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.) |
