Arsenic is a widely distributed, naturally occurring element in the Earth's crust and is present in trace amounts in all living organisms. Higher levels of arsenic tend to be found more frequently in ground water than in surface water. Because small water systems typically rely on wells for drinking water, while the largest systems typically rely on surface-water sources, arsenic tends to occur in higher levels more often in water used by small communities. In the United States, the average level measured in ground-water samples is less than or equal to 1 part per billion; however, higher...