Multiple-isotope (Sr, Rn, O, and H), hydrochemistry, and microbiology to analyze mixing processes in a shallow aquifer near a river

This study aims at identifying the groundwater-surface water mixing processes and flow characteristics in shallow aquifer under large river water fluctuations. Investigations have been carried out within an island, which is composed of Precambrian biotite gneiss terrain with various portions of mica-schist, quartzite, and gneiss in the Gyeonggi massif. The main aquifer is located at the depth from 13 m to 16 m below the ground surface. A multi-tracer approach combining isotopes (Sr, Rn, O, and H), hydrochemistry, and microbial community was conducted using 18 groundwater samples and 1 surface water during three years (from 2014 to 2016). The Sr and Rn isotopes were also used to quantify mixing ratios by mass balance equation. The pairs of hydrogen (δ²H) and oxygen (δ¹⁸O) isotopes indicated that all the groundwater samples were originated from meteoric water. Most groundwater samples were classified as a single chemical type of Ca-HCO₃. Bacterial community anomalies also showed the influence by the exchanges between groundwater and surface water. The ⁸⁷Sr/⁸⁶Sr ratios of groundwater samples were from 0.7243 to 0.7584 with relatively high Sr²⁺ concentrations, whereas the ratio of surface water was from 0.7197 to 0.7282 with relatively low Sr²⁺ concentrations. The result of two end member mixing model using Sr showed that the value of average groundwater ratio (50.11 percent) was similar to the result of Rn (50.09 percent). The mixing ratio result of Sr tracer also showed that it had linear relationship with the result of Rn tracer, suggesting the possibilities of dual isotope application. In order to delineate the flow systems of the study site, moreover, the PHREEQC inverse modeling was conducted with Sr isotopes data considering river discharge rate. As the discharge increased or decreased, spatio-temporal variations in Sr elements were formed to reflect that. This had a tendency to coincide with the results of other tracer data along the flow paths.