The term "total maximum daily load," or TMDL, refers to the calculation of the maximum amount of a pollutant that a water body can receive and attain or maintain water quality standards for its designated use.
The one major unresolved issue identified following publication of the Draft Environmental Impact Statement (DEIS) related to the application of road salt and the potential impairment of surface waters due to elevated chloride concentrations. From the winter of 2002/2003 through 2005, surface water quality monitoring in the I-93 corridor was conducted jointly by the New Hampshire Department of Environmental Services (DES) and the United States Environmental Protection Agency (EPA).
The monitoring indicated that during various times of the year chloride levels do not meet state water quality standards in four streams - Beaver Brook, Dinsmore Brook, an unnamed tributary to the western embayment of Canobie Lake, and Policy Brook – which triggered the requirement for a TMDL study for chlorides. The study was conducted by DES and funded by the New Hampshire Department of Transportation.
The largest source of chlorides in these watersheds was presumed to be road salt. However, winter road maintenance on I-93 may not be the primary source of chloride. Other chloride sources include deicing activities on other state, municipal and private roads and commercial parking lots, as well as the discharge from septic systems and water-softening systems. Therefore, in an effort to implement a regional solution, and in coordination with the TMDL study, DOT and DES established a Salt Reduction Workgroup. Town Departments of Public Works and Highways, Regional Planning Commissions, the DOT, EPA, DES, Federal Highway Administration, University of New Hampshire and Plymouth State University were represented at the first meeting on September 26, 2006. Four additional meetings were then scheduled. Furthermore, the DOT and DES have been conducting a training/outreach program to promote the best practices for the storage and application of road salt.