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Different Strategies for Biological Remediation of Perchlorate Contaminated Groundwater

  • Author(s): Wang, Yue
  • Advisor(s): Matsumoto, Mark R.
  • et al.
Abstract

Perchlorate (ClO4-) has gained attention recently due to its interference with thyroid gland function. In infants and unborn children, inadequate thyroid hormone production can cause mental retardation and thyroid tumors. Since new perchlorate standards will be proposed in 2013, and if a stricter standard is imposed, cost effective technologies will be in high demand. The overall objective of this research was to evaluate two perchlorate bioremediation strategies using indigenous soil bacteria: 1) an autotrophic strategy using zero-valent iron (ZVI) to generate hydrogen as the electron donor and alkalinity in the form of (bi)carbonate as the carbon source for cell growth and maintenance and 2) a heterotrophic strategy using organic substrates as the electron donor and the carbon source for cell growth and maintenance.

The first strategy was evaluated on perchlorate-contaminated groundwater from West Valley Water District Well #2 located in Rialto, CA (Chapter III). A mobile treatment system consisting of a water holding tank, a ZVI packed bed and two parallel sand filters was placed at the site. In the first three months, the system experienced excellent performance, as measured by the tested parameters meeting the California drinking water standards. The effluent concentration of perchlorate was non-detectable (below 4 µg/L), nitrate effluent concentration was less than 0.01 mg/L as N, effluent iron ranged from 0 to 0.05 mg/L. Coliforms, fecal coliforms and E. coli in the reactor effluent were below the detection limit of 2 MPN/100mL. However, significant loss of perchlorate performance was observed after 3 months operation. The reason was attributed to the reduction of hydraulic conductivity and flow channeling.

A laboratory column experiment was conducted to investigate the hydraulic condition change in the ZVI beds (Chapter IV). Effects of flow rate and (bi)carbonate on hydraulic condition were evaluated by performing hydraulic conductivity tests, SEM examination, and tracer tests. The results indicated that the decrease of hydraulic conductivity was more pronounced in the low flow reactors than in the higher flow reactors. This result appeared to contradict the hypothesis that increasing the flow rate will accelerate the hydraulic conductivity reduction. (Bi)carbonate was determined to be the primary cause of the hydraulic conductivity reduction. The decrease in hydraulic conductivity was most severe in the segment receiving the higher concentration of NaHCO3. Hydraulic conductivity decreased from 10-2.73 cm/s to 10 -7.33 cm/s after constantly feeding 24 mM of NaHCO3 for 41 days. The reduction of hydraulic conductivity was caused by the formation of mineral precipitates.

Because of the lack of long-term perchlorate reduction in the autotrophic ZVI-based system, an alternative strategy that utilized organics as both the electron donor and carbon growth source was tested for perchlorate bioremediation (Chapter V). Laboratory microcosm and column tests were employed to assess the effectiveness of selected organic substrates on reducing perchlorate from two different locations of a real perchlorate-contaminated site. One location (source area) had 70 mg/L of perchlorate in groundwater, and another one (plume edge, referred to "biobarrier") had 500 µg/L of perchlorate. The effect of adding nutrients was also examined. For the high concentration source area treatment, emulsified oil substrate (EOS) and glycerin were determined to be the most effective organics from the microcosm testing. Hence, they were selected for column testing. The results revealed that amending soil with EOS had significant advantages over using glycerin as a soil amendment. After a single injection of EOS, perchlorate can be reduced to less than 4 µg/L for 4 months. Perchlorate reduction was not initiated in glycerin-amended soil. Glycerin had to be constantly added into the influent to treat perchlorate to non-detectable level. For the low concentration biobarrier treatment, compost/mulch, EOS, and EHC (a combination of carbon plant-based carbon source and zero-valent iron) had similar perchlorate removal rates in microcosm tests. EOS appeared to have greater longevity than EHC in the column tests. The addition of nutrients had minor benefit on both sites treatments.

Comparing the two strategies, using organic substrate was more feasible for perchlorate bioremediation in terms of overall performance and longevity.

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