While the Clean Water Act, requires a plan to restore impaired surface waters, the Porter-Cologne Act, which governs water quality regulation in California, also guides this process as it requires a program of implementation for a TMDL to be included into the Basin Plan (CWC § 13050(j)(3)). The Porter-Cologne Act is distinct from federal regulations because it applies to surface waters, wetlands, and ground water and to both point and nonpoint sources of pollution. Both Porter-Cologne and the Clean Water Act are important in the development of TMDL’s and protecting water quality.
TMDLs should address all significant stressors which cause or threaten to cause waterbody use impairment. Within the North Coast Region, the stressors of concern are most often sediment, temperature, nutrients, low dissolved oxygen, turbidity, pathogens, and mercury.
A complete TMDL must contain all of the following elements:
- Problem Statement: Describes which water quality standards are not being attained, which beneficial uses are impaired, and the nature of the impairment.
- Numeric Targets: The Desired Future Condition: Defines measurements that will ensure recovery of the beneficial uses that are impaired, and attainment of standards. Numeric targets are usually not directly enforceable but are used to assess progress towards the attainment of standards.
- Source Analysis: Identifies the amount, timing, and point of origin of pollutants of concern. Source analysis may be based on field measurements and/or models and estimations.
- Allocations: Allocates responsibility, and identifies the parties to take the specified actions. The allocations may be specific to agencies or persons (businesses), or generally by source category or sector. Allocations of allowable pollutant burdens define TMDL endpoints (e.g., total sediment load from urban runoff). The sum of individual allocations must equal total allowable pollutant burden.
- Margin of Safety: Describes how the required margin of safety was incorporated into the TMDL. The margin of safety may be implicit (i.e., using conservative assumptions), or explicit (i.e., a discrete allocation assigned to the margin of safety).
- Implementation Plan: Describes what actions will be undertaken to alleviate the impairments. The Implementation Plan identifies enforceable features (e.g., prohibition) and triggers for Regional Board action (e.g., performance standards).
- Linkage Analysis: How the Numeric Targets relate to the Problem: Describes how the actions to be taken will result in achievement of the relevant standards.
These listed elements, compromising the scientific basis for the TMDL are documented in a Staff Report, which undergoes revision where necessary based upon both peer-review and public comment. The Action Plan component of a TMDL, also subject to public review, outlines the strategy to meet the TMDL load allocations. The Staff Report in combination with the Action Plan contain the documentation for a TMDL. These reports are often considered on a watershed-wide scale, in order to assure comprehensive source identification and control.
