Arsenic content in groundwater from the southern part of the San Antonio-El Triunfo mining district, Baja California Sur, Mexico

• The groundwater in the study area is affected by former mining-activity in respect to arsenic.
• The groundwater is partly affected by hydrothermalism in respect to boron and fluoride.
• The study area had less impact from mining activity than the northern part of the mining district.
• There is no significant impact of arsenic from peripheral hydrothermal fluids in the study area.
• Composition of thermal affected water differs from other known reservoirs in Mexico.

SummaryThe San Antonio-El Triunfo mining district is located in a mountain region 60 km southeast of La Paz, the capital of the Mexican state of Baja California Sur. Mining activities, focused on gold and silver extraction, which began in the second half of the 18th century; the main activity took place between 1878 and 1911. The minerals were treated through the cyanide method, burned, and smelted. In effect between 800,000 and 1 million tons of mine waste materials were scattered in an area of approximately 350–400 km2. This area contains today several byproducts as a result from arsenopyrite oxidation, such as arsenolite (As2O3).The aim of this study was to analyze groundwater composition in order to define the actual concentrations of relevant parameters to detect ore mining contaminations, especially in respect to arsenic. In the autumn of 2010, 29 groundwater samples were obtained using flow-through bailer or down-hole pump methods, and a total of 37 variables were measured in each sample. Arsenic (dissolved) exceeded the values established by the World Health Organization (WHO, 2011) (0.01 mg/L) in 41% of the monitored sites. The maximum concentration of arsenic found in one observation well was 0.45 mg/L.Four water samples, which were characterized by elevated concentration of boron (max. conc. 9.5 mg/L), fluoride (max. conc. 3.25 mg/L) and alkaline pH values (>9), showed a composition typical for hydrothermal water. Because all four samples had arsenic concentration under 0.008 mg/L, we conclude that no significant impact of arsenic from hydrothermal fluids is found in the study area.