Transport and degradation of perchlorate in deep vadose zone: implications from direct observations during bioremediation treatment

Abstract

Abstract. An in situ bioremediation experiment of a deep vadose zone (∼ 40 m) contaminated with a high concentration of perchlorate (> 25 000 mg L −1 ) was conducted through a full-scale field operation. Favourable environmental conditions for microbiological reduction of perchlorate were sought by infiltrating an electron donor-enriched water solution using drip irrigation underlying an airtight sealing liner. A vadose zone monitoring system (VMS) was used for real-time tracking of the percolation process, the penetration depth of dissolved organic carbon (DOC), and the variation in perchlorate concentration across the entire soil depth. The experimental conditions for each infiltration event were adjusted according to insight gained from data obtained by the VMS in previous stages. Continuous monitoring of the vadose zone indicated that in the top 13 m of the cross section, perchlorate concentration is dramatically reduced from thousands of milligrams per litre to near-detection limits with a concurrent increase in chloride concentration. Nevertheless, in the deeper parts of the vadose zone (< 17 m), perchlorate concentration increased, suggesting its mobilization down through the cross section. Breakthrough of DOC and bromide at different depths across the unsaturated zone showed limited migration capacity of biologically consumable carbon and energy sources due to their enhanced biodegradation in the upper soil layers. Nevertheless, the increased DOC concentration with concurrent reduction in perchlorate and increase in the chloride-to-perchlorate ratio in the top 13 m indicate partial degradation of perchlorate in this zone. There was no evidence of improved degradation conditions in the deeper parts where the initial concentrations of perchlorate were significantly higher.

Vadose Zone Monitoring as a Key to Groundwater Protection

“…The potential for in-situ monitoring of the unsaturated zone has been demonstrated through several studies where the VMS was used to measure flow velocities, solute transport, and chemical transformation of contaminants across the unsaturated zone. It has been implemented in a variety of studies on water flow and contaminant transport including: (a) rainwater infiltration and groundwater recharge (Rimon et al, 2007(Rimon et al, , 2011a, (b) floodwater infiltration from stream channels and reservoirs (Shani, 2006;Dahan et al, 2007Dahan et al, , 2008Dahan et al, , 2009, (c) impact of agricultural practices on groundwater quality (Baram et al, 2012;Dahan et al, 2014;Turkeltaub et al, 2015bTurkeltaub et al, , 2016Weissman et al, 2020), (d) leachate generation and migration in landfills (Aharoni et al, 2017(Aharoni et al, , 2020, and (e) interactive remediation of contaminated sites (Dahan et al, 2017;Moshkovich et al, 2017;Levakov et al, 2019). Scientific and environmental implications from a number of these studies are presented here to demonstrate the capability to continuously monitor water flow and contaminant migration in the unsaturated zone.…”

“…In-situ bioremediation of a contaminated unsaturated zone depends mainly on the ability to control the hydrological, physical, and chemical conditions in the subsurface. Application of vadose zone monitoring technology during a remediation operation enables optimizing the remediation strategy on the basis of real-time data on the unsaturated zone's chemical composition, saturation degree, and flow characteristics (Avishai et al, 2017;Dahan et al, 2017;Moshkovich et al, 2017;Levakov et al, 2019). Direct information from the unsaturated zone was used for real-time adjustment of water application cycles, nutrient augmentation, and supply of electron donors/acceptors to the treated soil.…”