Natural Solutes and Signals for Characterizing Advection and Dispersion during Tracer Tests

Abstract Traditional field tracer tests often utilize non‐reactive, non‐natural halides, such as iodide or bromide, added to injection water to characterize advection and dispersion via analyses of their concentration versus time profiles. However, non‐reactive, natural solutes and signals, such as chloride, conductance, and temperature, can be used, if their contrast between the injection water and groundwater is sufficient. Furthermore, the cost to utilize natural solutes and signals can be a fraction of traditional field tracers, for example, conductance is readily measured with a down‐well sensor. To test the accuracy and cost‐effectiveness of natural solutes and signals, eight tracer tests were conducted in a heterogeneous, unconfined, sand and gravel aquifer. Temporal moments analyses of breakthrough curves were conducted for added iodide or bromide and natural chloride, conductance, and temperature. Advection, based on added iodide or bromide, ranged from 0.5 to 2 ft/d and dispersivity ranged from 0.1 to 1 feet; advection and dispersivity based on natural chloride and conductance, in most cases, was within a factor of 3, whereas the temperature contrast was not sufficient. A negative power law relationship was observed between errors in advection and dispersivity versus natural solute/signal strength. These results suggested that a twofold contrast between natural solutes/signals in the injection water and groundwater will yield advection and dispersivity estimates within a factor of 2 of added halides. Furthermore, a cost analysis suggested a notable advantage in using natural solutes/signals when ten or more tracer tests are conducted. The results of this study clearly demonstrated the accuracy and cost advantage of natural solutes/signals to characterize advection and dispersion given a sufficient contrast between injection and groundwater levels (twofold) and total number of field tracer tests (more than or equal to 10).