Statistical Extrapolation Methods Research Articles

The relation between extrapolated risk, expressed as potentially affected fraction, and community effects, expressed as pollution‐induced community tolerance

The results of toxicity tests can be used to calculate the potentially affected fraction (PAF) of species in an ecosystem at a given pollutant concentration using statistical extrapolation methods. The PAF curve indicates the fraction of species from the original community that may become inhibited at each elevated pollutant concentration and is a measure of the ecotoxicological risk. Pollution-induced community tolerance (PICT) is a true community response that is measured under controlled conditions in the laboratory, using organisms from contaminated field sites. Microorganisms from experimental field plots with added Zn were exposed to various concentrations of Zn in the laboratory and the mineralization of 14C acetate was monitored. Microorganisms from plots with Zn concentrations above 124 mg/kg showed a significant increase in the effect concentration 10% (EC10) and, therefore, had a significant PICT. The pore-water concentrations of Zn in these field soils were in the same magnitude as the EC10 of the microorganisms from these soils. The PAF curve was calculated from previously reported toxicity tests with five different microbial species using the average and the standard deviation of the logarithmically transformed EC10 values. The average sensitivity of this PAF curve was similar to the EC50 of the acetate mineralization curve from the field plot without added Zn2+, but the PAF curve was less steep. Our experiments indicated that 27 to 84% of the original microbial species were inhibited at Zn concentrations from 334 to 1,858 mg/kg soil, respectively. Our results suggest that the PICT method can now also be used to quantify the fraction of the original species composition that is inhibited at a specific pollutant concentration.

THE RELATION BETWEEN EXTRAPOLATED RISK, EXPRESSED AS POTENTIALLY AFFECTED FRACTION, AND COMMUNITY EFFECTS, EXPRESSED AS POLLUTION-INDUCED COMMUNITY TOLERANCE

The results of toxicity tests can be used to calculate the potentially affected fraction (PAF) of species in an ecosystem at a given pollutant concentration using statistical extrapolation methods. The PAF curve indicates the fraction of species from the original community that may become inhibited at each elevated pollutant concentration and is a measure of the ecotoxicological risk. Pollution-induced community tolerance (PICT) is a true community response that is measured under controlled conditions in the laboratory, using organisms from contaminated field sites. Microorganisms from experimental field plots with added Zn were exposed to various concentrations of Zn in the laboratory and the mineralization of 14C acetate was monitored. Microorganisms from plots with Zn concentrations above 124 mg/kg showed a significant increase in the effect concentration 10% (EC10) and, therefore, had a significant PICT. The pore-water concentrations of Zn in these field soils were in the same magnitude as the EC10 of the microorganisms from these soils. The PAF curve was calculated from previously reported toxicity tests with five different microbial species using the average and the standard deviation of the logarithmically transformed EC10 values. The average sensitivity of this PAF curve was similar to the EC50 of the acetate mineralization curve from the field plot without added Zn2+, but the PAF curve was less steep. Our experiments indicated that 27 to 84% of the original microbial species were inhibited at Zn concentrations from 334 to 1,858 mg/kg soil, respectively. Our results suggest that the PICT method can now also be used to quantify the fraction of the original species composition that is inhibited at a specific pollutant concentration.

Maximum permissible and negligible concentrations for some organic substances and pesticides

The aim of the paper is to provide interested parties the methods that were used for generic hazard assessment in The Netherlands, and the resulting so-called maximum permissible concentrations (MPCs) and the negligible concentrations (NCs) for approximately 150 organic substances and pesticides. The MPCs and NCs were derived for water, sediment, and soil. The concentration in the environment above which the risk of adverse effects was considered unacceptable to ecosystems is called the MPC. The MPCs take into account that the substances are distributed among the different environmental compartments, and are harmonized accordingly. The MPCs served as a basis for the Dutch government to set generic environmental quality standards (EQS) in The Netherlands (IWINS,15). EQS in turn are used by the Dutch Government to assess the environmental quality and for other environmental policy purposes. Concentrations in the environment below which the occurrence of adverse effects is considered to be negligible are called NCs. Hazards must be reduced when the environmental concentration of a substance exceeds its MPC. In-between this limits reduction of hazards is preferable. The MPC is a scientifically derived hazard limit. The NC is simply defined as 1% of the MPC. In general, there is a great demand for ecotoxicological data that currently limits a more reliable estimate of many MPCs. For water, approximately half of the MPCs are derived on the basis of four or more NOECs (no observed effect concentrations). For the other half, MPCs are based on only a few chronic or acute tests. For soil and sediment, however, almost no ecotoxicological data are available, and MPCs for those compartments have, in many cases, been derived from MPCs in water applying the equilibrium partitioning method (EqP-method), resulting in MPCs with greater uncertainty. Some of the methods and underlying assumptions that have been used may need improvement. For example, the factor between MPC and NC, the statistical extrapolation method, the method that is used for secondary poisoning, the role of the background concentrations of ‘naturally’ occurring substances, and the bioavailability and the EqP-method. There is a great need for hazard limits, and the present compilation tries to provide those as well as identifying research gaps.

Application of species sensitivity distributions as ecological risk assessment tool for water management

Water quality monitoring programs concern measurements of numerous substances in different water compartments such as water, suspended matter and tissues of organisms. The obtained monitoring data are often used for compliance testing of environmental quality objectives. The quality assessment concerns either a binary result (data meet or exceed objective) or a factor by which the measured concentration exceeds the objective (times-to-objective). This factor is often applied in an arbitrarily chosen classification system. Major disadvantage of this method is that the factor of exceedance is meaningless with respect to expected ecosystem health risk (ecotoxicological effects). In the Netherlands, a statistical extrapolation method, which uses the variability in sensitivity (NOEC's) between various test species, is used to derive risk levels for ecosystems. The shape of the distribution curve for species sensitivity is assumed to be a log-logistic one. This statistical extrapolation method can also be applied to assess the potential ecotoxicological effect of environmental concentrations, which can, in that case, be expressed as the potentially affected fraction. A classification system following risk levels chosen in the extrapolation method, provides a more powerful tool for priority setting with respect to ecosystem risk than a times-to-objective or binary approach.

Derivation of water quality objectives for hazardous substances to protect aquatic ecosystems: Single‐species test approach

AbstractTwo methodological approaches used to derive quality objectives to protect aquatic communities based on single‐species tests will be described. Water quality objectives to protect aquatic life may be derived on the basis of ecotoxicological tests considering toxicity to bacteria, algae, (small) crustaceans, and fish. This spectrum of species comprises representatives of four trophic levels of water biocoenosis. The lowest no observed effect concentration value of the most sensitive specie is reduced by a safety factor (more appropriate: uncertainty factor) to extrapolate a “safe level” for the aquatic ecosystem. An alternative approach is the use of statistical extrapolation methods based on species sensitivity distributions to calculate maxium permissible concentrations. The approach using uncertainty factors will be compared with the approach using species sensitivity distribution to extrapolate safe levels. © 1994 by John Wiley & Sons, Inc..