"In November 2011 the Environmental Protection Agency (EPA) proposed two Clean Water Act (CWA) permits to regulate certain types of vessel discharges into U.S. waters. The proposed permits would replace a single Vessel General Permit (VGP) issued in 2008 that was due to expire in December 2013. As proposed, the permits would apply to approximately 71,000 large domestic and foreign vessels and perhaps as many as 138,000 small vessels. […] EPA proposed two permits, one for large vessels to replace the 2008 VGP, and one for smaller vessels covered by a congressionally enacted temporary moratorium. Both were proposed well in advance of the VGP's expiration to provide ample time for the regulated community to prepare for new requirements. On March 28, 2013, EPA issued a final version of the VGP for large vessels. It took effect December 19, 2013. The permit for smaller vessels, the sVGP, was issued on September 10, 2014, and was scheduled to take effect on December 19. […] The principal benefits of the permit will be reduced risk of introducing ANS [aquatic nuisance species] into U.S. waters and enhanced environmental quality resulting from reduced pollutant discharges, but the magnitude of benefits is not calculable, according to EPA. The agency acknowledged significant uncertainty about several assumptions affecting estimated costs of the permit. The revised VGP raises two key issues. One concerns inclusion of specific numeric ballast water discharge limits in the permit. At issue had been whether EPA would propose more stringent numeric limits, as some environmental groups have favored and a few states have already adopted. A second issue concerns the role of states in regulating vessel discharges."
CRS Report for Congress, R42142