5.2 Establishing Baseline Conditions and Water Level Trends

Pre‑test water levels should be measured prior to the initiation of the pumping test. The baseline data are used to document on‑going regional water level changes and determine if other pumping wells in the area are impacting background water levels (well interference). An observable rise or fall in water levels may also be caused by natural variation in recharge and discharge and/or barometric effects.

Water levels in the pumping and observation wells should be monitored for at least 48 hours before the test to establish natural regional trends. When nearby pumping wells are causing water level changes at the test site they should be shut off prior to testing and remain off during testing.

Barometric pressure changes influence heads within confined and semi‑confined aquifers. If tested formations are confined, a micro‑barometric sensor should also be operated during the entire test to examine if some small water‑level changes are the result of variations in atmospheric pressure (e.g., Todd & Mays, 2005; Wang, 2020). Though small changes in confined water levels due to changes in barometric pressure may be masked by large changes in test water levels at early times, they may become significant if test water level changes are small (Clark, 1967). Barometric effects do not affect unconfined aquifer heads as the water table is open to the atmosphere. Also, some hydrogeologic systems located near areas effected by ocean tides may show responses to the changing tides in the water level data (e.g., Todd & Mays, 2005).

When variations in pumping test water levels occur that are not directly related to formational properties, observed drawdown data need to be corrected for these perturbations prior to analysis (e.g., Todd & Mays, 2005; Kruseman & de Ridder, 2000; Sterrett, 2007).