5 Colo. Code Regs. § 1002-42.34
The provisions of C.R.S. sections 25-8-202; 25-8-203; 25-8-204; 25-8-402, provide the specific statutory authority for adoption of this regulation. The Commission also adopted, in compliance with section 24-4-103(4) the following statement of basis and purpose.
BASIS AND PURPOSE
Site-Specific Classification
As a result of this hearing, the Commission applied the classification of potentially usable quality to the unconfined alluvial ground water and upper 50 feet of Denver Aquifer ground water located beneath specified areas within the City and County of Denver and the City of Aurora, Arapahoe County, Colorado.
Site-Specific Numeric Standards
In this hearing, the Commission established site-specific TCE groundwater standards of 11 µg/L for the on-base specified areas and 12 µg/L for the off-base specified areas (the on-base specified areas are located within the historic boundaries of the Air Force Base, the off-base specified areas extend north beyond the historical Air Force Base boundary). The Commission considered the factors set forth in C.R.S. § 25-8-204(4) to establish site-specific standards for the Specified Areas. The parties presented the majority of the evidence in the record on two of the statutory factors: the feasibility of additional ground water remediation, and the level of risk to human health of residual groundwater contamination levels.
On the first factor, the Commission determined that the proponent, Lowry Assumption, LLC ("LAC"), had made significant reductions in the level of contamination in the groundwater, and that it was unclear from the record and testimony whether additional remediation would significantly reduce the remaining contamination. Therefore, the Commission made no conclusions about the practicality of additional remediation.
On the second factor, the Commission made two findings. First, the Commission found all exposure pathways other than indoor air had been eliminated either through recorded environmental covenants, site-specific characteristics, or other legal and practical limits. Second, the Commission determined as a matter of policy to establish the site-specific standard based on a human health chronic cancer risk level of 1 x 10-6 for the indoor air exposure pathway. Since 1989, the Commission has maintained the policy that, "the acceptable risk to human health should be 1 x 10-6," for both surface water and groundwater standards (Water Quality Control Commission Policy 96-2). Since 1989 the Commission has reaffirmed this policy decision at multiple standards hearings. The Commission determined that in order to protect public health and the environment, establishing a site specific standard that complies with the acceptable risk levels as outlined in Policy 96-2 is appropriate, and based its decision on a risk analysis that was limited to the only identified remaining exposure pathway, which was indoor air.
The Commission also adopted a site specific standard of 3.2 ug/l for 1,4-Dioxane for the On-Base and Off-Base Specified Areas. This site-specific standard was based upon a previous Interim Narrative Standard and the lack of exposure pathways (indoor air or drinking water). LAC also proposed site-specific standards for other parameters listed in Table A of Regulation #41 that were not adopted by the Commission.
PARTIES TO THE RULEMAKING HEARING
5 CCR 1002-42.34