Perry H. Rahn

Process and rate of dedolomitization: Mass transfer and 14C dating in a regional carbonate aquifer

Regional dedolomitization is the major process that controls the chemical character of water in the Mississippian Pahasapa Limestone (Madison equivalent) surrounding the Black Hills, South Dakota and Wyoming. The process of dedolomitization consists of dolomite dissolution and concurrent precipitation of calcite; it is driven by dissolution of gypsum. Deuterium and oxygen isotopic data from the ground water, coupled with regional potentiometric maps, show that recharge occurs on the western slope of the Black Hills and that the water flows northward and westward toward the Powder River Basin. A significant part flows around the southern end of the Black Hills to replenish the aquifer to the east of the Hills. Depth of flow was inferred from interpretation of the silica geothermometer based on the temperature-dependent solubilities of quartz and chalcedony in water. Chemical effects of warm water in the Pahasapa Limestone include changes in the solubility products of minerals, conversion of gypsum to anhydrite, solution and precipitation of minerals, and increases in the tendency for outgassing of carbon dioxide. Where sulfate reduction is not important, sulfur isotope data show that (1) in the Mississippian aquifer, most of the sulfate is from dissolution of gypsum and (2) some wells and springs have a hydrologic connection with overlying Permian and Pennsylvanian evaporites. Sulfate ion concentration, a progress variable, shows a strong correlation with pH as a result of the combined effects of the dedolomitization reactions. Mass-balance and mass-transfer calculations were used to adjust 14C values to determine a range of ground-water flow velocities between 2 and 20 m/yr. These velocities are characteristic of carbonate aquifers. The average rates of dolomite and gypsum dissolution are 1.7 × 10−4 and 3.4 × 10−4 mmol/kg of H2O/yr, respectively. The precipitation of calcite is occurring at the rate of 3.4 × 10−4 mmol/kg of H2O/yr. The close agreement among the model results demonstrates that dedolomitization is controlling water-rock interactions in this regional carbonate aquifer system.

Water quality impacts from mining in the Black Hills, South Dakota, USA

The focus of this research was to determine if abandoned mines constitute a major environmental hazard in the Black Hills. Many abandoned gold mines in the Black Hills contribute acid and heavy metals to streams. In some areas of sulfide mineralization local impacts are severe, but in most areas the impacts are small because most ore deposits consist of small quartz veins with few sulfides. Pegmatite mines appear to have negligible effects on water due to the insoluble nature of pegmatite minerals. Uranium mines in the southern Black Hills contribute some radioactivity to surface water, but the impact is limited because of the dry climate and lack of runoff in that area.

The Relationship between Natural Forested Slopes and Angles of Repose for Sand and Gravel

Students used Brunton compasses to measure the angles of repose in fresh sand and gravel talus exposed in artificial pits in a kame terrace near Willimantic, Connecticut. The angle of repose was found to average 33.08°, and is independent of exposure direction or material type. Measurements also were made on natural forested hillslopes of the same sand and gravel along a terrace scarp where the Nachaug River had cut into it. The hillslope averages 41.29° and is independent of exposure direction. The difference between the angle of repose and the natural hillslope is attributed to the stabilizing influence of the forest vegetation.

An Educational and Research Well Field

Since 1982 an operational well field consisting of a main well and 14 observation wells has served as an educational and research facility for students at South Dakota School of Mines and Technology. An alluvial aquifer 1.5 m thick is located at shallow depths near the Mineral Industries Building. Student projects include measurement of the static level, preparation of maps of the potentiometric surface, pumping, slug, and tracer tests, and flow and transport modeling. Drawdown during pumping tests follows the Theis nonequilibrium-type curve with transmissivity averaging 390 m2/d and a storage coefficient of 0.003. Tracer tests show the aquifer is more permeable in a NE-SW direction; this observation is supported by the spatial distribution of transmissivity values from the Theis analysis. Piezometer tests provide data about the variability of transmissivity and hydraulic conductivity within the aquifer. This information is useful in the development of digital models that are primarily used in graduate co...

Karstic gypsum problems at wastewater stabilization sites in the Black Hills of South Dakota

The Triassic Spearfish Formation contains numerous gypsum beds in which karstic conditions have developed in the Black Hills of South Dakota. The evaporite karst has caused severe engineering problems for foundations and water retention facilities, including wastewater stabilization sites. Two dramatic examples include the former sewage lagoons for the City of Spearfish and a proposed lagoon/wetlands facility for Whitewood, South Dakota.The Spearfish sewage lagoons were built in 1972 on alluvium above a thick gypsum layer, despite warnings from local ranchers. At one point during construction, a scraper became stuck in a sinkhole and required four bulldozers to pull it out. The lagoons started leaking badly within a year after completion; the southern lagoon was abandoned after four years because of leaks, and the northern lagoon could not provide adequate retention time for effective sewage treatment. Attempts at repairs, including a bentonite liner, were ineffective, and poorly treated sewage discharged beneath the lagoon’s berm into a nearby surface drainage. The lagoons were abandoned completely in 1980 after a large lawsuit, and a mechanical wastewater treatment plant was constructed nearby.The planned Whitewood sewage lagoon and artificial wetlands were proposed in 1988 at an area with troubling similarities to the failed Spearfish site. Field investigation showed gypsum outcrops within proposed wetland cells and 10-m deep sinkholes nearby. After an engineering report cited these difficulties, subsequent drilling at a proposed wetland cell showed a 9-m thickness of gypsum that was exposed at the surface. The wetlands project and later the lagoon project were tabled after controversy.

A Method to Mitigate Acid-Mine Drainage in the Shamokin Area, Pennsylvania, U.S.A.

The three anthracite coal fields of eastern Pennsylvania occur in large synclinal basins surrounded by sandstone hogback ridges. The streams draining the coal regions are heavily contaminated by sulfuric acid due to the weathering of pyrite in the abandoned mines and culm. Dams could be built in the water gaps of the streams leaving the basins, forming huge reservoirs. For example, in the western part of the Middle Coal Field (Shamokin-Mt. Carmel area), the construction of four dams would form a reservoir that would inundate practically all the culm banks, abandoned mines, and environmentally abused lands in this coal field. The reservoir (at approximately 410 m elevation) would be approximately 140 sq km in area and could be the source of water for pumped-storage hydroelectricity generation. The water should have moderately good quality and could serve the recreational needs of a vast population of the Middle Atlantic states.

Flood-plain management program in Rapid City, South Dakota

Following the devastating flood of June 9, 1972, Rapid City, South Dakota, embarked on a flood-plain management program. A “flood-way” was delineated and all homes and motels, as well as most commercial establishments, were removed from the floodway. Almost the entire area inundated by the 1972 flood is now a beautiful park, illustrating that man can live in harmony with natural processes without spending vast sums of money for hard engineering structures such as dams or levees. From a national perspective, it is desirable to reserve flood plains for agricultural use. Unfortunately, many flood plains are being urbanized, which not only takes agricultural land out of production, but also gives impetus to dam construction that results in the inundation of more land.

Interpolation of Horizontal Transmissivity Anisotropy from Head and Flowpath Observations

Rocks are rarely equally permeable in all directions. Ground water does not flow directly downgradient through anisotropic aquifers, but deviates at an angle determined by the direction and magnitude of the major and minor transmissivities. We show a mathematical model whereby the orientation and ratio of major to minor transmissivities can be estimated by an analysis of the deviation of ground-water flow from the hydraulic gradient. The solution is derived from the construction of transmissivity ellipse where the hydraulic gradient and flow direction are known. An example is given using an ethylene dibromide contaminant plume at Nemo, South Dakota, where the plume has been deflected because of bedrock foliation.