5.1 Well Hydrology, Aquifers & North Carolina Well Construction Standards
Key Takeaways
- Aquifers are unconfined (water table) or confined (artesian); a confined aquifer's potentiometric surface can stand above the top of the aquifer and even above ground, producing a flowing artesian well.
- Drawdown is the difference between the static water level and the pumping water level, and specific capacity equals yield in gallons per minute divided by drawdown in feet — a declining specific capacity is the earliest indicator of well or screen problems.
- North Carolina's Well Construction Standards (15A NCAC 02C) and the Public Water Supply rules require grouted casing, a watertight sanitary seal, a screened vent, and a casing terminating above grade and above flood level.
- Public water supply wells must be protected by minimum separation distances from sources of contamination — the gravity sewer design criteria require sewers to stay 100 feet from any water supply source, and never less than 50 feet from a public water supply well even with ferrous pressure-tested pipe.
- Wells must be disinfected under ANSI/AWWA C654-13 after construction, servicing, or any event that could contaminate the water, and may not return to service until bacteriological results from a certified laboratory are satisfactory, with records kept three years.
5.1 Well Hydrology, Aquifers & North Carolina Well Construction Standards
North Carolina's groundwater divides along the Fall Line. In the Piedmont and Mountains, water is produced from fractured crystalline bedrock — modest yields (often 10 to 50 gpm), soft and slightly acidic water, with localized radon, radium, arsenic, or uranium. In the Coastal Plain, thick sedimentary sand and limestone aquifers (Castle Hayne, Peedee, Black Creek) yield hundreds to thousands of gallons per minute of anaerobic, mineralized water high in iron, manganese, and sometimes hydrogen sulfide or chloride near the coast.
1. Aquifer types
| Type | Description | Water level behavior |
|---|---|---|
| Unconfined (water table) | Upper surface of saturation is at atmospheric pressure | Water stands at the water table; most vulnerable to surface contamination |
| Confined (artesian) | Saturated formation between confining layers | Water rises above the top of the aquifer to the potentiometric surface; a flowing artesian well occurs when that surface is above ground |
| Perched | Local saturated zone above an impermeable lens | Small, seasonal, unreliable |
Groundwater under the direct influence of surface water (GWUDI) is a regulatory determination, not a geologic one: a well showing rapid turbidity or temperature response to rainfall, or containing large-diameter pathogens or insect parts, is treated as surface water and must meet the Surface Water Treatment Rule.
2. Hydraulics of a pumping well
Static water level ______________________________
\ /
Drawdown \ cone of /
Pumping water level ________\ depression /______
\___________/
Well screen
- Static water level: the level with the pump off and the aquifer recovered.
- Pumping (dynamic) water level: the level with the pump running at a stated rate.
- Drawdown = static level − pumping level.
- Cone of depression: the conical drawdown surface around the well; its radius of influence determines whether nearby wells interfere with one another.
- Specific capacity = yield (gpm) ÷ drawdown (ft). A well producing 300 gpm with 25 feet of drawdown has a specific capacity of 12 gpm/ft. Trending this number is the single best diagnostic an operator has: a falling specific capacity at the same pumping rate means screen encrustation, biofouling, sand accumulation, or aquifer decline, and it shows up long before the pump fails.
- Well efficiency compares actual drawdown to theoretical aquifer drawdown; step-drawdown testing separates aquifer loss from well loss.
3. Construction and sanitary protection
North Carolina well construction is governed by the Well Construction Standards, 15A NCAC 02C, with additional public water supply requirements in 15A NCAC 18C .0402. The features an operator must be able to inspect:
- Casing extending from above grade into the producing formation, with a grouted annular space to seal out surface water and shallow contamination.
- Casing termination above grade — and above the regulatory flood elevation — so that surface water cannot enter, with a sloped concrete pad draining away from the well.
- Watertight, vermin-proof well cap or sanitary seal, with any vent turned downward and screened (24-mesh is the usual standard) to keep insects out while allowing pressure equalization.
- Sampling tap on the discharge, upstream of any treatment, plus a check valve, flow meter, and an air/vacuum relief where needed.
- Access port for water level measurement (airline, sounder, or transducer).
- No cross-connections: no pump primer connection, hose bib, or chemical injection point that could allow backflow into the well.
Separation distances. Wells must be sited away from contamination sources. From the wastewater side, North Carolina's Gravity Sewer Minimum Design Criteria require 100 feet between a sewer and any private or public water supply source, including WS-I waters and Class I or II impounded reservoirs; where that cannot be met, ferrous pipe with water-main-grade joints pressure tested to 150 psi is required, and the separation may never be less than 25 feet from a private well or 50 feet from a public water supply well.
4. Well maintenance, rehabilitation, and abandonment
- Symptoms of trouble: falling specific capacity, sand pumping, rising motor amperage, air entrainment, or sudden turbidity or bacteriological positives.
- Rehabilitation: mechanical brushing and surging, chemical treatment with acids for mineral encrustation or with chlorine and dispersants for iron bacteria slimes, then redevelopment and disinfection.
- Abandonment: unused wells are direct conduits to the aquifer and must be properly plugged under 15A NCAC 02C — an abandoned well left open is one of the most common causes of local aquifer contamination.
5. Disinfection and records (15A NCAC 18C .1002)
After construction, servicing, maintenance, or any other activity or event that might lead to contamination of the water, wells must be disinfected in accordance with ANSI/AWWA C654-13, "Disinfection of Wells." The well must not be placed back into service until bacteriological results from representative samples analyzed by a certified laboratory are satisfactory, and records demonstrating compliance with C654-13 must be available for three years for Department inspection.
[!WARNING] A pump pull is a contamination event. Any time the well is opened — pump replacement, column repair, video inspection, or a dropped tool — the well must be disinfected and cleared bacteriologically before it returns to service. This is one of the most frequently cited violations at small systems.
A well is tested at 240 gpm. The static water level is 42 feet and the pumping water level stabilizes at 82 feet. What is the specific capacity?
Under 15A NCAC 18C .1002, what must occur after a well pump is pulled and reinstalled?
Which situation describes a flowing artesian well?