T4 Arsenic and Other Geogenic Contaminants in Groundwater Resources: Linking Water Quality, Food Security, and Treatment I

Sunday, 22 October 2017
8:00 AM - 12:00 PM
Room 618/619/620

GEOCHEMICAL MECHANISMS EXPLAINING TOXIC CONCENTRATIONS OF DISSOLVED ARSENIC AND FLUORIDE IN THE INDEPENDENCE AQUIFER

Paper number: T4-7
Program: Technical Sessions
Time: 9:25 AM – 9:40 AM

Authors

Abstract

Groundwater from volcanic-sedimentary aquifers serve as the primary source of drinking water for the Alto Plano region in the northeast corner of the state of Guanajuato in Mexico. These aquifers are known to have high concentrations of arsenic (As) and fluoride (F), but the specific source rocks that release these elements are to some extent unexplored. To measure the distribution and extent of As and F, twenty-one drilled wells, two hand-dug wells, and a water treatment plant’s effluent waters were sampled in January of 2017 from the rural communities, agricultural lands, and urban areas within the Independence Aquifer. Arsenic and F values ranged from 0 to 80 µg/L and 0.25 to 15.24 mg/L with mean values of 15 µg/L and 2.5 mg/L, respectively. These results agree with prior investigations which indicate the persistent co-occurrence of As and F concentrations above the World Health Organization’s provisional guidelines of 10 µg/L and 1.5 mg/L, respectively. The deuterium and oxygen isotopic compositions measured from drilled wells indicate similar recharge conditions with mixing between relatively enriched urban areas (-72.6‰ δ2H and -9.4‰ δ18O) and depleted rural areas (-67.8‰ δ2H and -8.8‰ δ18O). The average groundwater temperature measured in the basin was 29 °C and the mean pH was 7.6. Drill cuttings from two wells >500 m in depth from the eastern and western sides of the basin, collected by the University of Guanajuato, were analyzed via x-ray diffraction. The 524 m eastern core is composed of felsic minerals through 160 m of depth followed by an intermediate mineralogical composition until the end of the core. The 500 m western core displays similar lithologies, but mafic minerals dominate below 452 m. A water – rock interaction study via batch experiment was implemented to investigate the dissolution of a sample from a depth of 384 m on the western side of the basin with de-ionized water (at 21 ± 1 °C and an initial pH of 6.2) at a 5% weight-to-volume ratio. After 720 minutes of reaction, the F concentration in the reactor displayed an exponential change from 0.224 to 1.162 mg/L. This leached F concentration resembles values measured within deep rural wells on the western side of the basin indicating the dissolution of volcanic rocks is a possible mechanism of releasing F. The sources and mechanisms releasing As are still under investigation.

Geological Society of America Abstracts with Programs. Vol. 49, No. 6

doi: 10.1130/abs/2017AM-305974

© Copyright 2017 The Geological Society of America (GSA), all rights reserved. Permission is hereby granted to the author(s) of this abstract to reproduce and distribute it freely, for noncommercial purposes. Permission is hereby granted to any individual scientist to download a single copy of this electronic file and reproduce up to 20 paper copies for noncommercial purposes advancing science and education, including classroom use, providing all reproductions include the complete content shown here, including the author information. All other forms of reproduction and/or transmittal are prohibited without written permission from GSA Copyright Permissions.