Durham is trying to close a 25-square-mile “donut hole” where no stormwater standards currently exist, by enacting a citywide stormwater standards ordinance. But state regulators worry that closing the regulatory loophole, as written, may come at the expense of existing stormwater requirements in the Neuse River Basin, which the ordinance would replace. Roughly 55 square miles in north Durham currently falls within this area, which is subject to the state’s Nutrient Sensitive Water Rules due to excess pollution.

In an e-mail dated March 2, N.C. Division of Water Quality community planner William Diuguid wrote, “We do not feel we can approve the proposed changes as presented because they appear to violate the existing Neuse River Basin Nutrient Sensitive Waters Rules.”

At issue is the method of reducing nitrogen, a potentially harmful pollutant contained in stormwater runoff, due to impervious surfaces such as roads and rooftops preventing its natural absorption. The proposed ordinance would allow developers to achieve nitrogen reduction through a combination of conservation land purchases and, based on a project’s density, a varying percentage of on-site treatment. For sites with impervious surfaces covering 90 percent of the property, no on-site treatment would be required.

Currently, sites within the Neuse River Basin cannot substitute on-site treatment with conservation land offsets until they reduce nitrogen to 10 pounds per acre per year, for commercial sites, or 6 pounds per acre per year, for single-family residential sites. All property owners must reduce their nitrogen output further to 3.6 pounds per acre per year, taking into account offsets. Under the proposed ordinance, properties with at least 37 percent impervious surfaces would be subject to no such bright-line pound per acre per year requirement for treating nitrogen on-site. Instead, a percentage of nitrogen reduction would have to occur on-site for most development projects, and a sliding scale would reward denser development with fewer on-site requirements.

In a letter to DWQ dated March 9 (PDF, 32KB), Durham Water Quality Manager John Cox writes, “Projects that are less than 75% impervious cover would require more treatment than under current requirements. Only projects that exceed 75% impervious cover would have lower onsite requirements than under the current 10 pound per acre per year limit. These projects would be allowed to use either land banking or offset payments to achieve 3.6 pounds per acre per year.”

“We’re offering a little bit more leeway above 75 percent, because there are very few areas where you can achieve that,” Acting Stormwater Manager Paul Wiebke said in an interview.

Since presenting the proposed ordinance to City Council, Wiebke said he had modified its language to specify that high-density projects eligible for on-site treatment exemptions must be located within the City’s Compact Neighborhood, Downtown, or Suburban Transit Development tiers, as specified by the Unified Development Ordinance. Also, redevelopment projects that increase the amount of impervious surfaces must treat runoff from those surfaces on-site.

“The state felt like it would be a free-for-all,” Wiebke said of his original proposal. “Standing on its own, if you don’t read the UDO, you don’t get the context of what is possible.”

Wiebke estimated that roughly 2.1 square miles within the Neuse River Basin–out of 54.8 square miles in Durham– would qualify for the exception. An additional 2 to 3 miles within the donut hole–where no stormwater protections currently exist–would qualify.

Previously, a handful of Durham developers complained to City Council that the proposed ordinance would disproportionately punish high-density developers, because it would subject them to technically unfeasible onsite treatments. In fact, the proposed ordinance would loosen restrictions on nitrogen controls for sites within the Neuse River Basin that have a density of 75 percent or higher. Projects within transit-oriented tiers, which developers had cited as particularly vulnerable, would gain additional exceptions.

DWQ will consider amendments to the proposed ordinance this week, according to DWQ environmental engineer Mike Randall.

More updates to come.