A Re-Circulating Tracer Well Test method for measuring reaction rates in fast-flowing aquifers: Conceptual and mathematical model

Reactive groundwater tracer tests provide a practical means by which rates of reaction processes that constitute natural attenuation can be measured in aquifer systems. We present a two-well Re-Circulating Tracer Well Test (RCTWT) method that we conceive applicable to determining in situ reaction rates in fast-flowing alluvial aquifers where conventional small-scale single-well tracer tests, such as the push–pull test, might be impractical. The RCTWT concept was analysed mathematically and breakthrough curve datasets were generated using a numerical model to simulate hypothetical aquifer systems from which the sensitivity of the RCTWT method to governing mathematical variables was analysed. A simplified data interpretation method for determining reaction rates from observed RCTWT breakthrough data was developed. It was demonstrated that the efficiency of the RCTWT is strongly affected by the degree of aquifer heterogeneity and the performance can be optimised by applying a high pumping rate along with a short doublet well-spacing. The simplified method provided reasonably accurate estimates of first-order reaction rate coefficients when evaluated using the hypothetical datasets, from which it was concluded that the RCTWT is a valid concept for determining potential in situ reaction rates. The findings are to be incorporated in the design of practical RCTWTs aimed at measuring denitrification rates in fast-flowing alluvial aquifers, in New Zealand.