Stormwater Runoff TMDL for Aquatic Life

Project Objectives

The purpose of this proposal is to establish an applicable TMDL approach to address aquatic life impairments associated with stormwater runoff and hydrologic alteration for streams in New Jersey with the goal of improving river systems by reducing the impact of hydrologic stress such that beneficial uses of a stream may be restored. We intend to identify a set of ecologically relevant flow parameters based on relations to known aquatic community impairment within pre-defined hydrologic regions or Watershed Management Areas in New Jersey. These attributes will be used to 1) create a composite stormwater runoff index or identify a series of critical flow parameters as a surrogate for defining aquatic life impairment and 2) compare normalized flow duration statistics between stormwater impaired streams with non-impaired reference streams to develop hydrologic targets (percent deviation from reference) for rehabilitation of streams.

Statement of Problem

Unlike impairments by pollutants such as phosphorus or metals, which can be specifically quantified, the cause for biological impairment is not readily apparent. Biological impairment is but an indicator that some type of anthropogenic process has occurred resulting in aquatic assemblage degradation. Therefore, directly measurable and quantifiable surrogate parameters need to be identified which provide state and resource managers with the ability to allocate a percentage of the remedial response necessary to address biological (aquatic life) impairment. Some state agencies have had success using sediment loads as a surrogate for biological impairment on a site specific basis, however, stormwater runoff more holistically encapsulates the full suite of potential environmental stressors and has a greater potential for TMDL application in New Jersey streams. Stormwater runoff can be viewed as a master variable or an "umbrella" surrogate representing the cumulative affects of multiple stressors contributing to aquatic life impairments is these systems. Because there is often insufficient information to isolate the relative strength of stressors independently to the aquatic life impairment, innovative approaches are required. The use of stormwater runoff as a measurable surrogate in the TMDL development process to address aquatic life impairment and represent the basis for implementing control actions for New Jersey streams.

Strategy and Approach

Numeric endpoints are typically based on surface water quality standards for a pollutant of concern (e.g., phosphorus or metals) and load reductions necessary to meet these endpoints are determined (ultimately, the goal is to meet NJ surface water quality standards for biological impairment -which would be a New Jersey impairment score of "good"). TMDL load allocations scenarios are then developed based on an analysis of the degree to which contribution sources can be reasonably reduced. In this case, however, the "loadings" are not a measurable pollutant, but a surrogate based on stormwater runoff or a composite stormflow index that deviates from that expected in comparison to a reference stream or from a modeled parameter that was assessed using a simulated hydrograph derived under less anthropogenically modified conditions. The numeric endpoints, therefore, are based on percent deviation from the expected hydrologic response under least modified conditions (reference) as opposed to the required minimal loading of a specific pollutant. For stormwater runoff-based TMDL's, "load" scenarios would be developed by using established baseline hydrology and developing models predicting aquatic community response along a gradient of stream degradation for specific hydrologic regions or Watershed Management Areas in New Jersey. As mentioned above, statistics based on flow duration processes (e.g., 5- and 95-percent exceedance flow) of days during the period of record which exceed critical high and low flow values, can be directly compared and used to estimate acceptable stormwater runoff targets that will be protective of instream and riparian habitat and meet aquatic life uses.