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Gotkowitz, Madeline B.; Roden, Eric E.; Schreiber, Madeline E.; Shelobolina, Evgenya S. / Mineral transformation and release of arsenic to solution under the oxidizing conditions of well disinfection
[DNR-192] (2007)

Introduction,   pp. 1-4 PDF (2.1 MB)


Procedures and methods,   pp. 4-7 PDF (2.0 MB)


Page 4


Mg-HCO3 type,.with a pH of 7.1 and low dissolved oxygen (DO) concentrations (about
0.35 mg/L). Groundwater is under reducing conditions with an oxidation reduction
potential (ORP) of -50 mV (Gotkowitz et al. 2004).
        Based on several pumping tests at the field site, Gotkowitz et al. (2004)
attributed variability in groundwater arsenic concentrations to the residence time of
groundwater in the well bore. During this previous cite characterization, As ranged from
1.8 to 22 tg/L in wells at the site, with higher concentrations related to longer residence
times. Samples collected during a pumping regime intended to simulate domestic use
(380 L, or about 1.3 well volumes, pumped every eight hours at a rate of 38 L/minute)
ranged from about 3 to 6 Ag/L. The well water became more reducing during this
domestic pumping schedule than under fully purged conditions, with DO decreasing to
0.25 mg/L and ORP to -100 mV. These findings indicate that pumping at a rate and
volume similar to domestic use does not fully purge the well of water with a long
residence time in the borehole. While oxidation of sulfide minerals appears to release
arsenic to ground water in zones within the aquifer, reduction of arsenic-bearing iron
(hydr)oxides is a likely mechanism of arsenic release to water having a long residence
time in the well borehole.
PROCEDURES AND METHODS
Laboratory studies of strongly oxidizing conditions
       The impact of high-dose chlorination on mineral transformation and arsenic
release was assessed through batch experiments conducted with two different samples of
the St. Peter formation: one from the Skunk Hill (SH) quarry (located about two miles
west of the Town of Freedom, Outagamie County, Wisconsin) and the other from the
Leonard Road Michaels Materials (LM) quarry (figure 1).
       For each sample, thin sections (12) were made (Spectrum Petrographics, Inc) for
characterizing the mineralogy and elemental composition before and after chlorination.
Because of the small surface area of the thin sections, additional experiments with
crushed sample were conducted to allow for calculation of rates of oxidation and arsenic
release. Portions of the crushed sample were also digested and analyzed for whole rock
geochemistry (Actlabs, Inc).
       Pre-characterization of the thin sections was completed at the Microbeam Lab at
USGS in Reston, VA. Quantitative analysis of sulfide minerals was run on the JEOL
8900 EMPA at 20kV and 3xl0-8 mA. Qualitative element maps were run at 20kV and
5x1 0-8 mA at a 30msec dwell time. Thin sections were characterized before and after
24-hr exposure to high (1200 mg/L) free chlorine (Cl2 + HOC] + OCI) solutions.
       Initial batch experiments using the SH material in high chlorine (1200 mg/L free
chlorine) solution released very low concentrations of arsenic in solution; later digestion
and analysis of this material revealed low (22 mg/kg) arsenic in the whole rock material.
Thus, remaining experiments focused on the LM quarry material, which from previous
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