URANIUM IN SURFACE WATERS AND SEDIMENTS AFFECTED BY HISTORICAL MINING IN THE DENVER WEST 1:100,000 QUADRANGLE, COLORADO

Geochemical sampling of 82 stream waters and 87 stream sediments within mountainous areas immediately west of Denver, Colorado, was conducted by the U.S. Geological Survey in October 1994. The primary purpose was to evaluate regionally the effects of geology and past mining on the concentration and distribution of uranium. The study area contains uranium- and thorium- rich bedrock, numerous noneconomic occurrences of uranium minerals, and several uranium deposits of variable size and production history. During the sampling period, local streams had low discharge and were more susceptible to uranium-bearing acid drainage originating from historical mines of base- and precious-metal sulfides. The spatial distribution of Precambrian granites and metamorphic rocks influences the concentration of uranium-and thorium-rich mineral grains in stream sediments. Dissolved uranium occurs predominantly as uranyl carbonate complexes, and concentrations ranged from less than 1 to 65 ppb. Most values were less than 5 ppb, which is less than the current drinking-water standard of 30 ppb and much less than locally applied aquatic-life toxicity standards of several hundred ppb. In streams affected by uranium-bearing acid mine drainage, dissolved uranium is moderated by dilution and sorptive uptake by stream sediments. Sorbants include mineral alteration products and chemical precipitates of iron- and aluminum-oxyhydroxides, the latter particularly subject to re-suspension during high flow. The ²³⁴U/²³⁸U activity ratio (AR) of acid drainage (0.95–1.0) is distinct from that of local surface waters (> 1.05). The study area includes a particularly large vein-type uranium deposit (Schwartzwalder mine) with past uranium production. Stream water and sediment collected downstream from the mine’s surface operations have locally anomalous concentrations of uranium. Fine-grained sediments downstream from the mine contain rare minute particles (10–20 micrometers) of uraninite, which is unstable in a stream environment and thus probably of recent origin related to mining. Additional rare particles of very fine grained (less than 5 micrometer) barite likely entered the stream as discharge from settling ponds in which barite precipitation was formerly used to scavenge dissolved radium from mine effluent.