Dept. Geology & Geophysics, Texas A&M University, College Station, TX 77843, United States.
Dept. Mines, Metallurgy and Geology Engineering, University of Guanajuato, Guanajuato 36000, México.
Water Res. 2020 Oct 15;185:116257. doi: 10.1016/j.watres.2020.116257. Epub 2020 Jul 31.
This study identifies causes of rising arsenic (As) concentrations over 17 years in an inter-montane aquifer system located just north of the Trans-Mexican-Volcanic-Belt in the Mesa central physiographic region that is extensively developed by long-screened production wells. Arsenic concentrations increased by more than 10 µg/L in 14% (3/22) of re-sampled wells. Similarly, in a larger scale analysis wherein As concentrations measured in 137 wells in 2016 were compared to interpolated, baseline concentrations from 246 wells in 1999, As concentrations rose more than 10 µg/L in 30% of wells. Between 1999 and 2016, the percentage of all wells sampled in each basin-wide sampling campaign exceeding the World Health Organization's 10 µg/L drinking water limit increased from 38 to 64%. Principal Components Analysis (PCA), step-wise multiple regression, and Random Forest modeling (RF) revealed that high As concentrations are closely associated with high pH and temperature, and high concentrations of fluoride (F), molybdenum (Mo), lithium (Li), sodium (Na) and silica (Si), but low calcium (Ca) and nitrate (NO) concentrations. Pumping-induced mixing with hot, geothermally impacted groundwater generates alkaline water through hydrolysis of silicate minerals. The rising pH converts oxyanion sorption sites from positive to negative releasing As (and Mo) to pore waters. The negative correlation between nitrate and As concentrations can be explained by conservative mixing of shallow, young groundwater with geothermally influenced groundwater. Therefore water carrying an anthropogenic contaminant dilutes water carrying geogenic contaminants. This process is enabled by long well screens. Over-exploitation of aquifers in geothermal regions for agriculture can drive As concentrations in water from production wells to toxic levels even as the total dissolved solids remain low.
本研究确定了在中美洲地盾中央地貌区一个山间含水层系统中，砷（As）浓度在 17 年内上升的原因，该系统位于跨墨西哥火山带以北，广泛开发了长筛生产井。在重新采样的 22 口井中，有 14%（3/22）的井的砷浓度增加了 10μg/L 以上。同样，在一项更大规模的分析中，将 2016 年 137 口井测量的砷浓度与 1999 年 246 口井的插值基线浓度进行比较，结果显示 30%的井的砷浓度增加了 10μg/L 以上。在 1999 年至 2016 年间，在每个盆地范围内的采样活动中，所有采样井中超过世界卫生组织 10μg/L 饮用水限值的百分比从 38%增加到 64%。主成分分析（PCA）、逐步多元回归和随机森林模型（RF）表明，高砷浓度与高 pH 值和温度，以及高浓度的氟化物（F）、钼（Mo）、锂（Li）、钠（Na）和硅（Si）密切相关，但与低钙（Ca）和硝酸盐（NO）浓度相关。抽水泵引起的与热、地热影响地下水的混合作用通过硅酸盐矿物的水解作用产生碱性水。pH 值升高会将含氧阴离子吸附位点从正电荷变为负电荷，从而将砷（和 Mo）释放到孔隙水中。硝酸盐和砷浓度之间的负相关关系可以用浅部年轻地下水与地热影响地下水的保守混合来解释。因此，携带人为污染物的水会稀释携带地球成因污染物的水。这一过程是通过长井筛来实现的。在为农业过度开采地热区含水层的情况下，即使总溶解固体含量保持在低水平，生产井中的水的砷浓度也可能上升到有毒水平。
