Geochemistry of molybdenum in some stream sediments and waters

Elevated concentrations of Mo are present in both the waters and sediments of Tenmile Creek, downstream from the large Mo deposit at Climax. Colorado. Concentrations of Mo reach a maximum of 10mg/1 in the water and 384μ/g in the (?) 80 mesh fraction of the sediment. The Mo anomaly extends for more than 80 km downstream from Climax, and results from the mining and milling at Climax. Background Mo concentrations in the nearby mountainous area are < 10μg/l (water) and < 5μg/g (sediment). Immediately below three small unmined Mo-rich orebodies elsewhere in Colorado < 3μg/l Mo are present in the waters and 20–30μg/g Mo in the fine fraction of the sediments.The Mo in the sediment of Tenmile Creek is chiefly adsorbed on coatings of amorphous Fe oxyhydroxide. and is similar to its form below two small, unmined Mo deposits. Mining has not changed the character of the chemical processes responsible for Mo dispersion from the Climax site.A modified version of the WATEQF computer program (Plummeret al., 1976) predicts that Tenmile Creek is undersaturated with respect to ferrimolybdite. molybdenite, powellite, and ilsemannite. The Mo in the stream water occurs as the molybdate ion which can be adsorbed on amorphous Fe oxyhydroxides. These predictions are supported by the absence of Mo minerals in the sediment of Tenmile Creek.     

A Field Test of Electromigration as a Method for Remediating Sulfate from Shallow Ground Water

Eloctromigraiion offers a potential tool for remediating ground water contaminated with highly soluble components, such as Na⁺, Cl, NO₃ and SO₄⁻. A field experiment was designed to lest the efficacy of electromigration for preconcontrating dissolved SO₄² in ground water associated with a fossil-fuel power plant. Two shallow wells, 25 feel apart (one 25 feel deep, the other 47 feet deep), were constructed in the upper portion of an unconfined alluvial aquifer. The wells were constructed with a double-wall design, with an outer casing of 4-inch PVC and an inner lube of 2-inch FVC; both were fully slotted (0.01 inch). Electrodes were constructed by wrapping the inner lulling with a 100-foot length of rare-earth metal oxide/copper wire. An electrical potential of 10.65 volts DC Was applied, and tests were run for periods of 12, 44, and 216 hours. Results showed large changes in the pH from the initial pH of ground water of about 7.5 to values of approximately 2 and 12 at the anode and cathode, respectively. Despite the fact that the test conditions were far from ideal, dissolved SO₄^2-; was significantly concentrated at the anode. Over a period of approximately nine days, the concentration of SO₄^2- at the anode reached what appeared to he a steady-state value of 2200 mg/L. compared lo the initial value in ground water of approximately 1150 mg/L. The results of this field lest should encourage further investigation of electromigration as a tool in the remediation of contaminated ground water.     

Hydrogeochemical exploration for uranium ore deposits: Use of the computer model wateqfc

Groundwaters in contact with hidden ore deposits may acquire chemical compositions that offer a guide for exploration, if the chemistry of the waters can be properly interpreted. Hydrogeochemical computer models allow significant progress to be made in the interpretation of the chemistry of all types of natural waters. The computer program, WATEQFC, an expansion and restructuring of WATEQF, is directed toward geochemical exploration for uranium and base metals. The expanded program can now solve the simultaneous equilibria involved in the aqueous geochemistry of 47 chemical elements, represented by approximately 540 minerals and solid compounds and 650 aqueous species.Calibration of the model with groundwaters from known deposits of uranium ore suggests that hidden ore bodies may be revealed by the state of saturation of the waters with respect to a suite of potential ore and gangue minerals. The saturation index (SI) is a reliable predictor of the presence of uranium ore from known deposits in Texas, Wyoming, and Czechoslovakia. The use of groundwater for regional reconnaissance strongly suggests that uranium mineralization is present in the subsurface near Colorado City, Colorado, whereas a similar modeling of groundwaters from a large area of Triassic sandstones in England virtually eliminates that area from consideration as a host for hidden uranium ore.     

Estimating ground water discharge by hydrograph separation

Iron Mountain is located in the West Shasta Mining District in California. An investigation of the generation of acid rock drainage and metals loading to Boulder Creek at Iron Mountain was conducted. As part of that investigation, a hydrograph separation technique was used to determine the contribution of ground water to total flow in Boulder Creek. During high-flow storm events in the winter months, peak flow in Boulder Creek can exceed 22.7 m3/sec, and comprises surface runoff, interflow, and ground water discharge. A hydrograph separation technique was used to estimate ground water discharge into Boulder Creek during high-flow conditions. Total ground water discharge to the creek approaches 0.31 m3/sec during the high-flow season. The hydrograph separation technique combined with an extensive field data set provided reasonable estimates of ground water discharge. These estimates are useful for other investigations, such as determining a corresponding metals load from the metal-rich ground water found at Iron Mountain and thus contributing to remedial alternatives.     

In Situ Electromigration as a Method for Removing Sulfate, Metals, and Other Contaminants from Ground Water

Electromigration is proposed as an in situ method for preconcentrating contaminants in ground water prior to pumping and treating. In earlier investigations by the senior author and co-workers, it was found that Cu in synthetic ground water migrated strongly to a Pt cathode and plated out as metallic copper. In the present study, carbon electrodes were inserted into a laboratory column of fine quartz sand that was saturated with a lower concentration of CuSO₄ solution. A fixed potential of 2.5 V was applied, causing dissolved Cu and SO₄ to accumulate strongly at the cathode and anode, respectively. Only minor plating-out of Cu took place on the carbon electrodes. In addition to the use of carbon electrodes, the present research also investigated the effects of a lower concentration of metal, accumulation of SO₄ adjacent to the anodes, adsorption of Cu on the sand, and competition by moving ground water.
At an imposed voltage of 2.5 V and in the presence of 65 mg/L of dissolved Cu and 96 mg/L of SO₄ (0.001 M CuSO₄ solution), electrolysis of water caused large changes in the pH and speciation of the aqueous components, as well as precipitation of solid Cu-hydroxides. Significant retardation of Cu occurred in the presence of ground water flowing at an average intergranular velocity of 0.2 m/day, but only minor retardation at water velocities of 1.9 and 2.9 m/day.
Sulfate tends to migrate strongly to the anodes, suggesting that in situ electromigration may offer a useful new method for preconcentrating such highly soluble ions as SO₄, NO₃, and CI that are difficult to remove by conventional pump-and-treat methods. A number of potential problems exist that should be addressed in a field test.     

Radiocarbon-dated ferricrete provides a record of natural acid rock drainage and paleoclimatic changes

 Ancient deposits of ferricrete (stratified iron oxyhydroxide or clastic sediment cemented by stratified iron oxyhydroxide) are present in headwater streams draining an historic mining district in the Rocky Mountains, USA. These deposits, which form under conditions of low pH and high metal concentrations, are present in streams draining many other mineralized areas. Radiocarbon dating of wood fragments entombed within the ferricrete indicates dates of deposition as old as 8840±50 years before present (b.p.). The ferricrete deposits provide evidence that natural, metal-rich acid rock drainage has been occurring in this mineralized region for thousands of years, due to the weathering and oxidation of exposed or near-surface massive sulfide ore deposits. The dating and chemical composition of the ferricrete deposits have applications to the environmental remediation of historic mining sites and to environmental issues related to natural background chemistry. In addition, the radiocarbon dates of the ferricrete deposits correlate closely with warm–wet periods in the Holocene record, showing that ferricrete deposits may represent a previously untapped source of paleoclimatic information. Received: 6 October 1997 · Accepted: 23 March 1998     

Baseline study of the concentration of molybdenum in bottom sediments of the major rivers in Colorado,U.S.A.

A study has been completed of the baseline concentrations of Mo in bottom sediments of the major rivers in Colorado. Three hundred and seventy-nine samples were taken from 14 rivers in the state. The geometric means for the concentration of Mo in the minus 80-mesh fraction of the sediment were found to vary from 1.2 to 14.0 ppm. The geometric mean for each river can be logically grouped according to the presence or absence of mineralization upstream in the drainage basin. This grouping results in three categories according to the extent and type of mineralization in the drainage basin. A grouping on the basis of mineralization accounts for 15% of the total variation in the concentration of Mo in the sediment, suggesting that Mo-rich sediment is transported downstream from mineralized areas and exerts a major influence on the baseline concentration. Multiple samples are required to characterize each river because the withinstream component of variance comprises 54% of the total variance.Bottom sediment (minus 80 mesh) is a sensitive indicator of Mo mineralization; values of molybdenum as low as 2 ppm were considered anomalous for several rivers. A precise analytical procedure permits detection of concentrations of Mo in sediment approximately 2 ppm below the detection limit of the usual potassium pyrosulfate-thiocyanate procedure.