An Analysis of Two-Well Tracer Tests
Creators: name: Wisniewski, Rebecca Davies
Publisher: name: The University of North Carolina at Chapel Hill University Libraries
Year: 1991

description: Due to the increasing number of lawsuits being filed related to groundwater contamination sites, predictions concerning the source of the contamination, its present course and future destination have become the bread and butter of the subsurface hydrology business. In order to procure funding for most contamination remediation efforts, culpable parties and those at risk by the migration of the plume must be identified. The tools most often used to make these predictions are groundwater models. Mathematical groundwater models require that the user input many site specific aquifer parameters that are used to describe the contaminant transport hydraulics and the subsurface conditions over a site. These parameters include hydraulic conductivity, hydrodynamic dispersion, and contaminant sorption and decay, either chemical or biological. The hydraulic conductivity of an aquifer is a property of a water bearing formation that is defined as the capacity of a porous medium to transmit water. The conductivity when used in model calculations with the gradient and porosity determines the average direction and rate of groundwater flow. The transport of solutes in the direction of the flowing groundwater is called advection. Hydraulic conductivity can vary over a site not only in a horizontal plane but also vertically. In order to obtain accurate modeling results over a large area with different conductivities, these variations should be taken into account. Hydrodynamic dispersion describes the spreading and dilution of a contaminant from the path that it would be expected to follow according to the bulk motion of the flowing groundwater. The spreading can occur in the longitudinal direction (the direction of the flowing groundwater) and in a transverse direction (perpendicular to the principle direction of flow). Hydrodynamic dispersion occurs due to mechanical mixing of the groundwater during advection and due to molecular diffusion of the contaminant. Aquifer tracer tests may be performed to identify the spatial variability of hydraulic conductivity within an aquifer, and to estimate the effective hydrodynamic dispersion using tracer breakthrough data obtained during the test. This can be accomplished by packing off sections of fully penetrating well screens and performing tracer tests at different vertical locations within the aquifer. By separately analyzing these data sets, a set of hydraulic parameters can be estimated for each selected screened interval. Purpose of Present Study: The main objective of this study was to obtain a better understanding of the groundwater flow pattern that is present at a particular area of a research site by measuring certain aquifer parameters. The groundwater at the site had become contaminated with gasoline due to the failure of underground storage tanks and associated piping. One of the first phases of the field study involved the installation of groundwater monitoring wells; these included two pairs of wells specifically designed for tracer studies. Two two-well field tracer studies were then performed using each pair of wells. The tracer tests had three objectives: (1) estimate aquifer parameters of the area in question, (2) compare the field results to literature values; and (3) to confirm the field results with modeling.