3.2 Willamette Basin TMDL and Oregon Water Quality Standards

Willamette River Basin TMDL & Temperature, Bacteria, Nutrient Standards

When standard technology-based controls (such as secondary wastewater treatment) prove insufficient for a waterbody to meet state water quality criteria, the Clean Water Act shifts regulatory mechanics from technology-based limits to water quality-based standards. This transition is executed through Section 303(d) of the Clean Water Act and the development of Total Maximum Daily Loads (TMDLs).

In Oregon, water quality standards are codified under OAR Chapter 340, Division 041. Every two years, Oregon DEQ conducts a statewide assessment of river basins, lakes, and estuaries. Waters that fail to satisfy applicable numerical or narrative criteria for one or more beneficial uses (e.g., resident fish and aquatic life, salmonid fish spawning, or contact recreation) are placed on the state's Section 303(d) List of Impaired Waters.

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|                                 THE TMDL ALLOCATION EQUATION                            |
|                                                                                         |
|  TMDL = ∑ WLA (Point Sources) + ∑ LA (Nonpoint Sources) + MOS (Safety) + RC (Growth)   |
|            |                                |                         |           |     |
|            v                                v                         v           v     |
|     Municipal POTWs &                Agricultural Runoff,         Scientific   Future   |
|     Industrial Discharges            Forestry, Urban Overland     Uncertainty  Capacity |
|     (Enforced via NPDES)             (Managed via DMAs)           Buffer       Reserve  |
+-----------------------------------------------------------------------------------------+

A TMDL defines the maximum quantity of a specific pollutant that a receiving waterbody can assimilate on a daily basis while still achieving ambient water quality standards. Mathematically, a TMDL divides pollutant loading into distinct allocations: $\text{TMDL} = \sum \text{WLA} + \sum \text{LA} + \text{MOS} + \text{RC}$

• Wasteload Allocations (WLAs): The portion of the receiving water's assimilative capacity allocated to existing and future point sources (e.g., municipal wastewater treatment plants, industrial outfalls, and regulated municipal separate storm sewer systems [MS4s]). WLAs are legally enforced through enforceable effluent limits written directly into NPDES permits.
• Load Allocations (LAs): The portion allocated to nonpoint sources (e.g., agricultural drainage, commercial timberlands, and unchanneled urban runoff) as well as natural background sources.
• Margin of Safety (MOS): An analytical buffer that accounts for scientific uncertainty in hydraulic modeling, water chemistry interactions, and biological response.
• Reserve Capacity (RC): An optional allocation set aside to accommodate future population expansion and industrial growth without violating water quality standards.

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The Willamette River Basin TMDL & Designated Management Agencies

The Willamette River Basin TMDL is one of the most comprehensive watershed management programs in the United States. Spanning 11,478 square miles across northwestern Oregon, the Willamette Basin extends from the Cascade Mountains to the Coast Range, encompassing twelve major subbasins (including the Upper Willamette, McKenzie, North and South Santiam, Yamhill, Molalla-Pudding, Tualatin, Clackamas, and Lower Willamette). The basin is home to approximately $70\%$ of Oregon's population, major industrial centers, productive agricultural valleys, and vital migratory routes for Endangered Species Act (ESA) listed fish.

Issued by DEQ and approved by EPA in 2006, the Willamette Basin TMDL set basin-wide allocations for:

1. Stream Temperature
2. Bacteria (Escherichia coli)
3. Mercury

It also set dissolved oxygen, phosphorus and other allocations for particular subbasins, and the Tualatin River has its own TMDLs for temperature, bacteria, dissolved oxygen and phosphorus. DEQ has since revised parts of the Willamette TMDL; for example, the mercury TMDL was revised in 2019.

Designated Management Agencies (DMAs)

Because DEQ directly regulates only point sources via NPDES permits, the implementation of nonpoint Load Allocations (LAs) requires coordinated statutory authority. Under Oregon's TMDL rules (OAR 340-042), DEQ names specific Designated Management Agencies (DMAs) that possess legal jurisdiction to implement load reductions:

• Oregon Department of Agriculture (ODA): Administers Agricultural Water Quality Management Area Plans under Senate Bill 1010, regulating agricultural operations, vegetative riparian buffers, and livestock waste containment.
• Oregon Department of Forestry (ODF): Enforces the Oregon Forest Practices Act on state and private timberlands, mandating stream shade preservation, tree-retention buffers, and logging road stability to curb sediment and thermal loading.
• Cities and Counties (Municipal DMAs): Enforce local land use codes, stream setback ordinances, erosion control standards on construction sites, and municipal separate storm sewer system (MS4) stormwater management programs.
• Federal Agencies (USACE, USFS, BLM): The U.S. Army Corps of Engineers (USACE) operates thirteen multi-purpose dams across Willamette subbasins that alter natural seasonal flow regimes, water release temperatures, and downstream fish habitat.

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Parameter 1: Stream Temperature & Cold-Water Salmonid Protection

Water temperature is a critical physical parameter governing aquatic life in Pacific Northwest watersheds. Native anadromous salmonids—including Upper Willamette River spring Chinook salmon (Oncorhynchus tshawytscha), winter steelhead (Oncorhynchus mykiss), and bull trout (Salvelinus confluentus)—require cold water across all life stages. Elevated water temperatures induce thermal shock, inhibit growth, elevate susceptibility to diseases such as Ceratomyxa shasta and Columnaris, form behavioral thermal migration blocks, and cause pre-spawning mortality.

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|               OREGON BIOLOGICAL TEMPERATURE CRITERIA (OAR 340-041-0028)                 |
|                    [Evaluated as 7-Day Average Daily Maximum (7DADM)]                   |
|                                                                                         |
|  12.0°C (53.6°F)  Bull Trout Spawning and Juvenile Rearing                              |
|  13.0°C (55.4°F)  Salmon and Steelhead Spawning, Egg Incubation, and Fry Emergence      |
|  16.0°C (60.8°F)  Core Cold-Water Juvenile Salmonid Rearing                             |
|  18.0°C (64.4°F)  Salmon and Trout Rearing and Migration                               |
|  20.0°C (68.0°F)  Designated Migration Corridors (with cold-water refugia)              |
+-----------------------------------------------------------------------------------------+

Oregon Biological Temperature Criteria

Under OAR 340-041-0028, Oregon water quality standards establish biologically based temperature criteria evaluated using the 7-Day Average of the Daily Maximum (7DADM) metric:

• Bull Trout Spawning and Juvenile Rearing: 7DADM $\le 12.0^\circ\text{C}$ ($53.6^\circ\text{F}$).
• Salmon and Steelhead Spawning: 7DADM $\le 13.0^\circ\text{C}$ ($55.4^\circ\text{F}$) during designated seasonal spawning, incubation, and emergence windows.
• Core Cold-Water Salmonid Rearing: 7DADM $\le 16.0^\circ\text{C}$ ($60.8^\circ\text{F}$).
• Salmon and Trout Rearing and Migration: 7DADM $\le 18.0^\circ\text{C}$ ($64.4^\circ\text{F}$).
• Migration Corridors (such as the lower Willamette and Columbia): 7DADM $\le 20.0^\circ\text{C}$ ($68.0^\circ\text{F}$), with cold-water refugia distributed for migrating fish.

Thermal Wasteload Allocations (WLAs) for Municipal POTWs

Municipal wastewater effluent during the late summer months (July through September) typically discharges at temperatures ranging between $18^\circ\text{C}$ and $24^\circ\text{C}$, reflecting domestic warm water usage and ambient solar heating in aeration basins and clarifiers. Because receiving river temperatures during low summer flows often exceed $18^\circ\text{C}$ naturally, the river has zero assimilative heat capacity.

Under the Willamette TMDL, DEQ established thermal Wasteload Allocations that restrict all point sources combined to a cumulative human-use allowance of no more than $0.3^\circ\text{C}$ above natural background temperature. To prevent plant shutdowns during summer, operators implement distinct thermal compliance pathways:

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Parameter 2: Bacteria Allocations & Pathogen Control

The bacteria TMDL protects water contact recreation (e.g., swimming, wading, rafting) under OAR 340-041-0009. The regulatory organism in freshwater is Escherichia coli (E. coli):

• 90-Day Geometric Mean: $\le 126$ E. coli per 100 mL (at least five samples in 90 days).
• Single Sample Maximum: $\le 406 \text{ MPN/100 mL}$.

Point Source Control

Municipal POTWs are assigned Wasteload Allocations requiring effluent disinfection to meet criteria prior to discharge. Disinfection is achieved primarily through Ultraviolet (UV) Irradiation (delivering a minimum germicidal fluence, typically $> 30-40 \text{ mJ/cm}^2$) or Sodium Hypochlorite Chlorination followed by complete chemical dechlorination using sodium bisulfite. Disinfection systems must operate continuously or seasonally (typically May 1 through October 31, though year-round disinfection is required on reaches with year-round primary contact recreation).

Nonpoint Source Control

Nonpoint load allocations are managed by agricultural DMAs through livestock exclusion fencing (preventing cattle from defecating directly in riparian zones), off-stream watering troughs, and vegetative filter strips. Urban municipalities implement illicit discharge detection and elimination (IDDE) protocols to detect cross-connected sanitary sewers, alongside regular street sweeping and pet waste education programs.

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Parameter 3: Dissolved Oxygen, Ammonia & Nutrient Dynamics

Adequate dissolved oxygen (DO) is vital to aquatic organism survival. Oregon DO criteria (OAR 340-041-0016) specify that waters supporting cold-water aquatic life must maintain a minimum DO concentration of $8.0 \text{ mg/L}$, which increases to $11.0 \text{ mg/L}$ (or $95\%$ saturation) during active salmonid spawning periods.

Upper Basin vs. Lower Willamette / Portland Harbor Estuary

The dynamics of dissolved oxygen vary dramatically along the Willamette mainstem:

• Upper Willamette: Characterized by shallow, braided, high-velocity gravel beds with rapid natural atmospheric reaeration across riffles.
• Lower Willamette (River Mile 0 to 26.5): From Willamette Falls at Oregon City downstream through Portland Harbor to the Columbia River confluence, the river transitions into a deep, dredged shipping channel (depths $>40 \text{ feet}$) with slow velocities, prolonged hydraulic retention times, and tidal reversals influenced by the Columbia River. In late summer, these lake-like hydraulics severely limit atmospheric reaeration, making the lower river acutely susceptible to catastrophic dissolved oxygen sags.

+-----------------------------------------------------------------------------------------+
|               LOWER WILLAMETTE HYDRAULIC SINK & DISSOLVED OXYGEN SAG                    |
|                                                                                         |
|  Upper Basin: High Velocity, Shallow, Gravel Riffles  ===> Fast Atmospheric Reaeration  |
|                                                                                         |
|  Lower Basin: Deep Shipping Channel (40+ ft), Sluggish ===> Minimal Surface Reaeration; |
|               Tidal Backwater from Columbia River          High Risk of DO Sag          |
|                                                                                         |
|   Primary Drivers of Oxygen Depletion in Lower River:                                   |
|   1. Nitrogenous Oxygen Demand (NBOD): Raw ammonia consumes 4.57 lbs O2 per lb NH3-N    |
|   2. Carbonaceous Oxygen Demand (CBOD): Residual organic matter decomposed by bacteria  |
|   3. Phytoplankton Decay: Excess phosphorus fuels algal blooms; night respiration and   |
|      subsequent cell death consume dissolved oxygen                                     |
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Ammonia Oxidation & Biological Nitrification

Ammonia ($NH_3 / NH_4^+$) discharged in wastewater effluent creates two major environmental hazards: it is directly toxic to fish at low concentrations, and it exerts severe Nitrogenous Biochemical Oxygen Demand (NBOD) on the receiving stream. Autotrophic nitrifying bacteria consume dissolved oxygen to oxidize ammonia into nitrate in a two-step biochemical reaction:

1. Nitrosomonas bacteria oxidize ammonia to nitrite: $\text{NH}_4^+ + 1.5 \text{ O}_2 \rightarrow \text{NO}_2^- + \text{H}_2\text{O} + 2\text{H}^+$
2. Nitrobacter bacteria oxidize nitrite to nitrate: $\text{NO}_2^- + 0.5 \text{ O}_2 \rightarrow \text{NO}_3^-$

Phosphorus Control & Algal Kinetics

In nutrient-sensitive subbasins like the Tualatin, Yamhill, and Pudding Rivers, phosphorus is the primary limiting nutrient governing algal growth. Excess soluble reactive phosphorus discharged in warm summer months fuels explosive blooms of planktonic algae. While actively photosynthesizing during daylight, algae supersaturate surface waters with oxygen; however, during darkness, algal cellular respiration consumes oxygen. When the bloom dies, bacterial decomposition of algal biomass strips dissolved oxygen from the water column, creating acute nocturnal DO depressions.

Where TMDLs address algae and dissolved oxygen, as in the Tualatin subbasin, DEQ permits can carry stringent seasonal Total Phosphorus (TP) limits (for example, around $0.10 \text{ mg/L}$). Operators achieve these low limits using:

• Chemical Precipitation: Dosing metal salts—such as alum (aluminum sulfate, $Al_2(SO_4)_3$) or ferric chloride ($FeCl_3$)—to precipitate soluble orthophosphate into insoluble aluminum or iron phosphate flocs, followed by polymer addition and tertiary cloth-media or sand filtration.
• Biological Phosphorus Removal (BPR): Cycling activated sludge through an anaerobic selector basin followed by an aerobic basin to select for Phosphorus Accumulating Organisms (PAOs), which take up luxury amounts of orthophosphate that are permanently removed with waste activated sludge (WAS).

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Oregon Water Quality Standards: OAR Chapter 340, Division 041

In addition to numerical pollutant criteria, Oregon operators must understand the broader legal framework governing water quality standards under OAR Chapter 340, Division 041:

Antidegradation Policy (OAR 340-041-0004)

The antidegradation policy prevents existing clean waters from being degraded. It establishes three tiers of environmental protection:

• Tier 1 (Existing Uses): Protects all existing instream beneficial uses and the level of water quality necessary to maintain those uses. No discharge may cause a loss of an existing use.
• Tier 2 (High Quality Waters): Applies to waterbodies where water quality is cleaner than the minimum standards. Water quality cannot be lowered unless the applicant proves through an extensive Alternatives Analysis that lowering water quality is necessary to accommodate important economic or social development.
• Tier 3 (Outstanding Resource Waters [ORWs]): Applies to designated pristine waters Oregon's designated ORWs are Crater Lake, Waldo Lake and the North Fork Smith River (OAR 340-041-0004(8)). Existing water quality must be maintained and protected, and the basin rules bar new or increased NPDES discharges to them except in narrow cases.

Regulatory Mixing Zones (OAR 340-041-0053)

A mixing zone is a designated volume of water adjacent to an outfall where effluent mixes with receiving water. Within the mixing zone, ambient water quality criteria may be exceeded, subject to strict boundaries:

• Inside the mixing zone: the discharge may not cause acutely toxic conditions, objectionable deposits, floating scum or nuisance growths. Lethality in 100 percent effluent may be allowed only for ammonia and chlorine when immediate dilution removes it, using a zone of immediate dilution DEQ sets case by case.
• At and beyond the mixing zone boundary: no chronic toxicity, and all other water quality standards must be met under normal annual low flow.
• Mixing zone limits (OAR 340-041-0053): DEQ defines each mixing zone in the permit to be as small as feasible, avoid overlapping other mixing zones, stay narrower than the stream to allow fish passage, minimize harm to the biological community, not threaten public health, and minimize effects on other beneficial uses. Temperature mixing zones also protect spawning areas.

Statewide Narrative Criteria

Oregon rules prohibit the discharge of wastes that produce visible aesthetic nuisances or environmental degradation: waters must be free from floating debris, oil, scum, and slicks; free from deposits that form putrescent or objectionable sludge; free from toxic substances in concentrations that produce lethal or sublethal effects in aquatic life; and free from biostimulatory nutrients that stimulate offensive fungal, weed, or algal growths.