Adaptive Management: Developing Staged TMDL Implementation in Rapidly Urbanizing Watersheds

The lower Boise River watershed is a physically complex tangle of river, tributaries, irrigation conveyances that are increasingly affected by a rapidly growing population and land use conversion. Furrow irrigated lands are being rapidly converted to urban/suburban uses (e. g., 3000+ acres annually). The rapid conversion of irrigated agricultural lands into urban uses will significantly impact development and implementation of new and existing Total Maximum Daily Loads (TMDLs) and the attainment of water quality goals. Specifically, land use conversion will cause most agriculture-related discharges to be replaced with new urban stormwater and sanitary sewer waste water discharges. A TMDL implementation plan that meets water quality standards is necessary for the existing and anticipated change from non-point source agricultural to point source urban land use over a long-term (70 year) timeframe. Similar to other rapidly urbanizing watersheds in the western United States, the lower Boise River watershed represents a case where it is important to establish a TMDL implementation plan in a timely manner, but the data and analysis required to support the allocations will be substantially improved in the future. The lower Boise River TMDL implementation plan relies on "staged implementation," as referenced in EPA's 2006 Phased TMDL Clarification memo. EPA (2006) asserts that where the wasteload allocation to point sources is predicated on long-term reductions from sources that are outside the control of the point sources (the EPA example in the 2006 memo is atmospheric mercury deposition), that the appropriate terminology is "staged implementation." In the case of the lower Boise River, "staged implementation" is the appropriate classification of this process, recognizing that the SR-HC TMDL target can only be achieved because of the land use conversion from agricultural to urban land uses, coupled with population growth. The SR-HC TMDL attainment strategy relies on immediate actions that reduce sanitary sewer discharges of phosphorus, followed by an array of long-term actions such as implementing stormwater management programs, reducing sanitary sewer discharges, monitoring to fill in current data gaps on source loading, as well as receiving water monitoring and experimentation for watershed model refinement. Thus, the staged implementation plan seeks to provide an allocation scheme which meets the SR-HC TMDL target through implementation actions that will be staged over a period of time. A watershed mass balance model was developed to help define the impact of land use conversion on allocations at multiple milestones during implementation stages. The model is used to analyze, develop, and hopefully obtain acceptance of a proposed allocation approach. This approach is based on meeting the SR-HC TMDL target through a hybrid approach that ensures allocations are provided in a fair, achievable and cost-efficient manner. This approach is expected to ensure that resources are spent wisely while achieving the water quality goals of the watershed. At the time of the writing of this paper, the proposed approach is included in a preliminary draft of the staged TMDL implementation plan that is currently under review by the Lower Boise Watershed Council.