Geochemical Relations between Surface Water and Groundwater In Fractured Rock In Nevada County, CA

The Sierra Nevada foothills in the South Yuba River basin Nevada County, CA is underlain by plutonic, metabasic rocks of pre-Tertiary age. Fifty-six water samples from five sites were analyzed for spatial and temporal variations using stable isotopes of water and major element chemistry. Hydrogeochemical data from surface water and groundwater indicate site specific, depth related spatial variations. Temporal variations are evident in most surface water data and absent in most groundwater data. Data for ¹⁸O and deuterium (D) plot close to the global meteoric water line confirming a meteoric source and minimal subsequent evaporative fractionation or effects of gas-water-rock interactions. An altitude effect presented in surface and groundwater data as a 0.1 per mil decrease in δ ¹⁸O and a 0.5- 0.7 per mil decrease in δ D per 30.5 meter (100ft) increase between 305 and 884 meters (1,000 and 2,900 feet) in elevation. Major element chemistry was dominated by sodium, magnesium, and calcium bicarbonate in dilute concentrations. Most groundwater data had higher Ca/Na (1.8:1), Ca/Mg (4:3), and HCO₃^- /Cl^- (3:1) ratios than surface water data from the same site, indicative of local gas-water-rock interactions or mixing of infiltrating water with other groundwater sources. Most deep groundwater data (>91 meters or 300ft) had similar Ca/Na (0.9:1), and higher Ca/Mg (1.6:1) and HCO₃^- /Cl^- (1.6:1) ratios than shallow groundwater data. Most groundwater samples were under saturated or close to saturation with respect to calcite. Aqueous speciation modeling, EQ3NR version 7.2c, adjusted for lab conditions, indicated acceptable charge balance relations. Most water data are consistent with fracture flow in the granitic and metamorphic rocks of the region. Data suggest a precipitation dominated hydrologic regime where local surface waters are related to local groundwaters. Most data indicate that infiltration by way of secondary porosity contributes to groundwater production in fractured rock. Figure 1. Stable Isotopes of Water Data, Nevada County, CA, relative to the Global Meteoric Water Line. Surface water (SW) (circle) and groundwater (GW) (square) data from fifty-six samples collected in the South Yuba River basin, Nevada County, CA during Spring 2010, Fall 2010, and Spring 2011 are plotted in relation to the GMWL. Colors correspond to the season in which each water feature was sampled. Outliers appeared as more enriched or depleted in ¹⁸O relative to the GMWL.