Natural contamination with arsenic and other toxic trace elements was studied on a 50,000 km(2) area of the Pampean Plain in Argentina. The locations where natural sources of arsenic are considered to be of concern continue to grow, and include those associated with soils developed on loess or loessic sediments zones and transported volcanic materials sites. Contents of total arsenic, vanadium, chromium, iron and barium higher than those recommended as maximum allowable levels in drinking waters have been measured. In the case of arsenic, analyses of raw groundwaters yielded levels as high as 600 microg As/l. Reported data are discussed in connection with geological and environmental processes involved in ground water contamination.