Supply (DUWS) reservoir by the Colorado Water Quality Control Commission (WQCC) due to the direct connection between the reservoir and the water treatment plants.

Disinfection byproducts (DBPs) in finished drinking water can form via a reaction between disinfectants used in the treatment process and organic matter in the source water. Certain groups of DBPs, such as total trihalomethanes (TTHMs) and haloacetic acids (HAA5s), are regulated by the Safe Drinking Water Act due to serious health risks. The current maximum contaminant level (MCL) for TTHM is 80 ug/L and 60 ug/L for HAA5. Data indicates that DBP MCLs have been exceeded in four of the six distribution systems that use Pueblo Reservoir as a raw water source. Studies have shown a strong linkage between increased chlorophyll a concentrations (algae biomass) in source water and the formation of DBPs.

In an effort to manage organic matter concentrations in Pueblo Reservoir and thus, DBP formation in the distribution systems, the Pueblo Board of Waterworks retained Hydros staff in 2012 to provide technical support in developing an appropriate site-specific chlorophyll a standard for the reservoir. Using site-specific data and general guidance from the Water Quality Control Division (WQCD), Hydros staff developed a technical rationale and a proposed chlorophyll a standard, which was supported by WQCD staff. Hydros staff wrote an expert report regarding the proposal and provided testimony during the 2013 rulemaking hearing. The standard was adopted by the WQCC and the reservoir continues to be in attainment.