NGWA CWD cheat sheet

Cable tool vs. rotary: cable tool breaks and bails rock by repeated impact; rotary destroys formation with a rotating bit while circulating fluid removes cuttings.

Hole volume in gallons per foot = borehole diameter squared (in) ÷ 24.5; a 6-in hole holds about 1.47 gal/ft, so annular velocity and fill volumes scale with diameter squared.

Annular velocity (ft/min) = 24.5 × pump output (gpm) ÷ (hole diameter² − pipe diameter²), both diameters in inches.

Hydrostatic pressure (psi) = 0.433 × fluid density (lb/gal) × true vertical depth (ft); a 9.0 lb/gal mud at 200 ft exerts about 779 psi.

Formation fracture gradient sets the upper bound on mud weight and grout pressure; exceeding it causes loss of circulation or formation breakdown.

Standard well construction sequence: drill pilot hole, ream to design diameter, set casing, grout annulus, develop, then test yield and water quality.

Grout volume (ft³) = annular capacity between casing OD and borehole wall × length; add 10-25% for washouts and fractures.

Well development methods include surging, jetting, air-lift pumping, and overpumping; each removes drilling fines and filter cake to restore formation permeability.

Abandonment requires sealing the entire borehole with low-permeability material (neat cement or bentonite grout) and a surface seal, not just backfill with cuttings.

Monitoring well screen slot size is typically selected to retain 90-100% of the filter pack, with the pack sized 4-5 times the d50 of the formation.

Filter pack extends at least 2 ft above the top of screen; the bentonite seal above the pack is typically 2-5 ft thick and hydrated before grouting.

Sand content of grout and filter pack must be low; neat cement grout is mixed at a water-to-cement ratio near 0.4-0.5 by weight (about 6-7 gal per 94-lb sack).

Grout is placed by tremie from the bottom up, keeping the discharge end submerged to prevent voids, bridging, and dilution.

Well casing and screen materials for environmental work are usually flush-threaded Schedule 40 PVC, stainless steel, or PTFE; solvent cement joints are avoided to prevent VOC contamination.

Static water level is measured from a surveyed reference point (top of casing), and depth to water plus measuring-point elevation yields hydraulic head for gradient calculations.

Hydraulic gradient = (h1 − h2) ÷ distance between wells; flow direction follows the head decrease, and Darcy velocity = K × gradient ÷ effective porosity.

Low-flow purging targets drawdown under 0.1 ft and stabilized water-quality parameters (pH, specific conductance, turbidity, DO, ORP) before sample collection.

Decontamination sequence for sampling equipment: wash with phosphate-free detergent, rinse with tap water, rinse with deionized water, then air-dry; dedicated or disposable equipment reduces cross-contamination.