By Dan Oancea

"Reconnaissance studies of mining districts in the central part of the Western Slope (CWS) of Colorado were undertaken to characterize mined materials, mine drainage, and the influence of historic mining on nearby streams...

These geochemical investigations show that contamination from historic mines is generally less than that asserted by some or suggested by geochemical models. Most of the studied mining districts have at least a few sources of contaminants, but, in the majority of cases, the metals and acidity of concern are naturally mitigated to acceptable levels within about 2 or 3 mi of the sources."

Read the Hydrogeochemical Investigations of Historic Mining Districts, Central West Slope of Colorado, Including Influence on Surface-Water Quality paper.

Also, did you know that metals dissolved in stream waters fluctuate significantly on a daily cycle? I didn't know that either but I found a site dealing with a topic that might influence the way that water samples are getting collected in mineral exploration:  Importance and Causes of Daily Variation in Metal Concentrations in Streams.