SYMPOSIA PAPER Published: 01 January 1998

An Approach to Defining a Control or Diluent Soil for Ecotoxicity Assays


Ecotoxicity assays must include controls, where there is no contaminant, and a dilution series, where the contaminated medium is diluted by uncontaminated medium. This has been a challenge for aquatic assays, but is usually satisfied by choosing water with the appropriate basic chemistry such as pH and hardness. It is a much greater challenge in soils, where the solid and biological components of the control material are dominant and crucial to the performance of the test organisms. Clearly, an ideal control and diluent soil would be collected from an uncontaminated point near the site of concern. However, proving the absence of contamination is difficult and costly, and natural variability of soils may make the exercise pointless anyway. We developed an alternative method. A suite of soils, very different in chemical and physical properties, were characterized and stored as defined stock materials. The same properties were measured in the contaminated soil. A blend-matching algorithm was used to define a blend of the stock soils that would have the same properties as the contaminated soil. At this stage, soil properties are assumed to mix linearly, but this could be improved. Importance factors are used to emphasize the properties thought to be most critical to the test organism or the contaminant. Blends of a few stock soils are thought to be preferable to blends of many soils, for simplicity. This method is the best surrogate to a true control and diluent soil that we have found. Results here include a dilution series of a blend and an (initially) unknown contaminated soil using a full-life-cycle plant, an earthworm survival and a lettuce emergence bioassay. Also included are pH and sorption characteristics for a large factorial blend of the five stock soils to evaluate the linear mixing assumption.

Author Information

Sheppard, SC
Whiteshell Laboratories, Pinawa, MB, Canada
Evenden, WG
Whiteshell Laboratories, Pinawa, MB, Canada
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Developed by Committee: E47
Pages: 215–226
DOI: 10.1520/STP12165S
ISBN-EB: 978-0-8031-5384-4
ISBN-13: 978-0-8031-1485-2