Background geochemistry of streams in mineralised
terranes is affected by (a) natural sulphide oxidation and associated metallic
discharges; and (b) previous mining history and associated discharges. Both of
these processes elevate background metal concentrations by (i) directly adding
dissolved metals to stream water; and (ii) adding sulphide-bearing sediments' to
the stream system, to be trapped in old channels and bars for later chemical
decomposition in situ or after flood reworking. Definition of downstream
geochemical background, involving sediments and low-flow dissolved chemistry
prior to mining is prudent, as this will provide a benchmark for any required
estimation of new inputs from, for example, accidental tailings discharges. Two
high-profile examples from western USA illustrate
these points. Summitville mine in southern Colorado is an environmental disaster left
when the operating company went bankrupt in 1991. Acid metal-rich discharges are
leaving the site into a river system used for agricultural irrigation. However,
nearby sub-catchments have substantial naturally occurring acid metal-rich
discharges and locally discharge natural sulphide-rich debris flows into the
same river. Distinction between natural and anthropogenic pollution in the
downstream reaches is difficult. Early mining at Cooke City, Montana (New World District) released large
volumes of sulphide-bearing tailings into the river system when a tailings
impoundment dam failed. The event was equivalent to a 100-year flood, and
tailings were deposited for 30 km downstream. These tailings are currently
oxidising, and are being reworked into the modem stream during high flow events,
thus perturbing background geochemistry.