The Biotic Ligand Model (BLM) takes into account many environmental conditions in order to determine if a metal exists in a form or concentration that is toxic to aquatic life. Toxicity is usually based upon the dosage of whatever it is that we are looking at. At some point or somewhere around the point (it differs for different species and even within the same species) a concentration/dosage of a substance will kill you — even plain old water. To be toxic to aquatic life a substance must be bio-available to the organism (meaning the organism can remove the substance from the environment and incorporate it into/onto itself, whether it is beneficial or not to its health).
The EPA and certain states began looking at toxic metals, like copper, to determine a concentration where 50% of the study population (fish, macroinvertebrates, macrophytes, etc.) died off. One of the major models scientists originally used took into account the hardness of the water to determine if metals were bio-available for uptake. It became apparent over time that bio-availability of metals was time and site specific and that hardness was not the only factor. Other factors presented better indicators for the bio-availability of metals, so scientists branched out from the hardness model and looked for other contributing factors, currently the following tests have been added under the BLM: temperature, pH, dissolved organic carbon, calcium, magnesium, sodium, potassium, sulfate, chloride, and alkalinity. BLM has been determined to really be sensitive to dissolve organic carbon and pH. After the data has been entered into the model a result that is greater than or equal to 1 Toxic Unit (TU) indicates that the level of copper in the environment may be deadly (I use copper because the data on other metals is very limited) to aquatic life.
The output from the BLM has given everyone the ability to test and interpret the data in real time. This testing is termed: Instantaneous Water Quality Criterion (IWQC). If you take the IWQC for any given location and time and try and reproduce it at a later time, you will find that the number does not stay the same because the environmental conditions have changed between sampling/testing dates or times.
What’s Toxic to Aquatic Life?
We have used various models overtime, but what we are missing is consistency in detection and reporting limits based upon location or other factors. The claim is the BLM provides the ability for real time modeling, but how do we the end user know what is toxic/deadly to aquatic life. Is it 1 ppb or 10 ppb of copper? Are these concentrations above or below the 1 TU mentioned above? And if not, why do some government agencies want to see numbers smaller than those they claim to be concerned about? If the conditions are always in flux then why are hard numbers (detection and/or reporting limits) assigned to different water bodies or stream orders (shouldn’t they be flexible to account for time and site factors)? It is not a bad thing to gather enough data to understand at what concentration metal availability is toxic based upon the above factors. As a side note, did you know that when nothing is detected, labs are still required to report a number other than zero (1/2 the MDL, MDL, RL…) – because the assertion is that something must be there?
Point Source Pollution
Usually sampling decisions target areas where pollution is potentially being introduced into a system (known as point source pollution) or at check points where a water system’s effluents can be readily monitored. The Clean Water Act uses the National Pollution Discharge Elimination System (NPDES) to make sure that Total Maximum Daily Loads (TMDL) for designated pollutants are tested so that we know when there are exceedances. The problem with sampling/testing under this system is that toxic metal levels cannot be directly interpreted like they can when the BLM is used. Remember the BLM takes into account the following parameters: temperature, pH, Ca2+, Na2+, Mg2+, DOC/DOM, Cl–, CaCO3, and sulfide to determine when toxic metals are available. BLM results help determine where the “stressors” to an environmental system may occur. This model saves time and money by bypassing another set of required tests used to determine toxicity called “Whole Effluent Toxicity” testing, which is both time consuming and expensive. The test is where small fauna are subjected to potential stressful environmental conditions/pollutants to see if/when half the test subjects perish. Again I will point out that we need a model like BLM to understand when metals are bio-available so that decisions can be made to mitigate the potential disruptions to ecosystems and food webs.
I would suggest that if you are conducting testing in a state/area under older permitting rules which don’t allow for the use of a BLM, that you actively pursue contacting your department of environmental quality (however named) to look at the benefits of using a BLM.
For further background on this topic I suggest: