Here is some info out there
Found this ( quotes below) old NPS bulletin floating around the internet still for some reason. The briefing is dated (92 I think) but does reference other studies that you may be able to get access to through Google Scholar or even the Park. It will probaly run you through Springerlink and such where you can get some things free, need to register for others, and then pay yet for others. If you contact the resource mgmt folks with the park, I wouldn't be surprised if the have scanned copies that maybe they'd share.
As far as amt of water to get to the point of dilution is the solution, it is probably dependent somewhat on seasons and rainfall. Low flow conditions and shortly there after those conditions probably will find effects further downstream as the baseflow would likely be more concentrated. After a good flushing, wetter periods, the effects probably don't reach as far. Hope this is useful.
"(13) = = = = Great Smokies Streams Acidified by Anakeesta Formation Exposures = = = =
by Darlene J. Kucken, Richard P. Maas, and Steven C. Patch
Exposed Anakeesta Formations, located within the Great Smoky Mountains NP (GSMNP) along Anakeesta Ridge, have long been known to cause negative impacts on aquatic ecosystems. Anakeesta Formations are composed of pyritic and carbonaceous slate and phyllite, which when exposed to air and water oxidize and form leachate containing sulfuric acid, iron, and other heavy metals such as zinc, manganese, and aluminum. In 1964, when a section of U.S. Highway 441 through the GSMNP between Cherokee, NC and Gatlinburg, TN was reconstructed, large Anakeesta Formations were disturbed and used as roadfill material at Newfound Gap. The chemical composition of Beech Flats Creek (BFC), previously a favored pristine trout stream, was severely and perhaps permanently altered by this construction disturbance, rendering it virtually lifeless.
The headwaters of BFC begin just below Clingman's Dome Road, flow under U.S. Hwy 441 at Newfound Gap, and eventually flow into the Oconoluftee River. The extent of stream chemistry alteration was first documented in 1975/76 (Bacon and Maas, 1979), when an attempt was made to determine stream recovery as a function of downstream distance from the source of Anakeesta. A follow-up water chemistry study was conducted from spring 1968 through summer 1990 (Maas et al., 1990) (Kucken, 1991) and the results compared to the 1975/76 study to determine whether significant changes in stream conditions had occurred over the 15-year period."
The abstract for a report by Mass is the following;
Contamination of Great Smoky Mountains Trout Streams by Exposed Anakeesta Formations1
J. R. Bacon and R. P. Maas2
Anakeesta formations, prevalent geologic deposits in the Great Smoky Mountains National Park (GSMNP), when exposed to air and water release an acid and heavy metal containing leachate into several GSMNP streams, rendering them virtually lifeless. Anakeesta pyrite affecting Beech Flats Creek was exposed during the construction of U.S. Highway 441, while exposures affecting Alum Cave Creek and Walker Prong are naturally occurring.
The road cut on U.S. 441 at Beech Flats Creek acts as point source of various contaminants. The acidity increases 100-fold (pH 6.7 to 4.7) in the several hundred meters where the stream flows through a road cut fill area. Manganese and zinc were the main metallic ions introduced at the road cut. The zinc increased from 6 to nearly 200 ppb and the manganese increased from an undetectable level to over 250 ppb. Seasonal variations below the road show a high concentration near 500 ppb in the summer to a low concentration near 100 ppb in the winter for both manganese and zinc. Both manganese and zinc showed the dilution effect expected going downstream away from the road cut. Alum Cave Creek and Walker Prong showed much lower levels of manganese and zinc and no point source could be located.
The results of the present study should prove useful to National Park personnel in making management decisions. This is especially important in view of several proposed new roads in the Great Smoky Mountains National Park.