Efficacy of bromide tracers for evaluating the hydraulic performance of denitrification beds

Authors: GHANE, EHSAN - Michigan State University; Feyereisen, Gary; ROSEN, CARL - University Of Minnesota

Submitted to: Journal of Hydrology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/18/2019
Publication Date: 7/1/2019
Citation: Ghane, E., Feyereisen, G.W., Rosen, C.J. 2019. Efficacy of bromide tracers for evaluating the hydraulic performance of denitrification beds. Journal of Hydrology. 574:129-137. https://doi.org/10.1016/j.dib.2019.103914.
DOI: https://doi.org/10.1016/j.dib.2019.103914

Interpretive Summary: A woodchip bioreactor is a conservation practice that removes nitrate from agricultural tile drainage water by the process of microbial denitrification. It is important to understand the internal hydraulics of woodchip beds in order to design them, maximize their efficiency, and model results. This study investigated the use of a conservative tracer, bromide, to test the hydraulics of seven field bioreactor beds. Average recovery of the bromide tracer was 82%, which was considered less than optimal. Additional laboratory study determined that the bromide did not sorb to the woodchips. The fate of the unaccounted-for bromide was not determined. The hydraulic characteristics for the seven beds were good, representing those of effective plug-flow bioreactors. The beds are being used for future research work. The findings of this research are pertinent to researchers, academics, and agency personnel with interest in developing structural solutions such as denitrifying bioreactors to improve tile drainage water quality.

Technical Abstract: One biotechnology that can reduce nitrate concentration in subsurface drainage water is called a denitrification bed. Bromide tracer testing is used to evaluate the internal hydraulics of these systems to be used in design and modeling, but efficacy of bromide tracers has not been investigated. The objectives of this study were to determine if bromide is a suitable tracer for woodchip media based on laboratory sorption experiments and based on field tracer tests. To achieve this goal, we conducted bromide sorption experiments in the laboratory and bromide tracer tests in seven denitrification beds near Willmar, Minnesota, USA. We did not find sorption (neither adsorption nor absorption) of bromide to woodchips in our laboratory sorption experiments. In contrast, bromide tracer tests showed an average bromide recovery of 82%, revealing that bromide was retarded in the woodchip denitrification beds. Thus, bromide did not meet the conservancy requirement of a tracer test. To the best of our knowledge, our experiment is the first study to estimate the in-situ effective porosity (average of 0.61) of a field-scale bed (i.e., with a diffuse-source inflow and outflow) using a tracer test. A diffuse-source inflow and outflow is the common design where the collector and distributor pipes extend across the entire width of a bed. A more accurate estimate of the actual hydraulic retention time (HRT) can be obtained by using the in-situ effective porosity rather than total porosity. In conclusion, our laboratory experiment did not show any sorption of bromide to woodchips, but in contrast and for unclear reasons, our field tracer testing showed bromide retardation.