Amanda Porfidio

"Chemical analysis of soils developed on the Henryville Member of the Devonian New Albany Shale: The effects of development time on soil chemistry " BSES Senior Research Project 2002
Original Poster (PPT)

Abstract

Introduction
(Devonian–Mississippian) New Albany black shale is an organic rich rock, abundant in pyrite sulfur and metals, such as Co, Zn, Cu, Ni, and Pb. Due to its physical and chemical nature, black shale is readily weathered by oxidation of many reduced compounds. As the shale is weathered, specifically by water, the sulfur and metals dissolve in solution as sulfate and metal oxides. Because of their high metal concentrations, New Albany shales are potentially valuable resources, yet they release sulfur to their surrounding environment, which may have detrimental effects on local watersheds and soils. For example, run–off from the rock and soil can possibly alter the pH of nearby lakes, resulting in eutrophication.

Previous research shows that soils overlying Devonian–Mississippian oil shale outcrops are relatively shallow, acidic, and infertile (Hower et al, 1992). Because these soils are characteristically infertile, the Indiana Department of Transportation finds it more difficult to establish and maintain groundcover. As a result, topsoil substitutes or supplements may need to be developed to establish sufficient groundcover on black shale outcrops. To facilitate the development of possible topsoil substitutes or supplements, it would be beneficial to understand the basic development of these overlying soils. In particular, whether the soils become more acidic and infertile as the weathering exposure time of the shale increases. Once this is established, it may be necessary to design soil supplements based on the relative age of the exposed rock and its overlying soil. To understand how the soils overlying these rocks develop with time, carbon, sulfur and trace metals concentrations were examined for coupled rock and soil samples. Borden tan–gray shales served as the control samples because they develop more normal soils, yet have similar basic shale characteristics. Results from this study confirm higher metal and sulfur concentrations in the Henryville versus the Borden shales.

Field collection and sample preparation

Rock, soil, and water samples (if possible) were collected for each site. Four Henryville outcrops with relatively different lengths of weathering time were sampled. Three Borden sites with similar slope and latitude to the Henryville samples were also sampled as the control. Borden shales were selected as the control group because they have similar shale properties but have lower sulfur and metal concentrations, so they develop more regular soils. When choosing the sites, considerations of slope and location to the outcrop profile were made as comparable as possible, to eliminate such soil development variables from the experiment. The sites range from approximately 20–10,000 years of exposure time. All rock and soil samples were dried at 105^oC and ground using a hardened steel ball–mill. If outcrop seeps or stream water samples were available, BaCl₂ was added to filtered samples to precipitate BaSO₄ in solution.

Water seep samples 

Sulfate precipitated from seeps is responsible for deposits on the surface of the outcrop, which are highly ephemeral, and are a result of sulfate left behind by evaporation on the outcrop face. Significant sulfate is found in the water of the seeps on the outcrop face.

Soil pH  

Soil pH varies widely between the sites of each bedrock type, and is acidic to near neutral. At site HEN–3, the B–horizon is far more acidic than the A–Horizon. Using dry core soil samples, a slurry was made with 0.01 M CaCl₂ in a ratio of 1:2.5 weight of sample to volume of CaCl₂ (Baize, 1993). An Orion pH meter was used to measure soil pH in the slurry. Calcium chloride is used instead of de-ionized water to give more realistic readings by inhibiting the release of calcium from the soil, which would buffer the pH measurements. These sites are ordered in relative lengths of exposure time from oldest to youngest in each sample group. The pH ranges for Henryville black shales and Borden gray shales are similar, ranging from 3.39–7.02 and 4.43–7.00 respectively.

Sequential Extraction and Sulfur Isotope Analysis
Samples were prepared for sequential sulfur extraction using a solvent soxhlet extraction procedure. The sequential extraction procedure (Canfield et al., 1986), was used to obtain the distribution of acid volatile sulfur (monosulfides), chrome reducible sulfur (pyrite) and soluble sulfur (sulfate). The extraction products of Ag₂S and BaSO₄ were then analyzed using mass spectroscopy to assess the processes at work in the soil.

Isocon plots
This representative Borden comparison of rock versus soil metal concentrations indicates that the metals that are weathering, are conserved in the soil environment at the site. Along with the overall higher concentrations in the Henryville samples, a comparison of relative metal concentrations in the rock versus the soil shows that there is a wider distribution, hence mobility, for the Henryville sites, especially HEN–4. As weathering time increases from HEN–1 to HEN–3 to HEN–2 to HEN–4 the distribution of the metal concentrations in the rock becomes more pronounced.

Trace metal analysis

Trace metal concentrations explored include Pb, Zn, Cu, Ni, Co, Cr, Mo, Nb, Zr, Y, Sr, U, Rb, Th, V, Ti, and Ba. Dried, ground samples were prepared as pressed pellets and analyzed using X-ray fluorescence. Further evaluation of the data concentrated on the more toxic species including Pb, Co, Zn, Cu, and Ni.

Total sulfur and carbon

Unacidified rock and soil samples were processed using the LECO C/S analyzer to determine percent total sulfur and total carbon. For total organic carbon measurements, 0.25g of each sample was treated with 25.0mL of 1N HCl, for 12 hours at 60^oC, filtered, and dried before the analysis with the untreated samples. USGS shale standard SDO–1 was used as an internal standard.

Percent total sulfur versus percent total organic carbon

Borden rock samples follow a normal marine distribution while the Henryville rock samples follow a euxinic II (iron limited) distribution, which was to be expected. Soil samples for the Henryville sites show more dramatically depleted values of sulfur compared to their rock counterparts.

Discussion 
The percent total sulfur versus total organic carbon for the Henryville samples appears to follow a euxinic II distribution (Leventhal, 1983) which would indicate the rocks formed under Fe–limiting conditions. Nickel was high in all of the Henryville soil samples and lead, zinc, copper, and cobalt were high in three of the four samples. Looking at the relative ages of the soils, the longer exposed soils (HEN–R4 and HEN–R2) show more extreme, high concentrations of the metals mentioned, while the younger soils show fewer extreme metal concentrations. According to (Ross, 1994 ), the HEN–S4 and HEN–S2 have toxic levels of multiple metals than the HEN–S4 and HEN–S3A (top of A horizon). In fact HEN–S3B (bottom of A horizon) has no toxic levels of these metals. This would indicate that as the rocks weather, most of the metals from the rock are leached into the soil, and then into the water system.   In contrast, the Borden shales developed under conditions are characterized as normal marine. Therefore, as expected, this study shows relatively average levels of metals for the Borden rock samples. Since the Borden rock samples fall within normal metal concentration ranges (as found in typical shales), there is not a toxic accumulation of metals in the resulting soils. Many of these metals in low concentrations are considered to be nutrients, and are apparently maintained at the site within the plant cycle. Only zinc was measured to be at levels considered toxic for all three sites and Cr in BOR–S2.   Looking at the soil pH values, there is not an extreme difference between the Henryville and Borden samples. However, isolating the third Henryville site, shows a great difference in pH between HEN–S3A at the top of the soil profile and HEN–S3B, which is below the HEN–S3A and above the underlying rock. HEN–S3A, while lower than the others, has a comparable pH to the other Henryville soil samples. HEN–S3B however is much lower than all of the soil samples at a pH of 3.39. This may indicate that the weathering of this soil has resulted in the loss of metals to the environment. While many metals prefer a slightly acidic medium, this soil is so acidic that the metals are easily mobilized and do not remain in the soil, which is obvious from the low metal concentrations found in the X-ray fluorescence data.