Effect of low-permeability layers on vadose well recharge rates

Abstract

Recharge in unconfined aquifers during Managed Aquifer Recharge (MAR) is influenced by the lithologic characteristics of the vadose zone. The capacity of vadose wells to achieve recharge rates above those possible with recharge basins is studied in heterogeneous unsaturated porous media. Although near-surface low permeability layers can be bypassed with vadose wells, subsequent low permeability layers below or adjacent to the well screen influence recharge rates by forming perched conditions. The numerical model VS2DTI was used to evaluate the effect of heterogeneous lithology on vadose well recharge. A sand aquifer was modeled using hydrogeologic characteristics from the Hueco-Bolson aquifer in Texas. Modeled recharge was studied for a 33 m deep vadose well with a 10 m high water column evaluated under conditions of both a homogenous aquifer and an aquifer with heterogeneity formed by a thin, low permeability clay layer. These low permeability layers were modeled at various depths alongside and below the well to study the impact of heterogeneity on MAR processes. Results contrast vadose wells a) without clay layers to recharge 200.6 m3/d, considered the well's maximum (100%) efficiency, and b) inclusion of clay layers creating perched conditions which are shown to reduce recharge. Specifically, recharge rates greater than 180 m3/d (90% efficiency) occur if the clay layer was aligned at or above the upper half of the well screen. Simulating the low permeability clay layer along the bottom half of the well caused recharge rates reduction to a minimum of 30 m3/d (15% efficiency). Rates above 100 m3/d (50% efficiency) occurred when the clay layer was 10 m below the base of the well. These results characterize the impact of low permeability layer distance adjacent to and below a vadose well. A low permeability horizon at a fixed depth relative to vadose wells at variable depths was also studied to demonstrate that recharge rate increases corresponding to deeper vadose wells due to additional screen length and greater pressure head. Results quantify the impact of vadose well placement relative to low permeability layer depth, and promote understanding to achieve design recharge rates using the fewest and shallowest wells possible.