Acid sulfate weathering in natural glacially derived soils of north western Indiana and the environmental implications
Northwestern Indiana has land surfaces and soils that are around 15,000 years old, resulting from sediments deposited from the Wisconsinan Glaciation during the late Pleistocene period. These geologically young soils exhibit depletion of base cations and low pH's (near 4.0) which are chemically similar to much older and more intensely weathered soils. We hypothesize that FeS2 (iron pyrite) within the shale fragments of the glacial deposits was oxidized upon exposure of oxygen near the earth's surface and resulted in naturally occurring acid sulfate weathering. To test our hypothesis, we isolated the area with ArcGIS, collected clast samples from gravel pits, utilized historic soils databases, and analyzed samples with X-ray diffraction to determine if associated properties and soil minerals would support acid sulfate weathering as the cause of the current soil chemical properties. Soil samples and glacial outwash cobble-sized clasts were collected and X-ray diffraction was used to determine the presence of pyrite or of pyrite weathering products jarosite and gypsum. Existing soil databases and lab data were examined to obtain further details of the occurrence of the potential acid sulfate weathering. A review of the geologic literature was also conducted to put the acid sulfate weathering processes into a landscape perspective and geologic context. The clast samples and some soil samples indicated the presence of jarosite and gypsum which are commonly associated with acid sulfate weathering. This research also found that ultic soil properties and low pH soils were associated with sandy glacial outwash and this information was displayed spatially. The sandy outwash has a lower CEC and lower buffering capacity when compared to nearby higher clayey glacial till. There was inferential evidence of acid sulfate weathering in calcareous glacial which is illustrated by iron oxide pseudomorphs. The lack of initial pyritic shale combined with the neutralizing potential for the calcareous till prevented these soils from developing ultic properties and low pH's. Additionally, this research assessed the potential for naturally occurring arsenic in the soils. Elevated arsenic levels were found in soils that occurred in the lowest part of the landscape. Understanding the process of naturally occurring acid sulfate weathering and associated elevated arsenic within this landscape will provide more information on use and management of these areas.^
Phillip R. Owens, Purdue University.
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