Introduction
Arsenic and selenium are two widely distributed semimetals which for different reasons have attracted
interest in connection with public health issues. Arsenic has been known for centuries as a toxicelement: in popular imagination, it has become a synonymous of poison. Besides the obvious behavior of some of its compounds in acute exposure, there is an increasing concern about the risk associated with longterm
exposure. There exists considerable evidence suggesting that consumption of arsenic-containing
water can cause skin, bladder and lung cancer (Goyer 1996).
One of the main problems in some geographical areas arises from groundwater contamination of either mineral or anthropogenic origin. A number of chronic intoxication episodes have occurred that have drawn attention to the risk to which the population of certain areas is exposed due to consumption of water from contaminated wells. Thus, it has been classified as a high-priority substance for screening in drinking water sources. Maximum acceptable levels of total arsenic in drinking water have been the object of some controversy, although there is a trend toward the establishment of lower values. WHO, in its Guidelines for