USING STABLE ISOTOPES TO EXAMINE THE RELATIONSHIP BETWEEN GROUNDWATER CHEMISTRY AND RAINWATER IN A SEMI-ARID BASIN: INDEPENDENCE AQUIFER, GUANAJUATO, MEXICO
The objective of the present study is to use stable isotopes (18 O/16O) and (2H/1H), major and trace element chemistry to identify distinct geochemical facies throughout the AI and identify important recharge areas in the Guanajuato Sierras. Nine rainwater samples were taken during 2015 at 4 locations: 2 were at intermediate and high elevations on the eastern flank of the Guanajuato Sierras and 2 were at low elevations within the basin. Twenty wells were sampled (70-500 m). Water levels were measured in the wells and well tops were surveyed using a Real-time kinematic (RTK) GPS survey (± 20 cm).
Hydraulic heads trended from the mountainous outer basin inward. Groundwater samples were very fresh, indicating the aquifer sediments contain slowly dissolving minerals. Total dissolved solids (TDS) increased with depth. High As (>50 ppb) and F (>4 ppm) concentrations were found across a wide range of depths and locations. Principal components analysis (PCA) on the major and trace elements and stable isotopes suggested 6 distinct hydrochemical facies. A more comprehensive groundwater and rain water sampling needs to be performed to identify important recharge zones and the source zones for As and F.