An Ecological Risk Assessment Framework for Effects of Onsite Wastewater Treatment Systems and Other Localized Sources of Nutrients on Aquatic Ecosystems

ABSTRACT

An ecological risk assessment framework for onsite wastewater treatment systems and other localized sources of nutrients is presented, including problem formulation, characterization of exposure, characterization of effects, and risk characterization. The framework is most pertinent to the spatial scale of residential treatment systems located adjacent to small ponds, streams, or lagoons and some parts of shallow estuaries. Freshwater and estuarine ecosystems are distinguished based on differences in nutrient dynamics. Phosphorus exposure is the major determinant of phytoplankton production in most North American lakes. Nitrate can be directly toxic to aquatic biota such as amphibians. In shallow estuaries or lagoons, nitrogen is the primary stressor, which can be directly toxic to vegetation or can interact with biota to produce secondary stressors (limited light penetration, oxygen limitation, reduction in habitat, or reduction in forage vegetation or prey). Algal production, macrophyte production, fish community abundance and production, benthic community abundance and production, and amphibian community abundance and production are examples of risk assessment endpoint properties. Models and measurement methods for the characterization of exposure and effects are discussed, as well as sources and quantification of uncertainty. Example weight-of-evidence tables are presented for failure scenarios involving traditional and emerging onsite wastewater system technologies.

Key Words:

• ecological risk assessment
• nitrogen
• nutrients
• phosphorus
• amphibian
• macrophyte
• eelgrass
• seagrass
• phytoplankton
• wastewater
• septic tank

ACKNOWLEDGMENTS

The integrated risk assessment project was conducted by Oak Ridge National Laboratory (ORNL), which is managed by the UT-Battelle, LLC, for the U.S. Department of Energy under contract DE-AC05-00OR22725. Funding support for the project was provided by the National Decentralized Water Resources Capacity Development Project (NDWRCDP) through a Cooperative Agreement between the U.S. Environmental Protection Agency and Washington University in St. Louis, MO (USEPA No. CR827887-01-0). Members of the Technical Advisory Committee included: Arthur Gold, University of Rhode Island; Charles Gerba, Arizona State University; Richard Pinkham, Rocky Mountain Institute; Robert Siegrist, Colorado School of Mines; and George Tchobanoglous, University of California at Davis. We also thank Andrea Shephard of the NDWRCDP for project management and the NDWRCDP Management & Economics Subcommittee and Glenn Suter II of the USEPA for insightful review comments on the report on which this article is based. The contents of this article do not necessarily reflect the views and policies of the NDWRCDP, Washington University, the U.S. Environmental Protection Agency, or the U.S. Department of Energy, nor does the mention of trade names or commercial products constitute their endorsement or recommendation for use.

The submitted manuscript has been authored by a contractor of the U.S. Government under contract DE-AC05-00OR22725. Accordingly, the U.S. Government retains a nonexclusive, royalty-free license to publish or reproduce the published form of this contribution, or allow others to do so, for U.S. Government purposes.

Notes

¹An exception is indirect effects from nutrient-induced algal blooms that reduce light penetration, which are discussed here.

¹Assessment endpoint entities, properties, and measures are not intended to be an exhaustive list. Entities and properties are the focus of the characterization of exposure and the characterization of effects in this ecological risk assessment framework.

²Particular populations of the phytoplankton community may be additional endpoint entities, such as cyanobacteria.

¹Many of these measures of exposure represent effects of nutrients that serve as stressors for secondary effects. Others represent indicators of effects.

²The reader is cautioned to consult each study to determine the applicability of relationships before using them for a particular risk assessment.

¹Ammonia could be a contributor to toxicity in studies where ammonium nitrate was used.