3.5 Wells, Water Levels & Sanitary Condition

3.5 Wells, Water Levels & Sanitary Condition

Most Class I distribution systems are groundwater systems, and in a small utility the same operator who exercises valves also reads the well. The Need-to-Know Criteria reflect that: wells appear in Distribution System Components ("understand wells and related equipment — measure static water levels and pumping water levels") and again twice in Equipment Installation, Operation, Maintenance, and Repair ("maintain well and related equipment" and "maintain the sanitary condition of the well"). You are not expected to design or drill a well, but you are expected to read its levels, keep it sanitary, and recognize when it is failing.

Well Components

Static Level, Pumping Level, and Drawdown

Three measurements describe how a well is performing, and the exam expects you to tell them apart:

• Static water level (SWL) — the depth from a fixed reference point (normally the top of the casing) down to the water surface when the pump has been off long enough for the level to recover and stabilize.
• Pumping water level (PWL) — the depth to the water surface while the pump is running at a steady rate.
• Drawdown — the difference between them: Drawdown = PWL − SWL, expressed in feet.

When a well pumps, water flows toward the well from all directions and the water table around it drops into a cone of depression. The horizontal distance out to where drawdown becomes negligible is the radius of influence. Both grow as pumping rate or duration increases, which is why nearby wells can interfere with each other.

Worked example. A well has a static water level of 60 ft and, after pumping steadily at 250 gpm, the level settles at 95 ft.

Drawdown = 95 − 60 = 35 ft.

Specific Capacity — The Well's Report Card

Divide the yield by the drawdown and you get specific capacity, the single most useful trend number an operator tracks on a well:

Specific capacity (gpm/ft) = Yield (gpm) ÷ Drawdown (ft)

For the well above: 250 gpm ÷ 35 ft = 7.1 gpm/ft.

Specific capacity is only meaningful as a trend. Record it at the same pumping rate each time. A steady decline — say from 7.1 to 4.5 gpm/ft over several years — means the well is losing efficiency, and there are only a few causes: mineral encrustation (calcium carbonate, iron) plugging the screen slots, biofouling by iron bacteria, sand infiltration filling the bottom, or a genuine aquifer decline that also shows up as a falling static level. The distinguishing clue is the static level: if static is stable but drawdown grows, the problem is at the well; if static itself is falling, the problem is the aquifer.

Measuring Water Levels

• Electric sounder (water-level tape) — a weighted probe on a marked tape that beeps or lights when it touches water. The most common and most accurate field method.
• Airline and pressure gauge — a small tube of known length runs down the casing; air is pumped in until the gauge stops rising, and the reading converts to submergence at 2.31 ft per psi.
• Pressure transducer — a submerged sensor reporting continuous level to SCADA, which is how larger systems trend drawdown automatically.

Always measure from the same reference point and record it. A level "60 ft" from the top of casing and "58 ft" from ground surface are the same water, described two ways, and mixing them corrupts the trend.

Maintaining the Sanitary Condition of the Well

A well is a direct opening into the aquifer, so sanitary defects are contamination pathways. The recurring requirements — checked on every sanitary survey — are:

• Casing height — terminates at least 12 in above finished grade, and well above any known flood level.
• Surface drainage — the ground slopes away from the casing so runoff cannot pond at the wellhead.
• Sanitary seal / watertight cap — gasketed and bolted; a pitless adapter installation uses a watertight cap.
• Screened vent — turned down, secured, tamper-resistant, and screened at 24-mesh or finer to exclude insects.
• Grouted annulus — no unsealed space alongside the casing.
• No cross-connections — the discharge, blow-off, and any chemical feed are protected against backflow.
• Setbacks — the required separation from septic systems, sewers, and fuel storage is maintained, and the wellhead protection area designated under the state's SDWA §1428 program is respected.

Maintenance and Disinfection

Routine well maintenance means trending water levels, yield, and motor amps; inspecting the cap, vent, and slab; testing the raw sample tap; and scheduling pump pulls on condition rather than after failure. Rehabilitation of a plugged screen uses mechanical brushing/surging, chemical treatment, or both.

Any time the well is opened — new construction, pump pull, screen work, or a suspected contamination event — it must be disinfected before it goes back into service. AWWA C654, Disinfection of Wells, is the governing standard: the water column is dosed to at least 100 mg/L chlorine initially, at least 50 mg/L free chlorine is maintained throughout the column, and contact is held for at least 8 hours. Afterward the well is pumped to waste until the residual matches normal distribution levels, and bacteriological samples must come back coliform-free before the source is returned to service. Do not confuse this with C651 (mains) or C652 (storage) — the exam mixes the three deliberately.