Tracer tests in a fractured dolomite 1. Experimental design and observed tracer recoveries

A series of tracer tests has been conducted in a 7-m-thick fractured dolomi te at two sites in southeastern New Mexico. The tests were designed to eval uate transport processes, especially matrix diffusion, in fractured, permea ble media. Both Single-well injection-withdrawal (SWIW) and multiwell conve rgent flow (MWCF) tests were conducted. Seventeen different organic tracers (the fluorobenzoic and chlorobenzoic acids) and iodide were used as conser vative tracers for the tests. The MWCF tests included repeated tracer injec tions while pumping the central well at different rates, injection of trace rs with different aqueous diffusion coefficients, and injection of tracers' into both the full and partial formation thickness. This paper describes t he tracer test sites and aquifer characteristics, the experimental methods, and the trader data produced. The tracer test results provide a high-quali ty data set for a critical evaluation of the conceptual model for transport . Both the SWIW and MWCF tracer test data showed gradual mass recovery ana breakthrough (or recovery) curve tailing consistent with matrix diffusion. However, the SWIW recovery curves did not display the -1.5 log-log slope ex pected from a conventional double-porosity medium with a single rate of dif fusion. The breakthrough curves from MWCF tests conducted at two different pumping rates showed similar peak heights, which is also not what was expec ted with a conventional double-porosity model. However, the peak heights we re different for two tracers with different aqueous diffusion coefficients that were injected simultaneously in one test, consistent with the effects of matrix diffusion. The complexity of the tracer test results suggests tha t a simple double-porosity conceptual model for transport iri the Culebra w ith a single rate of diffusion is overly simplistic.