Surface Coal Mine Soils: Evidence for Chronosequence Development

Round 1

Reviewer 1 Report

General comment:

This study analyzes the bulk density, organic matter loss and particle size of a series of spoil piles in Ohio, USA, and studies the relationship between determining the soil age of mines and these indicators, so as to provide guidance for inferring mine age and guiding subsequent ecosystem restoration and reclamation.However, some revisions are needed before it can be accepted.

 

Major comment:

1.      The distribution of mine spoil piles should be related to the location of the mine tracks, and this should be taken into account when selecting sites and making mine soil predictions.

2.      I have a doubt, by reading relevant literature such as Mining in the United States: overview, I found that as early as 1872 or before, the United States already had legislation against mining practices (mainly federal land), which means that you must be able to find past information to determine the specific location of historical mines, and the end of the introduction also detailed the law of spoil piles in mines. Therefore, from this point of view, it seems that relying on environmental detection methods to determine the formation time of mine spoil piles is more convenient than consulting historical records, and this paradox is not explained in the text.

3.      The corollary from this study points to the loss of elements such as AL and K, which may be caused by leaching, so the hydrological characteristics around the belt should be described in detail

 

Minor comment:

1.      In line 45, the description of changes in soil time scale here seems to be somewhat verbose

2.      In line 99, "non-point source" here may be better changed to a non-point source

3.      On line 176, since the text says that it is very uneven, is the three repetitions somewhat inadequate？

4.      In 2.2.2, the loss of crude ash in preparation does not seem to matter

5.      At the end of 4.1, changes in soil properties may be supplemented by microbial-mediated influences

Author Response

Reviewer #1

General comment:

This study analyzes the bulk density, organic matter loss and particle size of a series of spoil piles in Ohio, USA, and studies the relationship between determining the soil age of mines and these indicators, so as to provide guidance for inferring mine age and guiding subsequent ecosystem restoration and reclamation. However, some revisions are needed before it can be accepted.

 

Major comment:

1. The distribution of mine spoil piles should be related to the location of the mine tracks, and this should be taken into account when selecting sites and making mine soil predictions.

            There are limited records related to the specific location of mining operations, however based on maps from the Ohio Department of Natural Resources, most of the study area was impacted by surface mining and mine spoil. We have now noted this in the site information section.

 

2. I have a doubt, by reading relevant literature such as Mining in the United States: overview, I found that as early as 1872 or before, the United States already had legislation against mining practices (mainly federal land), which means that you must be able to find past information to determine the specific location of historical mines, and the end of the introduction also detailed the law of spoil piles in mines. Therefore, from this point of view, it seems that relying on environmental detection methods to determine the formation time of mine spoil piles is more convenient than consulting historical records, and this paradox is not explained in the text.

            Coal mines in the study area, like much of Appalachian coal mining, was not conducted on US Federal land. Further, smaller operations like those conducted in the Huff Run Watershed were often less rigorous with record keeping in the pre-SMCRA period. We agree that in the total absence of historical records, environmental indicators would be useful in determining the extent of mine soil distribution. However, when some historical data is available (as is the case in our field site) it is advantageous to at least be partially guided by historical maps.

 

3. The corollary from this study points to the loss of elements such as AL and K, which may be caused by leaching, so the hydrological characteristics around the belt should be described in detail

            There have been limited hydrologic studies of the watershed that have mainly focused on tributaries and passive remediation systems (these studies are cited as part of the description of the watershed). As we note in a response to Reviewer #3, we have now included more information about the drainage properties of the watershed and the relationship of the study site to the local water table.

 

Minor comment:

1. In line 45, the description of changes in soil time scale here seems to be somewhat verbose

            We agree; we have broken up this text into two sentences.

 

2. In line 99, "non-point source" here may be better changed to a non-point source

            We have revised this text to “…often non-point sources…”

 

3. On line 176, since the text says that it is very uneven, is the three repetitions somewhat inadequate?

            We have removed this text, as it added unnecessary confusion and cannot be supported.  The description of the bulk density calculations was moved into the previous section, and the subsequent sub-sections have been renumbered.  We have also addressed questions about heterogeneity below in response to Reviewer #3

 

4. In 2.2.2, the loss of crude ash in preparation does not seem to matter

            There was clearly a loss of sample mass during heating, which was used as a proxy for organic matter.  Determine the ash content was not an objective or relevant to the study.

 

5. At the end of 4.1, changes in soil properties may be supplemented by microbial-mediated influences

            We agree: we have revised the text in the last paragraph of this section to make note of this.

Reviewer 2 Report

In order to improve this article, I think it is necessary to respond to the questions and suggestions I propose:

 1-      In section 1 (Introduction), have mine spoils been dumped directly on soil areas that have not been previously impermeabilized?

 2-      In section 2.1 (Field Site and Soil Sampling), what was the thickness of overburden and was it necessary to use a drill hole? I would be necessary a schematic of how the samples were taken can help.

 3-      In section 2.2 (Sample Preparation and Analyses), in the XDR and XFR tests, did they present results of minerals or elements of coal?

 4-      In section 4 (Discussion), the figure number 6 is not shown.

 5-      In section 4.1 (The Relationship Between Soil Physical Characteristics and Spoil Pile Relative Age) how does soil compaction due to overburden contribute to bulk density? Why is bulk density higher in elevated piles?

 

Author Response

Reviewer #2

In order to improve this article, I think it is necessary to respond to the questions and suggestions I propose:

1. In section 1 (Introduction), have mine spoils been dumped directly on soil areas that have not been previously impermeabilized?

            This is unlikely, as no remediation occurred during or after mining activities. We now note in the site study description that no remediation took place where mine soils are developing.

 

2. In section 2.1 (Field Site and Soil Sampling), what was the thickness of overburden and was it necessary to use a drill hole? I would be necessary a schematic of how the samples were taken can help.

            As we note in the Introduction section, the spoil that the mine soils are developing on is a mix of residual coal shale and overburden; there was no overburden covering the mine soils.

 

3. In section 2.2 (Sample Preparation and Analyses), in the XDR and XFR tests, did they present results of minerals or elements of coal?

            The mine soils are dominated by coal and shale, as well as other materials, and we did not separate the coal fragments for separate XRD and XRF analysis. Reference #55 (Singer et al., GCA, 2020) has mineralogical and geochemical information about the coal from this location.

 

4. In section 4 (Discussion), the figure number 6 is not shown.

            The conceptual model (Figure 6) was inadvertently excluded and is now presented at the beginning of the Discussion section.

 

5. In section 4.1 (The Relationship Between Soil Physical Characteristics and Spoil Pile Relative Age) how does soil compaction due to overburden contribute to bulk density? Why is bulk density higher in elevated piles?

            As noted above, there was no overburden covering the spoil and mine soils. Further, surface sampling avoided issues of uneven compaction over time in piles of varying thickness.

Reviewer 3 Report

This is an interesting and well written paper related to soil development in coal mine spoil.  Although it is well written and has a lot of interesting data of various types, I do have a few concerns that could be addressed and some points to clarify.

One significant issue is the uniformity of the parent material (spoil in this case).  In a chronosequence, it must be assumed that the other soil forming factors are equal.  How can one be sure that the spoil is uniform in composition ?  I imagine that in some areas, more bedrock or shale may be included and in other areas, more of the unconsolidated residuum or loess above may be included in the spoil.  Secondly, the overburden rock lithology could change laterally.  Is there any field evidence on this or perhaps field descriptions of the lithology and color of the spoil ?   The authors do mention about the possibility of variations in the spoil lithology, but more needs to be added about this.  It could explain some of the variation in sand vs. silt vs. clay --- soil texture.

         Another soil forming factor that could change on the local scale is drainage.  Is there good evidence that the sites have similar drainage or water table depths ?  This would be important to be sure that the differences along the transect are due to time of soil formation, rather than drainage or parent material changes.

         My next concern is that the age of the spoil is not well known.  It would be good to have a more objective determination of the age of the surfaces ---- perhaps age of vegetation on the landscape from tree rings ?  Or historical maps / air photos ?

--- Mineralogy:   There is something strange about the mineralogy and grain size data in Figure 3.   Since Illite, kaolinite, and chlorite are mainly in the clay fraction, I am puzzled how the clay minerals can comprise > 50 % of many samples but the % clay in these same samples are < 10 %.  It seems that either the clay minerals (of the bulk sample) are overestimated greatly or that the % clay is underestimated greatly ----- It is true that % clay is generally underestimated in laser diffraction analyses, but this is a very significant different.  Was the XRD really on the bulk sample or was it a finer fraction ?   Something is awry here.

--  A Figure 6 is mentioned in the text, but I could not find a figure 6.

--  Figure 5 was mislabeled as Figure 1.

---- Muscovite vs. Illite:  some say that illite is just microcrystalline mica and so are these really much different on the XRD ?

---   are there any expandable clay minerals like smectite ?   I'm surprised that it is not present

-- clay movement:  the decadal scale seems very short for movement of clay within the soil profile to the extent noted:  is there other literature that could help support this ?

 

Comments for author File: Comments.pdf

Author Response

Reviewer #3

This is an interesting and well written paper related to soil development in coal mine spoil.  Although it is well written and has a lot of interesting data of various types, I do have a few concerns that could be addressed and some points to clarify.

 

One significant issue is the uniformity of the parent material (spoil in this case).  In a chronosequence, it must be assumed that the other soil forming factors are equal.  How can one be sure that the spoil is uniform in composition?  I imagine that in some areas, more bedrock or shale may be included and in other areas, more of the unconsolidated residuum or loess above may be included in the spoil.  Secondly, the overburden rock lithology could change laterally.  Is there any field evidence on this or perhaps field descriptions of the lithology and color of the spoil ?   The authors do mention about the possibility of variations in the spoil lithology, but more needs to be added about this.  It could explain some of the variation in sand vs. silt vs. clay --- soil texture.

            We have added a panel to Figure S1, showing the color of the soils samples which highlights the relatively similar color among the soil samples. There was no overburden covering the spoil, so potential heterogeneity is likely limited to variations in the mixture of mine coal shale and other lithologies, for which historical data is limited. However, most of the observed variation in physical and chemical properties can be explained without invoking heterogeneity in source material.

 

         Another soil forming factor that could change on the local scale is drainage.  Is there good evidence that the sites have similar drainage or water table depths ?  This would be important to be sure that the differences along the transect are due to time of soil formation, rather than drainage or parent material changes.

            We had added the following text to the site description: “Field-based observations indicated that the spoil piles had similar drainage properties; standing water has never been observed on or near the piles following large rain events, and all sites sit at least 2 meters above the local water table based on soil cores collected within the subwatershed [5].”

 

         My next concern is that the age of the spoil is not well known.  It would be good to have a more objective determination of the age of the surfaces ---- perhaps age of vegetation on the landscape from tree rings ?  Or historical maps / air photos ?

            We have added the following text to the site description after discussing the approximate ages of the spoil piles: “This is also consistent with the size and density of the trees at the field site, in comparison to trees dated with growth rings within the same subwatershed [31]”

 

--- Mineralogy:   There is something strange about the mineralogy and grain size data in Figure 3.   Since Illite, kaolinite, and chlorite are mainly in the clay fraction, I am puzzled how the clay minerals can comprise > 50 % of many samples but the % clay in these same samples are < 10 %.  It seems that either the clay minerals (of the bulk sample) are overestimated greatly or that the % clay is underestimated greatly ----- It is true that % clay is generally underestimated in laser diffraction analyses, but this is a very significant different.  Was the XRD really on the bulk sample or was it a finer fraction ?   Something is awry here.

            The clay content (from XRD) matches the combination of clay + silt (from the particle size analysis), and it is likely that the clay minerals are aggregating during the particle size analyses.  Further, previous work by our group and cited in the manuscript has shown evidence for these aggregates in mine soils are nearby sites.

 

--  A Figure 6 is mentioned in the text, but I could not find a figure 6.

            The conceptual model (Figure 6) was inadvertently excluded and is now presented at the beginning of the Discussion section.

 

--  Figure 5 was mislabeled as Figure 1.

            Fixed.

 

---- Muscovite vs. Illite:  some say that illite is just microcrystalline mica and so are these really much different on the XRD ?

            The reference patterns for muscovite and illite have distinct and differentiable peaks, so we feel comfortable identifying both phases.

 

---   are there any expandable clay minerals like smectite ?   I'm surprised that it is not present

            It is possible that expandable clays minerals like smectite are present, and preliminary XRD analysis indicated the potential presence of smectite in low abundance; however other clay mineral were dominant and no other phases were above the detection limit.

 

-- clay movement:  the decadal scale seems very short for movement of clay within the soil profile to the extent noted:  is there other literature that could help support this ?

            With respect to colloid transport of clays within soil pore water, we now note that this is happening “over similar time scales”, providing evidence for decadal time scales for clay movement.

Round 2

Reviewer 1 Report

I think it can be accepted now.