Pulsed Exposure Toxicity Testing: Method Development and Initial Evaluation for Stormwater Compliance

Preliminary research into modifying whole effluent toxicity (WET) testing protocols designed for continuous flow discharges as applied to episodic and/or ephemeral discharges such as those associated with storm water runoff.

Pulsed exposure experimental design.

This report describes a preliminary research effort to modify whole effluent toxicity (WET) testing protocols designed for continuous flow discharges for application to episodic and/or ephemeral discharges such as those associated with storm water runoff. The effort was undertaken in response to a Naval Base San Diego industrial stormwater National Pollutant Discharge Elimination System (NPDES) permit (R9-2013-0064) condition that allows the Navy to assess and propose alternative testing parameters. This research (at the time it was conducted) was done for Naval Base San Diego by environmental toxicologists at the Navy’s Space and Naval Warfare Systems Center Pacific (SSC Pacific).

The technical approach taken was to modify the WET testing method to simulate a range of exposure conditions found at the end-of-pipe. The test conditions matrix included: acute and chronic endpoints with commonly used test organisms; copper, zinc, and a combination of the two toxicants at various concentrations found to cause toxicity under standard WET testing; and short-term exposure conditions representing the 50th, 75th, and 95th percentile historical rainfall durations observed in San Diego over the past 55 years. The initial testing culminated in its application to multiple stormwater samples collected from Naval Base San Diego outfalls during a single rain event in March 2016. All testing was conducted concurrently with standard test method durations for comparison.

Chronic toxicity tests with purple sea urchin (Strongylocentrotus purpuratus) embryos and acute toxicity tests with the mysid shrimp (Americamysis bahia) were performed using standard Environmental Protection Agency (EPA) 96-hr continuous exposures alongside pulsed exposures of 3, 6 and 12-hr toxicant exposures, followed by transfer to uncontaminated seawater for the remainder of the 96 hr. Copper, zinc and a combination of the two were tested at concentrations ranging from 5.8 to 3,200 μg/L and 20 to 20,880 μg/L for copper and zinc, respectively. Copper and zinc were selected as these are commonly elevated constituents and often the cause of toxicity in stormwater at San Diego Naval Bases and other non-Navy stormwater discharges. Additionally, stormwater samples collected from Naval Base San Diego were tested in a similar manner using the standard and modified EPA methods.

Toxicity tests with single and mixed metals, and stormwater samples, resulted in progressively lower toxicity with reduced contact time to the sample when compared to standard static 96-hr exposures. The effect was more pronounced for zinc than copper for both test species. Median effective concentrations (EC50) ranged from a factor of 2 to 186 higher (less toxic) under the pulsed conditions relative to the standard 96-hr exposure. Stormwater samples collected from NBSD consisted of a wide range of copper and zinc concentrations. As with the copper and zinc tests, the stormwater results also showed a consistent progressively lower toxicity with reduced contact time to the sample.

The results of this study showed that modifying standard WET test methods is a feasible approach to accurately access short-term exposure conditions. The tests, which were conducted over a range of realistic conditions for both a chronic and acute endpoint presented consistent results lending confidence in their application. All the tests displayed a significant progressive reduction in toxicity with decreasing exposure time. The toxicity determined with standard 96-hr static tests overestimated that of short-term exposures over a wide range (<1 to 2 orders of magnitude) depending on the exposure duration, toxicant, and endpoint evaluated. The implication is that exposure duration is as critical a testing condition as the exposure concentration when evaluating toxicity.

Although the test procedures focused on exposure conditions likely to occur at the end-of-pipe, those conditions are still conservative in comparison to actual exposures that occur once the stormwater is discharged to, and mixes with, receiving waters.

This work was performed by Gunther Rosen, Molly Colvin, Chuck Katz, Jacob Munson-Decker, and Nicholas Hayman for the Naval Information Warfare Center Pacific (NIWC Pacific). For more information, download the Technical Support Package (free white paper) at here under the Test & Measurement category. NUWC-0012


This Brief includes a Technical Support Package (TSP).
Dense Transducer Array and Method

(reference NUWC-0012) is currently available for download from the TSP library.

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