4.1 Raw Water Sources, Intake Structures & Pretreatment

3.1 Raw Water Sources, Intake Structures & Pretreatment

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Oregon Raw Water Resources & Regional Hydrology

Oregon features four distinct hydrological provinces, each presenting unique water quality characteristics and operational challenges for drinking water treatment facilities:

1. Cascade Range Snowpack-Fed Basins

Rivers originating in the high Cascades—such as the Clackamas River, North and South Santiam Rivers, and McKenzie River—provide municipal drinking water to over a million Oregonians. These surface waters drain volcanic basalt, andesite, and glacial terrain:

• Baseline Water Quality: Exceptionally pure under baseflow conditions, characterized by ultra-low total dissolved solids (TDS $< 50 \text{ mg/L}$), low natural turbidity ($< 1.0-3.0 \text{ NTU}$), cold water temperatures ($3-10^\circ\text{C}$), and very low total alkalinity (often $10-20 \text{ mg/L as } \text{CaCO}_3$).
• Operational Challenges: During autumn rain-on-snow events and spring snowmelt freshets, these streams experience rapid, violent turbidity spikes ($> 200-500 \text{ NTU}$) composed of colloidal clay, volcanic ash, and glacial flour. Furthermore, their low baseline alkalinity provides virtually no buffering capacity against acidic coagulants like aluminum sulfate.

2. Willamette Valley Alluvial Aquifers

Groundwater systems in the Willamette Valley are situated within unconfined and semi-confined river gravel and sand deposits:

• Baseline Water Quality: Moderate hardness ($60-150 \text{ mg/L as } \text{CaCO}_3$), stable temperatures ($11-14^\circ\text{C}$), and high clarity.
• Operational Challenges: Anoxic geochemical conditions frequently dissolve substantial concentrations of reduced ferrous iron ($\text{Fe}^{2+}$) and manganous manganese ($\text{Mn}^{2+}$). Shallow unconfined aquifers are also vulnerable to anthropogenic contamination, including agricultural nitrate ($\text{NO}_3^-$) from fertilizer applications and localized volatile organic compounds (VOCs).

3. Coastal Surface Basins

Rivers draining the Oregon Coast Range—such as the Nehalem, Siletz, Yaquina, and Coquille rivers—flow through temperate rainforests with annual precipitation exceeding $80-120 \text{ inches}$:

• Baseline Water Quality: Naturally acidic to neutral pH ($6.0-7.0$), low hardness, and moderate turbidity.
• Operational Challenges: Abundant decaying forest litter yields exceptionally high concentrations of Natural Organic Matter (NOM), including humic and fulvic acids. This produces elevated True Color ($> 40-80 \text{ Pt-Co units}$) and high Total Organic Carbon (TOC) levels ($4.0-10.0 \text{ mg/L}$), creating extreme Disinfection Byproduct (DBP) precursor loading when exposed to free chlorine.

4. High-Desert Groundwater & Volcanic Aquifers

East of the Cascade crest, the Deschutes Basin, Harney Basin, and regional basalt formations govern drinking water supplies:

• Baseline Water Quality: Low organic content, excellent microbiological clarity, and stable year-round yields.
• Operational Challenges: Deep volcanic contact zones introduce high dissolved silica ($30-60 \text{ mg/L as } \text{SiO}_2$), which can foul membrane systems, as well as localized geothermal signatures including naturally occurring arsenic ($\text{As}$), boron, and elevated fluoride ($\text{F}^-$) that require specialized adsorption or ion exchange processes.

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Source Water Protection & Sanitary Surveys (OAR 333-061)

Drinking water source protection

DEQ and OHA jointly run Oregon's drinking water source protection program. They have delineated source areas and produced source water assessments that inventory potential contaminant sources (septic systems, fuel storage, agriculture, timber harvest, upstream wastewater outfalls, transportation corridors) and rate susceptibility. Local protection plans are voluntary. A groundwater system may also have a wellhead protection program certified by OHA (OAR 333-061-0057). For surface water, the supplier must survey the watershed for natural and human factors affecting quality before siting an intake (OAR 333-061-0050(3)).

Sanitary surveys (OAR 333-061-0076)

OHA conducts a sanitary survey of every community, non-transient non-community and transient non-community system at least every five years, at a frequency it determines. The report covers eight components:

1. Source: intake structures, well casings and seals, springs, protection from flooding and surface water.
2. Treatment: chemical feed equipment, separation of treated from untreated water, turbidity monitoring locations, CT, membrane integrity testing, UV alarms.
3. Distribution system: pressure of at least 20 psi at all service connections at all times, main repair practices, cross connection control, flushing.
4. Finished water storage: secure, watertight roofs and hatches, screened vents, flap valves or screens on drains and overflows.
5. Pumps, pump facilities and controls: suction-side pressure protection on booster pumps, standby power, controls.
6. Monitoring, reporting and data verification: sampling plans, analyzer calibration, monthly reports.
7. System management and operation: O&M practices, emergency planning, records.
8. Operator compliance: an operator in Direct Responsible Charge at the required level.

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Raw Water Intake Engineering

Intake structures must physically extract water while excluding debris, protecting aquatic organisms, and dampening raw water quality fluctuations.

+-----------------------------------------------------------------------------+
|                        RAW WATER INTAKE SPECTRUM                            |
+-----------------------------------+-----------------------------------------+
|      SURFACE WATER INTAKES        |          SUBSURFACE INTAKES             |
|  - Submerged Intake Cribs         |  - Ranney Collector Wells (Radial)      |
|  - Velocity Caps (< 0.5 ft/s)     |  - Infiltration Galleries               |
|  - Coarse Bar Racks (2-4 in)      |  - Natural Riverbank Filtration (RBF)   |
|  - Traveling Water Screens        |  - Zero Fish Impingement                |
|  - Vulnerable to Ice & Floods     |  - Attenuates Turbidity & Temp Spikes   |
+-----------------------------------+-----------------------------------------+

Surface Water River Intakes

• Submerged Intake Cribs: Concrete or steel structures anchored in deep, stable river channels. Equipped with velocity caps that redirect vertical suction into horizontal inflow planes. Where juvenile salmon and steelhead are present, National Marine Fisheries Service fish screen criteria limit the approach velocity at the screen face to about 0.4 ft/s for actively cleaned screens and 0.2 ft/s for passive screens, to prevent impingement and entrainment.
• Bar Racks (Trash Racks): Heavy, parallel steel bars spaced $2 \text{ to } 4 \text{ inches}$ apart at the intake mouth. Bar racks physically exclude tree limbs, root wads, river stones, and large debris transported during flood stages.
• Traveling Water Screens: Positioned inside the raw intake pump well downstream of bar racks. These comprise continuous loops of motorized wire cloth trays (mesh openings typically $1/4 \text{ to } 3/8 \text{ inch}$). When debris accumulates and causes a predetermined differential water level across the screen ($2-4 \text{ inches}$ of head differential), the screen automatically rotates upward. High-pressure spray headers ($60-80 \text{ psi}$) blast trapped leaves, algae, and river trash into a collection sluice trough.

Subsurface Intakes: Ranney Collectors & Infiltration Galleries

To avoid surface debris, fish entrainment, and volatile water quality, many Pacific Northwest utilities utilize subsurface extraction:

• Ranney Collector Wells (Radial Collector Wells): A central reinforced concrete caisson (typically $10-16 \text{ ft}$ internal diameter) is sunk into permeable alluvial gravels adjacent to or beneath a riverbed. Perforated stainless steel lateral well screens are jacked horizontally outward through wall ports deep into the alluvial aquifer formation.
• Infiltration Galleries: Horizontal perforated pipes embedded in engineered, graded gravel trenches excavated beneath the active riverbed.
• Operational Advantages: Subsurface systems exploit Riverbank Filtration (RBF). The riverbed sediment bed serves as a natural roughing filter that eliminates fish screens, removes macro-invertebrates and Giardia cysts, buffers sudden temperature swings, and dampens river turbidity surges from $> 300 \text{ NTU}$ down to $< 5 \text{ NTU}$.

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Aquatic Biosecurity & Cold-Weather Challenges

Invasive Mussel Control

Invasive dreissenid mussels—Quagga mussels (Dreissena bugensis) and Zebra mussels (Dreissena polymorpha)—pose an existential threat to water infrastructure throughout the western United States. Their microscopic free-swimming larvae (veligers) enter intake pipelines, settle onto interior surfaces, and form dense, multi-layered colonies that severely restrict pipe diameters, increase friction head loss, and foul raw water pumps. Water with dissolved calcium concentrations $> 15 \text{ mg/L}$ and pH between $7.4 \text{ and } 8.5$ presents severe risk. Protective strategies include continuous mechanical pigging, intake line thermal shocking, copper ion generator dosing, and intake tunnel chlorination (subject to strict environmental NPDES discharge limits).

Frazil Ice & Anchor Ice Management

During clear, subfreezing winter nights in the Pacific Northwest, surface water in turbulent, open rivers can become supercooled to temperatures between $-0.01^\circ\text{C}$ and $-0.05^\circ\text{C}$ without freezing solid:

• Frazil Ice: Supercooling triggers the rapid formation of microscopic, disc-shaped ice crystals suspended in the turbulent water column. When this supercooled water enters an intake structure, the frazil crystals adhere instantly to submerged metal surfaces, trash racks, and screen wire. Within minutes, the crystals agglomerate, bridging openings and blinding the intake completely, leading to pump cavitation and total loss of plant inflow.
• Anchor Ice: Submerged ice that freezes directly onto riverbed cobbles and submerged cribbing, detaching during daylight and floating downstream to clog screens.
• Operational Countermeasures: Operators combat frazil ice by:
  1. Installing pneumatic air-burst systems that release massive, instantaneous blasts of compressed air to shatter ice bridging on racks.
  2. Applying low-voltage electrical resistance heating or warm water/steam circulation to keep metal rack surfaces slightly above $0.0^\circ\text{C}$.
  3. Throttling intake suction velocities below the critical shear threshold that promotes crystal adhesion.

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Pretreatment Unit Operations

Pretreatment unit operations prepare raw water physically and chemically for subsequent coagulation and filtration, removing interfering substances and reducing chemical coagulant demand.

1. Aeration Systems

Aeration introduces ambient atmospheric oxygen into water and strips out undesirable dissolved volatile gases based on Henry's Law.

• Gas Stripping: Stripping dissolved hydrogen sulfide ($\text{H}_2\text{S}$) removes the offensive "rotten egg" odor and eliminates a potent consumer of chlorine. Stripping dissolved carbon dioxide ($\text{CO}_2$) raises raw water pH naturally, eliminating corrosive carbonic acid without requiring supplemental caustic chemical feeds. Aeration also strips combustible methane ($\text{CH}_4$) and radioactive radon ($^{222}\text{Rn}$).
• Dissolved Metal Oxidation: Aeration introduces atmospheric oxygen to convert soluble reduced metals into insoluble precipitates: $$4\text{Fe}^{2+} + \text{O}_2 + 10\text{H}_2\text{O} \rightarrow 4\text{Fe(OH)}_3\downarrow + 8\text{H}^+$$ Stoichiometrically, $0.14 \text{ mg}$ of $\text{O}_2$ is required to oxidize $1.0 \text{ mg}$ of $\text{Fe}^{2+}$, while $0.29 \text{ mg}$ of $\text{O}_2$ is required to oxidize $1.0 \text{ mg}$ of $\text{Mn}^{2+}$. While iron oxidizes rapidly at $\text{pH} > 7.0$, dissolved manganese oxidation kinetics with oxygen alone are extremely sluggish below $\text{pH } 9.5$, necessitating chemical pre-oxidants.

2. Chemical Pre-Oxidation

• Potassium Permanganate ($\text{KMnO}_4$): A powerful chemical oxidant widely dosed at the raw intake to rapidly oxidize dissolved manganese and iron: $$3\text{Mn}^{2+} + 2\text{KMnO}_4 + 2\text{H}_2\text{O} \rightarrow 5\text{MnO}_2\downarrow + 2\text{K}^+ + 4\text{H}^+$$ Reaction stoichiometry requires $1.92 \text{ mg/L}$ of $\text{KMnO}_4$ per $1.0 \text{ mg/L}$ of $\text{Mn}^{2+}$. Precise jar-test calibration is critical: under-dosing leaves dissolved manganese in the water, which oxidizes in distribution to produce black staining; over-dosing passes unreacted permanganate into finished water, producing a distinct pink or purple discoloration at consumer taps.
• Chlorine Dioxide ($\text{ClO}_2$): A dissolved gas generated on-site. It rapidly oxidizes iron, manganese, and taste-and-odor compounds without reacting with ammonia or generating trihalomethanes. Dosage is constrained by the strict Maximum Contaminant Level (MCL) of $1.0 \text{ mg/L}$ for the chlorite byproduct.
• Ozone ($\text{O}_3$): An aggressive oxidant that destroys algal odor compounds (geosmin and 2-MIB). However, ozone will oxidize dissolved bromide ($\text{Br}^-$) into carcinogenic bromate ($\text{BrO}_3^-$), which has an EPA/OHA MCL of $0.010 \text{ mg/L}$.

3. Presedimentation Basins & Grit Removal

Presedimentation basins and vortex grit separators are installed ahead of the flash mix basin to settle out heavy, settleable mineral solids—such as volcanic pumice, coarse sand, and heavy silts—during Oregon's high-turbidity winter freshets. These basins protect low-lift pump impellers from abrasive wear, prevent scouring of chemical feed lines, and keep rapid-mix tanks from accumulating dead grit deposits.

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Raw Water Quality Parameters Matrix

Reservoir Stratification, Turnover and Algae Control

Many Oregon systems draw from storage reservoirs or lakes, where water quality changes with the seasons:

• Summer stratification: the sun warms the surface layer (epilimnion), which floats on colder, denser bottom water (hypolimnion), separated by the thermocline. Bottom water is cut off from the air, and decaying organic matter uses up its oxygen.
• Anoxic bottom water dissolves iron and manganese from sediments and builds up hydrogen sulfide, ammonia and color.
• Turnover: in fall, the surface cools until it matches the bottom temperature, and wind mixes the whole lake. Poor-quality bottom water suddenly reaches the intake, bringing manganese spikes, odors, color and higher oxidant and coagulant demand. A smaller spring turnover can follow ice-out.
• Operator responses: use multi-level intakes to draw from the best depth, increase raw water monitoring before and during turnover, adjust oxidant and coagulant doses with jar tests, and in some reservoirs run hypolimnetic aeration or destratification mixers.
• Algae and cyanobacteria: watch for blooms in warm, calm, nutrient-rich water. Algaecides such as copper products must be used according to the label. In Oregon, pesticide applications in, over or within three feet of water fall under DEQ's 2300-A Pesticide General Permit. Treating an established cyanobacterial bloom can lyse cells and release toxins, so treat early or switch sources when possible.