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Natural Background and Superfund
Background refers to constituents or locations that are not influenced by the releases
from a site, and is usually described as naturally occurring or anthropogenic:
(1) Anthropogenic - natural and human-made substances present in environment
as a result of human activities (not specifically related to the CERCLA release in
question); and
(2) Naturally occurring - substances present in the environment in forms that have
not been influenced by activity.

From 42 U.S.C § 9604(a)(3): "The President shall not provide for a removal or
remedial action under this section in response to a release or threat of release -
(A) of a naturally occurring substance in its unaltered form, or altered solely
through natural occurring processes or phenomena, from a location where it is
naturally found.”

Approaches to Determine Background
--Historical data, analogs, or “type areas.”
--Geochemical equilibrium modeling.
--Stream overbank sediment sampling and age dating.
--Synoptic sampling, mass balance, and statistical analyses.
--Stable isotopes to distinguish mining-affected water.
--Ferricrete sampling, age dating, and relations to
  currently forming iron bogs.
--Stream tracer-injections, mass-loading studies, and stream reactive
  transport modeling.