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Allocations: Defintion and Options
Table of Contents
- Definitions and Allocation Options
- Case Study
- Modeling Framework
- Download Model
- Next Steps
Allocations describe how pollutant loads are distributed among different sources in the area surrounding the waterbody. Within the context of the TMDL program, allocations are the distribution or assignment of pollutant loads to entitites or sources, such that the sum of the loads does not exceed the maximum allowable load to the waterbody. Allocations are a component of legally approved Total Maximum Daily Loads (TMDL) and are composed of wasteload allocations (WLA) and load allocations (LA). WLAs are portions of the TMDL assigned to existing and future point sources and LAs are portions of the TMDL assigned to existing and future nonpoint sources, including background loads:
TMDL = ΣWLAi + ΣLAi + MOS
- Where:
- ΣWLAi: sum of waste load allocation (point sources)
- ΣLAi: sum of load allocations (nonpoint sources)
- MOS: Margin of Safety
TMDLs must also account for uncertainty about the relationships between loads and water quality using Margins of Safety (MOS). The MOS may be a reserved portion of the TMDL or may be provided implicitly by using conservative assumptions in the TMDL development process.
EPA's Technical Support Document for Water Quality-based Toxics Control (PDF) (26.6MB, 335 pages) (EPA/505/2-90-001) lists 19 allocation schemes for developing WLAs, but also indicates that any reasonable allocation scheme that meets the antidegradation provisions and other requirements of State water quality standards can be used. Examples of allocation schemes that can be applied to point and nonpoint sources are listed below. These allocation objectives focus on flexible endpoint measures such as source loads and cost.
- Equal Percent Overall Removal
- Equal Percent Incremental Removal
- Equal Overall Reduction of Raw Load
- Equal Incremental Reduction of Raw Load
- Equal Cost per Pound of Pollutant Removed
- Percent Removal Proportional to Raw Load per Day
- Seasonal Limits based on Cost-effectiveness
- Minimum Total Compliance Cost
As noted above, allocations should be technically feasible and should be consistent with other local, state, or federal programs that might apply (i.e., institutional constraints). In some cases, allocation options are constrained by technical feasibility, and sources must implement all possible management practices and available technologies to satisfy TMDLs or other regulatory limits. However, in other cases, regulatory limits are less constraining, and EPA encourages the use of allocations that are based on competing measures of desirability such as cost-effectiveness and equity and fairness. Other factors that should be considered when making allocation decisions include relative source contributions, ability of small entities to pay, and prior load reductions.
The marginal water quality impact of a pollutant load from one source may differ from the marginal impacts of another source, due to spatial and seasonal variability. If marginal impacts are assumed to be equal across sources, then TMDLs can be developed independently of allocations. However, if marginal impacts are not equal, then allocations and the TMDL must be developed simultaneously. Regardless of the situation, managers must keep in mind that:
Allocations must meet water quality standards
Not all allocation schemes or objectives are applicable in all situations, and some allocation objectives are likely to be more desirable than others for a particular watershed. The material in the next section demonstrates how measures of desirability, including cost, can differ across allocation objectives.
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