Rising Arsenic Concentrations in an Intensively Pumped Aquifer, Independence Basin, Central Mexico
Abstract
Sixty percent of Mexico's water supply comes from groundwater. The annual pumped volumes from Mexico's aquifers is high relative to groundwater in storage and recharge rates. In many regions, water tables have fallen by over 100 meters since the introduction of modern drilling technology in the 1940s. The impacts of water table declines on concentrations of arsenic (As) in wells in semi-arid regions is poorly documented. The objective of this study is to discover the processes driving rising As concentrations in an intensively pumped, inter-montane aquifer system in the state of Guanajuato, Mexico. We tested the impacts of geologic and geochemical factors and water table declines on changes in As concentrations between 1999 and 2016 using logistic regression. In 2016, the chemistry of water from 93 wells was analyzed. Twenty-two of these wells were re-sampled from a set of 243 wells sampled in 1999. These 243 wells were used to interpolate baseline (1999) As concentrations across the basin. Whereas in 1999, only 93 of 246 wells (38%) exceeded the World Health Organization (WHO) recommended limit of 10 μg/L, in 2016, 52 of 93 wells (56%) exceeded this limit. Arsenic concentrations increased by more than 10 μg/L in 3 of 22 re-sampled wells and 19 of 93 wells compared to local baseline 1999 concentrations in the aquifer. In 1999, wells with high As were confined to an area in the northeast of the Basin. In 2016, however, toxic levels of As were widespread throughout areas where this was not previously found. The most obvious explanation is the drilling of deeper wells accessing older, more mineralized groundwater. However, even excluding deep wells (over 400m) from the analysis, As concentrations are increasing. Proximity to geothermal heat sources, faults, surface geology and soil type, historic water level declines and geochemical covariates were all examined using logistic regression to inform the local processes driving As changes, and produce an As change risk map for the basin. The study reinforces a call by the United Nations for ongoing sentinel monitoring of As in drinking water wells, to inform stakeholders of their changing drinking water quality and enable governments and non-governmental Organizations (NGO's) to assess the costs of investments in water treatment and transportation for affected communities.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2018
- Bibcode:
- 2018AGUFM.H31J2024K
- Keywords:
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- 1829 Groundwater hydrology;
- HYDROLOGYDE: 1831 Groundwater quality;
- HYDROLOGYDE: 1832 Groundwater transport;
- HYDROLOGYDE: 1847 Modeling;
- HYDROLOGY