Documentation of Acidic Mining Exploration Drill Cuttings at the Pebble Copper–Gold Mineral Prospect, Southwest Alaska

Round 1

Reviewer 1 Report

Review for Chambers and Zamzow, “Documentation of acid rock drainage from mining drill cuttings at the Pebble copper-gold mineral exploration prospect, Southwest Alaska, 2016,” submitted to Environments

This article presents a description of a region in Alaska that was affected by mining operations, and details results from chemical analyses of copper-gold mine drill cuttings collected from various disposal sites. Although the findings of the field pH monitoring, lab-based chemical analysis of cuttings solids, and reporting on effects on vegetation are important to report in the technical literature, the paper needs substantial revisions to better clarify the motivation for the study and context for the results.

1.       Although pH was monitored in the field and shown to be acidic, and metals are measured in the drill cuttings collected, no data were presented for direct measurement of drainage from the disposal sites (if it was presented in the paper, that designation wasn’t clear within the current organization of the manuscript). I recommend modifying the title to focus more on documentation of field conditions and metals content at these sites.

2.       Were any x-ray diffraction analyses performed on the samples collected as part of this study, or as part of other studies associated with this site? Some information on the mineralogy of the samples is needed in order to make the connection that oxidation of pyritic samples can lead to acid rock drainage per the reactions presented in the introduction.

3.       The acid mine drainage context presented in the introduction would be better as part of a discussion of potential long-term issues associated with oxidized drill cuttings disposed of at the Pebble site. It is noted that documentation of pyrite oxidation and metal release from drill cuttings is absent in the literature; this isn’t entirely correct, and you may want to check publications associated with organic-rich shale drill cuttings  (Breuer et al., 1999, published by the Scottish Association for Marine Science; Stuckman et al., 2015 and 2018 conference papers from the Unconventional Resources Technology Conference; Mikos-Szymanska et al., Environmental Science and Pollution Research International, 2018). There also is another publication, Stuckman et al., 2019 in the Journal of Natural Gas Science and Engineering, that was published after submission of your article for review, that may be of interest to you.

4.       For paper organization, as someone unfamiliar with the Pebble mining operations, it would be helpful to include the topo map (figure 5) earlier in the manuscript.

5.       Were any field parameters collected that could be used as indicators of the redox state of the drill cuttings? Do any of the USGS publications present that type of data?

6.       For the analytical methods, it would be helpful if the description was organized to first describe the sample preparation methods, and then the chemical analysis techniques. Also, based on the reagents applied by the ASET and the USGS, and with both samples analyzed by ICP-MS, the results should be comparable. Do you have the quality control data that can be compared across both lab analyses to determine comparability?

Author Response

We thank the reviewer for their time and effort in reviewing our manuscript.  We have addressed the comments as follows:

Comment 1. Although pH was monitored in the field and shown to be acidic, and metals are measured in the drill cuttings collected, no data were presented for direct measurement of drainage from the disposal sites (if it was presented in the paper, that designation wasn’t clear within the current organization of the manuscript). I recommend modifying the title to focus more on documentation of field conditions and metals content at these sites.

    Response:  We have revised the title.

Comment 2. Were any x-ray diffraction analyses performed on the samples collected as part of this study, or as part of other studies associated with this site? Some information on the mineralogy of the samples is needed in order to make the connection that oxidation of pyritic samples can lead to acid rock drainage per the reactions presented in the introduction.

    Response:  No XRD or mineralogical samples were collected.  We believe our sampling indicates acid conditions have developed on a small scale at the mineralized drill cuttings wastes and the laboratory testing by the mining company PLP, which was cited in the manuscript,  indicates acid drainage will occur. If further field work is conducted, XRD and mineralogical analysis would be a valuable addition to the work we have done.

Comment 3. The acid mine drainage context presented in the introduction would be better as part of a discussion of potential long-term issues associated with oxidized drill cuttings disposed of at the Pebble site. It is noted that documentation of pyrite oxidation and metal release from drill cuttings is absent in the literature; this isn’t entirely correct, and you may want to check publications associated with organic-rich shale drill cuttings  (Breuer et al., 1999, published by the Scottish Association for Marine Science; Stuckman et al., 2015 and 2018 conference papers from the Unconventional Resources Technology Conference; Mikos-Szymanska et al., Environmental Science and Pollution Research International, 2018). There also is another publication, Stuckman et al., 2019 in the Journal of Natural Gas Science and Engineering, that was published after submission of your article for review, that may be of interest to you.

    Response:  Thank you for the references and for letting us know that research has been conducted on drill cuttings from hydraulic fracturing and oil drilling operations; this is an area of literature we should explore.  We were specifically referencing the lack of documentation at hard-rock mining sites, but there are certainly parallels in the oil industry worth investigating.

    We note upon our read of the Mikos-Szymanska 2018 article, that the drill cuttings were composed primarily of carbonate-based material (calcite, dolomite) and silicates, and were therefore not comparable to sulfide-based material such as the arsenopyrite and molybdenite at the Pebble mine.  We have requested the 2019 Stuckman article.

Comment 4. For paper organization, as someone unfamiliar with the Pebble mining operations, it would be helpful to include the topo map (figure 5) earlier in the manuscript.

    Response: We have moved the figure, and also added a site location map to the Introduction.

Comment 5. Were any field parameters collected that could be used as indicators of the redox state of the drill cuttings? Do any of the USGS publications present that type of data?

    Response:  We did not collect redox data, nor did we find redox data for water, sediment, or rock samples collected by the USGS, however if further field work is conducted it could be valuable to gather information on indicators of and extent of oxidation.  A description of USGS analytical methods applied can be found here: https://pubs.usgs.gov/ds/608/downloads/Geochemical%20Relational%20Database/PebbleGeochem_xls/AnalyticMethod.xls.  The USGS did calculate ferric iron on some samples, but only analyzed one sample of drill cuttings we are aware of and ferric iron calculation does not appear to have been performed on this single sample (sample PB 180).

Comment 6a. For the analytical methods, it would be helpful if the description was organized to first describe the sample preparation methods, and then the chemical analysis techniques.

    Response:  Please clarify -- we thought we did describe the sample collection and then the analytical methods.  We did not prepare the samples for analysis, but simply delivered them to the lab for preparation and analysis.

Comment 6b. Also, based on the reagents applied by the ASET and the USGS, and with both samples analyzed by ICP-MS, the results should be comparable. Do you have the quality control data that can be compared across both lab analyses to determine comparability?

    All our results appeared comparable, with the exception of Fe.  Our trip and equipment blanks showed no contamination. Our duplicates showed some variability in metal concentrations, but concentrations remained within an order of magnitude and were likely due to heterogeneity in samples. The QC from our laboratory appeared to be in order, with matrix spikes running between 96-107% recovery for Fe. Our results for Fe on sump covers appeared to be similar to USGS background soil results (Table 1).  The differences we observed were for drill cutting samples, which appeared to be higher in Fe than background collected by the USGS (Table 2).  This could  reflect differences in sample collection technique (USGS lab collected samples from 4-8" below ground and sieved the soil samples to -80 mesh prior to digestion, while our most oxidized samples were collected from the surface and all our samples were unsieved), but because the only real discrepancy was in Fe, we suspect this may be a result of acid development on cuttings causing Fe from pyrite to leach.  This argument is strengthened by the results from the single drill cutting sample analyzed by the USGS (PB 180, Table 2) which showed Fe concentrations similar to those from drill cuttings we collected.

Reviewer 2 Report

This is a very interesting work about the possible impact of drill cuttings which left during an exploration project at the Peble porphyry type deposit in SW Alaska several years ago. I find the paper well organized and very detailed. I have only some minor suggestions which may improve the presentation. My opinion is that an extra map of the whole area should be added in the Introduction, which will include the area of research, the location of Anchorage and the Nondalton village, which are discussed in the paper. Figure 5 should move in the Materials and Methods and so the readers realize the distribution and the density of the samples obtained for this research. 

Line 200: 24-inches and not imches

Lines 245-252: This paragraph should move to the Materials and Methods section since it does not describe any results of this study.

Author Response

Thank you for the review comments. We have addressed each of them as follows:

Comment 1: An extra map of the whole area should be added in the Introduction, which will include the area of research, the location of Anchorage and the Nondalton village, which are discussed in the paper.

    Response:  We have provided this now as Figure 1

Comment 2: Figure 5 should move in the Materials and Methods

    Response: We have moved this figure.

Comment 3: Line 200: 24-inches and not imches

    Response: We have made the correction

Comment 4: This paragraph should move to the Materials and Methods section since it does not describe any results of this study.

    Response: We have made this change.