Hydrogeochemical characteristics and genesis of seismic observation wells in Shandong Province, China

Hydrogeochemical characteristics, controlling factors, and recharge sources of the seismic observation wells in Shandong Province were investigated by analyzing cation and anion concentrations and hydrogen and oxygen isotope compositions in well waters. A total of 17 water samples were collected in April 25–29, 2018. The results show that water temperatures in the wells are in the range of 14.8 to 52.1 °C, and the values of δD and δ 18 O range from − 72.4 to − 37.9‰ and from − 9.4 to − 4.3‰, respectively. Seventeen water samples are classified into 4 types: HCO 3 -Ca·Mg, HCO 3 -Na, SO 4 ·Cl-Na, and SO 4 ·Cl-Ca·Mg. The hydrogeochemical characteristics in 17 seismic observation wells, with a certain spatial distribution pattern, are affected by several factors, such as the tectonic, topography, stratigraphy, hydrology, and meteorology. The analyses of ratio coefficients, hydrogen and oxygen isotope compositions, and Giggenbach and Gibbs diagrams suggest that the meteoric water is the main recharge source for 17 observation wells. The recharge sources of deep lateral runoff and sedimentation water, moreover, play a significant role in some observation wells. The factors of meteorology, topography, stratigraphy, hydrology, and tectonic control the recharge, runoff, and discharge groundwater, resulting in hydrogeochemical characteristics with complex spatial variations. Our results indicated that the hydrogeochemical observation items carried the reliable signals of earthquake activity. In order to obtain more accurate earthquake prediction results, therefore, multiple observation items should be considered comprehensively, and the co-seismic hydrogeochemical response characteristics of the observation items in different regions also should be taken into account.