Arsenic Removal: Lessons Learned from Pilot Studies on the Navajo Nation

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Arsenic Removal Lessons Learned from Pilot
Studies on the Navajo Nation
LCDR Gretchen A. Cowman, PE, MSPH Senior
Environmental Engineer Navajo Area Indian Health
Service
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Arsenic Rule

• Lowered MCL from 50 ppb to 10 ppb.
• New MCL reduces risk of bladder and lung cancer.
• Effective January 2006.
• Waivers up to 9 years available to small,
  disadvantaged systems.

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Arsenic in Four Corners Area Groundwater

• Four community water systems exceed the MCL.
• Six wells ranging from 11-30 ppb As.
• 1,020 homes affected.

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Initial Design Approach

• Inter-tie the four community systems.
• Construct a centralized, 350 gpm treatment system
  and distribute treated water throughout the area.
• More cost effective than 45 mile transmission
  line from Shiprock.

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Adsorptive Treatment Technology

• Simple design.
• Easy to operate, no chemical handling.
• Used adsorbents are not a hazardous waste.
• Cost-effective for small systems.

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Measuring Performance of Adsorptive Media

• Number of bed volumes (bv) of water treated
  before arsenic in the effluent exceeds 10 ppb
  (breakthrough).
• gt 50,000 bv is good performance.

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Initial Pilot Test Results

• Initial testing at 3 well sites.
• Six different medias tested.
• Poor media performance, lt10,000 bv, most likely
  due to high ambient pH of water sources (9.3-9.5)
• Poor media performance, lt20,000 bv, when pH was
  reduced to 7.0

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Known Factors Affecting Adsorption

• High pH
• Silica
• Phosphate

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Suspected Factors Affecting Adsorption

• Vanadium
• Uranium

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Sweetwater Master Well Water Quality

• Arsenic 20 ppb
• Uranium 30 ppb
• Vanadium 350 ppb
• pH 9.3

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Pilot Test Procedure

• Chlorinate water prior to treatment.
• Reduce pH to 7.1 using sulfuric acid.
• Test a titanium-based media (Metsorb) and an
  iron-based media (AD 33) concurrently.
• Measure influent and effluent concentrations of
  As, U, and V.

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Results

• Metsorb and AD 33 removed As, U, and V.
• Arsenic broke through at about 11,000 bv with
  Metsorb and at about 14,000 bv with AD 33.

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Uranium Accumulation

• A solid waste containing more than 500 ppm
  uranium is a low level radioactive waste.
• Both medias passed this limit at about 14,000 bv
  of treatment.

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Loss of pH Control

• Will a sudden rise in pH cause accumulated As, U,
  and V to leach off the media?
• How quickly and in what amounts will contaminants
  leach off the media?

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Results

• As, U, and V leach off both medias.
• Effluent concentrations exceed influent
  concentrations within the first half hour of loss
  of pH control.
• As, U, and V are released at high concentrations.

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Coagulation/Filtration Pilot Test

• Add iron in the form of ferric chloride
• Add chlorine and adjust pH
• Filter out ferric hydroxide floc
• Arsenic removed with iron floc
• Produces an iron sludge

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Results

• 90 arsenic removal.
• 50 uranium removal.
• Excessive acid consumption to reduce pH to 6.3.
• Calculations indicate uranium content of iron
  sludge is 4000 ppm.

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Conclusions

• High pH, uranium, and vanadium complicate arsenic
  removal.
• Uranium and vanadium are likely to compete with
  arsenic for adsorptive sites.
• Adsorption of uranium can generate a low level
  radioactive waste.
• pH adjustment with adsorption technology
  introduces the risk of exposing consumers to high
  concentrations of arsenic and other contaminants.

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Conclusions (cont.)

• Presence of uranium can cause sludge from a
  coagulation/filtration process to be a low level
  radioactive waste.
• Pilot testing is essential to evaluate the
  feasibility of a treatment system.

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Acknowledgements

• Adeline Johnson, NAIHS
• Greg LeFevre, Co-Step, NAIHS
• Tom Sorg, USEPA
• Green Analytical Laboratories