In-well time-of-travel approach to evaluate optimal purge duration during low-flow sampling of monitoring wells

Passive sampling of groundwater wells for determination of water chemistry

“…A common assumption for low-flow groundwater sampling was that low purge rates capture primarily lateral inflow (horizontal laminar flow) through the screened interval from the formation at depths coincident with the pump intake (Stone, 1997). However, because even low-flow sampling causes some drawdown in the well, convergent, in-well, vertical flow is induced toward the pump intake from inflow across the entire well screen (Harte, 2017). Varljen and others (2006) show that the entire well screen is sampled during low flow with preferential sampling of high permeability layers under steady-state transport.…”

“…Inherent in many active/purge sampling methods (volumetric and low-flow) is the requirement that, prior to collecting a sample, field physical and chemical characteristics such as pH, DO, SC, temperature, and turbidity achieve some degree of stability as an indicator of formation water recharging the well water. Harte (2017) found that stabilization of field physical and chemical characteristics monitored during purging was useful in diagnosing the contribution of in-well vertical flow and transport to the pump intake location. However, stabilization of field physical and chemical characteristics during purging may not always be a reliable indicator of the chemical stability for other chemical constituents.…”

Instructions for running the analytical code PAT (Purge Analyzer Tool) for computation of in-well time of travel of groundwater under pumping conditions

“…The Q H from the Dupuit-Theim equation is calculated within the associated M z using K avg and compared against the Q p' to assess if Q v outside M z can take place. A "NO" is shown in cells B15:C16 of the "Input" worksheet when it is unlikely that Q v in the well is dominant, as discussed in Harte (2017). Note that if the P L is in the casing, Q v is assured.…”

“…Transient storage effects may be significant in cases where the drawdown (s) takes significant time to stabilize, in which case the analysis will overestimate values of K avg . The impact of formation S on K avg was evaluated by comparing the calculated K avg from the Theis equation, which accounts for aquifer storage, to the calculated K avg from the Dupuit-Theim equation as used by this analytical model (Harte, 2017). For a 1-ft drawdown or less, the effect of S for values of < 0.001 results in a small error (approximately a factor-of-2 difference) in the K avg calculated with this model.…”

“…The time of travel relative to in-well flow is affected by the pump's intake location. The Purge Analyzer Tool (PAT) incorporates hydraulic calculations to help assess the optimal purge times required to vertically transport groundwater in the well to the pump intake (Harte, 2017). Harte (2017) includes a discussion on the rationale for determining in-well groundwater flow and time of travel and also discusses the limitations inherent in the PAT; an understanding of the limitations is important to ensure proper use.…”