Step-Drawdown Test and Well Efficiency

Total drawdown is B times the flow plus C times the flow squared. The linear term is formation loss -- a property of the aquifer, and not something any amount of work on the well will improve. The quadratic term is turbulent loss at the screen and the gravel pack, and it is the part that degrades over a well's life and the part rehabilitation recovers. Pumping at three or more increasing rates and plotting drawdown over flow against flow separates them: the intercept is B and the slope is C, fitted here by least squares. WELL EFFICIENCY IS THE AQUIFER LOSS OVER THE TOTAL, and the useful reading of a low number is not that the well is bad but that a specific, recoverable quantity of head is being spent on turbulence at the screen. Feet of well loss convert directly into pumping energy, every hour the well runs. THE RATE YOU TEST AT DETERMINES THE ANSWER, which is the trap. Well loss goes with the SQUARE of flow, so a well that is 62 percent efficient at its operating rate can be 83 percent efficient at a third of it. A light-duty test finds nothing wrong with a well that is in trouble at the rate it actually runs, and a test rate chosen for convenience describes a well nobody operates. THE TREND IS WORTH MORE THAN THE NUMBER. A well's efficiency when new is the baseline every later test is read against; the same well losing twenty points over five years at the same rate is incrusting, and that trajectory is the case for rehabilitation in a way an absolute figure never is. Which is why the test is worth running when the well is new, when nobody thinks they need it. This fits and evaluates entered step data. It does not conduct or design a step test (the steps must be long enough for drawdown to stabilise at each rate, which is the part most often done badly), determine aquifer properties -- transmissivity and storativity come from a constant-rate test, not this one -- diagnose WHY efficiency is low (incrustation, biofouling, sand pumping, plugging and a poorly developed pack all read the same here), select or design a rehabilitation, size a pump, or address well construction, screen selection or development. AWWA A100, the driller's log and the hydrogeologist govern.

the step-drawdown relation s = BQ + CQ^2, with B (aquifer loss) and C (well loss) fitted by least squares from the entered steps by plotting s/Q against Q; well efficiency = BQ / (BQ + CQ^2), and specific capacity = Q / s.

Step-drawdown testing as AWWA A100 and standard hydrogeologic practice state it. The steps are ENTERED from a field test in which drawdown was allowed to stabilise at each rate.

One published relation and a two-parameter least-squares fit.

Estimate. AHJ and licensed professional govern.

Field names used by the API: q1_gpm, s1_ft, q2_gpm, s2_ft, q3_gpm, s3_ft, operating_gpm, efficiency_threshold_pct, previous_efficiency_pct, b_ft_per_gpm, c_ft_per_gpm2, aquifer_loss_ft, well_loss_ft, total_drawdown_ft, efficiency_pct, specific_capacity_gpm_ft, light_rate_efficiency_pct

• Steps must be stabilised drawdown still moving at a step biases the fitthe field test procedure
• Test at the operating rate efficiency is rate-dependent because well loss is quadraticAWWA A100
• Not an aquifer test transmissivity and storativity come from a constant-rate testthe hydrogeologist