title: 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.
authors.affiliation: Water Sciences Group, School of Geography, Earth & Environmental Sciences, University of Birmingham, Birmingham B15 2TT, UK. Electronic address: LAM906@bham.ac.uk.
authors.forename: Lindsay A
authors.initials: LA
authors.lastname: McMillan
authors.affiliation: Water Sciences Group, School of Geography, Earth & Environmental Sciences, University of Birmingham, Birmingham B15 2TT, UK.
authors.forename: Michael O
authors.initials: MO
authors.lastname: Rivett
authors.affiliation: Water Sciences Group, School of Geography, Earth & Environmental Sciences, University of Birmingham, Birmingham B15 2TT, UK.
authors.forename: John H
authors.initials: JH
authors.lastname: Tellam
authors.affiliation: In-Situ Europe Ltd, Stratford Road Shirley, Solihull, West Midlands B90 3AU, UK.
authors.forename: Peter
authors.initials: P
authors.lastname: Dumble
authors.affiliation: Environment Agency, The Lateral 8 City Walk, Leeds, West Yorkshire LS11 9AT, UK.
authors.forename: Helen
authors.initials: H
authors.lastname: Sharp
mesh_terms: D004784:Environmental Monitoring / Q000379:methods*; D060587:Groundwater / Q000032:analysis*; D008962:Models, Theoretical; D014872:Water Movements; D060753:Water Quality; D060986:Water Wells
keywords: Ambient vertical flows; Groundwater monitoring; Low-flow sampling; Monitoring wells; Permeability-weighted samples; Well purging