4.2 Groundwater Sources and Wells: Hydraulics, Construction and Protection
Key Takeaways
Drawdown equals pumping water level minus static water level, and specific capacity equals well yield divided by drawdown, in gpm per foot.
With an air line, depth to water equals the air line length minus the gauge reading times 2.31 feet per psi.
OHA requires the water supplier to own or hold a recorded easement over the area within 100 feet of a well; listed hazards are barred from that zone, and gravity sewers and septic tanks must be at least 50 feet away.
Oregon well heads need a casing at least 12 inches above a sloped concrete slab, a screened vent, a sample tap, pump-to-waste piping, flow measurement and a way to measure water levels.
A properly built well whose water meets microbiological standards may be used without disinfection, but a total coliform-positive routine sample triggers source sampling under the Ground Water Rule provisions.
Groundwater Basics
Groundwater fills the pore spaces and fractures of geologic formations called aquifers. Two aquifer types appear on every source-water exam:
| Aquifer | Description | Operating implications |
|---|---|---|
| Unconfined (water table) | No low-permeability layer above; the water table rises and falls with recharge | More vulnerable to surface contamination; shallow alluvial wells along Oregon rivers may be evaluated as GWUDI |
| Confined (artesian) | Overlain by clay or unfractured rock; water rises in the well above the top of the aquifer | Better natural protection; the pressure surface is the piezometric surface, and a flowing artesian well rises above ground |
OHA defines confined and unconfined wells this way in OAR 333-061-0050(1)(j) and requires both to be built to the Oregon Water Resources Department's well construction standards in OAR 690-200 through 690-220.
Groundwater under the direct influence of surface water (GWUDI) is groundwater with significant surface-water characteristics, such as rapid shifts in turbidity, temperature or conductivity, or the presence of insects, algae or large-diameter pathogens. OHA evaluates every new well or spring for possible direct influence (OAR 333-061-0032(7)). A GWUDI source is treated like surface water and must meet filtration and disinfection requirements.
Well Hydraulics
When a pump starts, water levels near the well fall and form a cone of depression. The extent of that cone is the zone of influence.
| Term | Meaning |
|---|---|
| Static water level (SWL) | Distance from a reference point (top of casing) down to water when the pump has been off long enough to recover |
| Pumping water level (PWL) | Distance to water while the pump runs at a steady rate |
| Drawdown | PWL minus SWL, in feet |
| Specific capacity | Yield (gpm) divided by drawdown (ft), in gpm per foot of drawdown |
| Recovery | The return toward SWL after pumping stops |
A falling specific capacity over time is a classic sign that the screen is plugging (encrustation or biofouling) or that the aquifer is being overdrawn.
Worked examples
Specific capacity. A well pumps 300 gpm. The SWL is 42 ft and the PWL is 92 ft.
- Drawdown = 92 − 42 = 50 ft
- Specific capacity = 300 ÷ 50 = 6 gpm per ft
Air line. Many wells measure water level with an air line, a small tube of known length running down beside the pump. Air is pumped in until the gauge pressure stops rising; that pressure equals the water column above the tube bottom.
With a 200 ft air line reading 45 psi: 45 × 2.31 = 104 ft submerged, so the depth to water is 200 − 104 = 96 ft.
Oregon Well Construction and Protection Rules (OAR 333-061-0050(2))
| Requirement | Oregon standard |
|---|---|
| Control of the well site | The supplier owns, or holds a perpetual restrictive easement recorded with the county over, all land within 100 feet of the well (public land may use a permit instead) |
| Prohibited within 100 ft | Privies, drainfields, cesspools, solid waste sites, pressure sewer lines, buried fuel tanks, animal yards, chemical storage or application, fuel transfer, vehicle maintenance, cemeteries, unapproved or improperly abandoned wells, and similar hazards, unless OHA waives the restriction |
| Within 50 ft | No gravity sewer line or septic tank |
| Flooding | Avoid flood-prone sites; otherwise mound the area and extend the casing at least 2 ft above the 100-year flood level |
| Casing and slab | Reinforced concrete slab sloped away from the casing; casing top at least 12 in above the slab |
| Pump and wellhead | Turbine pumps sealed to the casing (water-lubricated or food-grade-lubricated bearings); submersibles with a watertight sanitary seal; screened, downward-facing casing vent (not required with pitless units) |
| Appurtenances | Water-level measurement provision; raw sample tap on the discharge; piping to pump the whole flow to waste; a way to measure each well's output |
| Before use | Lab analyses under OAR 333-061-0036, pump test reports for unconfined wells, pump performance data, any disinfection proposal, and the GWUDI determination |
Missing or failing items here are sanitary survey significant deficiencies: a casing or sanitary seal that is not watertight, or an unscreened vent.
Disinfection Decisions and Triggered Monitoring
Oregon does not require every well to be disinfected. Water from wells built to these standards that consistently meets microbiological standards may be served without treatment (OAR 333-061-0050(5)). When disinfection is required for groundwater, the design must provide 4-log virus inactivation or removal. Alternatively, it must achieve either 0.2 mg/L free chlorine after 30 minutes of contact, or 2.0 mg/L combined chlorine after 3 hours, before the first user under all flow conditions.
Ground Water Rule provisions add triggered source monitoring. After a routine distribution sample is total coliform-positive, the supplier collects raw-water samples from each well in use. A confirmed E. coli-positive source sample leads to corrective action: fixing the defect, providing an alternate source, eliminating the source, or installing treatment that reliably achieves 4-log virus inactivation.
Well Problems Operators Must Recognize
- Encrustation. Iron and manganese oxides, carbonate scale or iron-bacteria slime plug the screen and gravel pack. Specific capacity drops, and the pump may run near its shutoff head. Rehabilitation uses mechanical surging and brushing plus acids, chlorine or specialized cleaners.
- Sand pumping. Usually a sign of a damaged screen or poor development. It abrades pump bowls and fills distribution lines with sediment.
- Cascading water. Water entering above the pumping level tumbles down the casing, entraining air and promoting corrosion and biofouling.
- Pumping water level near the pump intake. Leads to cavitation or air entrainment; check whether drawdown has grown or the well is being over-pumped.
- Contamination after work. Wells are disinfected after construction, repair or pump pulls, following AWWA C654 methods, then flushed to waste and sampled for coliform before returning to service.
Many Oregon systems also run a wellhead protection program. OHA can certify a program that meets OAR 333-061-0057 and DEQ's groundwater rules in OAR 340-040. The program delineates the protection area and inventories potential contaminant sources, and it adds management and contingency plans coordinated with county emergency plans.
The Annular Seal
The most important protection for a well is a watertight seal between the casing and the borehole. Without it, surface water can run down the outside of the casing straight into the aquifer.
- Oregon Water Resources Department well construction standards (OAR 690-210) require water supply wells to have watertight, unperforated casing and an upper oversize drillhole four inches larger than the casing, both extending at least 18 feet below land surface. The annular space is filled with cement or bentonite grout. Deeper seals are required in some geologic settings, such as clay beds or consolidated rock.
- On inspection, look for a casing that ends at or below grade, a cracked or missing well cap or vent screen, a pump base that is not watertight, and ponding or slopes draining toward the wellhead. Any of these is a pathway for contamination and can become a sanitary survey deficiency.
A well produces 450 gpm. The static water level is 38 ft and the pumping water level is 128 ft below the top of casing. What is the specific capacity?
3.5 gpm/ft
11.8 gpm/ft
5.0 gpm/ft
90 gpm/ft
An air line in a well is 180 ft long and the gauge reads 52 psi while the pump runs. What is the pumping water level?
52 ft
300 ft
120 ft
60 ft
Under OAR 333-061-0050, which installation may be no closer than 50 feet to a public water system well, rather than 100 feet?
A confined livestock feedlot
A gravity sewer line or septic tank
A subsurface sewage drainfield
An underground fuel storage tank
Sections you finish are checked off in the contents.