THE TRANSPORT AND FATE OF FE, MN, CU, ZN, CD, PH AND SO4 IN A GROUNDWATER PLUME AND IN DOWNSTREAM SURFACE WATERS IN THE COEUR-DALENE MININGDISTRICT, IDAHO, USA

The controls on metal concentrations in a plume of acidic (pH 3.29-5.5 5) groundwater in the Moon Creek watershed in Idaho, U.S.A., were inve stigated with the use of property-property plots. A plot of Ca vs S de monstrated that a plume of contaminated groundwater was being diluted by infiltration of rain and creek water at shallow depths and by ambie nt groundwater near bedrock. The small amount of dissolved Fe (2.1 mg/ l) was removed while dissolved Pb was added, reaching a maximum concen tration of 0.37 mg/l. The other metals (Zn less than or equal to 16, A l less than or equal to 6.2, Cu less than or equal to 2.1 and Cd less than or equal to 0.077 mg/l) in the shallow groundwater were essential ly conserved until they emerged as a seep along the creek bank. Upon m ixing with the creek water, groundwater was diluted by factors between 11 and 50, and the pH of the mixture became neutral. Metals originati ng from the contaminated groundwater were removed in the creek in the following order: Fe > Al > Pb >> Cu > Mn > Zn = Cd. Pb and Cu continue d to be removed from solution even as the creek passed adjacent to a t ailings pile. In contrast, Zn concentrations in the creek increased ad jacent to the tailings area, presumably as a result of the reemergence of the upgradient plume as the creek lost elevation. Below the tailin gs dam, contaminated creek water (400-800 mu g Zn/l) was diluted by bo th smaller side streams and a creek of equal flow. The presence of 3 d istinctive water masses required the use of two tracers (dissolved Si and S) to distinguish between mixing and geochemical reactions. The re moval of metals was greater during low flow conditions. Pb was removed to the greatest extent, falling below detection limits (0.5 mu g/l) a t the first sampling location. Copper and Mn were removed to a lesser extent during low flow conditions and approached conservative behavior during high dow conditions. During a 5-km journey through two hydrolo gical regimes, less than 10% of the dissolved Zn and Cd was lost. (C) 1997 Elsevier Science Ltd.