2.1 Source Water Hydrology, Groundwater Protection & Raw Water Quality

Source Water Hydrology & Hydrogeology

Water treatment begins with understanding the origin, movement, and quality of raw water sources. The hydrologic cycle continuously circulates water through evaporation, transpiration, condensation, precipitation, surface runoff, infiltration, and groundwater percolation. Drinking water utilities extract water from two primary source classifications: surface water (rivers, streams, lakes, and impoundments) and groundwater (unconfined, confined, and karst aquifers).

Surface Water Hydrology & Reservoir Dynamics

Surface water supplies are characterized by open exposure to the atmosphere and terrestrial runoff. Their yield depends on watershed area, precipitation patterns, topography, vegetation, and land use. Rivers and streams exhibit rapid fluctuations in flow rate, turbidity, temperature, microbial pathogen density, and chemical composition following storm events.

In deep lakes and reservoirs, seasonal thermal changes induce vertical water column stratification:

• Epilimnion: The warm, less dense upper layer in direct contact with sunlight and atmospheric oxygen, maintaining high dissolved oxygen (DO) and active photosynthetic biological communities.
• Metalimnion (Thermocline): The transitional middle zone where temperature drops rapidly with increasing depth (at a rate $> 1^\circ\text{C}$ per meter of depth).
• Hypolimnion: The cold, dense, isolated bottom layer cut off from atmospheric re-aeration and sunlight.

During prolonged summer stratification, biological respiration and the decay of settling organic matter consume oxygen in the hypolimnion, driving it into anoxic (anaerobic) conditions. Under low redox potentials, insoluble oxidized minerals in bottom sediments are chemically reduced into soluble forms:

$\text{Insoluble } Fe^{3+} \text{ (ferric iron)} \xrightarrow{\text{anaerobic reduction}} \text{Soluble } Fe^{2+} \text{ (ferrous iron)}$ $\text{Insoluble } Mn^{4+} \text{ (manganic manganese)} \xrightarrow{\text{anaerobic reduction}} \text{Soluble } Mn^{2+} \text{ (manganous manganese)}$ $\text{Sulfate } (SO_4^{2-}) \xrightarrow{\text{bacterial reduction}} \text{Hydrogen Sulfide } (H_2S)$

Additionally, cyanobacteria (blue-green algae) proliferate in nutrient-rich epilimnetic waters, producing organic taste-and-odor metabolites such as geosmin (earthy odor) and 2-methylisoborneol (MIB) (musty odor), which have human sensory detection thresholds below $10\text{ ng/L}$ ($10\text{ parts per trillion}$).

During seasonal transitions in autumn and spring, the surface water cools or warms to $4^\circ\text{C}$ (maximum water density), destabilizing the water column. Wind-induced mixing creates reservoir turnover, abruptly distributing anoxic, mineral-rich, foul-smelling hypolimnetic water throughout the intake depth and requiring immediate treatment adjustments.

Groundwater Hydrogeology & Aquifer Dynamics

Groundwater resides in subsurface porous geological formations known as aquifers. The movement of groundwater through porous media is governed by Darcy's Law:

$Q = K \cdot A \cdot i = K \cdot A \cdot \left(\frac{dh}{dL}\right)$

Where:

• $Q$ = Groundwater volumetric discharge rate ($\text{ft}^3/\text{day}$ or $\text{m}^3/\text{day}$)
• $K$ = Hydraulic conductivity of the media ($\text{ft/day}$ or $\text{m/day}$)
• $A$ = Cross-sectional area perpendicular to flow ($\text{ft}^2$ or $\text{m}^2$)
• $i = \frac{dh}{dL}$ = Hydraulic gradient (head loss per unit flow length, dimensionless)

Geological formations are categorized based on their hydraulic properties:

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Public Water Supply Well Anatomy & Mechanics

A properly constructed public water supply well must deliver a reliable yield while preventing surface water, shallow contaminants, and undesirable aquifers from entering the wellbore.

                  Surface Pad / Sanitary Well Cap
                  [==============================] 
                                ||  <-- Casing Vent (Screened)
                      +---------||---------+
                      |  //// Grout ////   |  <-- Annular Grout Seal (>= 20 ft)
                      |  |              |  |
  Ground Surface =====|==|==============|==|=====
                      |  |              |  |
                      |  |  Steel or    |  |
  Unconsolidated      |  |  PVC Casing  |  |
  Formation           |  |              |  |
                      |  |              |  |
                      |  |              |  |
  --------------------|--|--------------|--|-----
  Static Water Level  :  :~~~~~~~~~~~~~~:  :  <-- SWL
                      |  |              |  |
  Confining Layer     |  |              |  |
  ====================|==|==============|==|=====
  Pumping Water Level :  :--------------:  :  <-- PWL (during pumping)
                      |  |              |  |
                      |  |  Submersible |  |
                      |  |  Pump Bowl   |  |
                      |  +==============+  |
  Aquifer Formation   |  | Well Screen  |  |  <-- Continuous Slot Screen
                      |  | oooooooooooo |  |
                      |  | oooooooooooo |  |  <-- Gravel Pack in Annulus
                      +--+==============+--+

Well Components & Protective Barriers

1. Well Casing: Heavy-wall carbon steel or NSF/ANSI 61 certified PVC pipe that lines the drilled borehole, supporting the earth walls and preventing borehole collapse.
2. Annular Grout Seal: The continuous void between the drilled borehole wall and the outer diameter of the casing. It must be pressure-grouted from the bottom up using neat cement grout (Class A Portland cement and clean water, $5-6\text{ gal water per 94-lb sack}$) or high-solids sodium bentonite slurry. The seal must extend at least $20\text{ feet}$ below the surface (or into the primary confining layer) to prevent surface runoff from channeling down the casing exterior.
3. Sanitary Well Cap & Casing Extension: The casing must extend at least $12\text{ to }18\text{ inches}$ above the finished ground grade or $100\text{-year}$ flood elevation. The top is sealed with a watertight sanitary gasketed cap equipped with a downward-facing, corrosion-resistant 24-mesh non-corrodible screen vent to allow air displacement without admitting insects, dirt, or vermin.
4. Pitless Adapter or Unit: A specialized mechanical connection installed below the frost line that allows horizontal discharge piping to penetrate the well casing through a watertight, pressurized O-ring seal while keeping the wellhead accessible from the surface.
5. Well Screen: A precision-fabricated, continuous-slot wire-wound screen installed in unconsolidated sand/gravel formations. The slot opening size is selected based on a laboratory sieve analysis of the aquifer formation to retain $90%$ of the surrounding gravel pack.
6. Artificial Gravel Pack: Sized, rounded, washed silica quartz gravel placed in the annular space between the screen and the borehole to stabilize the formation, prevent fine sand pumping, and lower entrance velocities.

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Well Hydraulics, Pumping Tests & Operating Formulas

When a well pump operates, water is drawn from the aquifer into the screen, creating an outward-expanding hydraulic depression in the water table or potentiometric surface.

                             Ground Surface
      =============================================================
             |                     | |                     |
             |                     | | <-- Well Casing     |
      -------+---------------------+-+---------------------+-------
      Static Water Level (SWL)     | |                     |
             \                     | |                     /  <-- Piezometric Surface
              \                    | |                    /       (Idle)
               \_                  | |                  _/
                 \_                | |                _/
                   \___            | |            ___/
                       \____       | |       ____/ <-- Cone of Depression
                            \_____ | | _____/
      ----------------------------\+-+/----------------------------
      Pumping Water Level (PWL)    |X| <-- Pump
                                   |X|
      <---------------------------> | <--------------------------->
          Radius of Influence (R)        Radius of Influence (R)

Fundamental Hydraulic Definitions & Math

• Static Water Level (SWL): The vertical distance from a reference datum (usually top of well casing) to the water surface when the pump has been idle and the aquifer is fully recovered.
• Pumping Water Level (PWL): The vertical distance from the reference datum to the stabilized water surface while the well is pumping at a steady discharge rate $Q$.
• Drawdown ($DD$): The physical decline in water level caused by pumping:

$DD = \text{PWL} - \text{SWL}$

• Specific Capacity ($SC$): The yield of the well per unit of drawdown, serving as the most vital diagnostic indicator of well performance and well screen efficiency:

$SC = \frac{\text{Pumping Rate } (Q)}{\text{Drawdown } (DD)} = \frac{\text{gpm}}{\text{ft of drawdown}}$

• Specific Yield & Specific Retention: In an unconfined aquifer, total porosity $\eta$ is divided into Specific Yield ($S_y$, water drained by gravity) and Specific Retention ($S_r$, water held against gravity by capillary forces): $\eta = S_y + S_r$.
• Cone of Depression: The three-dimensional, inverted funnel-shaped depression in the water table surrounding a pumping well.
• Radius of Influence ($R$): The horizontal radial distance from the center of the wellbore to the outer edge of the cone of depression where drawdown is zero ($DD = 0$).
• Well Interference: When multiple production wells operate in close proximity, their cones of depression overlap, causing compounded drawdown, reduced specific capacities, and increased energy consumption.

Well Diagnostic Math Example

A municipal well has a static water level of $42.0\text{ ft}$ below top of casing. After pumping continuously at $600\text{ gpm}$ for 24 hours, the pumping water level stabilizes at $87.0\text{ ft}$.

1. Calculate Drawdown ($DD$): $DD = 87.0\text{ ft} - 42.0\text{ ft} = 45.0\text{ ft}$

2. Calculate Specific Capacity ($SC$): $SC = \frac{600\text{ gpm}}{45.0\text{ ft}} = 13.33\text{ gpm/ft}$

Operational Insight: If a future test at $600\text{ gpm}$ yields a drawdown of $60.0\text{ ft}$ ($SC = 10.0\text{ gpm/ft}$), the $25%$ loss in specific capacity alerts the operator to well screen encrustation (iron/manganese deposits or calcium carbonate scaling), biofouling from iron bacteria (Gallionella), gravel pack siltation, or a declining regional aquifer water table.

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Raw Water Quality Parameters & Analytical Methods

Water operators must evaluate raw water parameters to establish baseline treatment targets and detect rapid environmental changes.

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Virginia Department of Health (VDH) Source Standards & Sanitary Surveys

In Virginia, the Virginia Department of Health (VDH) Office of Drinking Water (ODW) enforces the Waterworks Regulations (12 VAC 5-590) to protect public water supplies.

Well Classification & Sanitary Setbacks in Virginia

VDH classifies public water supply wells based on construction depth and geological isolation:

• Class I: Wells constructed in consolidated rock or deep protected confined aquifers with continuous casing and pressure-grouted annular space extending at least $100\text{ feet}$ deep.
• Class II (A & B): Wells constructed in intermediate geological strata with pressure-grouted casing extending at least $50\text{ feet}$ (Class IIA) or $20\text{ feet}$ (Class IIB).

VDH mandates strict horizontal separation distances (sanitary setbacks) from potential contamination sources within the dedicated wellhead protection property:

                                  PUBLIC WELLHEAD
                                         (*)
                                          |
                  +-----------------------+-----------------------+
                  |                                               |
           50-Foot Radius                                  100-Foot Radius
     [Clean Horizontal Zone]                         [Sanitary Protection Zone]
     - Property boundaries                          - Gravity sewer pipes (cast iron)
     - Stormwater retention ditches                 - Septic tanks & drain fields
     - Paved roadways / parking lots                - Manure storage & livestock pens
     - Barnyards & non-sewage drains                - Underground fuel/chemical storage

Critical Virginia Standards:

1. 50-Foot Radius: Absolute dedicated ownership/control zone around the wellhead free from all structures and non-sewage contamination sources.
2. 100-Foot Radius: Minimum horizontal setback from any septic tank, drainfield, sewer main, pit privy, or sewage conveyance pipe.
3. 200 to 500-Foot Setbacks: Applied to major chemical storage, pesticide mixing facilities, solid waste landfills, and industrial waste lagoons.

Groundwater Under the Direct Influence of Surface Water (GWUDI)

Under EPA and VDH rules, any groundwater source displaying direct hydraulic connection to surface water is designated as GWUDI and must comply with all filtration and disinfection requirements of the Surface Water Treatment Rules.

Criteria Triggering a GWUDI Assessment:

1. Well constructed with shallow casing ($< 50\text{ ft}$) in unconfined alluvial sand/gravel or karst limestone terrain.
2. Well located within $200\text{ feet}$ of a lake, reservoir, river, or perennial stream.
3. Water quality measurements exhibiting rapid fluctuations in turbidity, water temperature, conductivity, or pH that correlate directly with local precipitation and surface stream stages.
4. Microscopic Particulate Analysis (MPA) identifying surface bio-indicators: diatoms, green algae, rotifers, insect parts, or protozoan parasites (Giardia lamblia cysts or Cryptosporidium parvum oocysts).