Sediment record of mining legacy and water quality from a drinking-water reservoir, Aztec, New Mexico, USA
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ORIGINAL ARTICLE
Sediment record of mining legacy and water quality from a drinking‑water reservoir, Aztec, New Mexico, USA Johanna M. Blake1 · Jeb E. Brown1 · Christina L. Ferguson1 · Rebecca J. Bixby2 · Naomi T. Delay1 Received: 27 March 2020 / Accepted: 30 July 2020 / Published online: 27 August 2020 © This is a U.S. Government work and not under copyright protection in the US; foreign copyright protection may apply 2020
Abstract The record of mining legacy and water quality was investigated in sediments collected in 2018 from four trenches in the Aztec, New Mexico, drinking-water reservoir #1. Bulk chemical analysis of sediments with depth in the reservoir revealed variable trace-element (uranium, vanadium, arsenic, copper, sulfur, silver, lead, and zinc) concentrations, which appear to coincide with historical mining and milling operations. Cesium-137 age dating, which identified the location of the 1963 radioactive fallout maximum, combined with the known age of the bottom and top of the sediment trenches, was used to estimate a polynomial sedimentation rate (average rate = 1.7 cm/yr). The clay size fraction ( 1300 mg/kg), but are less than 65 mg/ kg in sediment collected downstream from the caldera (Church et al. 1997). Copper was not a primary commodity during mining. The primary commodities were gold and silver, which had associated Cu that may have been a
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waste product deposited in the Animas River. In addition, aqueous Cu in the Silverton area has been found to sorb to colloids of Fe- and Al-oxides (Church et al. 1997), which may be a mechanism of transport far enough downstream to reach AR1. Copper also occurs as the primary sulfide mineral chalcopyrite and was observed in the reservoir sediments. The reservoir sediment mineralogy is discussed in more detail in the ‘Mineralogy and reactivity’ section. The majority of Cu ore sediments in the Silverton area have been identified in Mineral Creek, located to the west of the Gold King Mine (Church et al. 2007b). The gentle slope of decreasing Cu concentrations in AR1 may be explained by a combination of events and geochemical processes. Given the focused area of known elevated Cu in the Silverton area, there may have been a decrease of input to surface waters over time that could have contributed to a decrease in Cu concentration downstream. The end of mining in the Silverton area in the early 1990 s may have reduced the amount of Cu being released from deposits. Typically, Cu adsorbs or co-precipitates on solids at nearneutral pH (Church et al. 2007b). The changing management strategies in the Animas River above AR1 could have affected how Cu was deposited in the reservoir. However, further investigation is necessary to identify specific processes associated with this trend. The concentrations of Zn, Ag, Pb, and sulfur (S) in the reservoir sediments are elevated in the 1947 samples and decrease until about 1970. After about 1970, the trace-element concentrations are variable. The highest concentrations of Zn (475 mg/kg) and Pb (140 mg/kg) oc
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