One of the major advances in water quality studies is the improvement in analytical techniques, both in the lab and the field, but especially in the lab. When I first started in this field, detection limits were usually in the low parts per million (ppm), but parts per billion (ppb) detection limits were starting to become typical for many ions and compounds. Now for some analyses detection limits are being reported in parts per trillion (ppt) with regularity, an increase in detection of 1000 times or more in just a couple of decades. Were finding things in water we were unable to detect before, such as pharmaceuticals and personal care products (PPCPs). Something new to worry about! But what does it mean to be detecting compounds at those levels?
One ppb of a substance is roughly equal to a teaspoon of material in an Olympic sized swimming pool. Why should we be worried about such a small amount? But units of measurement can be misleading. I once read that if you want to minimize the idea of something, present the results in terms of volume (such as how much is in a swimming pool). If you want to maximize the idea of something, report it as a distance, like laying something end to end. How often have we heard something described as how long it is with respect to the distance between the Earth and Moon?
For most compounds, if we ingest 1 ppb of it, it is not going to harm us. But there is abundant evidence from toxicological studies that some compounds, including toxic metals such as lead, antimony, beryllium, cadmium, thallium, and mercury, are toxic at levels less than 10 ppb. A number of organic compounds, such as some pesticides and chemical solvents, also are toxic at ppb (and lower) concentrations. And U.S. EPA has set drinking water levels for those compounds to low ppb (and lower) levels.
In the search for PPCPs using our new and improved analytical techniques, we have found a number of compounds, including antibiotics, anticonvulsants, antidepressants, and birth control medications, in ppt levels in surface waters, primarily coming from wastewater discharges or livestock waste. While it may be hard to imagine these compounds being toxic to humans, considering the concentrations are orders of magnitude lower than what you would be taking if you got it from the pharmacy, many studies have found compounds at these minute concentrations to have negative effects on aquatic biota.
One concern about our improved analytical detection limits, primarily from the treatment industry, is that regulators are going to use them to lower drinking water standards to ridiculous levels; i.e., if you detect it, you gotta remove it. This concern was tested in a recent article (Calder, R.S.D. and K.A. Schmitt. 2010. Role of Detection Limits in Drinking Water Regulation. Environmental Science and Technology, 44:8008–8014). Their goal was to determine how the different aspects of the regulatory process, including analytical capabilities and toxicological data, as well as cost-benet analysis and analysis of risk, affect the promulgation of drinking water standards. They concluded that improved detection limits do not lead to more stringent standards in the U.S. The primary control is U.S. EPA’s toxicological modeling.
