Variability

It amazes me how much variability exists in the natural world. Once I took a class where the subject was Mars - it was wraught with generalities, where one measurement was scaled up to the entire planet. For example, new spectral data that may have been consistent with acidic geochemistry was used to explain how the early oceans of Mars must have acid that dominated the geology. At one point I was forced to stand up and exclaim that Mars was a planet with the same area as the surface of Earth, surely the same kind of variability in geology we see here must exist on Mars, if to a reduced degree because of the lack of tectonics and pervasive water. 

The samle subject struck me on a much smaller scale when I was reviewing a SD DENR report on the Homestake Mine closure I remember from my work as a consultant for the state. You will recall that the mine was closed and allowed to flood. There was a huge effort to remove all the potentially water-contaminating materials from the mine to comply with the Clean Water act. Following that effort, consultants traveled throughout the open portion of the mine and collected water samples where they could. There was over 300 miles of tunnels below Lead as deep as 8,150 feet below a 3000 acre footprint. The volume of openings is approximately the same as one third of the volume of Pactola Reservoir. The locations that were sampled were much less extensive, but they did find a surprisingly large variation in their results.

The temperature of the water sampled varied from about 50 degrees F up to about 130 degrees F. This is not entirely surprising, since the lower levels are quite warm from geothermal heat and the upper levels have temperatures equilibrated with the regional average temperature of about 52 degrees F. 

The pH varied from 6.5, slightly acidic, to 8.5, moderately basic, with a clear mean at 8.0. While there is much more to be considered, these values are important regarding the acid mine drainage potential. If the mine began generating acid, then these pH values would go down dramatically. 

They also sampled for TDS, sulfur, and a suite of trace elements. 

One item that was not sampled in this report was dissolved oxygen. The general idea is that after time oxygen is depleted and the water becomes anoxic. I suspect that this would have a dramatic effect on the ultimate geochemistry of these waters and the biota that could survive at the bottom of a flooded 8000 foot deep mine. 

The ultimate geochemistry of the mine is now a moot point as the mine is currently being dewatered to these depths and the results can be inspected directly. My hope is that they are doing an adequate job describing the kind of variation they observe not only with the dewatering progress but also as time progresses across the entire accessible mine footprint.