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

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.