Methods to assess environmental impacts from episodic discharges on receiving water bodies need a more environmentally relevant and scientifically defensible toxicity test design. Many permittees are regularly required to conduct 96-hour toxicity tests on discharges associated with events that are generally less than 24-hour in duration. Current standardized methods do not adequately reflect these episodic discharge conditions at either the point of compliance or as it mixes with the receiving environment. In order to evaluate more representative biological effects, an alternative toxicity approach is described, incorporating pulsed exposures of effluents and subsequent transfer of test organisms to clean water for the remainder of the test. 

A more realistic assessment of the toxicological impacts of stormwater runoff, or other episodic discharges (e.g., intermittent discharges of stormwater and relief water from dry dock outfalls at Naval shipyards, chlorinator/dechlorinator cooling water from pier-side ship, etc.) on beneficial uses in the receiving waters is critical to support decisions related to the need and prioritization of appropriate best management practices, and meaningful compliance with Clean Water Act goals. 

The objective of this project was to derive, demonstrate, and validate a standardized environmentally relevant and scientifically defensible exposure design for laboratory toxicity testing (i.e., whole effluent toxicity) to assess impacts to receiving waters from episodic discharges.

Depiction of Discharge Point from Stormwater Runoff During (left)

After (right) Episodic Discharge Event

Technology Description

The laboratory study was designed to refine the pulsed exposure protocol for two marine species, the purple sea urchin, Strongylocentrotus purpuratus, and survival of the mysid shrimp, Americamysis bahia, and two freshwater species, the freshwater amphipod, Hyalella azteca, and the water flea, Ceriodaphnia dubia. These species/methods cover a range of acute and chronic endpoints required under various National Pollutant Discharge Elimination System permits. This study investigated the effects of various copper, zinc, or bifenthrin (a pyrethroid pesticide) concentrations, and exposure durations, on the observed toxicity for all species.

Demonstration Results

A total of 144 toxicity tests were conducted. Test acceptability criteria were met for all tests conducted. Point estimate values and confidence intervals for each species/constituent/ exposure duration were generated. A standard operating procedure was published for each of the species evaluated. 

A Pulsed Exposure Scientific Advisory Panel was established with roughly 21 participants from diverse backgrounds including toxicological experts, regulators, and end-users. Biannual updates were conducted each year of the project program. 

A survey was provided to laboratories requesting their feedback on their participation in this study and implementation of the pulsed exposure methods. Four laboratories provided feedback and the primary concern was the increased labor hours that would be required for performing the pulsed exposure method. An additional concern was the one-time cost of constructing the materials needed for performing organism transfers. However, the labs recognized that the cost was an appropriate investment if pulsed exposure methodologies were integrated into future monitoring efforts. 

Operational costs for performing standard toxicity test methods were compared to costs associated with conducting the pulsed exposure methodology. The main notable difference in costs between standard toxicity test methods and the pulsed exposure methodology was that the number of labor hours that were required to conduct the transfer of organisms from the test/effluent solution into receiving or laboratory control water at the appropriate time point.

Implementation Issues

The pulsed exposure methodology was a fairly straightforward adaptation of existing toxicity methods that are widely used for the evaluation of effluent samples. 

The primary issue in getting the method accepted as an alternate test method was that there is no specific Environmental Protection Agency (EPA) procedure for getting a Whole Effluent Toxicity test method evaluated through the EPA’s Alternate Test Procedure (ATP) process. It has also been a learning process determining the optimal way to approach getting the method submitted to the ATP process. It was discovered that the project team would be unable to submit the method for review for a nationally accepted method, but rather the pulsed method would be reviewed for regional and permit specific acceptance. 

A pulsed exposure assessment can also be an added tool for areas identified as having high risk with prior documented toxicity in the outfall samples using standard continuous exposure methods. The idea is not to replace existing methods, but rather to supplement them to provide a more representative and meaningful measure for subsequent management decisions. (Project Completion - 2023)


Colvin M., K. Kowal, N. Hayman, C. Stransky, J. VanVoorhis, S. Carlson, and G. Rosen. 2021. Pulsed Exposure Toxicity Testing: Baseline Evaluations and Considerations Using Copper and Zinc with Two Marine Species. Chemosphere, 277:130323. doi.org/10.1016/j.chemosphere.2021.130323.

Colvin M., G. Rosen, N. Hayman, B.C. Stransky, B. Phillips, T. Hoang, and K. Kowal. 2020. Standard Operating Procedures: Pulsed Exposure Methodology. NIWC Technical Document, 3397. DTIC: AD1092603.

Colvin M., G. Rosen, N.T. Hayman, and K. Kowal. 2020. Long-term Rainfall Analysis at Select DoD-relevant Sites for the Determination of Appropriate Toxicity Exposure Durations. NIWC Technical Document, 3392. DTIC: AD1091097

Rosen G., M. Colvin, C. Katz, J. Munson-Decker, and Hayman NT. 2019. Pulsed Exposure Toxicity Testing: Method Development and Initial Evaluation for Stormwater Compliance. NIWC Pacific Technical Document, 3393. DTIC: AD1082517