Abstract
Publicly available groundwater and soil data were screened for data quality and used to investigate spatial variations in major cations and anions in groundwater, and major cations in soil, in the Southeastern San Joaquin Valley in the vicinity of Fresno, California. The possible correlations and geochemical relationships between soil and groundwater were also examined. Statistical and graphical analyses indicate that significant spatial variation occurs with regard to the chemistry of both groundwater and soil within the study area. Groundwater in the western portion of the study area generally contains higher concentrations of most major constituents, including Ca, Mg, Na, Cl, and SO4, whereas the eastern portion contained somewhat higher concentrations of K. Variations in groundwater chemistry with depth was also observed with higher concentrations of most elements occurring in the shallow interval (within 60 feet of ground surface), however this may be attributed to the fact that most shallow wells were located in the western portion of the study area. Variation across the study area of element concentrations and element ratios with depth is much greater in the western portion of the study area and is attributed to the presence of an aquitard, the Corcoran Clay formation, on this side of the valley. Mineralogical differences between source rocks for soils and sediments comprising aquifer material in the east side compared to the west side of the valley likely explain observed patterns of element concentrations in both soils and groundwater. The marine, mafic, and metamorphic rocks of the Coast Ranges in the west provide a steady source of Mg in both groundwater and soil and a positive correlation between the two media. Higher concentrations of Mg in the Inter-fan region in the east is likely explained by the input of sediments from the nearby outcrop of ultramafic and mafic rocks in the Sierra Nevada foothills. The adjacent Kings Fan contained very low concentrations of Mg, and although it is also in proximity to the ultramafic/mafic outcrops, is likely a reflection of the more felsic source for the alluvial fans in the higher elevations of Sierra Nevada where granitic rocks dominate. The faster weathering rates and greater solubility of the rocks and sediments generated from the Coast Ranges compared to those from the Sierra Nevada contribute to inverse correlations between groundwater and soil for the elements Ca and Na, and for the Na:K ratio. These element concentrations and the Na:K ratio are highest in groundwater in the west and conversely are lowest in soil in the west, where the dominant source of Ca and Na is probably plagioclase (Na-Ca) feldspar. In the east, where the dominant source of Na and K is probably the slower weathering alkali (K-Na) feldspar, the Na:K molar ratio is lower, closer to 1:1.