Identification and correlation of sensitive indicators for groundwater quality in South water recharge areas

Groundwater level and water quality data in Tongzhou District, Beijing from 2011-2019 were used to construct ROC curves of some water quality indicators before and after recharge of South-to-North water in the area．The following were identified: highly sensitive indicator Fe³⁺ with AUC value > 0.7, moderately sensitive indicators \rmNO_3^- , \rmNO_2^- and F⁻ with AUC values between 0.6～0.7. Concentration of Fe³⁺ was found to have become lower after recharge, but concentrations of \rmNO_3^- , \rmNO_2^- and F⁻ increased. A R-V copula model was constructed taking index F⁻ as an example．Indicators closely related to the sensitivity of F⁻ were found to be Ca²⁺, Mg²⁺, Na⁺, Cl⁻, pH．Ca²⁺ and F⁻were found markedly negatively correlated．After groundwater recharge, area Ca²⁺ concentration was found to have decreased, which stimulated F⁻ activity; pH and F⁻ showed an indirect correlation via Ca²⁺．With increased pH and alkaline environment, water Ca²⁺ activity decreased, this was beneficial for F⁻ enrichment．F⁻showed positive correlations with Mg²⁺, Na⁺, Cl⁻．The correlation between Mg²⁺ and F⁻ was due to the lixiviation and intimate hydraulic connection．Simultaneous increases in Cl⁻ and F⁻ were due to displacement reaction．When Ca²⁺ concentration was low, Na⁺ would adsorb and desorb with F⁻, F⁻ could therefore be enriched．The sensitive enrichment of F⁻ in groundwater after recharge was due to 2 reasons. On the one hand, recharge stimulated dissolution of fluorine-containing minerals of primary geological origin and mixed dilution to promote dissolution of fluorine-containing minerals. On the other hand, recharge accelerated the mutual displacement of Cl⁻ and F⁻, plus adsorption and desorption of Na⁺ and Cl⁻．The present work helps deepen the understanding of the evolution mechanism of groundwater quality in South-to-North water recharge area, also provides theoretical support and decision-making basis regarding groundwater monitoring, managing and water supply security in Beijing municipal administrative center．