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.