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2Introduction and Background Stormwater impacted by airport and air carrier deicing operations is regulated under the Clean Water Act through the administration of the Multi-Sector General Permit (MSGP), state-issued general permit, or National Pollutant Discharge and Elimination System (NPDES) permit programs. These permits may contain monitoring requirements or numeric limitations to ensure these discharges do not impact or impair waters of the United States. However, critical to the assessment of the impacts of airport stormwater on the receiving water environment and the determination of com- pliance with permit limitations is the representativeness of the monitoring sample. Because stormwater discharges are episodic in nature and exhibit high variability in terms of flow rate, volume, and chemical characteristics, different results can be obtained depending on how the stormwater is sampled. The application of WET testing requirements within indi- vidual NPDES permit programs to airport stormwater dis- charges presents unique challenges in both the collection of representative stormwater samples and the interpretation of the test results. This document provides an overview of these challenges and serves as a tool to airport environmental man- agers and regulatory officials in the development of sound WET testing programs which recognize the unique circum- stances associated with airport deicing operations stormwater runoff. 1.1 Regulatory Setting Section 101 of the Clean Water Act establishes a national pol- icy that prohibits the discharge of pollutants in toxic amounts. To achieve this, the United States Environmental Protection Agency (EPA) has developed a 3-pronged approach to regu- late the discharge of toxic pollutants. The 3 approaches consist of 1) the implementation of chemical-specific controls, 2) the conduct of WET testing, and 3) the development of biologi- cal criteria and conduct of receiving water bioassessments. Under the first approach (implementation of chemical- specific controls), numeric water quality criteria, which are protective of aquatic life from chemical-specific acute and chronic effects, are utilized as a basis for permit limitations. However, numeric water quality criteria have only been estab- lished for a limited number of potential toxicants. In contrast, there are a multitude of analytes present in a wastewater, many of which do not have corresponding aquatic toxicity data or a water quality criterion. Further, the use of water quality cri- teria to assess the potential toxicity of a discharge does not account for the interactive (both synergistic and inhibitory) effects between pollutants. As a result, the regulator has little information by which to determine if the discharge is likely to be toxic or contribute to instream water quality impacts. The second approach (conduct of WET testing) avoids constraints associated with the limited chemical-specific tox- icity data and potential interaction between chemicals that occur by directly measuring the aggregate toxicity of an aque- ous sample using aquatic organisms representative of species likely to be present in the receiving water. Thus, while the specific toxicant may not be identified, discharges contain- ing contaminants in toxic amounts can be identified through standardized and systematic testing of wastewater and storm- water discharges. Complementary to the chemical-specific and WET approaches is the third approach, which involves the direct measure of the health of the aquatic community in the receiv- ing water. These can include but are not limited to the presence, condition, and number of fish, insects, algae, plants and other organisms present in the water column or residing within the bottom substrate. Biological criteria define the qualities that must be present to support a desired biological community and serve as the standard against which assessment results are compared. By surveying and assessing the biological commu- nity in the receiving water environment, the overall biological integrity of an aquatic community that integrates the effects of chemical-specific as well as non-chemical environmental S E C T I O N 1
3 stressors can be measured and described. This tool is typi- cally utilized by regulatory agencies to establish water qual- ity goals, detect degradation, prioritize management actions, and track improvement. Using the above tools, federal and state regulators have the discretion to determine if a reasonable potential exists for a dis- charge to contribute to deleterious effects in the receiving water. Depending on state regulations, discharge and receiving water flows may be utilized to predict effluent concentrations at the edge of the acute and chronic mixing zones (if allowed by state regulations). Using this information, an assessment is made to determine if a specific discharge may result in toxic pollutants in toxic amounts or otherwise contribute to instream impair- ment (such as nutrient enrichment). Based on the results of this analysis, limitations on WET and requirements for toxicity testing may be applied to industrial and municipal discharges. Similarly, testing requirements have been implemented in some, but not all, airport stormwater discharge permits specifically focused on stormwaters impacted by deicing operations. In contrast to many municipal and industrial facilities that discharge wastewater on a continuous basis, airport storm- water discharges are, by definition, episodic in nature. They exhibit variability in flow rate, total discharge volume, and chemical composition and are influenced by drainage basin infrastructure and local weather characteristics. For example, flow rate is typically a function of precipitation intensity; total discharge volume is a function of total precipitation depth; and chemical composition is influenced by the interval from the last precipitation event as well as airport operations during the storm event. The presence of stormwater management ponds or other infrastructure can moderate stormwater discharges both hydrologically and chemically. Further, under certain storm event conditions (e.g., snow) the discharge event may be disconnected in time from the precipitation event resulting in discharges that occur one or more days after the precipita- tion event. These conditions present unique challenges to the transportation industry as well as the regulatory interpretation of the resulting compliance monitoring data. In contrast to municipal and industrial discharge permits that specify when and how samples are to be collected (i.e., monthly grab sam- ples), the identification of storm events to be sampled and how they are to be sampled is both site- and event-specific. 1.2 Research Objectives To provide guidance on factors that may influence esti- mates of airport stormwater discharges toxicity, this research program had 4 primary research objectives. The first objec- tive was to better understand how aquatic toxicity testing has been implemented within the aircraft transportation industry. NPDES permits from around the United States were reviewed with a focus on those airports that have deicing programs. The findings of this review are detailed in Appendix A. Of the 21 permits collected, 62% (13) of the permits contained WET testing requirements. The testing requirements varied extensively, with differences in test frequency (annually versus monthly), test duration (24- versus 96-hour duration), test type (acute or chronic), sampling requirements (grab sample versus composite sample), limitations on toxicity (limits versus monitoring only), and the permitteeâs response to test failures or observed toxicity. Specific factors that could affect estimates of the toxicity of the same stormwater discharge include the duration of a test (i.e., 24-hour exposure versus 96-hour expo- sure), how a sample is collected (grab versus composite), and what constitutes a storm event to be sampled. The second research objective was to better understand how effluent sampling protocols affect the toxicity estimates of airport stormwater discharges. Collection of a stormwater sample that accurately represents the discharge is difficult and is the first step in the conduct of whole effluent aquatic toxicity testing. The implementation and the characterization of a stormwater discharge event necessitate a sophisticated approach requiring knowledge of watershed hydrology and pollutant transport. The vast majority of permits reviewed contain sampling requirements that would not likely accu- rately characterize the stormwater discharge. Grab sampling, the most frequent sampling type required, is likely to over- or underestimate stormwater quality and has a high potential to incorrectly characterize discharge conditions. Grab sam- ples are collected at one time and represent a âsnap shotâ of effluent toxicity. If the characteristics of an effluent are not expected to change over time, a single grab sample would be considered representative. However, releases of stormwater impacted by deicing operations are highly variable in both flow and chemical characteristics and are poorly represented by a single grab sample. For example, grab samples collected shortly after the initiation of a precipitation event may under- estimate pollutant loading as the deicer is unlikely to have reached the outfall. Similarly, a grab sample collected shortly after a peak in deicing operations is likely to contain elevated amounts of residual deicing fluid and overestimate the total contaminant loading. In much the same way, stormwater Given the high variability of stormwater discharges in terms of flow and chemical char- acteristics, the vast majority of airport NPDES permits reviewed contain sampling conditions and requirements that do not accurately characterize the stormwater discharge from both aquatic toxicity and water chemistry perspectives.
4of WET testing programs should be evaluated in terms of the representativeness of the sampling and the specific condi- tions of the discharge event. The objectives of this report are in accordance with the above objectives of the research project and include: A brief description of how and why testing is conducted, Identification of the critical elements of whole effluent test evaluation, and Guidance on environmentally representative sampling technologies with discussions on what variables should be considered. Section 2 of this report provides a short summary of aquatic toxicity testing procedures. A discussion of the appli- cation of WET testing to airport stormwater discharges is presented in Section 3. Section 4 provides guidance on con- ducting environmentally representative sampling at airports, and Section 5 provides conclusions and recommendations for additional research in this topic. A literature survey was conducted during the initial phase of this project to understand how the aquatic toxicity of storm- water discharges is measured and limited within airport discharge permits. This evaluation identified significant limi- tations of the existing guidance with respect to the most basic activity of stormwater characterizationâsample collection. Thus, the research component of this investigation focused on sampling and other factors which could affect the observed toxicity characteristics of stormwater discharges. While the body of this report provides guidance on variables to be con- sidered in establishing a stormwater characterization program, the results of the literature survey are included in their entirety as Appendix A for reference and as a resource for a more detailed understanding of the difficulties of characterizing a highly variable and largely unpredictable discharge event. discharge toxicity can be over- or underestimated. Because the sampling technician does not know and cannot reliably predict the pollutant concentrations that occur throughout a storm event, the error introduced through the collection of a single grab sample is unknown. This condition applies to both chemical-specific measurements and effluent toxicity measurements. The third objective was to develop improved sampling methods in support of WET testing programs at airports. As noted above, the cornerstone of a successful effluent char- acterization program is representative sampling. In contrast to grab sampling in which a single sample is collected over a short (15 minute) time period or time-weighted composite sampling strategies in which grab samples are periodically collected over time and combined into a single sample, recent stormwater sampling protocols have focused on character- izing the event mean concentration (EMC). To calculate the EMC, multiple samples are collected in proportion to the flow at various points in time during a runoff event and com- bined into a single sample for analysis. This type of sampling approach presents numerous technical challenges typically not experienced in municipal or industrial sampling. Haz- ardous weather conditions, remote sampling locations, and highly variable stormwater flows are but a few of the chal- lenges. While many of these challenges are site-specific in nature, this document provides guidance on the selection and implementation of a sampling program to facilitate the col- lection of an environmentally representative sample. The fourth and last objective is to develop guidance on the use and implementation of WET tests at airports. There are many site-specific factors that can influence the outcome of WET testing programs and the subsequent regulation of discharges. State and federal permitting agencies have the authority to impose testing requirements and limitations on WET. However, the conduct and interpretation of the results
