ESTIMATING VERTICAL CONCENTRATION PROFILES AROUND LONG-SCREENED WELLS IN THE PRESENCE OF INTRABOREHOLE FLOW

Concentrations measured on samples from wells represent flow-weighted averages of the pore-fluid concentrations in the surrounding formation, where the weights depend on (1) the exchange of water between the well and surrounding aquifer and (2) vertical flow in the well, i.e., intraborehole flow (IBF). Understanding the vertical concentration profile (VCP) in the formation, which is critical to the design of monitoring and remediation strategies, is thus challenging in the presence of IBF. This challenge is increased in long-screened wells within heterogenous aquifers due to large permeability contrasts in the formation and strong vertical hydraulic gradients between different aquifer layers or fractures intersected by the well.

We recently developed an analytical framework and inverse modeling approach to estimate the VCP using (1) borehole flowmeter logs and (2) concentrations sampled under one or more ambient and/or pumped conditions. The analytical model and inverse approach accept concentration data collected under equilibrium and/or non-equilibrium transport conditions, thus allowing for analysis of time-series concentration data collected during pumping. We present a series of examples that demonstrate integration of data collected under different hydraulic conditions (e.g., different pump intake locations, low-flow sampling from different depths, passive sampling, etc.) to yield more accurate estimates of the VCP. The mathematical framework is also used to generate a series of examples that provide insight into the implications of IBF for different sampling strategies.