New Methods for Calculating Ecological Risk

The risk quotient (RQ) for each combination of contaminant and receptor (plant or animal) of concern is calculated by dividing the estimated environmental concentration (EEC) by the toxicity reference value (TRY)  

This has been the classical model for calculating risk. This basic equation requires two factors EEC and TRV. The EEC can be determined by a number of means, direct measurement in the field being the best. The advantage of the field measurement is that not only is it a direct indication of exposure, but spatial and temporal variability can also be assessed. A variety of values have been used as TRVs, NOELS, and MATCs, but we prefer using a regression method to obtain a specified effective concentration or an ECX. 

An attempt to improve the RQ method has been made by using the 95% upper confidence limit (UCL) of the mean for all of the measured values for each medium or the maximum measured concentration, whichever is lowest. This will result in a conservative estimate of risk, particularly for a small site with relatively few environmental sampling points or for a site with one or more small areas of high contamination. This approach should only be used as a screening tool. If the RQ exceeds one, the value gives no indication of where on the site or at what time the exceedence occurred. 

A simple quotient value as in the above example neither takes into account the variability in exposure due to the movement of an animal nor the variability in exposure due to the uneven distribution of contaminant in the environment. Although spatial variability is not expressed in the classical quotient method above, the two methods can incorporate spatial variability into the risk calculations. 

---