October 2, 2012 by Dean Leverett


A Water Quality Limit value is a numerical concentration of a substance against which water quality objectives are measured. Such limits are termed differently in different countries and regions (e.g. Environmental Quality Standard, Water Quality Guideline, Water Quality Criteria, Threshold Limits, etc) but the derivation of all involves the assessment of ecotoxicological data to derive a concentration of a substance below which environmental effects are not expected to occur, or to occur in an acceptably small percentage of species or life stages. In undertaking assessments to derive a limit concentration certain assumptions are made concerning the aquatic environment which allow for an extrapolation to be made from single species acute toxicity data to ecosystem effects (e.g. ecosystem sensitivity depends on the most sensitive species, and protecting community structure protects community function).

We have recently completed a number of reviews and assessments of the guidance applied in different countries and regions (EU, Canada, USA, Australia, New Zealand) to derive limit values. This work included comparisons of the procedures and methods used to assess ecotoxicological data and derive different types of limit values, and also assessed the potential for limit values derived in one jurisdiction to be adopted or adapted for use elsewhere.

What is very clear is that there are considerable differences between countries and regions with respect to the specific procedures used in the derivation of limit values, specifically in relation to the use of deterministic or probabilistic determination methods, de minimis data requirements, study reliability assessments, the potential for taking water chemistry into account, the degree of prescription in guidance and requirement for expert judgment, and the allowance for the use of field data in the assessment.

While the guidance and procedures for the derivation of limit values is relatively mature in the so-called ‘developed’ countries and regions, the procedures applied in the developing regions of the world are relatively immature and, in some cases, non-existent. Arguably, it is in these regions that reliable limit values for those specific substances known to be an environmental issue are most needed. Faced with limited resources for the development of bespoke country-specific guidance, regulatory agencies in developing areas often turn to the guidance developed by other regions, and in most cases this amounts to the direct application of the procedures for deriving guidelines published by the USEPA (1985), or indeed the direct application of the US guideline values themselves. However, the USEPA guidelines were developed for specific application in the USA, and are dated in terms of technical developments and improved understanding of chemical fate and behaviour compared to procedures developed in Europe and elsewhere.

While the adoption of the USEPA’s procedures may be suitable for some non-US areas, it is likely that an optimal and most cost-effective (in terms of the development of the guidance and the implementation of the derived values) approach for countries seeking to develop their own set of water quality limit values may be to take account of the full complement of mature guidance (e.g. EU, North America, Australasia) and to adapt the most applicable strategies to local conditions.