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Boulder Creek, Colorado


The Case Summary

Considerations for Using Ecological Restoration: Elevated Concentrations of Un-Ionized Ammonia

From its headwaters in the Southern Rockies, Boulder Creek flows rapidly through narrow, relatively deep channels where its clear, cool waters provide ideal habitat for aquatic communities. As the creek flows west toward Boulder, Colorado, it enters the western high plains. On these plains, Boulder Creek assumes a shallow, meandering character and is relied on as a domestic and agricultural water supply, for swimming and other water-based recreation, and as habitat for warmwater aquatic life.

The City of Boulder's 75th Street wastewater treatment plant serves 95,000 inhabitants. On average, the plant discharges 17 million gallons per day into Boulder Creek. While base flow in the creek at the point of discharge ranges from 10 to 30 cubic feet per second over 9 months of the year (Rudkin and Wheeler 1989), during periods of high withdrawals (i.e., the summer months) the creek is wastewater-dominated. This has greatly influenced water quality.

In 1985, the city of Boulder Department of Public Works needed to renew the wastewater treatment plant's discharge permit. A total maximum daily load developed to determine a wasteload allocation for un-ionized ammonia indicated the need to tighten the plant's ammonia discharge limits, because monitoring downstream from the plant indicated that un-ionized ammonia concentrations increased as the creek flowed downstream and, at times, exceeded the state's standard of 0.06 mg/L for warm-water streams.

The critical zone, in which the un-ionized ammonia concentration reached a maximum, occurred approximately 8.5 miles downstream from the plant (Rudkin and Wheeler 1989). In addition, a biological inventory of the 15.5-mile river segment below the wastewater treatment plant found that few of the 33 species of fish expected to inhabit this segment, including the greenback cutthroat trout, were present. The river segment was not fully supporting its aquatic life uses.

Stressors of Concern

Wastewater treatment plant effluent data, collected monthly from January 1982 through March 1985, showed no violations of the total ammonia permit limit from November through May of each year and only three violations from June through October. Effluent data also showed no exceedance of pH effluent limits that would contribute to the ammonia problem. This indicated either that the wastewater treatment plant's permit limits were not stringent enough or that the wastewater treatment plant was not the only problem within the watershed (EPA 1993d).

Additional monitoring and analysis indicated that potential aquatic life uses could not be achieved, even if discharges at the wastewater treatment plant were improved, because of the already-degraded physical condition of the creek habitat. Runoff, erosion, agricultural return flows, channelization, destruction of the riparian zone, and mining discharge each contributed to the problem. For example, at the time, over 70 percent of the 15.5-mile stretch below the wastewater treatment plant was channelized. Ideally, a stream should contain about 50 percent riffle and 50 percent pool to support aquatic life uses, but channelization in Boulder Creek shifted this ratio to 97 percent riffle and 3 percent pool. The long riffle zones were smooth and shallow. With little or no canopy, the water temperature rose to extreme levels. The transition from riffle to pool also often involves a small drop that increases water turbulence. These drops had also been largely eliminated. This combination of conditions greatly reduced the ability of the stream to reaerate naturally. Channelization shortened the length of Boulder Creek below the wastewater treatment plant from 30 miles to 22 miles, changing the creek's hydrology and increasing erosion and sediment loading (Channel 28, 1990). In addition, the shallow water depth and lack of riparian shading encouraged a lush growth of photosynthesizing aquatic vegetation. This vegetation, in turn, caused higher water temperatures and increased pH, conditions that favor conversion of ammonia to its toxic un-ionized form. Low alkalinity permitted the relatively large pH fluctuations to occur.

These stressors had to be addressed in order to lower the creek's temperature and pH significantly, thereby reducing concentrations of un-ionized ammonia. A study to evaluate the effectiveness of best management practices and restoration measures concluded that best management practices would enhance the effects of advanced wastewater treatment (Windell and Rink 1987c). The study also indicated that aquatic life uses could be attained if the aquatic and riparian habitats were restored, nonpoint source pollution was controlled, and poor land use practices were corrected. As a result, resource managers decided to restore Boulder Creek first, then develop a total maximum daily load for un-ionized ammonia, basing the wastewater treatment plant's wasteload allocation on a properly functioning ecosystem rather than the existing degraded ecosystem.

Improving instream water quality by using restorative techniques in the riparian zone in conjunction with traditional treatment methods was appealing for several reasons. The estimated cost was far less than the cost of relying on wastewater treatment plant upgrades alone, and improving the physical condition of the stream and its riparian zone would enhance the aesthetics of the creek, making it more appealing and useful to property owners. Also, if part of the enhanced area could be acquired by the city for use as a public park or greenway, it would add a valuable asset to the community.

The Goals for Restoration

The goals of the Boulder Creek Enhancement Project are alleviating of the un-ionized ammonia problem, restoring of full use of the river reach as a warmwater fishery, and maximizing the impact of expensive modifications at the treatment facility.

Restoration Techniques

Controlling Point Sources to Restore Chemical Integrity

The first step of the project was to improve the quality of effluent at the wastewater treatment plant. This played an important role in restoring the chemical component of stream integrity. In 1991, the City of Boulder upgraded and expanded its 75th Street wastewater treatment plant to meet the stricter discharge limits required in its 1986 National Pollutant Discharge Elimination System permit. Solid and liquid waste treatment were improved to provide high-quality secondary effluent to the nitrification trickling filter, which was added to increase removal of ammonia from the liquid waste stream. The improvements also reduced total suspended solids and biological oxygen demand to levels significantly below permit requirements.

Riparian Zone and In-stream Habitat Restoration

The second and third steps of the Boulder Creek Enhancement Project improved the riparian zone along the river and restored instream habitats. These steps were completed in phases.

Phase I, which was completed in the spring of 1990, involved designing and implementing six best management practices over a 1.3-mile reach that passed through the center of a heavily grazed cattle ranch. These best management practices included constructing high-tensile, wildlife-compatible fencing to exclude cattle from the riparian habitat; stabilizing streambanks using log revetments; planting crack willow and cottonwood trees in the riparian zone; replacing channelized berms with sculpted or terraced streambanks; excavating one-half mile of the thalweg (i.e., the deepest part of the channel) on concave meander bends; and creating three boulder aeration structures (EPA 1992).

A monitoring program was established to evaluate the combined effect of the best management practices and the individual impact of each. Baseline data were collected prior to best management practice construction, during construction, and after implementation. Instream monitoring included monthly sampling for water quality, flow, and temperature, as well as fish inventories and evaluation of canopy density, ground water levels, and physical habitat.

Fencing off the riparian zone was critical. If cattle had not been excluded, the impact of all other best management practices would have been minimal. Under a protective easement from the landowner, 40 acres was fenced using stretched steel wire on hammered posts to provide a 120-foot-wide buffer between grazing land and the stream. Cattle crossings, designed as rigid, hinged double gates, excluded cattle entirely and could be opened to provide a temporary corridor across the creek. Sections of fencing, specifically the permanent cattle crossings, had to be redesigned since they were subject to water-borne debris and runoff. PVC mesh suspended from cables now allows debris and boaters to pass under the fence while acting as a visible barrier to cattle.

Phase II, which was completed in 1991, restored 1.1 miles of Boulder Creek. Phase II reduced the impact of return flow from an irrigation ditch by rerouting it through existing and constructed wetlands (EPA 1992). Although cattle grazing along the Phase II reach did not pose a serious problem, streambank revegetation was badly needed. Because the individual plantings used to revegetate Phase I were only moderately successful, Phase II tested "wattles" and "brush layering." Wattles are horizontal bundles of willow cuttings buried at or near the creek bank. Brush layering is the backfilling of willows into the streambank parallel to the water surface, with the growing tips projecting into the stream (Rudkin 1992). Construction of rock/willow jetties to break up erosive currents was also tested. This method was less expensive and time consuming than using riprap or other traditional construction methods.

Phase III added an additional one-half mile to the project. No cattle were trampling the creek and its banks in this section, and the channel was not as severely eroded, but the adverse effects of surface gravel mining posed a new challenge. The plan called for biotechnical streambank stabilization, revegetation, and creation of wetlands. A chief aspect of this phase was to reduce channel abrasion by creating low-flow channel over approximately 0.25 miles of the project area. In addition, the plant species and planting methods used in Phases I and II were reevaluated.

Phase IV of the demonstration project was under design in 1993. It involved a 1.7-mile reach that would bring the total length of restored creek to 4.6 miles. Phase IV plans included aspects of the first three phases and incorporated the design changes made after evaluating the effectiveness of previous methods. Results from the first three phases supported expanded use of riparian plantings combined with the use of rock buttresses placed to protect vegetation in the earlier stages of their development. A unique aspect of the Phase IV plan was the use of abandoned gravel mines to remove solids from runoff (EPA 1992). The basins would discharge to wetlands to polish the runoff water before it enters the stream.

Issues of Cost

Overall project cost has included the costs of gathering data for planning and evaluating results, construction, materials, labor, and time. Funding for these activities has come from federal, state, city, local, and private organizations. The value of the project has also been augmented by donations of labor, time, and materials.

Monitoring is being conducted by a variety of agencies. U.S. EPA Region 8 is assisting the cities of Boulder and Longmont with instream monitoring costs. City officials authorized funding for two long-range planning studies, a use attainability study, two water quality studies, and a feasibility study. Two monitoring studies were funded by the University of Colorado Undergraduate Research Opportunities Participation Program (Windell and Rink 1992). The first was a $700 study on the interaction of riparian vegetation and water temperature. The second study, costing $2,500, covered follow-up monitoring of nonpoint source pollution controls after implementation. One study on the interaction of riparian vegetation, temperature, and fish population in Boulder Creek was funded for $2,500 by the W.L. Sussman Foundation (Windell and Rink 1992). Monitoring data are also provided by the U.S. Geological Survey and the Colorado Water Quality Control Division. The wastewater treatment plant monitors and reports effluent flows and concentrations as part of the permitting process. A portion of the funding for modeling was provided by U.S. EPA.

The 1991 upgrade of Boulder's 75th Street wastewater treatment plant was the largest capital project in the city's history, costing $23 million. 76 percent of the total improvement cost ($17.5 million) was expended to remove additional ammonia; the remainder was spent on sludge processing and disposal. Costs of the treatment plant upgrade were covered by the City of Boulder, with some assistance from the U.S. EPA Construction Grants program.

The total funding for Phase I of the demonstration project was $125,000. Colorado provided 60 percent of this amount under the state's nonpoint source control program; the remaining 40 percent was provided by the city of Boulder. With donated time, labor, and materials, the total worth of Phase I is estimated at $426,000 (Windell et al. 1991). Phase II funding, at $125,000, was similar to that of Phase I (Windell and Rink 1992). Phase III of the project was funded for $75,000 (Windell and Rink 1992), and Phase IV is estimated as having an on-the-ground budget of $225,000. The total cost of the completed enhancement project is currently estimated at $1.3 to $1.4 million (R.E. Williams, Assistant Director of Public Works for Utilities, City of Boulder, personal communication, March 28, 1991).

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