High-Frequency Bromide Sensor Performance during Denitrifying Bioreactor Tracer Testing

Abstract. High-frequency bromide (Br-) sensors could resolve challenges during denitrifying bioreactor tracer tests. Deployment of a Br- sensor during a tracer test resulted in both under- and over-reported concentrations. Collecting high-resolution Br- concentrations generally did not add value to calculated tracer metrics. However, high-frequency Br- sensors were overall considered a convenient tool, in part, due to ease of deployment. Bromide (Br-) tracer tests are an important environmental monitoring method for assessing internal hydraulics in denitrifying woodchip bioreactors. The objective was to assess performance of relatively new high-frequency Br- sensing technology to help resolve challenges associated with sampling uncertainty and failure during bioreactor tracer tests. A Seametrics TempHion™ Bromide Logger was used to: (1) continuously log Br- concentrations at least every 30 s during three tracer tests in the field and (2) determine Br- concentrations in sample bottles post-hoc following eight tracer tests (sampling frequency in the field: every 2 to 300 min). Both were compared to lab-analyzed Br- concentrations. The sensor underreported Br- concentrations at two real-time tracer tests and overreported during one test, but this mainly impacted the tracer metric of percent recovery. While Br- sensor deployment during a tracer test is the ideal use of this convenient tool, use of the sensor post-hoc in collected bottles matched the lab-analyzed concentrations more closely based on the Root Mean Square Errors and the Mean Absolute Errors. The sensor’s ability to collect large amounts of high-resolution concentrations did not necessarily add value because most tracer metrics were interpolated with reasonable accuracy from the discrete lab-analyzed samples. The relatively continuous nature of the sensed concentrations, however, along with ease of deployment were important sensor benefits. Keywords: Conservative tracer test, Continuous sensor, Double peak, Subsurface drainage, Woodchip bioreactor.