USGS Frequently Asked Questions

Answer:

Regulatory limits for safe levels of elements in water and foodstuffs are established by the U.S. Environmental Protection Agency and the U. S. Food and Drug Administration. However, there are generally no regulatory limits that scientists can refer to when dealing with plants, soils, rocks, and sediments. Therefore, to determine whether a plant, soil, rock, or sediment contains a "high or unusual" quantity of a specific element, it is necessary to determine what quantity is "normal or usual." These levels are referred to as background or baseline measurements, but they may be somewhat different.

A background measurement represents natural concentrations of an element in natural materials that exclude human influence. This measurement represents an idealized situation and is typically more difficult to measure than a baseline.

A baseline measurement represents concentrations measured at some point in time and is not generally a true background. Baseline concentrations are typically expressed as a range, not a single value. (From USGS Circular 1105.)