Survey and Modeling Analysis of MS4 Highway Storm Runoff on Oahu, Hawaii

Survey and Modeling Analysis of MS4 Highway Storm Runoff on Oahu, Hawaii

Hawaii Department of Transportation

09/01/08 - 08/31/10

Under Section 402 of the U.S. Clean Water Act, highway storm runoff is considered to be part of urban point source pollution and is regulated by the National Pollutant Discharge Elimination System (NPDES). As a NPDES mandate, municipal separate storm sewer system (MS4) permit is required for the discharge of highway storm water. An MS4 permit specifies allowable levels of relevant water pollutants in the discharge, determined by waste assimilative capacity analysis and waste load allocation (WLA) of the receiving water. The current Hawaii Department of Transportation (HDOT) MS4 permit for highway storm water discharge was issued in February 2006 by the Hawaii Department of Health (HDOH) on behalf of the U.S. Environmental Protection Agency (EPA). The permit stipulates that HDOT work jointly with the City and County of Honolulu (CCH) to propose monitoring and implementation plans for the existing Ala Wai Canal, Kawa Stream, and Waimanalo Stream WLAs.

If the receiving stream of highway storm runoff is designated as a water-quality-limited stream, highway storm runoff is also regulated by Section 303(d) of the U.S. Clean Water Act. Water-quality-limited streams are those water bodies in which established water quality standards cannot be met even if mandatory minimum treatment facilities have already been constructed and put in operation for point-sources of pollution. Under Section 303(d), the total maximum daily load (TMDL), a water-quality based management tool, must be established by states for water-quality-limited streams within their jurisdiction. TMDL is defined as the sum of the individual WLAs for point sources, load allocation (LA) for nonpoint sources, and natural background pollutants.

Storm runoff from the H-1 freeway drains into the Ala Wai Canal. According to a recent Ala Wai Canal TMDL report, a 65% loading reduction of total nitrogen and total phosphorus from urban point source pollution produced by CCH and HDOT is required. However, the report did not specify how much load reduction HDOT would be responsible for.