Heat, B10-Enriched Boric Acid, and Bromide as Recycled Groundwater Tracers for Managed Aquifer Recharge: Case Study

Abstract

California guidelines for indirect potable recycled wastewater reuse projects currently require groundwater tracers to demonstrate subsurface residence time for pathogenic microorganism control. Residence times over 6 months from infiltration to drinking water extraction are required. Two prospective tracers were evaluated in this case study: boron-10 (as 10 B-enriched boric acid) and heat (with recharging water ∼10°C warmer than native groundwater). Bromide (Br − ) was also released as a control. 10 B is attractive as a deliberate tracer because (1) reasonably accurate and affordable measurements can be made on an inductively coupled plasma mass spectrometer (ICP-MS) system, and (2) isotopic tracers require significantly less mass to tag an equivalent water volume than concentration-based salt tracers like Br − . 10 B and Br − tracer breakthroughs were observed at seven of nine monitoring wells, although at one well the detection of 10 B was barely observable and may have resulted from a slight change in source water composition. 10 B arrived 25% later than Br − on average, showing retardation through exchange with clay surfaces. Heat flow, requiring no artificial input, was interpreted from temperature changes recorded hourly at well loggers. Residence times to all wells were successfully determined from temperature changes with the longest flow path of 6 months. This implies that dilution of Br − and 10 B is a limitation the geochemical tracer experiment.

Tracer test modeling for characterizing heterogeneity and local-scale residence time distribution in an artificial recharge site

“…Ironically, few tracer tests have been performed in the context of artificial recharge. Notable exceptions are the studies in Berlin, Germany (Massmann et al, 2008), which were restricted to environmental tracers due to the proximity to the water supply, and California (Clark et al, 2004;Becker et al, 2014), which used environmental and deliberate (SF 6 ) tracers. In both cases, the goal was to monitor the recharge water plume.…”

Investigation of Groundwater Flow Variations near a Recharge Pond with Repeat Deliberate Tracer Experiments

“…Tailing is also evident on breakthrough curves and can represent the tracer reaching the well by slower flow paths, back diffusion out of lower permeability strata, non-ideal tracer input function, or retardation, which in the case of gas tracers can be due to trapped gas [29,30]. As discussed by Becker et al [35], sampling biases can result in moderate and hard to quantify travel time errors. These biases are often caused by infrequent sampling due to budget limitation.…”

A reactive barrier to enhance the removal of emerging organic compounds during artificial recharge of aquifers through infiltration basins

“…Ironically, few tracer tests have been performed in the context of artificial recharge. Notable exceptions are the studies in Berlin, Germany (Massmann et al, 2008b), which were restricted to environmental tracers due to the proximity to the water supply, and California (Clark et al, 2004;Becker et al, 2014), which used environmental and deliberate (SF 6 ) tracers. In both cases, the goal was to monitor the recharge water plume.…”