For over three years, U.S. EPA had been moving toward a seismic shift in how it regulated stormwater run-off from construction sites.  For the first time, U.S. EPA tried to impose a numeric permit limit on the turbidity of water (sediment mixed with water) that leaves construction sites following rain events.   EPA ‘s efforts are the result of a 2004 lawsuit by environmental group who obtained a Court order requiring development of a numeric standard.

Adjacent is a picture which shows bottles containing various amounts of sediment mixed with water.  The more dirt the higher the NTUs.  The picture is a good graphic illustration of EPA’s proposed numeric standards.

Historically, U.S. EPA has never tried to impose a numeric limit to control stormwater discharges.   Rather, U.S. EPA  required the use of  "best management practices" (BMPs) as part of NPDES construction stormwater permits.  BMPs were engineering controls that were designed to collect or limit run-off to reduce sediment loading on streams and lakes.

In November 2008, U.S. EPA issued proposed effluent guidelines that contained a numeric turbidity standards of 13 NTUs.  EPA received a flood of comments questioning the data and assumptions behind that proposed standard.

On December 1, 2009, U.S. EPA issued its final effluent guidelines raising the propose numeric turbidity standard to 280 NTUs.  The standard would apply to construction sites 10 acres and larger.  It also included the requirement to take samples throughout the day.  The average of the samples would be compared to the 280 NTU standard for purposes of determining compliance.

Despite the dramatic increase from the proposed effluent guidelines, industry still was deeply concerned with the cost of compliance and questioned the benefits of such a standard.  Two petitions for reconsideration were submitted challenging the 280 NTU standard.  After raising the technical issues raised in those petitions, EPA decided to stay implementation of the provision.

The EPA submitted a proposed rule to revise the turbidity limit to the Office of Management and Budget (OMB) in December 2010. On August 17th, EPA announced it had withdrawn its most proposal from OMB. , EPA decided to seek additional treatment performance data from construction and development sites before proposing a revised numeric turbidity limit. Next step, EPA will publish a Federal Register notice soliciting data in the near future.

The battle over numeric limits is hardly over.  First, EPA still faces a court order to develop a numeric limit,  Second, on November 10, 2010, EPA issued memo that signaled its intention to shift toward numeric limits for most stormwater permits, not just construction sites.  Therefore, numeric limits are only a matter of time.