Articles | Volume 21, issue 8
Hydrol. Earth Syst. Sci., 21, 4011–4020, 2017
Hydrol. Earth Syst. Sci., 21, 4011–4020, 2017

Research article 08 Aug 2017

Research article | 08 Aug 2017

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

Ofer Dahan, Idan Katz, Lior Avishai, and Zeev Ronen Ofer Dahan et al.
  • Zuckerberg Institute for Water Research (ZIWR), The Blaustein Institutes for Desert Research, Ben-Gurion University of the Negev, Sede Boqer Campus, 8499000, Israel

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.

Short summary
In situ bioremediation of a perchlorate-contaminated vadose zone was conducted through infiltration of electron-donor-enriched water. A vadose zone monitoring system (VMS) provided real-time tracking of the hydraulic and chemical conditions across the unsaturated zone. Variations in concentration profiles of perchlorate, chloride, DOC and bromide in the vadose zone pore water showed limited migration capacity of biologically consumable carbon and significant mobilization of perchlorate.