Microbial and Chemical Zonation and Mineral Weathering in the Middendorf Aquifer, South Carolina

Park, Jungho

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Description

Title

Microbial and Chemical Zonation and Mineral Weathering in the Middendorf Aquifer, South Carolina

Author(s)

Park, Jungho

Issue Date

2008

Doctoral Committee Chair(s)

Bethke, Craig M.

Department of Study

Geology

Discipline

Geology

Degree Granting Institution

University of Illinois at Urbana-Champaign

Degree Name

Ph.D.

Degree Level

Dissertation

Keyword(s)

Hydrology

Abstract

I consider the nature and origin of the chemical and microbiological zonation of the Cretaceous Middendorf aquifer, in the Coastal Plain of South Carolina, in light of the chemical evolution of groundwater along a flow path there. Some types of microbial activity, most notably aerobic respiration and iron reduction, lead to a net generation of acid as CO2, but reaction of groundwater with minerals in the aquifer consumes the acid, driving up pH and controlling the evolution of the groundwater's major ion chemistry. The thermodynamic energy available to the various functional groups of microbes, except the autotrophic hydrogen-oxidizing methanogens, is everywhere along the flow path sufficient to drive microbial growth. The concentrations of acetate, formate, and lactate do not vary systematically from zone to zone along the flow path, as might be expected if each zone were dominated by a single functional group of microbes. The overall rate of microbial respiration in the aquifer appears to be limited by the rate of the initial fermentation of organic matter. I found no compelling evidence indicating that one functional group of microbes excludes others from any of the aquifer's zones. Sulfate reduction, for example, may be non-existent in the high-iron zone associated with iron reduction, or may account for up to 90% of the respiration occurring there. Zonation of the groundwater into high-iron and low-iron facies may result from a minor change along the aquifer in the balance between the activities of iron and sulfate reducers. Using the chemical data obtained, I constructed reactive transport modeling to better understand the kinetics of chemical weathering in the Cretaceous Middendorf aquifer of South Carolina, USA, and the relationship of this process to subsurface microbial activity. The model accounts for the kinetics of mineral dissolution and precipitation, ion exchange, and the CO2 and bicarbonate produced by iron reducing and sulfate reducing bacteria in the aquifer. I fitted the model to observed trends in the chemical composition of groundwater along the aquifer by adjusting the rate constants for the kinetic reactions considered. The modeling portrays weathering in the Middendorf as a slow process by which groundwater gradually reacts toward equilibrium with minerals in the aquifer. The rate constants predicted are 6 to 7 orders of magnitude smaller than measured in laboratory experiments and 3 to 4 orders of magnitude less than those inferred from weathering rates in soils. The rate constants are smaller even than expected by projecting observed trends with the duration of weathering to the geologic age of the Middendorf. Weathering is driven largely by biological activity: about half the acid consumed is CO2 derived from the recharge area, and about half is supplied by iron reducing bacteria in the aquifer; only about 1% of the acid is of atmospheric origin, from CO2 dissolved in rainwater.

Graduation Semester

2008

Type of Resource

text

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