Characteristics and Limitations of Screening-Level Assessments

Screening-level assessments are a first step toward addressing the question of whether or not pesticides are present at concentrations that may affect human health, aquatic life or wildlife. They provide a perspective on where effects are most likely to occur and what pesticides may be responsible. Screening-level assessments are primarily intended to identify and prioritize needs for further investigation and have the following characteristics and limitations:

• The benchmarks used in a screening-level assessment are selected to correspond to both the sampling medium (drinking water, ambient surface water, bed sediment, whole fish, or edible fish tissue) and the beneficial uses of the hydrologic system being studied (human health, aquatic life, or wildlife). More than one type of benchmark may be appropriate for some samples and some hydrologic systems (such as surface water from a stream that is used for drinking water and that also supports aquatic life).
• Most water-quality benchmarks selected for the screening-level assessment are estimates of no-effect levels, such that concentrations below the benchmarks are expected to have a low likelihood of adverse effects and concentrations above a benchmark have a greater likelihood of adverse effects, which generally increases with concentration.
• The presence of pesticides in environmental samples at concentrations that exceed benchmarks does not indicate that adverse effects are certain to occur. Conversely, concentrations that are below benchmarks do not guarantee that adverse effects will not occur, but indicate that they are expected to be negligible (subject to the limitations of concentration estimates and benchmarks).
• The potential for adverse effects of pesticides on humans, aquatic life, and fish-eating wildlife can only be partially addressed by NAWQA (or any other) studies because chemical analyses did not include all pesticides and degradates. In addition, some compounds analyzed by NAWQA do not have benchmarks.
• Most water-quality benchmarks for pesticides are based on toxicity tests of individual chemicals, whereas pesticides usually occur as mixtures. Comparisons to single-compound benchmarks may tend to underestimate the potential for adverse effects.
• Water-quality benchmarks for different pesticides and media are not always comparable because they have been derived by a number of different approaches, using a variety of types of toxicity values and test species.
• For some benchmarks, there is substantial uncertainty in underlying estimates of no-effect levels, depending on the methods used to derive them and the quantity and types of toxicity information on which they are based. This is especially true of fish-tissue benchmarks for the protection of fish-eating wildlife, for which there is no consensus on national-scale benchmarks or toxicity values.
• Estimates of pesticide exposure derived from either measured or estimated concentrations are also uncertain—particularly estimates of short-term exposure of aquatic organisms to pesticides in stream water.

Contact: Lisa Nowell
lhnowell@usgs.gov, (916) 278-3096