DEVELOPMENT OF SITE-SPECIFIC DISSOLVED OXYGEN STANDARDS IN SURFACE WATERS AT DENVER INTERNATIONAL AIRPORT

Denver International Airport (DIA) seasonally utilizes chemicals for aircraft deicing to enhance air travel safety. According to the State of Colorado, DIA has a state of the art aircraft deicing fluid (ADF) collection system and has implemented all best practical, available and economically achievable technology for the control of ADF. However, even with these systems in place, the open air application of deicing chemicals to aircraft creates a potential for migration of fugitive ADF to the storm sewer and thus to the local receiving waters.Concerns regarding the impact of ADF on airport receiving waters resulted in the implementation of a Receiving Water Study to assess potential impacts on surface waters, including potential effects on dissolved oxygen levels. Working with state and federal regulators, DIA conducted studies for more than three years to characterize ambient dissolved oxygen conditions and assess the aquatic communities in airport receiving waters. Five streams, which receive stormwater runoff from the airport property, were studied. A range of hydrological, biological and potential impact conditions were represented: (1) Second Creek, a highly intermittent stream; (2) Third Creek, an intermittent stream with enhanced flows resulting from airport stormwater discharge; (3) Box Elder Creek, a hydrologically variable stream, which received little or no impact from ADF; and (4) two ephemeral streams that only rarely receive stormwater runoff. A site on Second Creek upstream of airport development served as the study reference site because it represented natural conditions without airport impact.The Receiving Water Study demonstrated that DIA's receiving waters, regardless of the presence of ADF, routinely experience dissolved oxygen concentrations below the 5.0 mg/L standard adopted by Colorado to protect warmwater aquatic life. Moreover, the results of the biological assessment indicated the presence of a healthy aquatic community, that was most limited by habitat constraints rather than impacts from airport operations. Based on these findings, it was recommended that DIA develop site-specific dissolved oxygen standards for the airport's receiving waters.The State of Colorado water quality standards regulations authorize various approaches for establishing water quality criteria, including both toxicological and ambient-based methods. To facilitate the development of site-specific dissolved oxygen standards, DIA established a broadbased stakeholder group to provide both technical and policy guidance throughout the process. Using Colorado regulations as a guide, several approaches for developing standards were evaluated with stakeholders. Ultimately, it was recommended that DIA use an ambient-based approach to establish site-specific standards because the data indicated that the aquatic life use was attained at existing dissolved oxygen concentrations.Following the recommendations of the stakeholder group, ambient-based dissolved oxygen criteria were established for Second, Third and Box Elder Creeks by calculating the 15th percentile of continuous dissolved oxygen measurements from the daytime hours between 6:30 am and 6:30 pm. Accordingly, the following standards were recommended: Second Creek (3.3 mg/L), Third Creek (4.0 mg/L), and Box Elder Creek (4.7 mg/L). For the two ephemeral waterbodies insufficient data existed to calculate a numeric ambient-based dissolved oxygen standards. As a consequence, a narrative dissolved oxygen standard was developed for these waters.DIA submitted the proposed site-specific numeric and narrative dissolved oxygen standards to the Colorado Water Quality Control Commission (WQCC) for consideration in the South Platte River Basin triennial review of water quality standards. On July 13, 2004, the WQCC unanimously moved to adopt the DIA proposal. The standards were approved by EPA and became effective on January 20, 2005.