2.1 Source Water Hydrology, Groundwater vs. Surface Water & Protection

2.1 Source Water Hydrology, Groundwater vs. Surface Water & Protection

Quick Answer: Water treatment begins at the source. Groundwater generally provides stable temperature, low biological pathogen counts, and low turbidity, but often contains elevated concentrations of dissolved minerals, hardness, iron, and manganese. Surface water responds rapidly to weather and runoff, presenting elevated microbiological risks (Cryptosporidium, Giardia, viruses), volatile turbidity, and natural organic matter that forms disinfection byproducts. Maintaining groundwater capacity requires tracking specific capacity ($\text{gpm/ft}$ of drawdown), while managing surface water requires monitoring thermal stratification, dissolved oxygen profiles, and seasonal reservoir turnover.

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The Hydrologic Cycle & Sourcing Dynamics

The hydrologic cycle is the continuous movement of water above, on, and below the surface of the Earth, powered by solar energy and gravity. Water evaporates from oceans, lakes, and rivers, and transpires from vegetative canopies into the atmosphere. As moisture-laden air rises and cools, water vapor condenses into clouds and returns to the Earth as precipitation (rain, sleet, snow).

When precipitation reaches land, it divides along three primary physical pathways:

1. Infiltration and Percolation: Water sinks into the soil mantle and percolates downward under gravity through the unsaturated vadose zone until reaching the saturated zone, replenishing groundwater aquifers.
2. Surface Runoff: Precipitation exceeding the soil's infiltration capacity flows overland into streams, rivers, impoundments, and lakes, picking up suspended sediment, agricultural nutrients, and biological matter.
3. Evapotranspiration: Water is directly re-evaporated into the atmosphere or taken up by plant root systems and transpired through leaf stomata.

Comparative Water Quality: Groundwater vs. Surface Water

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Groundwater Hydrology & Aquifer Classifications

An aquifer is a saturated underground geologic formation composed of permeable materials—such as sand, gravel, fractured limestone, or sandstone—capable of yielding culinary quantities of water to wells or springs.

Aquifer Types

• Unconfined Aquifer (Water Table Aquifer): An aquifer whose upper boundary is the water table, which is in direct contact with the atmosphere through permeable overlying soil. The water table rises and falls in direct response to local precipitation and drought cycles. These aquifers are highly vulnerable to surface contamination (fertilizers, pesticides, fuel leaks).
• Confined Aquifer (Artesian Aquifer): An aquifer bounded above and below by low-permeability geologic strata known as aquitards (clays, shales, or dense unfractured rock). The water is confined under hydraulic pressure greater than atmospheric pressure. When a well penetrates a confined aquifer, water rises inside the casing to a level known as the potentiometric (piezometric) surface.
• Flowing Artesian Well: If the potentiometric surface lies at an elevation higher than the ground surface, water flows naturally from the well without mechanical pumping.
• Perched Aquifer: A localized lens of saturated groundwater supported by an isolated, discontinuous impermeable clay lens situated above the regional water table in the vadose zone. Perched aquifers yield limited water volumes and can run dry quickly during pumping.

+------------------------------------------------------------------------+
| Ground Surface                                                         |
|   \       \       \       \       \       \       \       \       \   |
|--------------------------------- Water Table --------------------------|
| UNCONFINED AQUIFER (Sand & Gravel)                                     |
|========================================================================|
| UPPER CONFINING UNIT / AQUITARD (Dense Clay Layer)                     |
|------------------------------------------------------------------------|
| CONFINED (ARTESIAN) AQUIFER (Water under pressure)                     |
|                                                                        |
|   Potentiometric Surface Line ----.                                    |
|                                   | (Piezometric Head)                 |
|===================================|====================================|
| LOWER CONFINING BED (Solid Bedrock)                                    |
+------------------------------------------------------------------------+

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Well Hydraulics, Pumping Mechanics & Specific Capacity

When a well pump operates, it extracts water from the aquifer surrounding the intake screen. Understanding well hydraulics is vital for tracking well health and avoiding pump burnout.

Key Hydrogeologic Terms

• Static Water Level (SWL): The equilibrium distance from the reference measuring point (typically the top of the well casing) down to the water surface when the pump has been shut off for an extended period (typically 12 to 24 hours).
• Pumping Water Level (PWL): The stabilized distance from the measuring point down to the water surface while the pump is operating at a constant discharge rate.
• Drawdown ($DD$): The vertical distance that the water level drops during pumping: $\text{Drawdown (ft)} = \text{Pumping Water Level (ft)} - \text{Static Water Level (ft)}$
• Cone of Depression: The three-dimensional, inverted funnel-shaped depression formed in the water table or potentiometric surface around a pumping well. The steepest gradient occurs immediately adjacent to the well casing.
• Radius of Influence ($R$): The horizontal radial distance from the center of the well casing to the outermost edge of the cone of depression where drawdown approaches zero.
• Residual Drawdown & Recovery: After the pump shuts down, the time required for the water level to return to its initial Static Water Level measures the aquifer's recharge rate.

Specific Capacity: The Ultimate Well Diagnostic

Specific Capacity measures the yield of a well per unit of drawdown. It reflects the hydraulic efficiency of both the well structure (casing, screen, gravel pack) and the surrounding aquifer formation:

$\text{Specific Capacity (gpm/ft)} = \frac{\text{Pumping Rate (gpm)}}{\text{Drawdown (ft)}}$

$\text{Specific Capacity} = \frac{Q}{\text{PWL} - \text{SWL}}$

Operator Alert: A progressive decline in a well's specific capacity over time (typically a drop of $15\%\text{ to }20\%$) signals well deterioration. Common causes include:

1. Mineral encrustation (calcium carbonate or iron/manganese deposits clogging the screen slots).
2. Biofouling by iron-related bacteria (Gallionella, Crenothrix, Leptothrix) producing gelatinous biological slimes.
3. Sand pumping and silt accumulation plugging the gravel pack.
4. Regional groundwater overdraft depressing the surrounding water table.

Worked Engineering Example: Well Performance Evaluation

Problem: A municipal supply well has a documented Static Water Level of $48.0\text{ feet}$ below the top of the casing. When the vertical turbine pump discharges at a continuous rate of $850\text{ gpm}$, the Pumping Water Level stabilizes at $116.0\text{ feet}$. Calculate the total drawdown and the specific capacity of the well. If five years ago the specific capacity was $16.5\text{ gpm/ft}$, determine the percentage reduction in well performance.

Step 1: Calculate Drawdown ($DD$): $\text{Drawdown} = \text{PWL} - \text{SWL} = 116.0\text{ ft} - 48.0\text{ ft} = 68.0\text{ ft}$

Step 2: Calculate Current Specific Capacity: $\text{Specific Capacity} = \frac{Q}{\text{Drawdown}} = \frac{850\text{ gpm}}{68.0\text{ ft}} = 12.5\text{ gpm/ft}$

Step 3: Calculate Percentage Decline: $\text{Decline} = \frac{16.5\text{ gpm/ft} - 12.5\text{ gpm/ft}}{16.5\text{ gpm/ft}} \times 100 = \frac{4.0}{16.5} \times 100 = 24.24\%$

Evaluation: The $24.2\%$ drop exceeds the $15\%$ threshold, confirming that chemical acidization, surge block scouring, or chlorination shock treatment is required to restore well screen permeability.

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Surface Water Hydrology & Reservoir Limnology

Surface water sources include flowing lotic systems (rivers, streams) and standing lentic systems (lakes, reservoirs). Operating a surface water plant requires understanding watershed characteristics, seasonal thermal layering, and limnological chemical dynamics.

Watershed Management & Runoff

A watershed (or drainage basin) is the geographic land area from which all precipitation drains to a common outlet. Land use within the watershed directly dictates raw water quality:

• Forested Watersheds: Act as natural sponges, dampening peak runoff velocities, filtering silt, and yielding lower raw water turbidity.
• Agricultural Watersheds: Introduce animal waste, fertilizers (nitrates, phosphorus causing eutrophication), pesticides, and topsoil silt.
• Urban/Industrial Watersheds: Generate flashy hydrographs with rapid peak flows, elevated hydrocarbons, road salts, heavy metals, and sudden stormwater runoff surges.

Thermal Stratification in Reservoirs

During late spring and summer, solar radiation heats the surface of deep lakes and reservoirs. Because warm water is less dense than cold water, deep reservoirs stratify into three distinct thermal zones:

1. Epilimnion: The warm, circulating upper water layer. It receives abundant sunlight, experiences wind-driven atmospheric aeration, supports active photosynthesis, and exhibits high dissolved oxygen ($> 7\text{ mg/L}$) and elevated pH ($7.5\text{ to }8.8$).
2. Metalimnion (Thermocline): The middle transition layer characterized by a rapid temperature drop with depth—defined physically as a temperature decline of at least $1^\circ\text{C}$ per meter of depth ($0.55^\circ\text{F}$ per foot). The thermocline acts as a physical density barrier preventing vertical mixing between the upper and lower layers.
3. Hypolimnion: The cold, dense, dark bottom layer isolated from the atmosphere. Solar radiation cannot penetrate here, preventing photosynthesis. Bacterial decomposition of settling organic matter (dead algae, leaf litter) consumes dissolved oxygen.

+------------------------------------------------------------------------+
| EPILIMNION: Warm, Wind-Mixed, High DO, Photosynthesis (Algae)          |
|   Water Temp: 24°C - 28°C | DO: 8 - 10 mg/L | pH: 7.8 - 8.5            |
|~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~|
| THERMOCLINE / METALIMNION: Rapid Temperature Drop (>= 1°C per meter)   |
|~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~|
| HYPOLIMNION: Cold, Dense, Dark, Stagnant                               |
|   Water Temp: 4°C - 10°C | DO: Drops to 0.0 mg/L (Anoxic Zone)         |
|   Anaerobic Release: Fe2+ (Soluble), Mn2+ (Soluble), H2S (Rotten Egg)  |
|________________________________________________________________________|
| BENTHIC SEDIMENT LAYER                                                 |
+------------------------------------------------------------------------+

Anoxic Hypolimnion Chemistry

When thermal stratification persists throughout the summer, bacterial respiration exhausts all dissolved oxygen in the hypolimnion, creating an anoxic (anaerobic) environment ($\text{DO} < 0.5\text{ mg/L}$). This causes a sharp drop in the oxidation-reduction potential (redox potential), triggering chemical reduction of insoluble sediment minerals into soluble forms:

1. Iron Reduction: Insoluble oxidized ferric iron ($\text{Fe}^{3+}$) precipitates are chemically reduced into soluble divalent ferrous iron ($\text{Fe}^{2+}$): $\text{Fe(OH)}_3\text{(s)} + e^- + 3\text{H}^+ \to \text{Fe}^{2+} + 3\text{H}_2\text{O}$
2. Manganese Reduction: Insoluble manganese dioxide ($\text{MnO}_2$) is reduced to soluble manganous ions ($\text{Mn}^{2+}$): $\text{MnO}_2\text{(s)} + 4\text{H}^+ + 2e^- \to \text{Mn}^{2+} + 2\text{H}_2\text{O}$
3. Hydrogen Sulfide Generation: Sulfate-reducing bacteria reduce sulfate ions ($\text{SO}_4^{2-}$) into dissolved hydrogen sulfide gas ($\text{H}_2\text{S}$): $\text{SO}_4^{2-} + 8\text{H}^+ + 8e^- \to \text{S}^{2-} + 4\text{H}_2\text{O} \xrightarrow{+\text{H}^+} \text{H}_2\text{S}$
4. Nutrient & Byproduct Release: Organic nitrogen breaks down into ammonia ($\text{NH}_3$), increasing chlorine demand, while anaerobic fermentation generates methane ($\text{CH}_4$).

Lake Turnover (Thermal Destratification)

In autumn, ambient air temperatures drop and solar radiation decreases. The epilimnion cools, becomes denser, and sinks. Autumn winds mix the upper water column down into the thermocline. Eventually, the entire water column reaches a uniform temperature and density (isothermal condition, typically $4^\circ\text{C}$ to $10^\circ\text{C}$).

When density resistance vanishes, wind action initiates fall turnover (lake overturn). The stagnant, anoxic hypolimnion water mixes completely throughout the reservoir.

Operational Consequences of Turnover:

• Sudden surge in raw water turbidity.
• Dispersal of soluble iron, manganese, and hydrogen sulfide throughout all intake levels.
• Extreme chlorine demand spikes caused by ammonia and reduced inorganic compounds.
• Widespread taste and odor episodes from decaying organic matter, geosmin, and 2-methylisoborneol (MIB).

Multi-Level Intake Towers: Water utilities manage these challenges using multi-level intake structures equipped with gates at varying depths. Operators selectively draw water from the intermediate layer—avoiding surface algal scums and deep anoxic mineral layers.

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Source Water Protection: WHPA & SWAP

Preventing contamination at the source is significantly cheaper and more reliable than attempting chemical removal during treatment. Federal and state frameworks govern source protection.

Wellhead Protection Areas (WHPA)

A Wellhead Protection Area (WHPA) is the surface and subsurface area surrounding a public water supply well or wellfield through which contaminants are reasonably likely to move toward and reach the well.

Source Water Assessment Program (SWAP)

Under the 1996 Safe Drinking Water Act (SDWA) Amendments, states established a Source Water Assessment Program (SWAP) for every public water system. The SWAP process entails three mandatory phases:

1. Delineation: Mapping the geographic boundaries of the source water protection area for all wells, reservoirs, and river intakes.
2. Contaminant Inventory: Identifying existing and potential sources of contamination within the delineated zone (gas stations, dry cleaners, agricultural feedlots, chemical storage facilities).
3. Susceptibility Determination: Assessing the physical vulnerability of the intake or well to identified contaminants based on hydrogeology, well casing depth, soil permeability, and intake construction.

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Operational Troubleshooting: Source Water Anomalies