2.2 Well Construction, Sanitary Protection & Groundwater Hydraulics

2.2 Well Construction, Sanitary Protection & Groundwater Hydraulics

Groundwater wells serve as the primary drinking water source for hundreds of public water supplies across Missouri. Proper well design, rigorous sanitary sealing, routine hydraulic monitoring, and strict adherence to setback requirements are essential to maintain mechanical reliability and protect aquifers from surface contamination.

---

Anatomy of a Public Water Supply Well

A municipal or community water supply well is an engineered hydraulic structure extending from above ground grade into a water-bearing aquifer formation. Each component performs a specific sanitary or hydraulic function.

                     Sanitary Wellhead Cap
                       [=====|=====] ◄── Screened Vent (18 Mesh)
                             │
      Ground Level ══════════╪══════════ Ground Level (Sloped Pad)
                   │ █ █ █ █ │ █ █ █ █ │
                   │ █ █ █ █ │ █ █ █ █ │ ◄── Pressure Grout Seal
                   │ █ █ █ █ │ █ █ █ █ │     (Neat Cement / Bentonite)
  Frost Line ──────┼─█─█─█─█─┼─█─█─█─█─┼──────
                   │ █ █ █ █ │ █ █ █ █ │
                   │ █ █ █ █ ├─────────┼──► Approved Pitless Unit
                   │ █ █ █ █ │         │    (Watertight Subsurface Discharge)
                   │ █ █ █ █ │         │
                   │ █ █ █ █ │ Steel   │
                   │ █ █ █ █ │ Casing  │
                   │ █ █ █ █ │         │
  Bedrock / ───────┼─────────┼─────────┼──────
  Confining Layer  │ : : : : │         │
                   │ : : : : │  Pump   │ ◄── Submersible / Lineshaft Turbine
                   │ : : : : │ [====]  │
                   │ : : : : │   │     │
                   │ : : : : │ ╔═╧═══╗ │
                   │ : : : : │ ║     ║ │ ◄── Intake Screen
                   │ : : : : │ ║     ║ │     (Continuous Wire-Wrapped)
                   │ : : : : │ ╚═╤═══╝ │
                   │ : : : : │   │     │ ◄── Gravel Pack (Graded Sand)
═══════════════════╧═════════╧═══╧═════╧══════ Impermeable Base

Core Structural & Sanitary Components

• Well Casing: Permanent steel pipe installed in the borehole in accordance with PUB2489. The casing prevents the surrounding geological formation from collapsing into the borehole, seals out shallow, contaminated groundwater, and provides a clean conduit for housing the downhole pump and column pipe.
• Well Screen (Intake Screen): Where a screened completion is used, the stainless-steel screen openings are selected from sieve analyses of the formation and gravel pack. PUB2489 limits entrance velocity to $\le 0.1\text{ ft/s}$ and requires the pumping water level to remain above the screen, reducing head loss, sand movement and incrustation.
• Gravel Pack (Filter Pack): High-purity, uniformly graded, rounded silica sand placed within the annular space between the exterior of the well screen and the native borehole wall. The gravel pack stabilizes the unconsolidated aquifer formation, prevents fine sand grains from migrating through the screen, and increases the effective hydraulic diameter of the well.
• Grout Seal & Pressure Grouting: An impermeable sanitary barrier constructed by pumping neat cement grout (Type I/II Portland cement mixed with water) or high-solids bentonite slurry into the annular space between the well casing and the surrounding drill hole.
  • Placement Technique: Grout must be placed under continuous positive pressure from the bottom of the casing upward using a tremie pipe (the positive displacement method). Bottom-up placement prevents air pocket bridging, channeling, and voids, ensuring a continuous seal that prevents surface runoff and shallow non-potable groundwater from migrating down the outside of the casing into the aquifer.
• Pitless Adapter & Pitless Unit: For a Missouri community system, this must be a department-approved, commercially manufactured pitless well unit that extends the production casing to its upper terminal, excludes contamination and conducts water from the well. PUB2489 does not allow pitless adapters for community water systems. The approved unit must retain access for disinfection, water-level measurement and well removal.
• Sanitary Wellhead Seal & Overlapping Cap: A watertight, gasketed, and bolted cover sealing the top of the well casing. PUB2489 requires permanent casing to project at least 12 inches above the pump-house floor, platform floor or concrete apron and at least 18 inches above final ground. At flood-prone sites, casing left in place must terminate at least 4 feet above the 100-year flood level or highest known flood elevation, whichever is higher, or as the department directs.
• Screened Casing Vent: A downward-turned ("gooseneck") pipe penetrating the sanitary wellhead seal that equalizes atmospheric pressure inside the casing during pumping drawdown and water level recovery. The vent opening must be shielded with an 18-mesh corrosion-resistant screen to prevent insects, spiders, rodents, dust, and airborne debris from entering the well.

---

Groundwater Hydraulics: Principles & Operational Calculations

Operators must track well hydraulic parameters to assess well performance, evaluate pump efficiency, and detect early signs of well screen clogging, biofouling, or aquifer depletion.

                  Static Water Level (SWL)
  ═══════════════╤═══════════════════════════════════════ Ground Surface
                 │ ◄─── Drawdown (DD = PWL - SWL)
  - - - - - - - -▼- - - - - - - - - - - - - - - - - - - - Pumping Water Level (PWL)
               ╱   ╲
              ╱     ╲
             ╱       ╲ ◄─── Cone of Depression
            ╱    ▲    ╲
           ╱     │     ╲
  ────────┴──────┼──────┴──────────────────────── Aquifer Base
                 │
          Radius of Influence (R)

Hydraulic Definitions

1. Static Water Level (SWL): The resting distance from a fixed reference datum (usually the top of the well casing, TOC, or finished ground level) down to the water surface inside the well when the pump has been shut off and the water level has fully recovered to equilibrium.
2. Pumping Water Level (PWL): The stabilized vertical distance from the reference datum down to the water surface inside the well while the pump is operating at a constant, steady discharge rate ($Q$).
3. Drawdown ($DD$): The total vertical distance the water level drops inside the well during pumping: $\text{Drawdown (ft)} = \text{Pumping Water Level (ft)} - \text{Static Water Level (ft)}$
4. Cone of Depression: The three-dimensional, inverted funnel-shaped depression formed in the water table (or potentiometric surface) surrounding an active pumping well. The hydraulic gradient slopes inward toward the well, driving groundwater toward the pump intake.
5. Radius of Influence ($R$): The horizontal radial distance from the center of the pumping well to the outer boundary of the cone of depression, where drawdown diminishes to zero.
6. Specific Capacity ($SC$): The volume of water produced by the well per unit of drawdown at a given pumping rate: $\text{Specific Capacity } (\text{gpm/ft}) = \frac{\text{Pumping Discharge Rate } (Q\text{ in gpm})}{\text{Drawdown } (DD\text{ in ft})}$

[!IMPORTANT] Specific capacity is the single most valuable diagnostic indicator of well health. If an operator notes that specific capacity decreases over time at a constant pumping rate (meaning drawdown has increased), the well is experiencing physical problems—such as screen incrustation by calcium carbonate, biofouling from iron-oxidizing bacteria (Gallionella or Sphaerotilus), gravel pack clogging, or sand collapse.

1. Well Yield: The maximum sustainable volumetric flow rate (in gpm) that a well can pump continuously without exceeding available drawdown or drawing water levels below the pump intake bowls.

---

Step-by-Step Groundwater Hydraulic Calculations

Calculation 1: Determining Drawdown and Specific Capacity

Scenario: A municipal deep well in central Missouri has a measured Static Water Level of $62.0\text{ feet}$ below the top of the casing. After operating continuously for 6 hours at a steady discharge rate of $450\text{ gpm}$, the Pumping Water Level stabilizes at $116.0\text{ feet}$ below the top of the casing.

Step 1: Calculate Total Drawdown $\text{Drawdown} = \text{PWL} - \text{SWL} = 116.0\text{ ft} - 62.0\text{ ft} = 54.0\text{ feet}$

Step 2: Calculate Specific Capacity $\text{Specific Capacity} = \frac{Q}{\text{Drawdown}} = \frac{450\text{ gpm}}{54.0\text{ ft}} = 8.33\text{ gpm/ft of drawdown}$

---

Calculation 2: Diagnosing Well Deterioration via Historical Data

Scenario: When newly commissioned, a public well yielded $600\text{ gpm}$ with a drawdown of $20.0\text{ feet}$ (Specific Capacity = $30.0\text{ gpm/ft}$). Three years later, the operator pumps the well at $600\text{ gpm}$ and measures a Static Water Level of $50.0\text{ feet}$ and a Pumping Water Level of $90.0\text{ feet}$. Evaluate the well's condition.

Step 1: Calculate Current Drawdown $\text{Drawdown} = 90.0\text{ ft} - 50.0\text{ ft} = 40.0\text{ feet}$

Step 2: Calculate Current Specific Capacity $\text{Specific Capacity} = \frac{600\text{ gpm}}{40.0\text{ ft}} = 15.0\text{ gpm/ft}$

Step 3: Evaluate Performance $\text{Performance Loss} = \frac{30.0\text{ gpm/ft} - 15.0\text{ gpm/ft}}{30.0\text{ gpm/ft}} \times 100\% = 50.0\%\text{ reduction}$ Conclusion: The well's specific capacity has dropped by $50%$. Because the static water level remained constant, the increased drawdown indicates severe well screen clogging or aquifer incrustation, requiring acid rehabilitation, polyphosphate dispersant surging, or mechanical brushing.

---

Well Interference Dynamics

When two or more production wells are installed in close proximity, their individual cones of depression may overlap during simultaneous pumping. This phenomenon is known as well interference.

           Well A                          Well B
             │                               │
  ═══════════╪═══════════════════════════════╪═══════════ Ground Level
             │                               │
   Static WL ┼ - - - - - - - - - - - - - - - ┼ - - - - - - - - - -
            ╱ ╲                             ╱ ╲
           ╱   ╲       Individual Cones    ╱   ╲
          ╱     ╲                         ╱     ╲
         ╱       ╲                       ╱       ╲
  ──────┼─────────╲─────────────────────╱─────────┼─────
        │          ╲                   ╱          │
        │           ╲  Overlapping    ╱           │
        │            ╲ Interference  ╱            │
        │             ▼ Area        ▼             │
        │            ░░░░░░░░░░░░░░░              │
        │           Compounded Drawdown           │

Operational Consequences of Well Interference

• Additive Drawdown: Total drawdown in each well equals its own pumping drawdown plus the additional drawdown induced by adjacent pumping wells ($DD_{\text{total}} = DD_{\text{self}} + DD_{\text{adjacent}}$).
• Reduced Specific Capacity: Lower effective water levels force pumps to operate against higher total dynamic head (TDH).
• Increased Electrical Energy Costs: Pumping from deeper pumping water levels requires higher motor horsepower and electrical power consumption per million gallons produced.
• Risk of Pump Cavitation: Excessive cumulative drawdown can expose pump intake bowls or cause cascading water within the casing, leading to air entrainment, mechanical vibration, and impeller damage.

---

MoDNR Source-Water Protection & New-Well Isolation

Community-water wells are governed by Missouri public-drinking-water requirements and PUB2489. The separate 10 CSR 23 well-construction program primarily addresses non-public wells. For public systems, the department reviews the proposed site, casing and well construction before work begins.

Source-Water Protection Area Delineation

Missouri source-water protection planning is locally implemented and voluntary. A state source-water assessment delineates the area that may influence a well and inventories potential contaminant sources. Depending on the available hydrogeologic information, delineation may use a fixed radius or a calculated capture area; the department guidance describes a calculated 20-year groundwater-flow period. Do not substitute a generic three-zone diagram or an assumed 100-foot ownership zone for the system-specific assessment.

PUB2489 Table 3 — New-Well Isolation Radii

Unless geology and aquifer hydraulics justify a different distance, or the department approves a lesser distance from the engineering report, new community-well sites use these minimum radii:

The owner should control enough land to preserve these distances and should pursue a wellhead-protection program. The department may require greater radii for unconsolidated formations or site-specific hydrogeology.