Effects of aquifer heating on groundwater chemistry with a review of arsenic and its mobility
Date
2008
Authors
Journal Title
Journal ISSN
Volume Title
Publisher
Abstract
This research project was designed to assess changes to solution chemistry due to localized heating of a fresh water aquifer. Sediments collected from the test aquifer were subjected to controlled laboratory experiments to assess arsenic release at increasing temperatures. Release occurred at, or near, 50°C. An activation energy of 11.6 kcal/mol was determined - consistent with physical desorption, mineral dissolution and mineral dissolution via surface reaction controls. When exposed to fresh aquifer sediment arsenic was removed from aqueous solution, indicating attenuation capacity for the sediment. Sequential soil extraction indicated amorphous Fe and Al hydrous oxides as the controlling fraction. Sediment mineralogy identified Fe-rich smectite and amorphous Goethite as the likely sources and sinks. Release of arsenic attenuated to fresh sediment occurred more easily at lower temperature (20°C) indicating formation of weaker bonds with mineral solids as opposed to natural conditions. Monitoring wells installed as part of the field experiment identified increasing concentrations of sodium, calcium, potassium, magnesium, barium and strontium at elevated groundwater temperatures. Similar results were noted for chloride, silica, sulphate, bicarbonate, boron, arsenic and molybdenum. Average Ea values ranged from 1.2 to 10.3 kcal/mol, with a value of 8.1 kcal/mol being noted for arsenic. In contrast, iron, manganese, pH and redox decreased. These findings, in conjunction with dissolved gases, stable isotopes and microbial enumeration tests, support the occurrence of ligandpromoted and microbially-enhanced mineral dissolution and precipitation reactions. Arsenic mobility in the test aquifer was identified to be at a slower rate than the groundwater flow. Similar results were indicated for other mobilized constituents; however, relative rates were slightly different. A retardation factor (Rr) of 1.6 was identified for arsenic, which is considerably lower than calculated values (8.4 to 31) derived from laboratory tests. Geochemical modeling supports release and adsorption of arsenic by active clay mineral surfaces; therefore, lack of these attenuating minerals in portions of the aquifer facilitating active groundwater flow is the likely reason. Arsenic was also identified in an iron monosulphide mineral retrieved from one well. In addition to hydrodynamic dispersion, sorption and mineral precipitation reactions are thus implicated in the attenuation process.
Description
Bibliography: p. 313-330
Keywords
Citation
Fennell, J. W. (2008). Effects of aquifer heating on groundwater chemistry with a review of arsenic and its mobility (Doctoral thesis, University of Calgary, Calgary, Canada). Retrieved from https://prism.ucalgary.ca. doi:10.11575/PRISM/2311