Sample concentration response to laminar wellbore flow: Implications to ground water data variability

Ground water data variability and sample representativeness 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.