Effects of anisotropic transmissivity on a contaminant plume at Nemo, South Dakota

In 1996 low concentrations of ethylene dibromide (EDB) were found in thirteen domestic water wells in Precambrian metamorphic rocks at Nemo, South Dakota. The source of the contaminant is believed to be the result of pesticides disposed at a U.S. Forest Service Work Station area in the 1970s. Monitoring wells were installed and ground water from nine of them contained EDB. Sixteen additional wells within three kilometers of Nemo were sampled but EDB was not detected. Water in two domestic wells one kilometer south-southeast of Nemo had high concentrations, 13 and 2.2 micrograms per liter of EDB. The impacted landowners were initially provided bottled water by the USFS. The USFS then drilled new wells outside of the contaminant plume which now provide water to the impacted landowners. An equipotential map indicates that ground water should flow easterly. However the contaminant plume extends towards the south-southeast, in the direction as the foliation in the Precambrian phyllite and quartzite at Nemo. This foliation strongly influences the direction of contaminant migration. Based on an equipotential map and the known plume orientation, a graphical solution of the transmissivity anisotropy indicates that the major transmissivity is oriented N 10 degrees W and is approximately 0.73 m 2 /d. The minor transmissivity is 0.13 m 2 /d. A computer solution utilizing anisotropic transmissivity generated a plume which agrees with the field data.

Mise‐a‐la‐masse survey for an auriferous sulfide deposit

A mise‐a‐la‐masse survey was carried out in Bhukia area, Banswara district, Rajasthan, India for auriferous sulfide occurrences. This area was originally surveyed for copper mineralization. Exploratory drilling, however, proved it to be economically not viable. The area was reopened for geophysical surveys when grab samples indicated the presence of gold. Initial geophysical surveys for copper mineralization showed electromagnetic, induced polarization, and resistivity anomalies. At first, one borehole was drilled for gold exploration on the basis of initial geophysical surveys. It encountered massive sulfide mineralization in association with gold. Borehole logging and a mise‐a‐la‐masse survey were carried out in this borehole. Three further boreholes drilled on the basis of the mise‐a‐la‐masse results encountered massive sulfide mineralization in association with gold. One of the three boreholes, 100 m from the first borehole along strike, was used for another set of mise‐a‐la‐masse measurements. A composite equipotential map was prepared using the results of mise‐a‐la‐masse results of both the boreholes. The equipotential contours show a north‐northwest‐south‐southeast trend of mineralization. The boreholes drilled on the basis of the mise‐a‐la‐masse results have delineated a strike length of more than 500 m of gold‐bearing sulfide mineralization. The sulfide content ranges from 10 to 40% and gold concentration ranges from 2 to 6 ppm. The dip and plunge of the lode, as anticipated from the mise‐a‐la‐masse results, are toward the west and north, respectively. Mise‐a‐la‐masse surveys are continuing in the adjoining areas.