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 ave rage concentrations and modeling laminar wellbore flow, this study uses dev iations from those representative concentrations as a measure of data varia bility. The laminar wellbore flow model relates ground water concentration and permeability distributions to the concentration response at a pump duri ng purging or sampling, Given laminar wellbore flow, even if the concentrat ions within the well are not chemically altered, transient responses of con centrations to pumping lead to various amounts of variability depending on the screen length and position relative to hydraulic and chemical heterogen eities. in the simplest case, uniform inflow and inflowing concentrations, the concentration response to pumping is the same as the concentration resp onse with thorough mixing. In more complex situations involving ground wate r concentration and permeability heterogeneities, pumped concentrations are controlled by those heterogeneities and wellbore flow, A model that descri bes concentration responses in the presence of linear heterogeneities allow s a first-order approximation of variability during pumping. Using field sc reening of concentration and permeability heterogeneities, this first-order approximation can be used to design screen lengths that will limit the var iability to study-specific tolerances, In the case of existing wells, measu rement of inflows and concentration variability will allow the assessment o f variability during pumping. This investigation demonstrates that, in orde r to decipher sources of data variability, investigations must examine inte rnal wellbore processes in detail as well as the relationships to external conditions characterized in the field.