Watershed Management and NPDES Permitting Complexities with Nutrients for the City of Medford, Oregon

One of US EPA's priorities for the National Pollutant Discharge Elimination System (NPDES) program is to establish technology-based effluent limits (TBELs) and water quality-based effluent limits (WQBELs) for total phosphorus (TP) and total nitrogen (TN). As detailed in a National Association of Clean Water Agencies' report that reviewed US EPA methods for establishing WQBELs for nutrients, these limits are one of the chief regulatory and economic issues that wastewater utilities have and continue to face (NACWA, 2014). While there have been numerous attempts to evaluate TBELs, none of the studies have identified an approach that can be broadly applied and state program managers settle on various levels of control such as one milligram per liter (mg/L) monthly average and 1.5 mg/L weekly average for TP (Ohio Lake Erie discharges); 0.5 mg/L seasonal geometric mean for TP (Illinois), etc. Similarly, state managers recognize that nonpoint source contributions and other environmental conditions must be considered when establishing WQBELs (Illinois). In other instances, very stringent WQBELs have been established by nutrient total maximum daily loads (TMDLs) including the Chesapeake Bay, Truckee River, and Boise and Snake Rivers or even in the absence of a TMDL, such as the Blackstone River which was one of the first permitting cases for translation of narrative criteria in a NPDES permit.

The City of Medford, Oregon (City) was the first utility where the Oregon Department of Environmental Quality (DEQ) applied the state's biological criteria (biocriteria) to establish TP and TN WQBELs. DEQ used the results of various water quality studies of the Rogue River to establish a linkage between the recognized limiting nutrient, TN, but also for TP. Due to the complex relationship between nutrient loads from the regional water reclamation facility (RWRF), non-point sources, and other environmental conditions on the biota (macroinvertebrates), the proposed limits were challenged by the City of Medford and the Northwest Environmental Advocates. Ultimately, the final permit included interim and final TN and TP limits; an optimization study for the RWRF (due September 2022); a draft facility plan for the RWRF upgrades (due September 2023); construction of projects to achieve the final TN and interim TP limits (due September 2028); another optimization of the RWRF (due September 2029); achievement of the final TN and interim TP limits (by September 2030); and subsequent upgrades to achieve the final TP limit (by September 2035).

The permit also included requirements for a stressor analysis in accordance with the US EPA's Causal Analysis/Diagnosis Decision Information System (CADDIS,) which must be submitted by September 2025. The CADDIS study is designed to evaluate various physical and chemical stressors (including nutrients) that could be managed to address the macroinvertebrate based biocriteria impairment (see Figure 1). The study is also required to address the impact of stressors on periphyton biomass (cell density and biovolume) downstream of the RWRF's outfall. Consistent with the CADDIS approach, a data gap analysis was performed on the existing studies that DEQ relied on for the permit decision (see Table 1). Based on the analysis, co-located biological, chemical, and physical data was collected between March and October of 2023 both upstream and downstream of the RWRF outfall. In analyzing the data, there are 12 candidate causes and 11 types of evidence, for a total of 132 lines of evidence. Results of the analysis of the lines of evidence are shown in Figure 2. At the time of this submittal, data analysis is ongoing.

In addition to the CADDIS study, the City is also required to conduct additional monitoring of the river through the Rogue River Ambient Monitoring Plan (RRAMP). The RRAMP monitoring frequency, parameters, and methods will be informed by the CADDIS study and the timeline for monitoring must include both pre- and post-implementation final TN and TP limits. While the permit does not require monitoring for pre- and post-implementation of projects to achieve the interim TN and interim TP limits, the City plans to incorporate this into the RRAMP.

The City also participates in temperature trading in the Rogue Basin which was initiated in 2011. In 2019, US EPA disapproved Oregon's natural condition for temperature which serves as the foundation of temperature TMDLs and trading programs in the state. DEQ is therefore embarking on an update of temperature TMDLs and the Rogue River is schedule for 2026. The challenges associated with meeting the future temperature TMDL remain to be determined; however, the linkage between temperature, nutrients, and attainment of the biocriteria are difficult to understand. This presentation will provide an overview of the work conducted to date and the plans for the future. It will also include a discussion of the pros, cons, and challenges of embodying watershed studies within the context of an individual NPDES permit. The City's experience will help other dischargers and permit writers consider options for navigating permit conditions associated with nutrient limits intended to protect biological uses in surface waters.