Statistical characteristics of aquitard hydraulic conductivity, specific storage and porosity

• Laboratory experiments tend to result in lower aquitard K and higher aquitard S s . • Elastic S s is about one order of magnitude lower than inelastic S s. • For ϕ below 0.25, aquitard K tends to increase with ϕ and decrease vice versa. • Inelastic S s generally corresponds to ϕ > 0.37 and elastic S s to ϕ < 0.37. • ϕ outperforms other factors in predicting S s and is significant for predicting K. Aquitard hydraulic conductivity ( K ), specific storage ( S s ) and porosity ( ϕ ) are critical to the modeling of flow and mass and heat transport in groundwater systems, yet comprehensive insights into their statistical characteristics remain limited. This study compiles 456 K values from 47 studies, 202 S s values from 49 studies and 239 ϕ values from 18 studies, followed by rigorous statistical analysis of aquitard K , S s and ϕ with dependence on the features of test method, lithology and burial depth. Additionally, the interrelationships among K , S s and ϕ are analyzed. A predictive model for aquitard K and S s is developed utilizing the random forest (RF) approach. The statistical findings unearth that laboratory experiments tend to yield lower K while higher S s , particularly when burial depths are less than 50 m. Clay, silt clay, and glacial till exhibit comparable K values, in contrast to significantly lower K values observed in shale formations. Aquitard K , S s , and ϕ exhibit depth-decaying characteristics. Between two adjacent depth intervals, 0 – 50 m, 50 – 150 m and >150 m, there is nearly an order of magnitude difference in either K or S s . Aquitard K tends to increase with ϕ when ϕ is below 0.25 and decreases vice versa. This phenomenon is probably attributed to the significant amount of immobile-bound water adhering to clayey particles within the aquitard with a high ϕ value, which reduces the pore space available for water flow. Elastic S s , which indicates recoverable aquitard groundwater depletion, is about one order of magnitude lower than inelastic S s , which indicates permanent aquitard groundwater depletion, implying that the majority of groundwater depletion from aquitards under long-term groundwater exploitation is non-recoverable. The RF-based predictive model indicates that ϕ is a highly important feature for predicting K and outperforms lithology, test method and burial depth in predicting S s . The statistical findings of this study present a valuable basis for aquitard hydraulic parameters.