by: James R. Norris, R.G.

In August, U.S. Environmental Protection Agency (EPA) proposed significant revisions to the Total Maximum Daily Load (TMDL) program. Changes to the National Pollutant Discharge Elimination System (NPDES) and Water Quality Standards programs were also proposed to facilitate implementation of the revisions. You may be affected by the load reductions required by a TMDL if you are a point or nonpoint source to, or are in the same watershed as a water body that does not meet water quality standards.

Section 303(d) of the Clean Water Act requires states to

• identify waters that do not meet water quality standards,
• prioritize the water-quality-limited reaches based on severity of pollution and water use, and
• establish a TMDL for the impaired segments.

The proposed revisions redefine the TMDL in terms of a process which develops a mass balance accounting that quantifies all point and nonpoint source loads, determines the load reduction needed to meet water quality standards, and allocates the reduction between the various sources. TMDLs must also consider seasonal variations and include a margin of safety which takes into account any uncertainties in how sources may impact water quality. Some of the major TMDL revisions are

1. Waterbodies will be categorized within one of four parts:
   • Part 1 - Impaired or threatened by pollutants or unknown causes;
     
   • Part 2 - Impaired or threatened by pollution but not impaired or threatened by pollutants;
     
   • Part 3 - TMDLs have been completed, but water quality standards have not been attained; and
     
   • Part 4 - Expected to meet water quality standards by the next listing cycle.
     
     

2. TMDLs will be established only for Part 1 waterbodies.
3. All source categories, subcategories, and individual sources for which wasteload allocations are being established must be identified.
4. A margin of safety must be included, considering seasonal variations and allowances for future growth.
5. States must establish schedules for establishing TMDLs within 15 years, or, for high priority waters, 5 years.
6. States must also submit an implementation plan for approval by EPA, provide for public comment, and allow the public to petition EPA to establish TMDLs where a state has failed to do so.

The TMDL program requires a strict reduction and allocation of mass loads for sources to an impaired or threatened waterbody. Potential direct and indirect costs will vary according to situation and depend on the level of effort needed to allocate and monitor chemical loading, the requirements of NPDES permit holders and nonpoint sources to meet the mass load reduction, limitation of new discharges in fully allocated watersheds, public participation, and legal actions. Because of their broad scope, TMDLs should be based on a comprehensive evaluation of environmental fate and transport so that the load allocation is defensible and equitably applied to all stakeholders, and the expected benefit of cleaner water is realized.

A “load” may be expressed as a pollutant mass, M, such that:

M = C_tQ_t

where C is the pollutant concentration and Q is its discharge rate during time, t. The simplest way of allocating pollutant loads in a stream is where stream flow conditions are easy to understand, when all of the dischargers are known, and when the pollutants are conservative (nonreactive). Chloride and nitrate are good examples of fairly conservative pollutants. The total load in the stream is the sum of the individual loads:

M = Q₁C₁ + Q₂C₂ + . . . + Q_nC_n

Although the easiest method, this analysis is not necessarily the most conservative, fairest to stakeholders, or protective of the stream. A more realistic mass balance accounting includes multiple point and nonpoint sources, and naturally occurring chemical processes that can add or remove pollutants. This case can be represented as:

M = SPS + SNPS + Instream Sources
- Instream Sinks + Margin of Safety

where SPS equals the sum of all point sources, SNPS equals the sum of all nonpoint sources including atmospheric deposition and natural background, Instream Sources equals the load that may be derived from the waterbody by such processes as mineral dissolution or biological activity, and Instream Sinks equals the load removed from the waterbody by processes such as volatilization, biodegradation, mineral precipitation, and sorption.

The quantification of any instream sinks is important because, otherwise, the load reductions imposed on point and nonpoint sources would be greater. Similarly, reliable data are needed because if the TMDL analysis contains a high level of uncertainty, a larger margin of safety would require even larger reductions from point and nonpoint sources to meet the TMDL.

TMDL analysis requires careful assessment of data quality, the environmental chemistry of the system, and the extent to which varying temporal and spatial properties need to be accounted for. For example, if little information is available on nonpoint sources, the value of collecting additional data should be determined. Otherwise, uncertainties can result in an unnecessarily large margin of safety being allocated between the point and nonpoint sources.

Similarly, environmental data on the natural assimilative capacity of the water may warrant consideration. Volatilization, biodegradation, transformation, and sorption are potential sinks for organic chemicals, whereas precipitation and sorption are important for removing inorganic constituents. Analysis of these processes may require additional monitoring data and numerical modeling to avoid unnecessary load restrictions and develop a realistic margin of safety.

Regardless of the complexity of the analyses required, stakeholders should take an active roll to assure that technical data on which a TMDL is based are adequate, that the proper environmental processes are considered, and that the TMDL fully addresses the technical and policy issues of load allocation.