Sample Concentration Response to Laminar Wellbore Flow: Implications to Ground Water Data Variability

Abstract

Ground water data variability and sample repsentativeness ness are controlled by internal monitoring‐well flow and mixing, processes that require further investigation. By defining a representative sample using flow weighted average concentrations and modeling laminar wellbore flow, this study uses deviations from those representative concentrations as a measure of data variability. The laminar wellbore flow model relates ground water concentration and permeability distributions to the concentration response at a pump during purging or sampling. Given laminar wellbore flow, even if the concentrations within the well are not chemically altered, transient responses of concentrations to pumping lead to various amounts of variability depending on the screen length and position relative to hydraulic and chemical heterogeneities. In the simplest case, uniform inflow and inflowing concentrations, the concentration response to pumping is the same as the concentration response with thorough mixing. In more complex situations involving ground water concentration and permeability heterogeneities, pumped concentrations are controlled by those heterogeneities and wellbore flow. A model that describes concentration responses in the presence of linear heterogeneities allows a first‐order approximation of variability during pumping. Using field screening of concentration and permeability heterogeneities, this first‐order approximation can be used to design screen lengths that will limit the variability to study‐specific tolerances. In the case of existing wells, measurement of inflows and concentration variability will allow the assessment of variability during pumping. This investigation demonstrates that, in order to decipher sources of data variability, investigations must examine internal wellbore processes in detail as well as the relationships to external conditions characterized in the field.

An Assessment of Aquifer/Well Flow Dynamics: Identification of Parameters Key to Passive Sampling and Application of Downhole Sensor Technologies

“…Many of the purging curves in the field tests showed deviations similar to those predicted by the analytical models of Martin-Hayden (2000a). Those analytical models were based on several simplifying assumptions including, inviscid flow, predominantly vertical flow within the well bore, and pumps positioned at either end of the screen.…”

“…No-purge techniques are also being explored and are being adopted where applicable (Vroblesky, 2001, Parker andClark, 2002;Mulherin, 2007, Parsons Engineering Science, 2003;ITRC, 2004ITRC, , 2006ITRC, , 2007. These alternative techniques can solve some problems like elevated turbidity and VOC loss caused by bailer agitation or high pump rates, but they do not solve problems with vertical flow (Elci, et al, 2001 and, pumping-induced variability (Martin-Hayden, 2000a and2000b;Gibs, et al, 2000, Martin-Hayden and Britt 2006), or well convection (Martin-Hayden and Britt 2006Vroblesky et al 2007). …”

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

“…Assuming different initial conditions within the wellbore would clearly influence earlytime (< 1SV) sampling predictions, however, the analytical solution of Martin-Hayden (2000) suggests that the relationship between early time sampling variability and near-screen vertical stratification in contamination remains, even if the wellbore is assumed to be fully mixed. This raises the possibility of using early time variations in sample concentration as an indicator of contaminant variability.…”

“…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 .…”

Measuring Salinity within Shallow Piezometers:Comparison of Two Field Methods

“…These data have poor relationships with the effective environmental concentrations of chemicals [6]. Flow in the borehole (Figure 1) does not only affect the chemical concentrations in the well, but also affects the near-borehole aquifer acting as a preferred flow channel for solutes [11][12][13]. Shallow conventional wells and piezometers are, especially, prone to this problem [14].…”