T112 A Showcase of Undergraduate Research in Hydrogeology (Posters)

Monday, 2 November 2015
9:00 AM - 6:30 PM
Exhibit Hall

USING STABLE ISOTOPES TO EXAMINE THE RELATIONSHIP BETWEEN GROUNDWATER CHEMISTRY AND RAINWATER IN A SEMI-ARID BASIN: INDEPENDENCE AQUIFER, GUANAJUATO, MEXICO

Paper number: T112-4
Program: Technical Sessions
Time: 9:00 AM – 6:30 PM

Authors

Abstract

The Independence Aquifer (AI) is a significant water resource with >500,000 people relying on it for drinking, irrigating and a growing industry. The AI underlies the Independence basin which covers 6,840 km2. The basin is semi-arid and high relief with elevations ranging from 1,850 to 2,850 masl. Bound by three mountain ranges to the east, west and north and volcanic peaks to the south, the basin contains volcanic and fluvial deposits. Since 1940 overexploitation of the AI by agriculture has greatly depleted the aquifer. People have increasingly become exposed to toxic levels of Fluoride (F) and Arsenic (As) as deeper wells are drilled (>200 m). Upwelling of deeper groundwater is possible in intensively pumped municipalities. The varying hydrogeochemistry of the AI reflect distinct recharge areas and/or flow pathways. The estimated minimum amounts of water being extracted from >2,500 wells from the years 1980, 1991, 2000 are 412, 542, 700 MMm3/year, respectively.

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.