Missouri Water Science Center
Lead-zinc mining has been occurring along the Viburnum Trend since the 1960s. Ground-water pumpage from the St. Francois aquifer for mine dewatering along the Viburnum Trend was estimated to be 26 million gallons per day (Mgal/d) in 1971. In 1999, the reported pumpage was slightly larger at 27 Mgal/d. A study was begun in 1999 to determine whether continued pumpage for mine dewatering has affected water levels in the surficial Ozark aquifer along the Viburnum Trend.
The ground-water study compared the pre-mining potentiometric surface (ground-water surface) of the Ozark aquifer with the 1999 potentiometric surface. The pre-mining potentiometric surface was mapped using 115 static water-level measurements made before 1960. The 1999 potentiometric surface was mapped from 59 water-level measurements collected by USGS personnel during the summer and fall, 1999, along the Viburnum Trend. When the two data sets were compared, no large cones of depression were identified in the 1999 data for the Ozark aquifer along the Viburnum Trend.
A digital analysis also was performed by computer interpolation of the pre-mining and 1999 data sets. Using a regular grid with 0.5-mile centers, each grid cell was assigned a water-level value based on nearby water-level measurements, potentiometric-contour altitudes, streambed altitudes of perennial streams, and perennial spring altitudes. The grid-cell values for pre-mining data were subtracted from the corresponding grid-cell values for 1999 data to quantify the water-level changes. The results of this analysis indicate that the differences in the two data sets are small and are within the accuracy of the determined potentiometric altitudes (plus or minus 15 feet). The few areas where calculated water-level declines were greater than 15 feet in the Ozark aquifer were scattered throughout the study area, and generally were not associated with the mines or municipal public–water supplies. No large cones of depression are apparent in the Ozark aquifer along the Viburnum Trend as a consequence of mining activity; however, there may be localized areas of small drawdowns occurring near shafts, ventholes, and inadequately plugged exploration drill holes.
Because much of the Viburnum Trend is under the Mark Twain National Forest, the area is sparsely populated. During the 1999 study, no wells located directly over active mining sites were available for water-level measurements; therefore, the analysis was not as thorough as desired. Although no large cones of depression were observed in the Viburnum Trend during the study, the data did not necessarily infer that small water–level declines have not occurred in the aquifer because of mining. Evidence existed for the lowering of water levels near active mine sites. Because of this, monitoring wells were drilled directly over active mining areas along the Viburnum Trend to continually monitor water levels. Through the cooperation of the U.S. Department of Agriculture, U.S. Forest Service, and The Doe Run Company, locations of possible well sites were determined and access to these areas was approved. Eight monitoring wells were installed during 2002.
The monitoring wells were drilled to a depth about 30 feet below the top of the water table and equipped with water-level recorders so measurements could be made at 1-hour intervals. These wells will be maintained for several years to determine if water levels over the mines are declining. A complication that has occurred since the wells were installed is that much of southern Missouri, including the Viburnum Trend area, has been experiencing drought conditions during the last couple years. Climatic conditions could lower ground-water levels naturally without the influence of mine dewatering. The drought may prolong the duration of this study, until the affect of the drought can be determined.
The Missouri Department of Natural Resources maintains an observation well near Bixby, Missouri, which transmits real-time data. Current hydrographs for this well can be viewed at:
For more information contact:
David C. Smith, Hydrologist
U.S. Geological Survey
Missouri Water Science Center
1400 Independence Rd., MS 100
Rolla, MO 65401
Telephone: (573) 308-3675