- Mine Interior
- O-18/Tritium
- S-35
- Tritium/Helium
- Uranium
The Mary Murphy lies on the east side of the Continental Divide, approximately 19 miles southwest of
Buena Vista, CO. The Mary Murphy Mine discharges into Chalk Creek; which drains 14 tributaries,
ten lakes, and an area of 61,730 acres into the Arkansas River. The mine was worked through drifts on
14 main levels and several intermediate levels in Chrysolite Mountain, part of the Mt. Princeton
Batholith, a Tertiary quartz-monzonite intrusion (Stover, 1997). The primary mined vein of the
Mary Murphy Mine was the Mary Vein. The Mary Vein strikes north-east 35°, and varies in width up
to 15m (Dings and Robinson, 1957).
The Mary Murphy Mine came under scrutiny in 1986 after a fingerling trout kill of 800,000 in Chalk Creek
was traced back to high zinc outputs during spring runoff. The mine has since been studied and is a United States
Environmental Protection Agency (EPA) Nonpoint Source Program Site, administered by the Colorado Division of Minerals
and Geology since 1990. The Mary Murphy Mine adversely affects water quality in Chalk Creek, and presents a unique
opportunity for continued research.
Research is being conducted primarily from two levels of the Mary Murphy Mine, the 1400' level and the 2200' (Golf) level. Currently,
the Colorado Division of Minerals and Geology
is working to stablilize mine workings up to the 1100' level. At the 1400' level there is the main shaft which
falls 800 vertical feet directly to the Golf adit level, and upward toward the 1100' level and beyond. The 1400' level has
been the focus of our inquiry, as much of the water that is being discharged from the Golf adit flows from the 1400' level.
The mine interior is a lesson in geology and geochemistry. As one moves further inward, slight alterations in pH cause minerals to
precipitate, while
remaining in solution otherwise.
This schematic (thanks to Jen Hazen) provides a birds-eye view of the 1400' level and sampling locations:
The entrance is to the east. The Mary Vein strikes north-east 35°, and has different hydrologic controls moving from the south to the
north and vice-versa. The south vein is considered to be dominated by groundwater, as isotope values from MVS-3 do not fluctuate seasonally.
However, the north vein shows seasonality in O-18 values. Sampling locations in cross-cuts (XC-n) have not been sampled since 2000,
due to a lack of water in these locations.
The conceptual model of the Mary Murphy Mine shows where the contamination is believed to occur, and the paths it takes to reach Chalk
Creek. Recharge is thought to affect only portion of the discharge, while most of it is moved through near-surface fractures toward Chalk
Creek.
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Tritium and O-18 values tend to show some similarities within sources of water. Snow parses itself out by
having the most depleted O-18 values and tritium values around 9. Thus, the seasonal tritium value is taken
as ~9 TU. O-18 isotopes show a snowmelt signal in the contaminated mine waters, as do alluvial wells.
Bedrock wells and some mine samples show a groundwater signal, with an O-18 signal which remains nearly
constant through runoff.
Tritium values within the mine show that though much of the system is influenced in part by snowmelt, that
there is an older water signal present, indicated by higher tritium values than annual snowmelt values.
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Sulfur-35 has been an elusive isotope to study in this setting. Due to the high concentrations of sulfate in the system (most mining
systems have high sulfate, due to hydrothermal activity), the method to collect and analyze for S-35 is quite different. In low
sulfate systems, a large volume (20L) of water is collected and passed through an anion-exchange column, where the sulfate is concentrated
in the column. However, when extremely high concentrations are present, then a large volume of water would overwhelm the column, and the
majority of the sulfur would pass through. So, we collect a smaller volume of water (~1-5L) and process the water directly with Barium Chloride.
This allows us the achieve the desired amount of Barium Sulfate (final product) for analysis. The issue in this technique
is whether or not we are capturing enough water to see sulfur-35 in the results due to a drastically reduced sample volume.
The analytical problem lies not in the sample volume as much as interference by a rogue contaminant which compromises the scintillation
counts. The barium sulfate is suspended in a scintillation cocktail and placed in low-level scintillation counter and is analyzed initially
and after several weeks to observe the proper decay curve. However, as our samples have been processed, the counts have risen in the
several weeks rather than decreased. Therefore, there must be another source of radioactive decay in addition to the sulfur-35.
Initial work focused on radium, bismuth and lead. Results improved, but not to any degree of satisfaction.
Now the focus has shifted to uranium, as it has been found in the water, and removing uranium may be done rather simply in the lab.
Sulfur-35 results have been limited to stream, snow, some monitoring wells, and an occasional mine sample. It has been noted that some
locations give results during different times of the year, which is being looked into.
S-35 and O-18 are plotted with each other to look for any correlations. It is hypothesized that a snowmelt O-18 signal would show S-35,
as new water would contain S-35. Alternatively, a groundwater O-18 value would not show S-35, as the water would have a residence time
of greater than one year, and be devoid of S-35.
What is interesting about the graph is that mine water shows a groundwater signal, and initially, there has been no S-35 detectable, though
it may be a product of analytical problems. Conversely, wells do show S-35 and a groundwater O-18 signal, which causes us to believe that
new water is mixing into the wells rather rapidly.
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