『Abstract
We use 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. We constructed a model accounting 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. We then
fit 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.
Keywords: Mineral alteration; Aquifer geochemistry; Groundwater
microbiology; Sulfate reducing bacteria; Iron reducing bacteria』
1. Introduction
2. Weathering and microbial activity
2.1. Middendorf aquifer
2.2. Microbial activity
2.3. Upper coastal plain
2.4. Lower coastal plain
2.5. Respiration rates
2.6. Possibility of sulfate reduction in upper coastal plain
3. Reactive transport model
3.1. Chemical and physical setting
3.2. Microbial activity
3.3. Weathering reactions
3.4. Ion exchange
3.5. Modeling procedure
4. Results
4.1. Upper coastal plain
4.2. Lower coastal plain
5. Discussion
5.1. Intrinsic rate constants
5.2. Role of microbial activity
5.3. Source of acid
5.4. Coexistence of iron and sulfate reduction
6. Closing remarks
Acknowledgments
Appendix A. Supplementary data
References