Understanding Trace Organic Chemical Attenuation During Groundwater Recharge by Means of a 2D Synthetic Aquifer

The successful operation of a managed aquifer recharge (MAR) system must be based on a sound understanding of hydrologic, biological, and chemical processes, and their interactions, that are currently not understood to the point of providing useful predictions at all relevant scales of interest, especially regarding the attenuation of trace organic chemicals (TOrC). Attenuation of certain TOrC is highly dependent on key controlling conditions such as biodegradable dissolved organic carbon, redox conditions, and residence time in the subsurface. Although rate constants are considered useful to provide first estimates of the fate of TOrC at field-scale, which is essential for the design and operation of MAR sites, the question can be asked: How accurate rather simplified first-order rate constants are for model-based prediction of contaminant fate and transport considering the variability of environmental conditions.

We designed and constructed a laboratory-scale two-dimensional (2D) synthetic aquifer equipped with an array of automated sensors (temperature, water pressure, conductivity, soil moisture, oxidation reduction potential) and adjacent water and soil sampling ports to test and model fundamental subsurface processes that occur during MAR more closely representing field settings. This biologically active 2D synthetic aquifer consisting of technical sand and defined pockets of field soil measures 5 m long by 2 m tall and allows for simulation of an unsaturated infiltration zone and a saturated zone with underlying groundwater flow. Tracer experiments using conservative inorganic tracer as well as five spiked TOrC indicated significant differences in contaminant transport that were not explained by compound specific soil water distribution coefficients but related to the charge of molecules.