Hydrogeochemistry and isotopic analyses of the fractured volcanic aquifer system of Mount Shasta, Siskiyou County, California

Mount Shasta is one of the largest stratovolcanoes in the world and consists of andesitic and basaltic volcanic rocks. Water enters the volcanic rocks and Mount Shasta aquifer system via precipitation and surface flows. Water discharges from the aquifer system through springs, underflow, wells, and evapotranspiration. As groundwater flows through the aquifer system, the chemistry changes by chemical actions and reactions and chemical weathering of soils and rocks and mixing with younger more dilute waters. This thesis consisted of analyzing the hydrogeochemistry of the area by using inorganic and isotopic chemistry data available for three regional and local flow systems. A total of 11 springs, one snowfield, and four wells were utilized as water sampling locations. Mixing of different types of groundwater is prevalent at lower elevations and distances farther away from the peak (i.e., the greater the distance to the peak from the groundwater sampling point, the greater the mixing). Both radioactive and stable isotopic analyses of the water samples corroborate the results of inorganic chemistry studies. In general, two flow paths occur, namely a deeper regional flow path with greater than 30 year old waters and several distinct shallow flows that have local-recharge areas. These shallow zones can discharge through springs, which then generally re-infiltrate and recharge the aquifer a short distance from the emergence point. Analysis of water quality differences within each regional and local flow systems are supported by variations in water quality data. Variations in topography, rocks and minerals, human influence, precipitation, infiltration rates and hydrogeologic characteristics can be shown to explain these differences: In general, gentler sloping topography allows an increase in infiltration arid recharge to the aquifer, precipitation increases from lower elevations to higher elevations and from the west to the south, and groundwater associated with deeper regional flow is older.