Paper No. 2
Presentation Time: 1:50 PM
GEOCHEMICAL RELATIONS BETWEEN SURFACE WATER AND GROUNDWATER IN NEVADA COUNTY, CALIFORNIA
Characterization of geochemical relations between surface water and groundwater contribute to understanding recharge mechanisms and sustainability of groundwater resources. Stable isotope and major element chemistry data were collected from surface water (ditches and streams) and groundwater (wells) at five sites in the South Yuba River Basin in Nevada County, California in the spring and fall of 2010 and in the spring of 2011. All O-18 and D data for water samples fall close to the global meteoric water line confirming a meteoric source and minimal subsequent evaporative fractionation or effects of water-rock interactions on the stable isotopes. An altitude effect is evident in both surface and groundwater indicating a 0.1 per mil decrease in delta O-18 and a 0.7 per mil decrease in delta D with 100 foot elevation increase between 1000 and 2900 feet in elevation on the western slope of the Sierra Nevada. Major element chemistry data show that the waters are dilute and dominated by sodium, magnesium, and calcium bicarbonate. Most groundwater samples show higher Ca/Na, higher Ca/Mg, and higher HCO3- /Cl- ratios than surface waters from the same sites indicating effects of gas-water-rock interactions during infiltration or mixing of infiltrating water with other groundwater sources. Aqueous speciation modeling indicates reasonable charge balance relations. Most groundwater samples are under saturated or close to saturation with respect to calcite. Similarities in stable isotopes and major element chemistry between surface waters and groundwaters at individual sites suggest local recharge. Seasonal variations in major element chemistry indicate greater recharge during the wet season and less recharge during the dry summer. Corresponding seasonal effects on both surface and groundwater data indicate rapid recharge. These relations are consistent with groundwater recharge by local fracture flow in the granitic and metamorphic rocks of the region.