Hydraulic Issues Concerning Injection of Harvested Rainwater to the Subsurface Through Drywells: Insight From Numerical Simulations of Flow in a Realistic Combined Vadose Zone-Groundwater Flow System

This study focuses on the analyses of field-scale water flow originating from a drywell, using the 3-D Richards equation for the local description of the flow in a spatially heterogeneous, combined vadose zone-groundwater flow system. Realistic features of the flow system, that is, height of the groundwater table, records of the time-dependent rainfall data, spatial heterogeneity of soil hydraulic properties and flow-controlled attributes, as well as man-controlled characteristics of the drywell, were considered in the analyses. The effect of (a) the depth-dependency of the soil type, (b) the vertical position of the drywell's screen, and (c) the mode of water injection to the drywell (constant or time-dependent), on the performance of the drywell, were examined. Results, and in particular those related to the importance of the soil type depth-dependency with regard to the drywell performance, can be explained by the physics of flow in unsaturated soils of differing soil texture. Of specific significance is the transition zone between the two different soil types in the case in which the spatially-heterogeneous coarse-textured soil overlies the spatially heterogeneous fine-textured soil. In this case, when most of the drywell screen is located within this transition zone and when the flow is transient and non-monotonous, the downward flow from the drywell is maximized. Although this study is site-specific, we believe that its results are novel and relevant for sandy to sandy-clay soils associated with managed aquifer recharge technology based on a device whose vertical axis is perpendicular to the soil layering.