Basin
reactivity: Determination of solute
fate on a basin scale
Temple
McKinnon, Department of Geology, University of Montana, Missoula, MT
59812
The
Clark Fork River basin
in western Montana is an excellent area to study solute metals in contaminated
systems. Past flooding events have deposited mine tailings in the river's
floodplain. Surface runoff flowing over, and groundwater flowing through, these
floodplain deposits provide non-point sources to the dissolved-phase (<0.2
mm), presenting the problem of where to focus remediation resources and efforts.
A basin-wide sampling of the surface water was conducted during the
summer of 1998 to determine the transport processes of dissolved-phase metals.
Tributaries of the primary drainages to the basin and with varied mining
histories were included in the sampling event.
The headwaters region produced elevated concentrations
of As, Cu, Fe, and Mn in surface waters with relatively high pH values (8.5 to
9.2). Treatment ponds downstream from the river's headwaters precipitate trace
metals due to the addition of liming agents to the surface water. Most trace
element concentrations decrease through this treatment system, with the
exception of As. On a basin-wide
scale, the concentration trends for trace elements in the system vary from
reactivity to domination by dilution with the surface water discharge of the
Clark Fork and its tributaries. Major
ions appear to have downstream sources associated with tributaries or
groundwater. This contrast in
concentration trends illustrates the difficulty in studying geochemical trends
on the large scale of a basin.
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