3.1 Groundwater Sources, Well Components & Water Level Measurement

3.1 Groundwater Sources, Well Components & Water Level Measurement

Roughly forty percent of Arizona's water supply comes from groundwater, and for small and rural systems the share is close to one hundred percent. The Need-to-Know Criteria explicitly require operators to "measure static water level and pumping levels of wells," "inspect ground water sources for issues that may affect water quality," and understand "wells and related equipment." These are not abstract hydrogeology questions — they are measurements you take with a tape and a stopwatch.

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Aquifers and How Water Gets Into Them

An aquifer is a saturated geologic formation that yields usable quantities of water to a well. Arizona's basin-and-range geology produced deep alluvial aquifers: valleys filled with hundreds or thousands of feet of sand, gravel, silt, and clay eroded from surrounding mountains.

• An unconfined (water table) aquifer has its upper surface open to atmospheric pressure through permeable material. The water table rises and falls with recharge and pumping, and the aquifer is comparatively vulnerable to surface contamination.
• A confined (artesian) aquifer is sandwiched between low-permeability confining layers. Water in it is under pressure greater than atmospheric, so the level in a well rises above the top of the aquifer. A flowing artesian well is one where that pressure surface sits above ground level.
• The potentiometric surface is the level to which water rises in wells tapping a confined aquifer.

Porosity is the fraction of a formation that is void space; permeability (or hydraulic conductivity) is how readily water moves through it. The distinction matters operationally: clay has high porosity but very low permeability, so a clay layer holds water without yielding it and functions as a confining layer.

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Well Construction and the Sanitary Barrier

A properly constructed well is a sealed conduit from a protected aquifer to your pump — nothing else should get in.

The casing should extend a minimum height above grade (commonly 12 to 18 inches, and higher in floodplains), and the surrounding surface should slope away so stormwater drains off rather than pooling at the wellhead. During a sanitary survey these are exactly the items an inspector photographs.

[!WARNING] A well vent that points upward without a screen, a cracked concrete pad, or a casing terminating at or below grade are all sanitary defects that can result in a violation and, more importantly, a real contamination event after a monsoon storm.

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Static Level, Pumping Level, and Drawdown

These three measurements, taken consistently, tell you more about well health than any other routine data you collect.

• Static water level (SWL): depth from a fixed reference point (usually the top of casing) to the water surface when the pump has been off long enough for the water level to fully recover. A level taken ten minutes after shutdown is not a static level.
• Pumping water level (PWL): depth to the water surface while the pump operates at a steady, known rate, after the level has stabilized.
• Drawdown: the difference.

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

Worked example. A well has a static water level of 185 ft below the top of casing. With the pump running at 450 gpm the level stabilizes at 242 ft.

$\text{Drawdown} = 242 - 185 = 57\text{ ft}$

Specific Capacity — the Number That Trends

$\text{Specific Capacity} = \frac{\text{Pumping Rate (gpm)}}{\text{Drawdown (ft)}}$

$\text{Specific Capacity} = \frac{450}{57} = 7.9\text{ gpm/ft}$

Specific capacity normalizes yield against drawdown, so it is comparable over time even if you pump at slightly different rates. A steady decline in specific capacity at a constant pumping rate is the classic signature of a fouling well — mineral incrustation (calcium carbonate, iron and manganese oxides) or biofouling (iron bacteria slimes) plugging the screen and gravel pack. Rehabilitation options include mechanical brushing and surging, acid treatment, and chlorine shock, generally in escalating order.

If specific capacity holds steady but both levels drop year over year, the problem is not the well — it is regional aquifer decline, an issue of real consequence across Arizona's pumped basins.

Measuring the Levels

• Electric sounder (water level meter): a marked cable with a probe that closes a circuit and beeps at contact. The standard method.
• Air line: a fixed tube of known length; applied air pressure at the point where bubbling begins converts to submergence at 2.31 ft per psi.
• Steel tape with chalk: a reference method; chalked tape lowered past the water shows a clean wet mark.
• Pressure transducer: continuous logging, usually feeding SCADA.

Always measure from the same reference point, and record it. A level "185 feet" means nothing if the next operator measures from ground surface instead of top of casing.

Groundwater Under the Direct Influence of Surface Water

Not every well is legally groundwater. Groundwater under the direct influence of surface water (GWUDI) is a source that, despite being pumped from a well, receives surface water quickly enough that surface pathogens can reach it. Indicators include:

• Significant occurrence of insects, other macroorganisms, algae, or large-diameter pathogens such as Giardia lamblia or Cryptosporidium
• Significant and relatively rapid shifts in turbidity, temperature, conductivity, or pH that track nearby surface water conditions
• Shallow wells, wells in alluvium adjacent to a river, infiltration galleries, and springs

The consequence is large: a GWUDI source is regulated as surface water. It must comply with the Surface Water Treatment Rules — filtration, disinfection with demonstrated log inactivation, and continuous turbidity monitoring — rather than the far lighter Ground Water Rule regime. Reclassification from groundwater to GWUDI can obligate a small system to build a filtration plant, so the determination is consequential and states evaluate it carefully.

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Wellhead and Source Water Protection

The Need-to-Know Criteria require operators to "follow source water protection plans and watershed management plans" and "inspect ground water sources for issues that may affect water quality (e.g., contamination, flooding, well head protection)."

A wellhead protection area is delineated as concentric zones based on the time of travel of groundwater toward the well:

Routine operator tasks inside these zones include maintaining a potential contaminant source inventory (fuel tanks, dry wells, septic systems, agricultural chemical storage, abandoned wells), inspecting after flooding, and ensuring abandoned wells are properly plugged — an unplugged abandoned well is a direct vertical conduit from the surface into the aquifer and one of the highest-risk features on any inventory.

Operators are also expected to educate the community on source water protection and conservation, a task the NTK lists explicitly. In practice this means well tours, school programs, consumer confidence report messaging, and coordinating with county planning on land use near wellfields.