Accuracy and Time Stability of Arsenic Redox Species in Acid Mine Waters
Accuracy of Arsenic(T) Determinations
Acid mine water may contain up to 31 major metal species with concentrations that can vary over several orders of magnitude (Nordstrom and Alpers, 1999). Consequently, interferences are likely during As redox determinations by HGAAS for acid mine waters. Samples of water collected from the Reynolds Adit and a monitoring well near the Chandler Adit on the Summitville Mine site, the Richmond Mine, and the Penn Mine were analyzed for many dissolved constituents including As(T) and As(III). Concentrations of potential interfering metals for these samples are listed in Table 2. The samples contain elevated concentrations of metals, as determined using inductively coupled plasmaoptical emission spectroscopy (ICP-OES) and colorimetric determinations for Fe redox species (To and others, 1999). Accuracy of the As(T) method was evaluated by performing a standard addition on samples collected from the Summitville Mine, Colo. (Reynolds Adit and a monitoring well near Chandler Adit); the Richmond Mine, Calif.; and the Penn Mine, Calif. Spike recoveries for the samples ranged from 96 to 101 percent (fig. 9). Twenty samples from the Summitville Mine site, 6 from the Richmond Mine, and 3 from the Penn Mine were analyzed by ICP-OES for As(T). Concentrations compared well with As(T) concentrations determined by hydride generation (fig. 10). All As(T) determinations were performed without cation-exchange separation. As another measurement of accuracy for As(T) determinations, USGS standard reference water samples (SRWS) AMW4, T143, T155, and T159 (Farrar, 2000) were analyzed as unknowns along with the acid mine water samples. Measured concentrations compare well to the most probable values (MPV) (table 3).
Accuracy of As(III) determinations, both with and without cation exchange, was estimated by spiking samples collected from the Summitville Mine (Reynolds Adit and Chandler Adit), the Richmond Mine, and the Penn Mine with As(III). Iron(III) and Cu(II) interfere with the determination of As(III) in samples collected from Summitville, but not with the Richmond or Penn Mine samples (fig 11). Spike recoveries for Summitville Mine samples not separated by cation exchange were 44 to 53 percent, whereas recoveries for the same samples separated by cation exchange were 102 to 103 percent. The Richmond and Penn Mine samples were not separated with cation exchange resin because the ratios of Fe(III) and Cu(II) to As(III) were low enough that samples could be diluted to Fe(III) and Cu(II) concentrations that did not interfere with the determination. The As(III) concentrations were underestimated in the Summitville Mine samples not separated by cation exchange (fig. 12). No USGS standard reference water sample exists for As redox species.
Time Stability of Arsenic Redox Species The time stability of As(III) was monitored in 45 surface and ground water samples from Yellowstone National Park, Wyo.; Questa Mine site, N. Mex., Summitville Mine site, Colo., Richmond Mine, Calif., Penn Mine, Calif., Ester Dome, Alaska, Fallon, Nev., Mojave Desert, Calif., and Kamchatka, Russia. Samples were filtered through a 0.1-μm membrane, acidified to pH <2 with HCl, and stored in opaque bottles at 4°C. Samples were reanalyzed as many as 15 months after the initial determination. The samples containing Cu(II) or Fe(III) concentrations that interfered with the As(III) determination were separated using cation exchange. The change in As(III) is represented by plotting percent difference in As(III) concentration as a function of the initial As(III)/As(T) ratio. The equation used to calculate percent difference (Δ%) is: The As(T) concentration ranged from 0.006 to 33 mg/L and the initial As(III)/As(T) ratio ranged from 0.01 to 1.0 (fig. 13). The curved line is a Gaussian Fit. The average percent difference (Δ%) for all samples was 0.2 with a standard deviation of 7.
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