NGWA CWD flashcards

32 cards. Read the term and the definition here. The deck at the top of this page is optional.

Annular velocity formula

In feet per minute, annular velocity equals 24.5 times pump output in gallons per minute divided by the difference between hole diameter squared and pipe diameter squared, both in inches.

Hydrostatic pressure formula

Hydrostatic pressure in psi equals 0.433 times the fluid density in pounds per gallon times true vertical depth in feet. It depends on vertical depth, not measured length.

Hole volume per foot

Gallons per foot of borehole equals diameter in inches squared divided by 24.5. A 6-inch hole holds about 1.47 gallons per foot.

Cable tool drilling action

A heavy bit is repeatedly lifted and dropped to crush formation, and a bailer removes the cuttings. The method works well in unconsolidated sand, gravel, and boulder formations.

Rotary drilling circulation

A rotating bit cuts formation while drilling fluid circulates down the drill pipe and up the annulus, carrying cuttings to the surface and cooling the bit.

Fracture gradient limit

The pressure at which the formation breaks down and accepts fluid. Mud weight and grout placement pressure must remain below this value to avoid loss of circulation.

Well development purpose

Surging, jetting, air lifting, or overpumping removes drilling fines and filter cake from the formation near the screen, restoring permeability and improving yield and water quality.

Grout volume estimate

Multiply annular capacity between casing outside diameter and borehole wall by the length to be sealed, then add 10 to 25 percent to allow for washouts and fractures.

Proper abandonment seal

The full borehole is sealed with low-permeability material such as neat cement or bentonite grout, plus a surface seal. Backfilling with cuttings is not an acceptable abandonment method.

Water level measuring point

Static water level is measured from a surveyed reference, usually top of casing. Adding measuring-point elevation converts depth to water into hydraulic head.

Casing setting depth

Casing is set into a competent, low-permeability confining layer so the annulus seal isolates the target aquifer from shallow groundwater and surface contamination.

Drilling fluid functions

Fluid cools and lubricates the bit, lifts cuttings, stabilizes the borehole wall by forming filter cake, and controls hydrostatic pressure against formation fluids.

Screen slot selection

Slot openings are sized to retain most of the filter pack, commonly keeping 90 to 100 percent of the pack. The pack itself is sized from the formation grain distribution.

Filter pack sizing rule

Filter pack grains are typically four to five times the d50 grain size of the formation. The pack must be clean, well rounded, and free of fines.

Filter pack placement

The pack extends at least two feet above the top of the screen. A bentonite seal two to five feet thick is placed above the pack and hydrated before grouting.

Tremie grouting method

Grout is pumped through a tremie pipe from the bottom of the annulus upward. The discharge end stays submerged in grout to prevent voids, bridging, and dilution.

Neat cement grout mix

Neat cement is mixed near 0.4 to 0.5 water-to-cement ratio by weight, roughly six to seven gallons per 94-pound sack, producing a dense, low-permeability seal.

Monitoring well materials

Flush-threaded Schedule 40 PVC, stainless steel, or PTFE are common. Solvent-cemented joints are avoided because solvents can contaminate groundwater samples.

Hydraulic gradient formula

Gradient equals the head difference between two points divided by the horizontal distance between them. Groundwater flows from higher head toward lower head.

Darcy velocity

Darcy velocity equals hydraulic conductivity times hydraulic gradient. Dividing by effective porosity gives average linear seepage velocity, which is faster than Darcy velocity.

Low-flow purging criteria

Purging at low flow aims to keep drawdown under 0.1 foot while pH, specific conductance, turbidity, dissolved oxygen, and oxidation-reduction potential stabilize before sampling.

Equipment decontamination

Wash with phosphate-free detergent, rinse with tap water, rinse with deionized water, and air-dry. Dedicated or disposable sampling equipment reduces cross-contamination between wells.

Bentonite seal hydration

Bentonite pellets or chips are hydrated with clean water before grout is placed above them, so the seal swells and prevents grout from migrating down the annulus.

Sand content of grout

Excess sand or fines in cement grout increase permeability and shrinkage. Grout should be mixed from clean water and neat cement unless a specified additive is required.

Top of screen setting

The screen is positioned to intersect the target saturated zone, with the top below the expected water table so the screen remains submerged during seasonal low-water conditions.

Surging and jetting

Surging moves water back and forth through the screen with a plunger; jetting directs water through the screen. Both flush fines from the filter pack and formation.

Air-lift development

Compressed air injected through a eductor pipe lifts water and fines from the well. It is effective in deep wells but can damage weak formations if flow is uncontrolled.

Well yield test

A step-drawdown or constant-rate pumping test measures yield and specific capacity. Drawdown and pumping rate are recorded until water levels stabilize.

Specific capacity

Specific capacity equals pumping rate divided by drawdown, usually in gallons per minute per foot. Higher values indicate a more productive well and formation.

Surface seal requirement

A low-permeability seal from land surface to the first confining layer prevents surface water and shallow contamination from moving down the annulus.

VOC sample handling

Samples for volatile organic compounds are collected with minimal agitation, zero headspace, and chilled storage. Field blanks and trip blanks document possible contamination.

Water level sounder use

A graduated cable with a conductivity probe is lowered until the probe contacts water and the meter signals. Depth is read from the surveyed top-of-casing reference.