2.1 Raw Water Sources & Pretreatment

South Carolina Hydrology & Raw Water Sources

Water treatment operators in South Carolina manage raw water derived from two distinctly different hydrogeologic provinces: the Piedmont/Blue Ridge region in the northwestern portion of the state and the Coastal Plain region extending southeast to the Atlantic Ocean.

Piedmont & Blue Ridge Surface Waters

The Upstate and Midlands rely primarily on surface water impoundments and river systems. Key raw water sources include large reservoir impoundments such as Lake Murray, Lake Hartwell, Lake Keowee, and Lake Wylie, alongside major river networks including the Saluda, Broad, Savannah, and Catawba Rivers.

Surface waters in the Piedmont region are characterized by:

• Variable Turbidity: Baseline turbidity ranges from 5 to 25 Nephelometric Turbidity Units (NTU) during dry periods but can rapidly surge past 500 NTU during heavy rainfall and soil erosion events.
• Low Alkalinity: Natural total alkalinity is typically low, ranging between 10 and 30 mg/L as $\text{CaCO}_3$, resulting in soft water with low buffering capacity.
• Low Total Dissolved Solids (TDS): TDS concentrations range between 30 and 80 mg/L.
• Natural Organic Matter (NOM): Contains humic and fulvic acids derived from leaf litter and forest runoff, serving as precursors for disinfection byproducts (DBPs).
• Seasonal Thermal Stratification: Deep reservoirs stratify during summer months, leading to oxygen depletion in bottom layers.

Coastal Plain Groundwater & Blackwater Rivers

The Coastal Plain relies heavily on deep confined and unconfined aquifers, including the Middendorf, Black Creek, Castle Hayne, and Floridan aquifers, as well as blackwater surface river basins like the Edisto and Pee Dee Rivers.

Coastal Plain water sources display distinct characteristics:

• Groundwater Quality: Deep wells produce water with exceptionally low turbidity ($< 1.0 \text{ NTU}$), stable year-round temperatures ($18^\circ\text{C}$ to $22^\circ\text{C}$), but elevated levels of dissolved minerals—specifically iron ($\text{Fe} > 0.3 \text{ mg/L}$), manganese ($\text{Mn} > 0.05 \text{ mg/L}$), and hydrogen sulfide gas ($\text{H}_2\text{S}$).
• Blackwater Surface Rivers: Drainage through coastal cypress swamps yields water with high true color (50 to 200 Platinum-Cobalt units) caused by dissolved tannins and lignins, low pH (5.5 to 6.8), and elevated Total Organic Carbon (TOC).

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Critical Raw Water Quality Parameters

Effective plant operations depend on continuous monitoring of five primary physical and chemical raw water parameters:

Thermal Stratification & Lake Limnology

During late spring and summer, deep South Carolina reservoirs stratify into three distinct temperature zones:

1. Epilimnion: Upper warm, well-mixed, oxygen-rich layer where photosynthesis occurs.
2. Metalimnion (Thermocline): Middle transition zone where temperature drops rapidly with depth ($> 1^\circ\text{C}$ per meter).
3. Hypolimnion: Cold, dense bottom layer isolated from atmospheric reaeration.

In the hypolimnion, bacterial decomposition consumes dissolved oxygen (DO), creating an anaerobic (anoxic) environment. Under zero-DO conditions, insoluble ferric iron ($\text{Fe}^{3+}$) and manganic manganese ($\text{Mn}^{4+}$) in lake sediments are reduced to soluble ferrous ($\text{Fe}^{2+}$) and manganous ($\text{Mn}^{2+}$) ions. Sulfate-reducing bacteria generate hydrogen sulfide ($\text{H}_2\text{S}$) gas, causing severe rotten-egg taste and odor issues. Multi-level intake structures allow operators to draw raw water from intake ports located above the anoxic hypolimnion.

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Reservoir Algae Control & Chemical Treatment

Algal blooms in surface reservoirs present major operational challenges, including taste and odor complaints, filter clogging, diurnal pH swings, and elevated DBP precursor loading.

Algal Impacts

• Taste & Odor Compounds: Actinomycetes and blue-green algae (cyanobacteria) produce Geosmin and 2-Methylisoborneol (MIB), which produce earthy/musty odors detectable by humans at concentrations as low as 5 to 10 nanograms per liter (ng/L or ppt).
• Filter Clogging: Diatoms such as Synedra, Melosira, and Asterionella possess rigid silica cell walls that bind filter media and reduce filter run times by over 50%.
• Diurnal pH Fluctuations: During peak daylight hours, active algal photosynthesis consumes dissolved carbon dioxide ($\text{CO}_2$), shifting chemical equilibria and driving raw water pH above 9.0. At night, algal respiration releases $\text{CO}_2$, dropping pH back to near 7.0.

Copper Sulfate Application Guidelines

Copper sulfate pentahydrate ($\text{CuSO}_4 \cdot 5\text{H}_2\text{O}$) is the standard algaecide applied to raw water impoundments. Dosing must be precisely controlled based on raw water alkalinity:

$\text{Total Alkalinity} \ge 50 \text{ mg/L as } \text{CaCO}_3 \implies \text{Standard Dose} = 1.0 \text{ mg/L } \text{CuSO}_4 \cdot 5\text{H}_2\text{O} \quad (8.34 \text{ lbs/MG})$ $\text{Total Alkalinity} < 50 \text{ mg/L as } \text{CaCO}_3 \implies \text{Restricted Dose} = 0.25 \text{ to } 0.50 \text{ mg/L } \text{CuSO}_4 \cdot 5\text{H}_2\text{O}$

[!WARNING] Exam Trap: In soft, low-alkalinity waters ($< 50 \text{ mg/L as } \text{CaCO}_3$) common in the South Carolina Piedmont, copper ions do not readily precipitate as insoluble copper carbonate. Applying standard $1.0 \text{ mg/L}$ doses in low-alkalinity water leads to toxic concentrations of free cupric ions ($\text{Cu}^{2+}$), causing catastrophic fish kills. Doses must be reduced, or sodium carbonate peroxyhydrate (hydrogen peroxide derivative) must be substituted.

Copper sulfate applications must be confined to the top 5 to 10 feet of the water column (the photic zone of the epilimnion). Treatment should be conducted when algal counts exceed 500 to 1,000 cells/mL or when chlorophyll-a levels rise.

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Intake Structures & Mechanical Pretreatment

Raw water intake facilities protect downstream high-service and chemical feed systems while removing coarse solids.

Raw Water Source ──► Coarse Bar Screens ──► Traveling Screens ──► Presedimentation ──► Pre-Oxidation ──► Chemical Flash Mix
                     (1-3 inch openings)   (3/8 inch mesh)      (1-3 days detention)   (KMnO4 / ClO2)

1. Coarse Bar Screens (Trash Racks)

Incline bar racks with clear openings of 1 to 3 inches set at a $60^\circ$ to $80^\circ$ angle prevent large logs, branches, debris, and aquatic fauna from entering the intake channel. Mechanical rakes periodically clear accumulated debris.

2. Traveling Water Screens

Continuously revolving wire mesh screen trays with openings of 3/8 inch (9.5 mm) or 1/4 inch (6.3 mm) capture smaller debris, fish, and leaves.

• Velocity Control: Clean Water Act Section 316(b) regulations mandate that the maximum through-screen approach velocity must not exceed 0.5 ft/s to prevent fish impingement.
• Spray Wash System: High-pressure spray jets operating at 60 to 80 psi flush trapped screen debris into a wash trough for collection and off-site disposal.

3. Presedimentation Basins

Presedimentation (presed) basins are un-chemicized earthen or concrete impoundments that provide 1 to 3 days of hydraulic detention time. They allow heavy sand, gravel, and coarse silt particles to settle out by gravity before raw water reaches chemical flash mix units. This protects pump impellers from abrasive wear and prevents sudden turbidity spikes from overloading coagulant feed systems.

4. Raw Water Pre-Oxidation

To handle dissolved metals and taste and odor compounds prior to primary coagulation, operators inject pre-oxidants at the intake intake structure:

• Potassium Permanganate ($\text{KMnO}_4$): Dosed at 0.5 to 2.0 mg/L, permanganate rapidly oxidizes soluble ferrous iron ($\text{Fe}^{2+}$) to insoluble ferric hydroxide ($\text{Fe(OH)}_3\downarrow$) and manganous ($\text{Mn}^{2+}$) to manganese dioxide ($\text{MnO}_2\downarrow$). It provides pre-oxidation without forming regulated Trihalomethanes (THMs).
• Chlorine Dioxide ($\text{ClO}_2$): Dosed at 0.5 to 1.5 mg/L, $\text{ClO}_2$ oxidizes organic color, iron, and manganese while destroying taste and odor compounds without generating THMs or Haloacetic Acids (HAAs).