LESSONS LEARNING FROM MONITORING THE LONG-TERM PERFORMANCE OF DRYWELLS

A drywell is a vadose zone infiltration system that has a small footprint at the land surface that is installed and releases water at varying depths into the vadose zone. This presentation highlights results from a six-year study that monitored the performance of two drywells in Fort Irwin, CA. A newly installed drywell was connected to a pre-existing drywell using connections pipes. Several water level and turbidity measuring sensors were installed in the drywells, and data was continuously collected. A monitoring well was installed with several vadose zone monitoring sensors (water content sensor, perched water table level sensor, and groundwater level sensor). The site hydraulic conductivity was determined using falling head tests, soil characterization, and numerical modeling. Daily irrigation water input from nearby houses caused the drywells to fill up and drain consistently. The drywells received a significant amount of water and clay from daily inflow and major storm events, and some sediments escaped into the lower chamber of the drywells. The infiltration rate from the old drywell is prolonged due to clogging. The new drywell infiltrated water very fast, and the hydraulic conductivity has not changed much since installation. A site geology investigation revealed an extensive low permeability clay layer below the drywells that created a persistent perched water table and inhibited direct recharge over the monitoring zone. The groundwater monitoring data show significant changes in daily response to pumping activity that inhibited the determination of recharge using only water level information.