The Pailaviri Tailings Deposit, Potosí, Bolivia: Extreme Acid Rock Drainage Generation

A.A. Neptune, K.J. Palmer, F.S. Llanos López, R. Ramos Callapa, R.W. Nairn, J.Z. Bandstra, W.H. Strosnider Introduction:

Potosí, Bolivia’s rich mining history can be traced back to the 15th century. However, mining and milling ores creates solid waste (tailings), which can be sources of ecotoxic metals, often from dust and/or acid rock drainage (ARD). The mountain of Cerro Rico has many tailings piles and the Pailaviri tailings deposit is one of the largest and most concerning. Starting in 1975 until the Sn market crash of 1985, a conveyer belt was dumping tailings onto the pile from the gravimetric concentration of Sn. A better understanding of the chemical and physical features of a tailings deposit can guide future remediation actions. The purpose of this research was to characterize the acid rock drainage (ARD) flowing from the Pailaviri tailings deposit. From the water samples analyzed and results from comparable ARD sites it can be presumed that the Pailaviri deposit contains some of the most extreme ARD characteristics encountered in the world today.

Study Area:The Pailaviri tailings pile is on the north slope of Cerro Rico near several abandoned and active mines, including the Pailaviri mine. The runoff and leachate discharges from the deposit drain into the Rio Huayna Mayu, which transports stormwater, wastewater, acid mine drainage (AMD), ARD, and ore processing effluent to the Rio de La Ribera de Vera Cruz. The Rio de La Ribera forms the Rio Tarapaya, a key branch of the upper Rio Pilcomayo, a crucial South American water resource for agricultural irrigation.

Figure 1: Cerro Rico and the Pailaviri tailings pile in Potosí, Bolivia

Figure 2: Pailaviri tailings deposit sample location map

Data Collection:Water and geologic samples were taken from the tailings pile for subsequent analysis at the University of Oklahoma and Saint Francis University (Figure 2). A Hach Sension 156 multimeter was used to obtain dissolved oxygen, specific conductivity, pH, and temperature for the water samples. Anion, dissolved metal, and total metal samples were collected using 60 mL HDPE bottles. An AMS soil corer and a shovel were used to collect geological samples.

Water Samples:Due to the extremely acidic nature of the water, the pH probes quickly degraded, rendering accurate measurements impossible. Previous research by Strosnider et al. estimated a pH of 0.9 during the rainy season of 2007. Similarities in sulfide and metal concentrations of other extreme mine waters (e.g., Iron Mountain, Iron Duke Mine) indicate that Pailaviri produces uniquely extreme ARD.

LocationAverage

Temperature °CpHs.u.

Specific Conductivity mS/cm

PTWB 17.5 1.00* 25.6PTW1 19.9 0.00* 23.0PTW2 13.9 No Result* 33.4

Table 1: Pailaviri water physiochemical parameter measurements

Geologic Samples:The tailings from Pailaviri were dried for long term storage in preparation for subsequent analysis. An oven maintained at approximately 110°F was used to dry the tailings to constant weight (difference < 0.5% between drying periods), thus establishing moisture content. The presence of water in unsaturated conditions, as was noted, allows for further oxidation of metal sulfides (i.e., ARD generation). However, the relatively low moisture content would indicate that the upper portion of the pile may be transported for reprocessing without pre-drying. Economic reprocessing should be an option.

Geology:Cerro Rico de Potosí was created by volcanic eruptions of the Neogene period intruding through an Ordovician base of shales and slates. The lithologic units of Cerro Rico consist of Pailaviri conglomerate, Venus breccias, Caracoles tuff, and Cerro Rico dacite dome, from the bottom to the top. The Pailaviri unit contains a breccias conglomerate with shale, quartzite and igneous rocks. The ore veins throughout Cerro Rico are mostly encased in pyrite and polymetallic sulfides, which are the “fuel” for ARD.

Figure 3: Geologic sampling along the top of Pailaviri

Figure 4: Water sampling locations along the Pailaviri tailings pile 1

Distance from Bottom North South West South East0 inches 5.04% 6.17% 6.40%6 inches 6.81% 5.84% 5.55%

12 inches 6.76% 5.77% 5.54%18 inches 4.96% 4.39% 4.78%24 inches 4.88% 5.33% 4.64%30 inches 6.38% 6.85% 4.74%36 inches 5.89% 10.16% 3.64%

Location Avg. Moisture ContentPTG1 5.60%PTG2 5.33%PTG3 5.70%PTG4 6.02%

Figure 5: Collecting, Drying and Storing Soil Samples

Table 4: Moisture content at sample PTG1

Table 5: Average moisture content for sample locations

Location Al As Cu Fe Mn Pb Zn

PTWB 3250 573 142 43970 100 20 2140

PTW1 23050 1830 932 90200 71 35 3220

PTW2 14100 308 453 25800 108 24 2790

Location Al As Cu Fe Mn Pb Zn

PTWB 2860 570 142 42900 100 21 1910

PTW1 25300 1800 850 103000 65 36 2980

PTW2 13970 310 470 24300 116 25 2800

Table 2: Total metal concentrations in mg/L for water samples at n = 1

Table 3: Dissolved metal concentrations in mg/L for water samples at n =1 References:

Cunningham, C.G., R.E. Zartman, E.H. McKee, R.O. Rye, C.W. Naeser, O.V. Sanjinés, G.E. Ericksen and F.V. Tavera. 1996. The age and thermal history of Cerro Rico de Potosí, Bolivia. Mineralium Deposita. 31:374- 385. Nordstrom D.K., Alpers C.N., Ptacek C.J., Blowes D.W. 2000. Negative pH and extremely acidic mine waters from Iron Mountain, California. Environ Sci Technol. 34:254-258.Strosnider, W.H., Llanos Lopez F.S., Nairn R.W. 2011. Acid mine drainage at Cerro Rico de Potosí I: unabated high-strength discharges reflect a five century legacy of mining. Environ Earth Sci. 64:899-910. Molson, J.W., Fala, O., Aubertin, M. and Bussiere, B. 2005. Numerical simulations of pyrite oxidation and acid mine drainage in unsaturated waste rock piles. J. Contam Hydrol. 78:343-371.

Funding and Support 1:From left to right: PTW2 taken at an elevated angle from atop Pailaviri and PTWB roughly 4 m in average diameter

*No accurate pH measurements obtained due to pH probe rapidly degrading.