In November 2014, EPA updated its 2002 memorandum, “Establishing Total Maximum Daily Load (TMDL) Wasteload Allocations (WLAs) for Storm Water Sources and NPDES Permit Requirements Based on Those WLAs.” The new memorandum is a final version of a 2010 memo on the same topic that guided EPA regional offices on updates related to the role of stormwater runoff in total maximum daily loads (TMDLs).
This article explains the evolution of regulated stormwater flows in the TMDL program and what this means moving forward.
Stormwater in TMDLs: background
How regulated stormwater runoff has been addressed in the TMDL program requires a review of regulations and recent guidance from EPA.
Initially, stormwater runoff wasn’t included in the Clean Water Act (CWA). That changed, however, with the Water Quality Act of 1987. Those amendments led to the establishment of stormwater entities, known as municipal separate storm sewer systems (MS4s).
The TMDL program identifies daily limits of regulated discharges (wasteload allocations, or WLAs) and unregulated discharges (load allocations, or LAs) allowable in a watershed while still meeting water quality standards for receiving waters. In the program’s early days, regulated and unregulated stormwater discharges were often lumped together into LAs. A 2002 EPA memo sought to change this policy by proposing that:
- Discharges associated with MS4s should no longer be addressed as an LA but could be aggregated into the WLA component of a TMDL.
- While WLAs and LAs are to be expressed numerically, stormwater WLAs and LAs could be “fairly rudimentary because of data limitations and variability in the system” and
- numeric limits should be used “in rare circumstances.”
- Water quality-based effluent limits (WQBELs) could be expressed as technology-based (in terms of best management practices used) rather than performance-based.
EPA’s 2010 memo suggested disaggregating discharges from MS4s in WLAs, providing numerically based WQBELs “where feasible” for regulated stormwater discharges, and allowing for the use of surrogates when establishing targets for TMDL loads (more on surrogates later). The reasoning behind these proposed policy changes was that “the technical capacity to monitor stormwater and its impacts on water quality has increased” and that “TMDL writers have better data…and have gained more experience since 2002 in developing TMDLs and WLAs in a less aggregated manner.”
Next page: Reaction to 2010 memo: lawsuits