MODELING THE EFFECT OF LOW-PERMEABILITY LAYERS ON VADOSE WELL RECHARGE RATES

Lithologic characteristics of the vadose zone influence methods for recharge to unconfined aquifers during Managed Aquifer Recharge (MAR). In heterogeneous unsaturated zones, vadose wells can achieve recharge rates not possible with infiltration basins. Although near surface (generally <10m) low permeability layers affecting infiltration basins are bypassed using vadose wells, subsequent layers below or adjacent to the well screen influence infiltration rates by forming perched conditions. The numerical model VS2DTI solving the Richard’s equation was used to evaluate heterogeneous lithology’s effect on vadose well recharge. A sand aquifer was created in VS2DTI using hydrogeologic characteristics from data published on the Hueco-Bolson aquifer in far west Texas. A 33m deep vadose well with a 10m high water column was added and models run with (a) no clay layer and (b) with a 2m thick clay layer set at various depths alongside and below the well. The clay layer created perched conditions. The vadose well model with no clay layer results in 200.6 m³/d recharge. This was considered 100% efficiency. Results show the vadose well performed above 180 m³/d (90% efficiency) if the clay layer is along the upper half of the well screen. Once the clay layer is adjacent to the bottom half of the well, recharge rate drops rapidly to minimum recharge when the clay layer is below and alongside the bottom 2m of well. The minimum recharge rate was 30 m³/d, only 15% of the no clay layer recharge rate. Recharge rose above 100 m³/d (50% efficiency) when the clay layer was 10m below the base of the well.

(c) The effects of a fixed perching layer relative to vadose well depth was also studied. Models run simulating multiple well depths screened from 5m below ground to well bottom show recharge rate increase in deeper vadose wells are negatively affected by perched water. These results allow a quantitative comparison of recharge from vadose wells relative to perching layer depth, allowing target recharge rate to be achieved using the fewest and shallowest wells possible. A microgravity field survey was conducted at El Paso’s recharge basins during a low recharge period to measure perched conditions. A second survey is planned during high recharge condition in the fall. Comparing measurements will evaluate changes in perched water and validate model inputs.