HYDROGEOLOGY AND HUMAN EXPOSURE ASSESSMENT

"All substances are poisons; there is none which is not a poison. The right dose differentiates a poison and a remedy." This quote from the 15th century Swiss physician Paracelsus, is credited as the origin of toxicology and the basis for the practice of exposure assessment, which identifies the pathways and doses to which an individual is likely to be exposed. When substances are present in potentially toxic concentrations in ground water, public health and the practice of exposure assessment are inextricably linked with the science of hydrogeology. The relationship between public health and hydrogeology is illustrated by the ongoing controversy over the revision of the arsenic drinking water standard. In 1999, the NRC recommended lowering the EPA MCL as a result of a re-assessment of cancer risk from ingestion of arsenic in drinking water. The NRC based its recommendation on a re-interpretation of epidemiological studies of bladder cancer in an area of elevated ground water arsenic concentrations in Taiwan. The incidence of cancer and other diseases in this area has been studied since the 1950s, there are different interpretations of the health effects of arsenic exposure. Villages served by non-artesian wells do not have similarly elevated rates of bladder cancer (relative to the artesian source villages), even though some of those non-artesian wells have median arsenic concentrations similar to the artesian source villages. As the arsenic well concentration alone is not sufficient to predict bladder cancer incidence, some studies have proposed that humic compounds, (alone or in combination with the arsenic) may be responsible for the induction of bladder cancer. From a hydrogeologist’s perspective, these epidemiological studies have several limitations. The estimates of arsenic concentration are based on median village well concentrations which do not consider the variability of concentrations for different wells or potential seasonal or long term effects for each well. The pending revision of the arsenic MCL represents a lowering of the threshold for what is considered a “safe” dose. As with every substance, arsenic is toxic at high doses and can cause a variety of adverse health effects. While such dose-response studies are inherently controversial, it is clear that additional hydrogeologic data is needed to resolve this controversy.