SOURCE WATER QUALITY PLANNING FOR BOULDER, COLORADO

The Phase 1 Source Water Quality Planning Study (Phase 1 Study) was completed for the City of Boulder (City) in April 2003. The purpose of the Phase 1 Study was to identify and recommend alternative approaches to improving and protecting source water for the Boulder Reservoir Water Treatment Facility (BRWTF). A large impetus for this study is the emerging regulations concerned with reducing the potential for occurrence of microbial pathogens, such as Cryptosporidium.The primary source of BRWTF raw water is Carter Lake. Raw water is delivered through a 21-mile open canal known as the Boulder Feeder Canal (BFC). The BFC discharges water either directly to the BRWTF or to Boulder Reservoir for later pumping to the BRWTF.The BFC generally follows a north to south route traversing the lower slopes of the foothills. As such, it tends to capture a significant amount of surface runoff that originates uphill and to the west. Although a riparian habitat along the canal could, to some extent, naturally attenuate contamination, the channel bottom and banks are regularly maintained to prevent growth of vegetation. Therefore, although the raw water at its source in Carter Lake is of a very high quality, significant degradation generally occurs as the water travels the length of the BFC.Boulder Reservoir is a shallow, Class 1 warm water reservoir with a surface area of approximately 700 acres and a storage capacity of about 13,270 acre-feet. It is used year-round for a variety of recreational activities, and therefore is subject to water quality degradation resulting from both body and non-body contact activities such as swimming and boating.During the summer, a natural temperature stratification occurs and a hypolimnetic layer low in dissolved oxygen forms at the bottom of the reservoir. This condition causes the uptake of soluble manganese from the reservoir sediment into the water and results in taste and odor treatment concerns at the BRWTF. In addition, the reservoir water is high in total dissolved solids (TDS), especially sodium, sulfate, and hardness. Wind action across the lake can increase the raw water turbidity up to 150 nephelometric turbidity units (ntu). Due to the shallow nature of the reservoir, algal blooms can occur throughout the year.The resulting variable water quality delivered to the BRWTF can pose significant treatment challenges to the City. The BRWTF staff must frequently adjust treatment operations in response to raw water quality changes from weather and human or animal activities. Operational changes in treatment are also required each time the raw water supply is switched between the BFC and Boulder Reservoir.Several source water management alternatives were explored as part of this study. These included:Utilizing Boulder Reservoir as a year-round terminal reservoir. Management approaches associated with this alternative included:Installing an aeration system to raise the dissolved oxygen concentration in the reservoir to above 5 milligram per liter.Constructing bioretention facilities at all the creek confluences.Installing structural storm water treatment systems on all pipe outfalls draining into the reservoir.Using landscape-based treatment facilities to treat runoff from developments and golf courses and irrigation start-up flows.Planting riparian vegetation along the banks of the reservoir and creeks.Protecting the reservoir outlet from recreational use impacts by implementing a 500-foot protective zone.Preparing a control program to establish appropriate guidelines for reservoir activities.Construction of a new 21-mile raw water pipeline that would contain, protect, and deliver Carter Lake water directly to the BRWTF on a year-around basis using gravity flow.Construction of a pipeline through the more highly developed areas to the north of the BRWTF. Above the pipeline, the BFC would remain in service as an open canal. Under this approach the pipeline would only be in operation during the months when the BFC was in operation. During the remainder of the year, raw water would have to be pumped from Boulder Reservoir, as is the current practice.Construction of a new earthen terminal reservoir upstream of the Boulder Reservoir and the BRWTF. This approach would avoid the water quality conflicts inherent in using Boulder Reservoir for both recreation activities and a drinking water supply.Construction of a new earthen forebay upstream of the Boulder Reservoir and the BRWTF. This approach would provide some amount of dilution and settling to reduce the contaminant load entering the BRWTF.The alternatives were evaluated based on providing the highest quality and most cost-effective means of delivering source water to the BRWTF.