Influence of vertical flows in wells on groundwater sampling

Abstract

Pumped groundwater sampling evaluations often assume that horizontal head gradients predominate and the sample comprises an average of water quality variation over the well screen interval weighted towards contributing zones of higher hydraulic conductivity (a permeability-weighted sample). However, the pumping rate used during sampling may not always be sufficient to overcome vertical flows in wells driven by ambient vertical head gradients. Such flows are reported in wells with screens between 3 and 10m in length where lower pumping rates are more likely to be used during sampling. Here, numerical flow and particle transport modeling is used to provide insight into the origin of samples under ambient vertical head gradients and under a range of pumping rates. When vertical gradients are present, sample provenance is sensitive to pump intake position, pumping rate and pumping duration. The sample may not be drawn from the whole screen interval even with extended pumping times. Sample bias is present even when the ambient vertical flow in the wellbore is less than the pumping rate. Knowledge of the maximum ambient vertical flow in the well does, however, allow estimation of the pumping rate that will yield a permeability-weighted sample. This rate may be much greater than that recommended for low-flow sampling. In practice at monitored sites, the sampling bias introduced by ambient vertical flows in wells may often be unrecognized or underestimated when drawing conclusions from sampling results. It follows that care should be taken in the interpretation of sampling data if supporting flow investigations have not been undertaken.

Monitoring well utility in a heterogeneous DNAPL source zone area: Insights from proximal multilevel sampler wells and sampling capture-zone modelling

“…Prior studies suggest purging of 3 SV is likely sufficient to obtain a quasi-steady state sample from a monitoring well; for example, the modelling of Martin-Hayden et al (2014) to account for partial mixing and transport times to the pump, and the modelling of McMillan et al (2014) to account for the delayed arrival of casing water at the pump. Our predictive model simulations support these prior conclusions.…”

“…Although monitoring wells will typically detect the presence of a DNAPL source area via the observation of elevated concentrations relative to the wider site, pumped groundwater samples will provide an averaged concentration drawn from across the well-screen length. Theoretically under a steady-state flow condition, a pumped sample should provide a flow-weighted average (FWA) concentration, i.e., an average of the formation's vertical heterogeneous concentration distribution weighted by the vertical distribution of flow rates, or formation hydraulic conductivities (K)i naflow system characterised by horizontal gradients (Martin-Hayden and Britt, 2006;McMillan et al, 2014;Puls and Barcelona, 1996). A wide spectrum of methods has become available to sample monitoring wells with potential to deliver a FWA concentration (Britt et al, 2010;ITRC, 2007;McHugh et al, 2016;Parsons, 2005;Puls and Barcelona, 1996;US EPA, 2010, 2013, 2017Vroblesky, 2001).…”

“…Formation vertical hydraulic gradients may induce up/downward in-well flows. In-well mixing (redistribution) may also arise from convection due to temperature gradients and fluid density effects, lateral or vertical flow turbulence and contaminant diffusion gradients (Martin-Hayden and Britt, 2006;McMillan et al, 2014;ITRC, 2006ITRC, , 2007Vroblesky et al, 2006). A FWA concentration may only arise for a zero/minimal-purge sample if full mixing of just groundwater flowing through the screen section occurs.…”

“…Whether purge times (and hence purged volumes) are adequate to attain a steady-state FWA sample concentration, however, is a key issue (Martin-Hayden, 2000;Martin-Hayden et al, 2014;McMillan et al, 2014). The variation in delayed arrival times within the sample taken of groundwater entering the monitoring well screen at distance from the pump intake becomes of critical significance here .…”

Inactive supply wells as conduits for flow and contaminant migration: conditions of occurrence and suggestions for management

“…Such occurrences can be important for contaminated site cleanup and research has focused on understanding concentration data collected from monitoring wells (Reilly et al 1989;Church and Granato 1996;Elci et al 2001;Ma et al 2011;McMillan et al 2014;Vitale and Robbins 2016). However, the differences in construction between water supply wells and monitoring wells, primarily casing diameter and length of well screen, create the potential for greater flows through supply wells where there is less resistance to flow as well as greater head difference between shallow and deep strata.…”