| Summary: | 博士 === 國立臺灣大學 === 地質科學研究所 === 91 === Precipitation of iron sulfides reduces iron mobility of groundwater in anaerobic aquifers. Iron sulfides occur in various allotropic forms from amorphous FeS to pyrite, the solubility of which can differ up to 13 orders of magnitude. Thermodynamically, pyrite is the most stable phase among iron sulfides in anoxic water. However, high concentrations of sulfide and iron were found oversaturated with respect to pyrite in anoxic groundwaters in Choshui fan-delta aquifers. In this thesis, groundwater chemistry, including iron and sulfide in water and acid volatile sulfide (AVS) in sediments, are determined for studying the dissolution and precipitation of iron sulfides in aquifers.
Aquifers of the Choshui fan-delta can be divided laterally into three redox zones based on the contents of dissolved oxygen, nitrate and sulfide in groundwaters along flow paths. Groundwaters in the first zone, i.e., the unconfined aquifer of the proximal zone (recharge area), are oxic with Eh > 0 mV, dissolved oxygen > 0.5 mg/l or nitrate-N > 0.5 mg/l. The second zone, a sulfate reduction zone, in the middle fan is anoxic with detectable dissolved sulfide species and Eh < 0 mV. The third zone is anoxic and without detectable dissolved sulfide species, probably undergoing methane production. The medians of the Eh values for the first, second and third zones decrease down gradient from +55, -138 to -172 mV, respectively. The mean value of the negative logarithms of ion activity products (-log IAP) for a total of 111 samples, of which 46 samples from the Choshui fan-delta and 65 samples from other areas worldwide, is 2.98 ± 0.45 (1σ), which is consistent with the solubility constant of amorphous FeS (2.95 ± 0.09).
Low concentrations of AVS (<61 nmole AVS/g) were found in freshwater aquifer sediments of the Hu-Wei borehole (156 m depth) in the Choshui fan-delta. The evidences that AVS were found to occur in sediments and IAP values show saturation with respect to amorphous FeS imply the anaerobic aquifers are still undergoing active sulfate-reduction processes. It is suggested that vertical variations of AVS concentrations are mainly caused by the change of redox potentials of the groundwater system and groundwater table fluctuations induced by Quaternary sea level change.
|
|---|
