From Ground Water Monitoring in the Tucson Copper Mining District September 1983.
Prepared by Pima Association of Governments and Upper Santa Cruz Task Force
Chapter 2. Study Methodology
A. Program Development (pg. 20)
Chapter 4. Data Interpretation, Conclusions, and Recommendations - Effects of Recharge from Tailings Ponds for Copper Milling Wastes in the Tucson Copper Mining District.
E. Duval [Sierrita] Study Area
6. Conclusions and Recommendations (pg. 114)
F. Summary of Conclusions (pg. 117)
A. Program Development (pg. 20)
Studies concerning tailings pond seepage and eventual aquifer recharge in the Tucson Copper Mining District began over 12 years ago, prompted by two law suits over water rights and water quality. Data collected in the early 1970's by the United States Geological Survey showed increased concentrations of sulfate and total dissolved solids in downgradient well monitored over time, suggesting tailings pond recharge at one mine. Information from the water rights case suggested significant ground-water recharge was occurring through tailings ponds. (Thuss, 1978)
Several soils and water-quality studies followed with varied conclusions. A materials investigation of the tailings ponds south of Tucson helped characterize the water quality, soil density, particle size, and permeabilities of the ponds (Engineers Testing Laboratory Inc., 1973). The rather high sulfate and total dissolved solids contents in the tailing ponds suggested they were a possible source of the degraded water in wells immediately downgradient of the ponds. Two additional studies (W. A. Wahler and Associates, 1973) (Hail, 1974) for the City of Tucson included a drilling exploration program at the four mines' tailings ponds with soil sampling and laboratory analyses of the soils. These studies also included drilling four monitor wells at the bas of four different tailings ponds. Water of a degraded nature, exceeding suggested Arizona State sulfate and TDS drinking limits, was found in three of the four wells.
6. Conclusions and Recommendations [For Duval/Sierrita Duval mine]
Pumping of the interceptor wells since 1979 has removed a large volume of degraded ground water near the [Duval] Sierrita pond. The interceptor wells have been effective in altering the direction of the ground-water flow [of tailings ponds seepage], particularly in the area east of the southeast corner of the pond. Based on water-level elevation contours for 1981-82, two public-supply wells east of the ponds (GV-1 and GV-2) are no longer downgradient of the ponds, as they may have been in 1976-77.
The interceptor wells have had little noticeable impact on the part of the plume near two other public-supply wells (CW-1 and CW-3). Also contents of hardness and sulfate near most of the east edge of the pond are remaining relatively constant. The volume of pond recharge is believed to be about 50 to 100 percent more than the volume of interceptor well pumpage at present. Also, pumpage of wells along the south edge of the pond has been relatively less effective than other interceptor wells because induced ground-water underflow is mixing with recharged tailings pond water in that vicinity.
It appears that pumpage from interceptor wells should be increased by about 50 to 100 percent. In addition, more water should be pumped in the area where sulfate and hardness contents exceed 1,200 mg/l. Wells I-4, I-5, and I-6 are in this area, but additional wells are probably needed between about one-quarter and one-half mile to the northeast of these wells.
Monitoring to provide protection for Green Valley public-supply wells appears to be inadequate. At least one monitor well should be installed between well H-ll and public-supply well CW-l, and the monitor well should tap a similar depth interval to CW-1. At least one monitor well should be installed between well M-13 and public-supply well CW-3, and the monitor well should tap a similar depth interval to CW-3. Both public-supply wells and the two monitor well should be sampled on a monthly basis, at least for indicator constituents (electrical conductivity, sulfate, and hardness). In addition, other constituents that are believed to be present to significant levels in tailings pond water should be determined in water from wells within the plume. These include cyanide, molybdenum, and trace organics. In addition, selenium should be routinely monitored.
Presently, the vertical distribution of calcium and sulfate in the aquifer is poorly known downgradient of the [tailings] pond. It is unlike that the plume present is present in the entire saturated thickness of the aquifer, particularly near wells M-11 and M-13. Before new deep monitor or interceptor wells are drilled and perforated over large depth intervals, a careful depth sampling program for water quality should be undertaken. This would allow delineation of the exact vertical distribution of recharged [tailings] pond in the aquifer. Potential vertical transfer of pollutants along the annular spaces of future wells could be minimized through the use of annular seals.
If exact amounts of [tailings] pond recharge are unknown, efforts should be intensified to make this determination. This would appear to be necessary to fully evaluate the effectiveness of the interceptor well system.
F. Summary of Conclusions
1. Recharge from tailings ponds at the ASARCO [Mission], Anamax, and Duval [Sierrita] mines has locally degraded the quality of downgradient ground water. High contents of sulfate and hardness are indicative of this recharge.
2. Migration of recharged water from ASARCO tailings ponds appears to be limited at present by pumpage of the ASARCO mine-supply wells.
3. Recharged water from the Anamax pond No. 2 appears to be moving toward at least one public-supply well. Pumpage of an interceptor well or wells may be desirable.
4. Pumpage of the interceptor wells near the Duval Sierrita pond is not adequately controlling the movement of recharged pond water. At least two downgradient public-supply wells appear to be threatened. A substantial increase in the volume of pumpage from interceptor wells appears to be necessary.
5. Enhanced monitoring programs appear to be necessary at each of the three study areas. These include additional characterization of the pond water and determination of water budgets for pond recharge. Also, contents of cyanide, molybdenum, selenium, and selected trace-organic chemical constituents should be determined in water from wells affected by tailings pond recharge.
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