Analysis of unsaturated-saturated flow induced by a vadose zone well injection

Abstract:

Vadose zone well (VZW) injection is an effective method of managed aquifer recharge (MAR) and it plays an important role in semiarid and arid regions where groundwater table is deep or sufficiently permeable soils and/or sufficiently large land areas for surface infiltration are not available. To improve VZW injection management, it is of great importance to describe unsaturated zone properties accurately. Several analytical models have been developed for VZW injection based on Gardner (1958) constitutive model (G model). As the 3-parameter model and 4-parameter model have been proposed, it is of interest that whether the application of more flexible constitutive models is able to improve the analyses of unsaturated-saturated flow induced by VZW injection and ground surface flux (GSF). This study adopts the 4-parameter model and numerically solves the problem by COMSOL Multiphysics. The results are utilized to investigate the unsaturated-saturated flow induced by VZW injection for different values of unsaturated zone properties and analyze the influences of GSF. A synthetic VZW injection test is conducted to further compare the hydraulic responses based on different exponential constitutive models. The relative hydraulic conductivity exponent affects the unsaturated hydraulic conductivity and the moisture retention exponent affects the storage capacity of the unsaturated zone. GSF generated by surface infiltration can enhance hydraulic responses while GSF generated by evapotranspiration has the opposite effect. Such influences of GSF depend on values of unsaturated properties. The analyses of the synthetic VZW injection test demonstrate that G model can roughly describe the water retention and relatively hydraulic conductivity of the unsaturated zone while the 3- and 4- parameters model provide better representation of unsaturated zone properties. These results indicate that to get a better description of the coupled unsaturated-saturated flow induced by VZW injection, it is necessary to use a more flexible constitutive model to represent relationships between pressure head, relative hydraulic conductivity and water content.