Discharge limits for specific toxicants are based on water quality standards.  Most states promulgate water quality standards based on national water quality criteria issued by EPA.  The national criteria are calculated from databases consisting of numerous toxicity tests performed on a variety of species and life stages.  These tests are generally performed using a spiked laboratory dilution water consisting of filtered well or tap water or “synthetic” fresh or salt water (deionized water plus essential salts).  However, a variety of physical and chemical characteristics of natural surface waters (e.g. dissolved organic carbon, pH, hardness, suspended solids) may affect toxicity.  For example, the toxicity of many metals in stream water is often much less than the toxicity predicted from the EPA criterion or state standard.  Each stream is different depending on the drainage basin, geology, etc.

A site-specific water quality standard can be derived by generating sufficient toxicity data using site water and a variety of resident species representing different taxonomic groups.  However, this approach is relatively expensive because of the amount of data that must be generated and the difficulties often encountered in testing non-standard organisms.  Another, more economical, approach is to perform side-by-side toxicity tests in both laboratory dilution water and receiving water using standard species.  The calculated water-effects ratio (WER) is then used as a multiplier to correct the national or state standard for site-specific conditions.  Further modifications to the standard may also be made by accounting for differences in water chemistry (e.g. dissolved-to-total ratio and hardness for metals) between the site water and regulatory default values.