1.3 Source Water Protection, Limnology & Raw Water Intakes

Reservoir Limnology and Thermal Stratification

Deep raw water lakes and impoundments undergo seasonal physical and chemical transformations driven by solar heating, atmospheric cooling, and wind mixing. These cycles depend directly on the unique density profile of water, which reaches its maximum density at 3.98°C (approx. 4.0°C / 39.2°F).

Surface ---------------------------------------------------
         EPILIMNION       Warm, Sunlit, Aerobic (DO > 8 mg/L)
                          Active Photosynthesis, Algae Blooms
        ---------------------------------------------------
         THERMOCLINE      Rapid Temperature Drop (>= 1°C/meter)
         (Metalimnion)    Density Barrier to Vertical Circulation
        ---------------------------------------------------
         HYPOLIMNION      Cold (4–10°C), Dense, Dark (Aphotic)
                          Anoxic (DO -> 0 mg/L), Soluble Fe2+/Mn2+, H2S
Benthic Sediments =========================================

The Three Stratification Strata

During late spring and summer, solar radiation heats the surface waters while deep water remains cold. Because warm water is less dense than cold water, wind energy becomes insufficient to mix the water column, establishing three stable horizontal layers:

1. Epilimnion: The uppermost thermal stratum. It is warm, wind-circulated, well-illuminated (euphotic), and rich in dissolved oxygen (DO typically 8–10 mg/L) through atmospheric re-aeration and algal photosynthesis. However, it is vulnerable to thermal warming, high pH swings, and severe cyanobacterial scums.
2. Metalimnion (Thermocline): The intermediate transition zone characterized by a rapid vertical temperature drop of at least 1.0°C per meter (approx. 0.55°F per foot) of depth. This rapid temperature-induced density change creates a physical barrier (the pycnocline) that completely arrests vertical mixing between surface and bottom strata.
3. Hypolimnion: The lowest, cold (4°C–10°C), dense, dark stratum. Isolated beneath the thermocline from atmospheric aeration and light, its dissolved oxygen is rapidly consumed by bacterial respiration decomposing descending dead biological biomass from the epilimnion, driving the entire zone into an anaerobic (anoxic) state.

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Anaerobic Hypolimnion Geochemistry and Reduction Cascade

When hypolimnetic dissolved oxygen drops below 0.2 mg/L, aerobic decomposition halts, and benthic microorganisms turn to alternative terminal electron acceptors. This initiates a sequential thermodynamic reduction cascade that transforms harmless oxidized mineral precipitates into hazardous soluble species:

1. Nitrate Reduction ($NO_3^-$): Facultative anaerobic bacteria reduce nitrates to nitrites, nitrogen gas, and toxic ammonia ($NH_3/NH_4^+$), establishing sudden raw water chemical disinfectant demand.
2. Manganese Reduction ($Mn^{4+} \rightarrow Mn^{2+}$): Insoluble manganese dioxide ($MnO_2$) bound in bottom muds reduces to highly soluble manganous ions ($Mn^{2+}$).
3. Iron Reduction ($Fe^{3+} \rightarrow Fe^{2+}$): Insoluble ferric hydroxide ($Fe(OH)_3$) precipitates reduce to highly soluble ferrous ions ($Fe^{2+}$).
4. Sulfate Reduction ($SO_4^{2-} \rightarrow H_2S$): Strictly anaerobic sulfate-reducing bacteria (Desulfovibrio) reduce sulfate into hydrogen sulfide ($H_2S$), which imparts a pungent rotten-egg odor, reacts with trace metals to form black colloidal precipitates, and imposes enormous chlorine oxidant demand.
5. Acidification: Anaerobic bacterial fermentation releases carbon dioxide and organic acids, suppressing hypolimnetic pH from 7.8 down to 6.2–6.8, which further accelerates mineral dissolution.

Downstream Distribution Hazard: 
Soluble Fe2+ and Mn2+ pass directly through media filters. When post-filter chlorine 
is injected, oxidation precipitates insoluble red/brown Fe(OH)3 and black MnO2, 
triggering severe distribution system dirty-water emergencies.

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Reservoir Turnover and Treatment Plant Operating Shocks

Summer Stratification      Autumn Cooling / Wind Mixing         Fall Turnover (Overturn)
   [ Warm Epilimnion ]     --->   Surface water cools to 4°C  --->   Complete Destratification
   [ Thermocline     ]            Density increases; sinks           Whole lake mixes uniformly
   [ Cold Hypolimnion]            Wind overcomes density shear       Anoxic water hits intakes!

Turnover Mechanics

• Fall Overturn: In late autumn, declining air temperatures cool the epilimnion. As surface water cools toward 4°C, its density increases, causing it to sink and displace deeper water. When epilimnetic water reaches the temperature of the hypolimnion, thermal density resistance collapses. Prevailing autumn winds then circulate the entire water column from top to bottom.
• Spring Overturn: In cold climates, following winter ice thaw, surface water warms from 0°C toward 4°C. Becoming denser, it sinks, and spring winds completely mix the lake before summer stratification sets in.

Operational Shocks and Countermeasures

Turnover violently brings weeks of accumulated anoxic, mineral-laden bottom water up to the treatment plant intake:

• Plant Symptoms: Instantaneous surge in raw water chlorine demand (jumping from 2 mg/L to >8 mg/L within hours), sudden plunge in raw pH, spikes in dissolved manganese and iron, intense septic odors, and rapid filter bed blinding from precipitated sulfides and organic debris.
• Corrective Actions:
  1. Feed Chemical Pre-Oxidants: Apply potassium permanganate ($KMnO_4$) at 1.0–2.0 mg/L ahead of rapid mix to oxidize soluble $Fe^{2+}$ and $Mn^{2+}$ to filterable precipitates before they reach the filters: $2KMnO_4 + 3Mn^{2+} + 2H_2O \rightarrow 5MnO_2(s) + 2K^+ + 4H^+$
  2. Add Powdered Activated Carbon (PAC): Feed 10–25 mg/L PAC at the intake to adsorb $H_2S$, geosmin, and 2-MIB before chemical coagulant addition.
  3. Supplemental Alkalinity: Feed hydrated lime or caustic soda to offset turnover pH depression and maintain optimal coagulation.
  4. Artificial Reservoir Destratification: Install continuous diffused-air bubble aerators or mechanical draft-tube mixers in the reservoir to circulate bottom waters continuously, maintaining aerobic hypolimnetic conditions throughout the year.

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Raw Water Intake Infrastructure and Selective Withdrawal

Intake structures constitute the first engineered barrier in the treatment chain. Their design directly influences raw water quality and equipment protection.

Multi-Level Intake Towers

Multi-level intake towers feature selectively valved intake ports situated at discrete vertical elevations (upper, intermediate, and lower depths). Operators adjust valve configurations based on limnological monitoring:

• Avoid the Surface Port: During summer cyanobacteria scums, warm temperatures, or surface oil spills.
• Avoid the Bottom Port: During summer and fall stratification when the hypolimnion is anoxic and laden with dissolved $Fe^{2+}$, $Mn^{2+}$, and $H_2S$.
• Select Intermediate (Upper Metalimnion) Ports: Draws cool, well-oxygenated water that avoids surface algal scums while remaining safely above anoxic mineral precipitates.

Intake Screens and Pretreatment

Raw Water Flow ---> [ Coarse Bar Screen / Trash Rack ] ---> [ Traveling Water Screen ] ---> Low-Lift Pumps
                     (Spaced 2 - 4 inches apart)             (3/8" woven wire mesh)
                     Intercepts logs, limbs, sheets of ice   Rotates automatically; 80 psi wash

1. Bar Screens / Trash Racks: Heavy-duty structural steel bars spaced 2 to 4 inches (50–100 mm) apart across the intake mouth to intercept large waterborne debris, logs, vegetation mats, and ice floes.
2. Traveling Water Screens: Continuous vertical loops of overlapping wire mesh baskets (typically 3/8-inch or 1/4-inch mesh) situated in intake wet wells. They rotate automatically when differential level sensors measure head loss exceeding 2 to 4 inches across the screen, or on timed intervals. Spray wash headers operating at 60 to 100 psi blast trapped fish, leaves, and debris into a disposal sluice trough.

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Invasive Aquatic Nuisance Species: Zebra and Quagga Mussels

Freshwater dreissenid bivalves—zebra mussels (Dreissena polymorpha) and quagga mussels (Dreissena bugensis)—represent severe biological threats to raw water intake facilities.

• Life Cycle: Adult mussels spawn millions of microscopic, free-swimming planktonic larvae termed veligers (40–70 micrometers). Carried into intake conduits by gravity flow, veligers anchor irreversibly to concrete and metal surfaces using high-tensile proteinaceous byssal threads.
• Infrastructure Fouling: Colonies form dense encrustations up to several inches thick (>100,000 organisms/$m^2$), drastically reducing effective pipe cross-sectional diameter, collapsing the Hazen-Williams friction coefficient ($C$-factor drops from 130 to <80), starving low-lift intake pumps, and generating massive sulfide/ammonia odors when colonies die.
• Control Strategies:
  • Intake Pre-Oxidation: Injecting continuous low-dose chlorine (free residual 0.5–1.0 mg/L) at the remote intake crib prevents veliger settlement and kills adult mussels. (Note: Must be balanced against raw water DBP precursor formation).
  • Potassium Permanganate: Continuous feed of 0.5–2.5 mg/L $KMnO_4$ at the intake crib acts as an effective molluscicide without forming halogenated trihalomethanes.
  • Physical Controls: Limiting intake entrance velocities to under 0.5 ft/s (0.15 m/s) to minimize veliger draw-in, applying non-toxic silicone foul-release polymer coatings, or performing periodic high-temperature thermal flushes (>40°C).