4.5 Water Level and Discharge Measurement Schedules
A schedule for measuring water levels at the pumping well and observation wells should be developed before initiating the test. Pumping causes water levels in pumping and nearby observation wells to respond rapidly at first and then more slowly as the test proceeds (often a logarithmic trend). The initial changes in water levels must be measured frequently as techniques used to analyze field data often rely on early‑time data trends. Most often researchers suggest measuring the water level changes as rapidly as possible in the pumping well and nearby observation wells in the first 30 minutes to an hour of the initiation of each drawdown and recovery period. Less frequent measurements are required the longer the pump is operated. When transducers are used to record water level changes, they can be programmed to measure water levels every fraction of a second if needed. If measurements are being completed by hand (e.g., mechanical, or electronic tapes) it is recommended that measurement intervals follow a logarithmic pattern. Sterrett (2007) provides information on the minimum number of data points required to create a useful water level data set (Table 1).
Table 1 ‑ Minimum drawdown measurement time intervals for pumping and observation wells (modified from Sterrett, 2007; Kruseman and de Ridder, 2000).
| Pumping Well | ||
|---|---|---|
| Time Since Pumping Started (or Stopped) (min) |
Time Intervals Between Measurements (min) | |
| 0‑5 | 0.5‑1.0 | |
| 5‑15 | 1 | |
| 16‑60 | 5 | |
| 60‑12 | 20 | |
| 120 to termination of test | 60 | |
| Observation Well | ||
| Time Since Pumping Started (or Stopped) |
Time Intervals Between Measurements | |
| 0‑2 min | Approx. 10 sec | |
| 2‑5 min | 30 sec | |
| 5‑15 min | 1 min | |
| 15‑50 min | 5 min | |
| 50‑100 min | 10 min | |
| 100 min‑5 h | 30 min | |
| 5 h to 48 h | 60 min | |
| 48 h to 6 days | Every 8 h | |
| 6 days to shutdown | Once a day | |
Flow rates should be recorded with sufficient frequency to document a constant rate or to monitor planned rate changes (Duffield, 2022). Most often pumping tests are conducted using a constant pumping rate. Kruseman and de Ridder (1994) recommend checking and adjusting the flow rate at least once every hour. USEPA guidance suggests that that a planned constant pumping rate should not vary more than five percent. Though no standard pumping rate monitoring schedules are commonly used, discharge measurements should be collected every 10 minutes in the first few hours of a pumping test and then every 20 to 40 minutes for the next three or four hours. As the drawdown rate slows, monitoring intervals can be extended. These recommendations are based on the use of standard electric pumps that operate at a constant number of revolutions per minute.