2.1 Water Sources, Characteristics & Intake Management

2.1 Water Sources, Characteristics & Intake Management

Water treatment begins with raw source evaluation and intake management. The physical, chemical, and biological characteristics of raw water govern chemical coagulant dosages, treatment processes, and finished water compliance. Certified operators must understand the hydrological origins of raw water, seasonal reservoir dynamics, algal hazards, and intake infrastructure controls.

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1. Hydrological Cycle & Raw Water Sources

Water cycles continuously through evaporation, transpiration, condensation, precipitation, surface runoff, infiltration, and percolation. Municipal supplies draw from two hydrological categories:

Surface Water Supplies

Exposed directly to atmospheric and terrestrial runoff, surface waters subdivide into:

• Lotic Systems (Flowing Rivers and Streams): Exhibit high velocity, high dissolved oxygen (DO), and extreme water quality volatility. Heavy rainfall causes rapid spikes in turbidity, suspended silt, natural organic matter (NOM), and microbial pathogens.
• Lentic Systems (Standing Lakes and Reservoirs): Characterized by longer retention times, particulate sedimentation, solar warming, algal blooms, and seasonal thermal density stratification.

Groundwater Supplies

Groundwater resides in subsurface geological formations called aquifers:

• Unconfined Aquifers: Bounded at the top by a fluctuating water table open to atmospheric pressure and direct recharge. While rapidly replenished, they are vulnerable to agricultural fertilizers, pesticides, and surface chemical spills.
• Confined (Artesian) Aquifers: Geologically sealed beneath impermeable aquitards (dense clay or rock) under positive hydrostatic pressure (piezometric head). Water is naturally filtered and bacteriologically secure, but highly mineralized.
• Alluvial Aquifers & GWUDI: Shallow sand and gravel deposits hydraulically linked to adjacent rivers. When recharge occurs without adequate filtration, regulatory bodies classify the supply as Groundwater Under the Direct Influence of Surface Water (GWUDI), requiring full surface water filtration and disinfection.

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2. Comparative Water Quality Characteristics

Raw water quality determines chemical dosing and filtration requirements. Surface and deep groundwater exhibit fundamentally contrasting baselines:

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3. Reservoir Thermal Stratification & Seasonal Turnover

Water reaches maximum physical density at 3.98°C (~4°C). In deep impoundments (>20 feet deep), this property drives seasonal thermal density stratification.

Summer Stratification Structure

Solar radiation heats upper waters while bottom waters remain cold and dense, establishing three horizontal zones:

1. Epilimnion: Warm, circulating, sunlit upper zone. Wind aerates the layer, while algal photosynthesis consumes dissolved $\text{CO}_2$, elevating pH (often 8.0–9.0).
2. Metalimnion (Thermocline): Dynamic transition zone where water temperature drops by at least 1.0°C per meter (0.55°F per foot) of depth.
3. Hypolimnion: Cold, dense, stagnant bottom layer. Isolated from solar irradiance and atmospheric reaeration, biological decomposition of settling organic matter depletes all dissolved oxygen, producing anoxia (DO = 0 mg/L).

Hypolimnetic Chemistry & Turnover Dynamics

Under reducing conditions in an anoxic hypolimnion, anaerobic bacteria reduce insoluble minerals:

• Insoluble ferric iron ($\text{Fe}^{3+}$) reduces to soluble ferrous iron ($\text{Fe}^{2+}$).
• Insoluble manganic manganese ($\text{Mn}^{4+}$) reduces to soluble manganous manganese ($\text{Mn}^{2+}$).
• Sulfate ($\text{SO}_4^{2-}$) reduces to dissolved hydrogen sulfide gas ($\text{H}_2\text{S}$), yielding rotten-egg odors.
• Organic nitrogen mineralizes into ammonia ($\text{NH}_3 / \text{NH}_4^+$).

When autumn cools the epilimnion toward 4°C, density equalization destabilizes the thermocline. Wind induces fall turnover, abruptly mixing anoxic, metal-laden hypolimnetic water throughout the lake. This creates massive oxidant and coagulant demands at the intake. In freezing regions, winter stagnation establishes inverse stratification (0°C ice at surface, 4°C dense water at bottom), followed by spring turnover upon thaw.

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4. Algal Blooms, Cyanotoxins & Chemical Control

Nutrient enrichment (phosphorus $>0.02\text{ mg/L}$ and nitrogen), warm temperatures, and sunlight trigger cyanobacterial (blue-green algae) blooms (Microcystis, Anabaena, Aphanizomenon).

• Taste and Odor: Cyanobacteria produce Geosmin (earthy) and 2-Methylisoborneol [MIB] (musty). Both are detectable by humans below 10 nanograms per liter (ng/L or ppt) and require powdered activated carbon (PAC) or ozone/AOP, as conventional coagulation does not remove dissolved T&O compounds.
• Cyanotoxins: Microcystis produces Microcystin, a hepatotoxin damaging liver tissue. EPA Health Advisories establish drinking water limits of 0.3 µg/L for infants and 1.6 µg/L for adults.
• Filter Blinding: Diatoms (Asterionella, Synedra) deposit rigid silica frustules on rapid sand filters, causing rapid head loss.

Algicide Dosing & Water Chemistry Constraints

Algae are controlled using copper sulfate pentahydrate ($\text{CuSO}_4\cdot5\text{H}_2\text{O}$), which contains 25.4% active copper ($\text{Cu}^{2+}$) by weight.

• Alkalinity $\ge 50\text{ mg/L as }\text{CaCO}_3$: Standard dose is $1.0\text{ mg/L}$ copper sulfate ($0.25\text{ mg/L as }\text{Cu}^{2+}$), calculated for the upper 6-foot photic zone.
• Alkalinity $< 50\text{ mg/L as }\text{CaCO}_3$: Toxic free cupric ions remain dissolved rather than precipitating as copper carbonate. Dosage must be capped at $\le 0.5\text{ mg/L}$ copper sulfate ($\le 0.12\text{ mg/L as }\text{Cu}^{2+}$) to prevent fish kills.
• Operational Rule: Treat reservoirs in staged sections (no more than one-third to one-half at a time). Decaying algal biomass consumes dissolved oxygen; treating entire water bodies simultaneously leads to total oxygen depletion and catastrophic fish mortality.

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5. Raw Water Intake Structures & Invasive Species Control

Intakes abstract raw water while excluding debris, fish, and sediment.

• Intake Types: Shore intakes sit along riverbanks (inexpensive but vulnerable to shoreline litter, ice, and spills). Submerged offshore cribs rest 4 to 10 feet above the lake bed in deep water, drawing cooler, cleaner supply. Multi-level intake towers feature selective withdrawal gates at multiple depths, allowing operators to draw water above the anoxic hypolimnion and below surface algal blooms.
• Intake Velocity Standards: Clean Water Act Section 316(b) mandates that through-screen approach velocity must not exceed 0.5 feet per second (ft/s) (0.15 m/s). This threshold enables juvenile fish to overcome suction currents, preventing impingement on screens and entrainment into pumps.
• Debris Screens: Bar racks (2–4 inch spacing, sloped 30°–45°) intercept logs and heavy drift. Downstream traveling water screens (3/8-inch mesh panels) rotate automatically when differential head loss exceeds 6 to 12 inches, utilizing high-pressure wash sprays to clear debris.
• Invasive Mussel Control: Zebra mussels (Dreissena polymorpha) and quagga mussels (Dreissena bugensis) colonize intake conduits in layers feet thick. Feeding potassium permanganate ($\text{KMnO}_4$, 0.5–1.5 mg/L) continuously at the crib mouth prevents veliger larvae settlement without generating trihalomethanes (THMs). Pre-chlorination is also biocidal but creates regulated disinfection byproducts (DBPs) when reacting with raw organics.