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Title: Chemical Evolution of Acid Precipitation in Unsaturated Zone of the Pennsylvanian Siltstones and Shales of Central Ohio

Author
item ECKSTEIN, YORAM - KENT STATE UNIVERSITY
item LEWIS, VALERIE - KENT STATE UNIVERSITY
item Bonta, James - Jim

Submitted to: Hydrogeology Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/11/2007
Publication Date: 12/1/2007
Citation: Eckstein, Y., Lewis, V.E., Bonta, J.V. 2007. Chemical Evolution of Acid Precipitation in Unsaturated Zone of the Pennsylvanian Siltstones and Shales of Central Ohio. Hydrogeology Journal. 15(8):1489-1505.

Interpretive Summary: The North Appalachian Experimental Watershed in Coshocton, Ohio has recorded over a 30-yr period average pH of precipitation of 4.7. The area lies within the unglaciated east-central part of Ohio in which geologic beds are composed of sandstone, shale, coal, and clay. A study was conducted to determine the fate of acid rainfall after it enters the soil surface, and percolates through an unsaturated soil zone to a saturated ground-water zone. The study area, known as Urban's Knob has a thick clay layer within the hilltop, and is exposed at the soil surface in the form of weathered soil. Two sets of suction porous devices were established at the top of the hill in an area that precludes lateral flow and seepage. These porous ceramic devices were placed at different radii from a monitoring well (and at different depths) that had its bottom on the clay layer that supported a perched water table. The results from water-rock chemical reactions modeled using a geochemistry model (PHREEQM) demonstrate the percolating precipitation water is neutralized to pH 7.5 within the top 1.5 m. The model suggests that along with the mineral calcite, the dissolving of minerals albite, illite, and kaolinite are the dominant mechanisms of neutralization. The cation exchange capacity (measure of the capacity of the soil to hold positively-charged ions) of the siltstone and shale, ranging 54.6 - 386 meq/100g, appears to be a function of high organic carbon content of 2.0 - 3.2% of the weathered soil and bedrock. While cation exchange is responsible for some of the sodium ion in solution, it is not the primary source of ions of calcium, magnesium, or potassium. Exchange onto clays is occurring, but is secondary to exchange on organic matter. Chemical composition of ground water perched within a coal seam is controlled by oxidation and dissolution of pyrite, returning pH to approximately 4.0. These results will be useful to scientists interested in chemical balances within watersheds and the effects of acid precipitation on water quality.

Technical Abstract: The North Appalachian Experimental Watershed in Coshocton, Ohio has recorded over a 30-yr period average pH of precipitation of 4.7. The area lies within the Pennsylvanian siltstones and shale dominated by aluminosilicates and <5% calcite. A study was conducted to determine the evolution of acid deposition through an unsaturated to saturated zones composed of siltstone and shale in an isolated hill, precluding lateral flow and seepage. The results from water-rock chemical reactions modeled using PHREEQM demonstrate the percolating precipitation water is neutralized to pH 7.5 within the top 1.5 m. The model suggests that along with calcite, dissolution of albite, illite, and kaolinite are the dominant mechanisms of neutralization. The cation exchange capacity of the siltstone and shale, ranging 54.6 - 386 meq/100g, appears to be a function of high organic carbon content of 2.0 - 3.2%. While cation exchange is responsible for some of the Na+ in solution, it is not the primary source of Ca2+, Mg2+, or K+ ions. Exchange onto clays is occurring, but is secondary to exchange on organic matter. Chemical composition of groundwater perched within a coal seam is controlled by oxidation and dissolution of pyrite, returning pH to approximately 4.0.