3.4 Wells, Aquifers & Groundwater Hydraulics

Why Groundwater Hydraulics Is Heavily Tested

The Source Water category carries 25 questions at T1 and T2, 20 at T3, and 15 at T4, and the Expected Range of Knowledge lists these as explicit T1-T4 abilities:

• Ability to calculate well drawdown
• Ability to calculate well specific capacity
• Ability to calculate well head pressure
• Ability to determine water level in a storage facility, reservoir, or well
• Knowledge of well components, well depth measurement procedures, well drawdown measurement techniques, the characteristics of aquifers, and the hydrologic cycle

On the distribution side, Wells (New and Abandoned) and Groundwater and Wells appear in two separate categories. In a state where roughly 40 percent of drinking water in a normal year and far more in a drought year comes from groundwater, this is core operator knowledge.

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Aquifer Types

Porosity is the fraction of aquifer volume that is void space. Specific yield is the fraction that actually drains under gravity. Permeability (hydraulic conductivity) is how readily water moves through the material. Coarse sand and gravel have moderate porosity but excellent permeability; clay has very high porosity and almost no permeability - which is why clay makes an aquitard, not an aquifer.

The cone of depression is the inverted cone of lowered water surface around a pumping well. Its radius of influence expands with pumping rate and duration and is much wider and flatter in a high-permeability aquifer. Well interference occurs when cones of depression overlap, increasing drawdown in both wells and reducing yield.

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Well Components

Working from the aquifer up:

• Borehole and gravel pack (filter pack) - graded sand or gravel placed in the annulus opposite the screen to stabilize the formation and reduce sand pumping
• Well screen (perforated casing) - the intake; slot size is chosen against the formation gradation
• Blank casing - the conduit from screen to surface
• Annular seal (sanitary seal) - cement or bentonite grout in the annulus, the single most important contamination barrier; a failed or short seal is the most common route for surface contamination into a well
• Surface seal and well pad - sloped concrete apron shedding water away
• Wellhead - terminates a minimum of 18 inches above finished grade under 22 CCR 64560
• Pump - typically a vertical turbine (motor at the surface, line shaft to bowls) or a submersible (motor below the bowls, downhole)
• Sounding tube (air line or transducer access) - the port used to measure water levels
• Check valve, air/vacuum relief, flow meter, sampling tap (non-threaded), chlorination point, and blow-off to waste

The Four Calculations You Must Be Able to Do

1. Drawdown

$\text{Drawdown (ft)} = \text{Pumping Water Level} - \text{Static Water Level}$

Static water level is measured with the pump off long enough to fully recover. Pumping water level is measured while the pump runs at a steady rate. Both are usually reported as depth below a fixed reference point at the wellhead.

Example: static level 84 ft; after 4 hours of pumping the level stabilizes at 152 ft. Drawdown = 152 - 84 = 68 ft.

2. Specific Capacity

$\text{Specific Capacity (gpm/ft)} = \frac{\text{Well Yield (gpm)}}{\text{Drawdown (ft)}}$

Example, continuing above: the well produces 850 gpm at 68 ft of drawdown. Specific capacity = 850 / 68 = 12.5 gpm/ft.

Specific capacity is the single best trend indicator of well health, because it normalizes yield against drawdown. Track it on the same schedule every year.

3. Well Head Pressure and Head Conversion

$\text{Head (ft)} = \text{Pressure (psi)} \times 2.31 \qquad \text{Pressure (psi)} = \text{Head (ft)} \times 0.433$

Example: a discharge gauge reads 62 psi. Head = 62 x 2.31 = 143.2 ft.

4. Well Volume (for disinfection and sanitary sampling)

$\text{Volume (gal)} = 0.785 \times D^{2}_{(\text{ft})} \times \text{Depth of water (ft)} \times 7.48$

For a casing diameter in inches, the shortcut $0.0408 \times D^{2}_{(\text{in})}$ gives gallons per foot of water column. A 12-inch casing holds 0.0408 x 144 = 5.88 gallons per foot. With 200 ft of water standing in the casing, the well holds about 1,175 gallons - the number you need to compute a chlorine charge for AWWA C654 disinfection.

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Reading Well Performance Trends

GWUDI Determinations

Groundwater under the direct influence of surface water must be treated as a surface water for regulatory purposes, which brings the Surface Water Treatment Rules, filtration, and CT requirements into play. Evidence includes:

• Microscopic particulate analysis (MPA) finding surface bio-indicators: diatoms, green algae, rotifers, insect parts, plant debris, Giardia or Cryptosporidium
• Rapid, correlated response of temperature, turbidity, conductivity, or pH to precipitation or surface water stage
• Shallow construction near a surface water body, or a screen set in a hydraulically connected alluvial formation

Stable temperature and mineral chemistry argue against direct influence. A high TDS or hardness result is a mineral fingerprint, not a surface water indicator.