fractions have already oxidized or because the 

 reactive zone is deeper than it used to be. How- 

 ever, the argument is probably specious. We have 

 no evidence that the oxidation rate is slower now 

 than it was in 1 974. And if the depth to the reactive 

 zone was a strong variable, the area covered with 

 topsoil would be expected to be different from the 

 untreated area. A more plausible suggestion is 

 that the areas that have been revegetated since 

 1975 (virtually all of the waste dump except the 

 untreated area of this study) have reduced sulfide 

 mineral dust production to the point where real 

 changes in the soil-water quality are occurring. 

 Dust is an important component of the acid cycle 

 on this mine (Farmer and Richardson 1 980). 



Regarding the second conclusion, the biologi- 

 cal oxygen demand created by roots and microbes 

 in the topsoil apparently has no detectable effect 

 on the oxidation rate of the sulfide minerals. The 

 suggestion that a thin layer of topsoil, and a heavy 

 stand of grass, can reduce or stop the oxidation of 

 waste sulfide minerals no longer appears justifi- 

 able. 



These conclusions do not reduce the value of a 

 heavy vegetative cover on mining waste piles. The 

 principal value of vegetation on mining wastes has 

 always been to build the edaphic qualities of the 

 spoil and to control surface erosion by water. This 

 work suggests that dust control by vegetation is 

 also important. 



The soil-water ionic concentrations measured 

 in this work are many times greater than would be 

 expected for undisturbed conditions. For in- 

 stance, an adjacent watershed unaffected by min- 

 ing shows mean streamflow concentrations of 

 copper, iron, and sulfate of 0.10, 0.12, and 4.99 

 mg/liter, respectively. While streamflow values 

 may not be completely comparable with soil-water 

 values, soil water(both saturated and unsaturated 

 flow) is the basis for perennial streamflow in moun- 

 tain channels. 



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