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New Arsenic Criteria Expected to Impact More Than 66 Idaho Municipal NPDES Permittees

Posted By Johanna Bell, Tuesday, October 11, 2016
Updated: Friday, October 28, 2016

EPA and Northwest Environmental Advocates (NWEA) entered into a settlement agreement on the Idaho Arsenic water quality standards on May 19, 2016.  In response to the May settlement agreement, the EPA Region 10 issued a disapproval letter on September 15, 2016 to the Idaho Department of Environmental Quality (attached) disapproving both Idaho's most recent arsenic criteria of 10 micrograms per liter (µg/L) or 0.01 milligrams per liter (mg/L) and older criteria of 50 µg/L that EPA had never approved or rejected.  The May agreement requires EPA to:

  • review and issue permits in Idaho using Idaho’s approved designated uses and EPA’s current recommended Arsenic criteria (6.2 µg/L or 0.0062 mg/L for consumption of fish only and 0.02 µg/L or 0.00002 mg/L for consumption of fish and water)
  • including water quality based numeric permit limitations for Arsenic where needed
  • require additional monitoring where detectable concentrations of Arsenic are greater than 0.5 µg/L using sufficiently sensitive analytical methods, and
  • for permittees that cannot meet permit limits, the permit will include a compliance schedule and where appropriate, a treatability study

The "fish plus water" criteria will affect at least half of the NPDES permits statewide.  Preliminary review of the existing 135 EPA-issued Idaho municipal NPDES permits for drinking water treatment and wastewater permittees along with the receiving waters designated uses show that:

  •  66 permittees (49%) would be subject to the 0.02 µg/L Arsenic criteria:
  • facilities located primarily in the Henry’s Fork, Pend Oreille, Spokane, Payette, Snake, Clearwater, Paradise Creek, Big Lost, Kootenai, Big Wood, Selway, St Maries, Potlatch, Weiser and Salmon watersheds
  • 69 permittees (51%) would be subject to the 6.2 µg/L Arsenic criterion:
  • facilities located primarily in the lower Boise, Little Wood, South Fork Palouse, Portneuf, South Fork Clearwater, South Fork CdA, South Fork Teton, and tributaries/canals to major rivers across the state

Preliminary assessment of the ability of permittees to meet the new criteria suggests that many NPDES permittees, including all of the permittees required to meet the "fish plus water" 0.02 µg/L arsenic criteria and potentially some of the permittees required to meet the "fish only" 6.2 µg/L criteria, will face significant permitting requirements and actions.  

Primary concerns include the ability of treatment technologies to reliably meet the criteria, high capital and operation and maintenance costs, and negative operational and environmental impacts (additional energy, additional solids, brine disposal, additional treatment space, etc.).  A 2013 toxic(s) Treatment Technology Review and Assessment funded for Washington State found that, for arsenic, benzo(a)pyrene, mercury, and polychlorinated biphenyls, no facilities currently treat to the revised human health water quality criteria; and that compliance with arsenic criteria of 0.018 or 0.02 µg/L appears unlikely. Click HERE for more information.

A statewide approach to the permitting and compliance issues raised by the May agreement would likely result in significant cost savings (i.e., implement one treatability study for the 66+ facilities and submit one statewide variance request; subsequently reducing the number of 401 certifications and variance reviews by EPA and IDEQ, etc.).  Benefits to a statewide approach also include increased consistency and technical rigor of the resulting treatability study. 

AIC will coordinate a statewide approach to address the limits, monitoring requirements, treatability study, and if needed, a variance request for EPA issued permits.  For additional information contact Johanna Bell at  or (208) 344-8594.

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