20.2 Wildfire Impacts on Watersheds, Post-Fire Water Quality & Emergency Operations

10.3 Wildfire Impacts on Watersheds, Post-Fire Water Quality & Emergency Operations

Municipal drinking water utilities across Oregon and the Pacific Northwest rely heavily on forested, mountainous surface water watersheds draining the Cascade and Coast ranges. Historically, these heavily vegetated basins acted as natural filtration systems, producing pristine raw water characterized by low turbidity ($< 5\text{ NTU}$), low organic carbon, and high chemical stability.

However, a changing climate, prolonged summer droughts, and dense fuel loading have catalyzed catastrophic wildfire events. Operators must understand how severe wildland fires disrupt watershed hydrologic regimes, alter water treatment chemistry, pyrolyze distribution infrastructure, and require rigorous emergency operations planning.

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Wildfires in Oregon Watersheds: The 2020 Labor Day Fires

In September 2020, an unprecedented meteorological event—historic sustained east winds exceeding $50\text{ mph}$ coupled with single-digit relative humidity—ignited and spread massive wildfires across western Oregon. In less than 72 hours, the 2020 Labor Day Fires burned more than $1.0\text{ million acres}$ of prime forest lands.

                      [Major 2020 Oregon Watershed Fires]

       Fire Complex          Burned Area      Impacted Municipal Drinking Water Supply
       ─────────────────────────────────────────────────────────────────────────────
       Beachie Creek Fire    193,500 acres    North Santiam River (City of Salem)
       Lionshead Fire        204,400 acres    North Santiam & Deschutes Basins
       Holiday Farm Fire     173,000 acres    McKenzie River (Eugene - EWEB)
       Riverside Fire        138,000 acres    Clackamas River (North Clackamas & Providers)
       Archie Creek Fire     131,500 acres    North Umpqua River (Roseburg / Glide)

The Hydrophobic Soil Phenomenon

High-severity wildfires consume forest canopy and burn the protective organic duff layer down to bare mineral soil.

• Intense heat (temperatures exceeding $400^\circ\text{C}\text{ to } 800^\circ\text{C}$) vaporizes aliphatic hydrocarbon waxes present in burning plant material.
• These vaporized organic compounds migrate downward into the soil profile along the temperature gradient.
• Upon hitting cooler subsurface soil layers ($2\text{ to } 5\text{ cm}$ below the surface), the hydrocarbons condense and coat the mineral soil grains, forming an impermeable, waxy hydrophobic (water-repellent) layer.
• Hydrophobic soils eliminate natural stormwater infiltration. Rainwater cannot soak into the ground, causing almost $100\%$ of rainfall to become rapid surface runoff, generating catastrophic overland sheet erosion, mudslides, and destructive debris flows during subsequent autumn and winter rains.

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Post-Fire Water Quality Degradation & Treatment Challenges

Post-fire runoff delivers complex chemical and physical contaminants into streams and terminal reservoirs, creating severe treatment challenges.

1. Extreme Turbidity Swings & Colloidal Ash

• Pre-fire baseline turbidity in Cascade streams typically ranges from $0.2\text{ to } 3.0\text{ NTU}$.
• Following autumn rains on burned slopes, raw water turbidity surges instantaneously to $> 1,000\text{ NTU}$ (and occasionally $> 5,000\text{ NTU}$ during major debris flow events).
• Post-fire ash particles are ultra-fine, colloidal, and possess a high negative surface charge. These particles do not settle naturally and will blind raw water intake screens, silt up river intake wet wells, and rapidly overwhelm conventional sedimentation basins.

2. Dissolved Organic Carbon (DOC) Surge & Disinfection Byproducts

• Leaching of partially burned organic duff, pyrolized vegetation, and ash releases massive concentrations of Dissolved Organic Carbon (DOC) into surface streams.
• Water quality is characterized by elevated Specific Ultraviolet Absorbance at $254\text{ nm}$ ($\text{SUVA}_{254}$):

$\text{SUVA} = \frac{\text{UV}_{254}\text{ Absorbance (cm}^{-1}\text{)}}{\text{DOC (mg/L)}} \times 100\text{ L/mg}\cdot\text{m}$

• High $\text{SUVA}$ values ($> 3.0\text{ L/mg}\cdot\text{m}$) indicate that the organic matter is predominantly composed of complex, aromatic humic and fulvic macromolecules.
• When operators add chlorine for primary pathogen inactivation, these aromatic organics react aggressively with free chlorine, generating severe spikes in regulated Disinfection Byproducts (DBPs):
  • Total Trihalomethanes (TTHMs): Chloroform, bromodichloromethane, dibromochloromethane, bromoform (EPA / OHA MCL: $0.080\text{ mg/L}$ or $80\text{ ppb}$).
  • Five Haloacetic Acids (HAA5): Monochloro-, dichloro-, trichloro-, monobromo-, and dibromoacetic acids (EPA / OHA MCL: $0.060\text{ mg/L}$ or $60\text{ ppb}$).

3. Nutrient Loading & Secondary Harmful Algal Blooms (HABs)

• Forest ash contains high concentrations of orthophosphate ($\text{PO}_4^{3-}$) and nitrogen compounds (nitrate $\text{NO}_3^-$ and ammonium $\text{NH}_4^+$).
• Ash deposited into terminal water storage reservoirs spikes nutrient loading by orders of magnitude.
• The loss of riparian forest shade elevates stream and lake water temperatures by $2^\circ\text{C}\text{ to } 5^\circ\text{C}$. The combination of high nutrient availability, sunlight, and warm water creates ideal conditions for late-summer Harmful Algal Blooms (HABs).
• Proliferation of cyanobacteria (such as Microcystis aeruginosa and Dolichospermum) generates potent cyanotoxins (Microcystins, Cylindrospermopsin, Anatoxin-a). In Oregon, systems with sources OHA finds susceptible must follow the cyanotoxin rules (OAR 333-061-0510 to 0580), whose health advisory level for total microcystins is $0.3\ \mu\text{g/L}$ for vulnerable people.

4. Heavy Metals Mobilization

Severe soil heating leaches heavy metals naturally bound in soils. Post-fire runoff carries elevated levels of iron, manganese, aluminum, arsenic, and mercury. Soluble manganese ($\text{Mn}^{2+}$) oxidizes inside distribution mains, producing black water customer complaints, while dissolved aluminum precipitates on filter media, causing short filter runs and media cementing.

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Plastic Infrastructure Pyrolysis & Distribution VOC Contamination

A critical discovery in municipal water resilience—first documented after California's 2017 Tubbs Fire and 2018 Camp Fire, and since tested for in burned Oregon systems after the 2020 Labor Day fires—is the widespread chemical contamination of distribution networks caused by plastic pipe pyrolysis.

                    [Mechanics of Distribution VOC Contamination]

       High Ambient Heat (500°C - 1000°C) ──► Melts / Pyrolyzes Plastic Piping (HDPE, PVC, PEX)
                                                        │
       Severe System Depressurization     ──► Broken services & hydrant drafting create negative head
                                                        │
       Back-Siphonage Vacuum               ──► Draws pyrolyzed gases & smoke condensate into mains
                                                        │
       Polymer Absorption & Leaching       ──► Benzene permeates pipe walls; desorbs over months

Pyrolysis and Back-Siphonage Mechanics

1. Thermal Degradation: When a wildfire enters wildland-urban interface (WUI) neighborhoods, structural fires generate radiant heat exceeding $500^\circ\text{C}\text{ to } 1,000^\circ\text{C}$. Shallow-buried plastic service lines (high-density polyethylene - HDPE), residential water meters, cross-linked polyethylene (PEX) plumbing, and PVC distribution mains undergo thermal degradation (pyrolysis) without complete combustion.
2. System Depressurization: Thousands of melted customer service lines, destroyed home plumbing fixtures, and heavy fire department drafting at hydrants cause distribution line pressure to drop to zero or create severe negative hydraulic pressure (vacuum).
3. Back-Siphonage: The negative pressure sucks pyrolyzed gases, toxic vapors, and liquid smoke condensate backward from burned building footprints deep into the municipal distribution water mains.
4. Primary Contaminant: Laboratory testing reveals a complex cocktail of Volatile Organic Compounds (VOCs), dominated by Benzene, a known human carcinogen, alongside styrene, toluene, ethylbenzene, naphthalene, and xylenes.
5. Massive Exceedances: While the federal and Oregon Maximum Contaminant Level (MCL) for Benzene is $0.005\text{ mg/L}$ ($5\ \mu\text{g/L}$ or $5\text{ ppb}$), burned distribution systems have recorded benzene concentrations ranging from $0.100\text{ to }> 40.0\text{ mg/L}$—thousands of times higher than safe drinking water standards!
6. The Desorption Problem: Volatile hydrocarbons adsorb into the polymer matrix of remaining, unburned plastic pipes and elastomeric gaskets. Simple distribution flushing fails to clear contamination because benzene slowly desorbs from the plastic pipe walls back into stagnant water over weeks and months.
7. Remediation: Utilities must isolate damaged zones, excavate and replace all plastic service lines with copper or ductile iron, and perform extensive testing via EPA Method 524.2 (Purge and Trap Gas Chromatography/Mass Spectrometry) before restoring service.

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Operational Response & Water Treatment Adaptation

To treat post-wildfire raw water successfully, treatment plant operators must adapt coagulation, clarification, and adsorption processes dynamically.

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Emergency Management, ICS/NIMS & AWIA Compliance

Operational resilience requires structural integration into federal and state disaster response frameworks.

America's Water Infrastructure Act (AWIA) Section 2013

AWIA Section 2013 mandates that every Community Water System (CWS) serving more than 3,300 people must complete and certify two comprehensive planning documents to the EPA every 5 years:

1. Risk and Resilience Assessment (RRA): Evaluates system vulnerabilities to malevolent acts and natural hazards (including wildfires, earthquakes, floods, and severe drought). Assesses physical assets, piping, water treatment facilities, electrical/SCADA systems, financial infrastructure, and chemical storage.
2. Emergency Operations Plan (EOP): Details mitigation strategies, alternate water supplies, emergency power islanding, mutual aid activation, and physical/cybersecurity procedures to withstand assessed threats. In Oregon, emergency plans should be coordinated with local and county emergency management and with OHA Drinking Water Services.

Incident Command System (ICS) & NIMS Integration

During a major wildfire threatening a water utility, operations must transition into the Incident Command System (ICS) under the National Incident Management System (NIMS):

• Incident Commander (IC): Directs overall emergency response and policy.
• Safety Officer: Assesses hazardous conditions (wildfire smoke particulate $\text{PM}_{2.5}$, radiant heat, active evacuation routes) to safeguard field staff.
• Operations Section: Manages plant treatment adjustments, distribution valve isolations, water hauling, and fire flow distribution.
• Planning Section: Tracks fire spread models, monitors watershed weather forecasts, coordinates water quality sampling schedules, and plans resource needs for future operational periods.
• Logistics Section: Procures emergency diesel fuel, mobile generators, chemical deliveries, and food/lodging for operating crews.
• Finance / Administration Section: Meticulously logs operational labor hours, equipment usage, and contract expenses according to FEMA Disaster Assistance standards for post-event reimbursement.

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Public Health Drinking Water Advisories

When water quality or distribution integrity is compromised, utilities must coordinate with the Oregon Health Authority (OHA) to issue clear, tiered public health advisories.

                      [Comparison of Public Drinking Water Advisories]

       [Boil Water Advisory]   ──► Microbiological Pathogens (Pressure loss < 20 psi, coliform)
                                   Instructions: Boil rolling for 1 minute; kills microbes

       [Do Not Drink Notice]   ──► Chemical Toxins / Cyanotoxins / Nitrates
                                   Instructions: DO NOT BOIL (concentrates toxins); use for toilet/wash

       [Do Not Use Notice]     ──► Severe VOC Contamination (Benzene from plastic pyrolysis)
                                   Instructions: DO NOT USE TAP WATER (inhalation & skin absorption risk)

1. Boil Water Advisory (BWA)

• Trigger: Issued when microbiological contamination is confirmed or suspected (e.g., system pressure drops below $20\text{ psi}$, presence of E. coli or fecal coliform, or physical distribution breaches).
• Mandatory Public Instruction: Bring tap water to a vigorous, rolling boil for at least 1 full minute (3 minutes at elevations $>6,500\text{ ft}$) before consuming, cooking, or making ice.
• Scientific Principle: Boiling rapidly thermal-inactivates enteric pathogenic bacteria, viruses, and protozoan parasites (Giardia and Cryptosporidium).

2. Do Not Drink Notice

• Trigger: Issued when non-volatile chemical contaminants, cyanotoxins (HABs), heavy metals, or radiological contaminants exceed health advisory levels.
• Mandatory Public Instruction: Do not drink tap water, use it for beverage preparation, or brush teeth. Water may still be used for flushing toilets, showering (if non-irritating), and laundry.
• CRITICAL WARNING: DO NOT BOIL THE WATER! Boiling chemical- or cyanotoxin-contaminated water evaporates clean steam, which concentrates the toxic chemical in the pot, dramatically increasing ingested poison dosages.

3. Do Not Use Notice

• Trigger: The most extreme emergency advisory, issued when volatile, toxic chemical contamination—such as Benzene from plastic pyrolysis—is present in the distribution system.
• Mandatory Public Instruction: DO NOT USE TAP WATER FOR ANY PURPOSE. Do not drink, cook, wash hands, shower, bathe, or wash clothes.
• Scientific Principle: Benzene and volatile aromatic hydrocarbons rapidly volatilize into household indoor air during showering or washing, creating lethal toxic inhalation hazards. Furthermore, benzene penetrates skin via transdermal absorption. Consumers must not open plumbing fixtures; utilities must establish emergency bulk potable water dispensing stations or distribute bottled water.