Statistical characteristics of aquitard hydraulic conductivity, specific storage and porosity

Aquitard hydraulic conductivity (K), specific storage (Ss) 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 Ss values from 49 studies and 239 ϕ values from 18 studies, followed by rigorous statistical analysis of aquitard K, Ss and ϕ with dependence on the features of test method, lithology and burial depth. Additionally, the interrelationships among K, Ss and ϕ are analyzed. A predictive model for aquitard K and Ss is developed utilizing the random forest (RF) approach. The statistical findings unearth that laboratory experiments tend to yield lower K while higher Ss, 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, Ss, 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 Ss. 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 Ss, which indicates recoverable aquitard groundwater depletion, is about one order of magnitude lower than inelastic Ss, 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 Ss. The statistical findings of this study present a valuable basis for aquitard hydraulic parameters.