OPTIMAL TRACER TEST DESIGN USING THE EFFICIENT HYDROLOGIC TRACER-TEST DESIGN METHODOLOGY

Hydrological tracer testing is the most reliable diagnostic technique available for identifying and quantifying hydrodispersive transport processes. However, tracer test design can be difficult because of a lack of prior knowledge of the basic transport characteristics. The Efficient Hydrologic Tracer-test Design (EHTD) methodology facilitates the design of tracer tests by root determination of the one-dimensional advection-dispersion equation using a preset average tracer concentration which provides a theoretical basis for an estimate of necessary tracer mass. The method uses basic measured field parameters such as discharge, distance, and cross-sectional area that are combined in functional relationships to describe solute-transport processes related to flow velocity, travel time, and dispersion. EHTD is extremely robust in that it is applicable to tracer tests in open- and closed-conduits and porous media. In the case of a porous media, EHTD allows for natural-gradient tracer tests, forced-gradient tracer tests, and recirculation tracer tests. It also allows consideration for impulse, pulse, and continuous tracer releases. Initial tracer concentration, decay and retardation are included parameters. The EHTD methodology has been utilized at several sites to improve tracer test designs. These sites included flow to a pumping well in porous media near a river to determine the contributing area and flow to a pumping well in and a karstic terrane near a dry channel to assess sinkhole development. The porous media test consisted of a forced gradient tracer test with tracer released in nearby wells. Rapid tracer migration was estimated by EHTD and did occur; the contributing area was subsequently defined. The karstic terrane test consisted of an instantaneous tracer release through the dry channel that was simulated by EHTD as a pulse release of tracer and as a flowing stream test. A second test was run as a forced gradient tracer test using an injection well. Analyses of the tracer tests suggested that sinkholes would most likely develop in a NE–SW direction primarily along a nearby dry stream channel. Homes located orthogonal to this axis may not be adversely affected at reasonable pumping rates. Sinkholes did develop along the channel which also conformed to aquifer testing and tensor analyses.