New Mexico Geological Society Annual Spring Meeting
April 7, 2017

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Legacy Molybdenum Mine Tailings in the Context of the Questa Caldera: Challenges in Distinguishing Anthropogenic From Background Water Types

Kylian Nathiel Robinson

New Mexico Tech, 1704 SE Columbia, Albuquerque, NM, 87196,

Questa, New Mexico is located near the confluence of the Rio Grande and Red River in north central New Mexico. Domestic supply wells in the area access waters collected and transmitted by the watershed in the adjoining Questa caldera, which also hosts an inactive molybdenum mine. Most wells are located in an overlying alluvial basin fill aquifer, while a considerable subset monitors groundwater around a mine tailings facility. Other wells access the volcanic aquifer. Constituents of concern at the Questa mine tailings facility superfund site include molybdenum, uranium, and sulfate. Although the mine is inactive and the facility no longer receives tailings, it continues to receive water from mine dewatering. Significant volumes of water infiltrate into the aquifer system beneath the mine tailings surface pond (NMED, Arcadis). We have evaluated three sets of data: (1) from NMBGMR sampling during 2015 in which p > n (or the number of parameters sampled exceeded the number of sample sites), (2) current tailings facility monitoring from the New Mexico Environment Department (NMED), and (3) 2005 USGS mountain block aquifer sample data. With regard to water chemistry, across the region, sulfate is highly correlated with TDS. Wells completed in the mountain block have mineralized weathering products with elevated concentrations of various trace metals. Wells completed in the alluvial aquifer up gradient of the tailings facility generally have low TDS, but may have elevated levels of some metals including uranium, complicating separation and mixing analysis in relation to contaminant loading. We hypothesize that there is preferential flow through highly heterogeneous rift- fill alluvial deposits that are well connected with mountain block aquifer fluxes. Data analysis is challenging because of sample size decay at each grouping hypothesis (not enough sample sites within the remaining group) and parameters chosen for testing. Traditional water source evaluation methods in conjunction with grouped regression led to identification of important chemical tracers for local hydrogeological processes. A map based on End Member Mixing Analysis (EMMA) retains significant uncertainties. Incorporating stable isotopes and additional dissolved constituents uniformly at all sample sites would be helpful to increase the number of covariance groups available for analysis.

pp. 60

2017 New Mexico Geological Society Annual Spring Meeting
April 7, 2017, Macey Center, New Mexico Tech campus, Socorro, NM