Well Casing Storage, Purge Volume, and Disinfection

The volume is the standard 0.0408 times the diameter in inches squared, giving gallons per foot, times the standing column -- the depth below the static water level rather than the drilled depth, which is the term most often got wrong. THE TIME IS THE USEFUL OUTPUT. Three casing volumes at a modest purge rate turns a sampling visit from a twenty-minute job into a two-hour one, and knowing that before driving out is the difference between a planned day and a rescheduled one. The purge exists because water standing in a casing is not what the aquifer is producing: it has exchanged gases with the air in the well, may have picked up metals from the casing, and has had time to change temperature and chemistry. Purging until field parameters stabilise is a better criterion than a fixed count of volumes, and a low-flow sample drawn at the screen avoids most of the argument. THE DISINFECTION DOSE HAS A UNIT TRAP AND A SCOPE TRAP. The unit trap is the solution: available chlorine is a percentage BY WEIGHT of a solution that is denser than water, so converting pounds of solution to gallons at 8.34 lb/gal overstates the volume by roughly a fifth. Strength and density are both entered here rather than assumed. The scope trap is larger. A dose computed on the casing volume treats the CASING. The gravel pack and the near-well formation hold a comparable volume of water, and chlorine that never reaches them disinfects the part of the well that was already cleanest -- so the practical dose is increased, agitated, and given contact time, and a well disinfected on the casing figure alone can return a clean sample while the source of the contamination sits untouched a few inches outside the screen. This computes volumes and doses from entered dimensions. It does not determine a purge protocol or a sampling method, size the formation and pack volume (which depends on the borehole diameter, the pack gradation and its porosity), specify a disinfection procedure, contact time or the neutralisation and discharge of the purge water, evaluate a bacteriological result, or address well construction or the annular seal that most often explains a contaminated well. AWWA A100 and C654, the applicable state well code, and the licensed driller or water system operator govern.

casing volume = 0.0408 x diameter(in)^2 gal/ft x the standing column (depth less static water level); chlorine demand = MG x 8.34 x mg/L, converted to solution volume at an entered strength and density.

Well casing volumes and disinfection as AWWA A100 and C654 state them. Hypochlorite strength and density are ENTERED because available chlorine is a percentage by weight of a solution denser than water.

Two standard volume relations and one dose conversion.

Estimate. AHJ and licensed professional govern.

Field names used by the API: casing_diameter_in, well_depth_ft, static_water_level_ft, purge_volumes, purge_rate_gpm, target_dose_mg_l, solution_strength_pct, solution_lb_per_gal, gal_per_ft, standing_column_ft, casing_volume_gal, purge_volume_gal, purge_minutes, chlorine_lb, solution_gal

• Standing column, not drilled depth the volume above the static level holds no waterthe well log
• Solution density is entered hypochlorite is denser than water; 8.34 lb/gal overstates by ~20%the product data sheet
• Treats the casing only the pack and formation hold a comparable volumeAWWA C654