Arsenic Cycle at the Harvard Mine Pit Lake, Mother Lode Gold District, CaliforniaEPA Grant Number: U915154
Title: Arsenic Cycle at the Harvard Mine Pit Lake, Mother Lode Gold District, California
Investigators: Savage, Kaye S.
Institution: Stanford University
EPA Project Officer: Hahn, Intaek
Project Period: December 1, 1997 through June 1, 2001
Project Amount: $102,000
RFA: STAR Graduate Fellowships (1997) RFA Text | Recipients Lists
Research Category: Fellowship - Geology , Academic Fellowships , Ecological Indicators/Assessment/Restoration
The objectives of this research project are to: (1) identify the mineral sources of arsenic contributing to elevated arsenic concentrations in the Harvard Mine pit lake; and (2) evaluate the effects of seasonal variations in rainfall and lake stratification on arsenic distribution and speciation in the alkaline aquatic environment.
Mapping focuses on possible controls for secondary mineral development, including modal abundance, composition and grain size of sulfides, carbonate content of wall rocks (which can control local fluid composition in contact with mineral surfaces), and outcrop surface area. To quantify the distribution and composition of arsenian pyrite in the pit mine walls, representative samples of lithologies exposed in the wall rock are collected and analyzed by electron probe microanalysis. False color digital maps of backscattered electron images quantify modal abundance of sulfides at the thin section scale.
Detailed mapping and collection of secondary minerals will be undertaken at the end of the dry season, prior to the onset of winter storms. Techniques for secondary mineral identification will include x-ray diffraction and bulk chemical analysis via digestion/inductively coupled plasma-mass spectrometry ICP-MS. Synchrotron x-ray powder diffraction will be utilized for identification and characterization of the fine grained and/or poorly crystalline weathering products of pyrite. X-ray absorption spectroscopy will be utilized to determine the oxidation state (from x-ray absorption near edge structurenear-edge structure, XANES) and atomic coordination environment (from extended x-ray absorption fine structureextended fine structure, EXAFS) of arsenic in the naturally occurring weathering products and in natural and synthetic model compounds. Comparison with theoretical models (generated using the ab initio computer code FEFF6) enables distinction between adsorption and substitution/coprecipitation modes of arsenic attenuation by secondary minerals.
Lake water and seep samples are collected using a Kemmerer-style sampler. Depths for sampling are chosen based on depth profiles of temperature, pH, Eh, and conductivity, and dissolved oxygen is determined using a Hydrolab multiparameter water quality monitoring instrument. Filtered (0.45 µm) subsamples of all waters are analyzed for metals, anions, alkalinity, and ferrous iron. Arsenic(III) and total inorganic arsenic are determined by hydride generation atomic absorption spectroscopy. As(V) is determined by the difference.