2.6 Groundwater Sources, Well Hydraulics, Static and Pumping Levels & Specific Capacity

Why groundwater knowledge is on the exam

Roughly one in seven scored questions on the WPI Water Treatment Operator Class I exam falls under Source Water Characteristics, and the outline is explicit: measure static water level and pumping levels of wells, inspect groundwater sources for issues that may affect water quality such as contamination, flooding, and wellhead protection, and evaluate biological, chemical, and physical source characteristics. In Colorado, where the majority of small public water systems are groundwater systems, these are daily tasks rather than theory.

Aquifers and the vocabulary of a well

An aquifer is a saturated geologic formation that will yield usable quantities of water to a well. An aquitard or confining layer is a low-permeability unit that restricts flow.

• Unconfined (water table) aquifer. The upper surface of the saturated zone is at atmospheric pressure. It is recharged directly from precipitation and surface infiltration above it, which also means it is the most vulnerable to surface contamination.
• Confined (artesian) aquifer. Saturated material bounded above and below by confining layers. Water in the aquifer is under pressure, so the level in a well rises above the top of the aquifer to the potentiometric surface. Where that surface is above ground, the well flows without pumping.
• Perched aquifer. A localized saturated lens above the regional water table, sitting on a discontinuous confining layer. Perched zones often produce erratic yields and poor water quality.

Key measurements, all taken from a fixed measuring point such as the top of the casing:

Water levels are read with an electric sounding tape (a probe closes a circuit at the water surface and a light or buzzer sounds), an air line and pressure gauge (gauge reading in psi times 2.31 gives feet of submergence), or a pressure transducer feeding SCADA.

Worked example: specific capacity

A well has a static water level of 84 ft below the measuring point. Pumping at 320 gpm, the level stabilizes at 146 ft.

• Drawdown = 146 ft − 84 ft = 62 ft
• Specific capacity = 320 gpm ÷ 62 ft = 5.2 gpm per foot

Six months later the same pump delivers 300 gpm with a pumping level of 178 ft:

• Drawdown = 178 − 84 = 94 ft
• Specific capacity = 300 ÷ 94 = 3.2 gpm per foot

Specific capacity has fallen by nearly 40 percent. That is the diagnostic signal. Because static level is unchanged, the regional aquifer has not dropped; the loss is at the well itself — incrustation of carbonate or iron and manganese scale on the screen, biofouling by iron bacteria, or sand plugging of the gravel pack. The corrective actions are mechanical (surging, brushing, jetting) and chemical (acidization or chlorination) well rehabilitation, not simply installing a bigger pump.

Groundwater quality and the GWUDI question

Groundwater is filtered by the formation, so it is usually low in turbidity and free of Giardia and Cryptosporidium. That advantage disappears if the well is actually drawing surface water through fractures, karst, or shallow alluvium. A source determined to be groundwater under the direct influence of surface water (GWUDI) must be treated as a surface water: filtration and disinfection under the Surface Water Treatment Rules. Indicators the Division evaluates include rapid turbidity, temperature, or conductivity response to precipitation and runoff; the presence of insect parts, algae, or Giardia cysts; shallow depth; and proximity to a stream. A Colorado Class T certificate is valid only on groundwater not under the direct influence of surface water — that is one reason the determination matters to operators.

Characteristic groundwater quality problems are chemical and physical rather than microbial:

• Iron and manganese. Dissolved in the reduced aquifer, they oxidize on contact with air or chlorine and produce red or black water and stained laundry. Secondary standards are 0.3 mg/L iron and 0.05 mg/L manganese.
• Hardness. Calcium and magnesium dissolved from limestone and dolomite; drives scaling and softening decisions.
• Hydrogen sulfide. Rotten-egg odor from sulfate-reducing bacteria; removed by aeration or oxidation.
• Nitrate. From septic systems, fertilizer, and feedlots; the MCL is 10 mg/L as nitrogen and an exceedance is acute.
• Radionuclides, arsenic, and fluoride. Naturally occurring in parts of Colorado, particularly in some deep bedrock aquifers.
• Methane and carbon dioxide. Cause gas binding in pumps and aggressive, corrosive water.

Sanitary protection of the wellhead

Inspecting groundwater sources for issues that may affect water quality is a listed job task. A compliant wellhead has:

1. Casing extended above grade — commonly at least 12 inches above the finished floor or ground, and above the highest known flood elevation.
2. A sanitary well seal or welded cap that prevents entry of surface water, insects, and vermin along the casing and around the discharge piping and wiring.
3. A downward-facing, screened vent using corrosion-resistant 24-mesh screen.
4. A grouted annular space to seal the borehole against downward migration of surface water along the outside of the casing.
5. Ground sloped away from the well and a concrete pad or floor drained to daylight.
6. A raw-water sampling tap ahead of any treatment, and a means of measuring water level.
7. An air gap on any discharge to waste so that no cross connection exists.

When an operator finds the casing at grade in a pit that floods, a missing well seal, an unscreened vent, or a pressure tank drain submerged in a sump, those are the significant deficiencies a sanitary survey will cite. Wellhead protection extends the same idea outward: delineating the contribution area, inventorying potential contaminant sources such as fuel tanks, septic systems, and chemical storage, and managing land use inside the protection zone.