A study of perched mound growth and dissipation: potential effects on artificial recharge efficiency

Abstract

A quantitative study of perched groundwater mounding and dissipation was conducted beneath RB-I of the Sweetwater Underground Storage and Recovery Facility in Tucson, Arizona. The purpose of this study was to ascertain the effect of perched water on local artificial recharge. Field measurements indicated that perched water intersected the basin floor, thereby reducing infiltration rates and recharge efficiency within the basin. Lateral perched mounding from an adjacent recharge basin (RB-2) did not influence infiltration rates. However, recharge in RB-2 was limited to short wetting intervals during this study. Although focused on perched water, the study also suggested that recharge efficiency was dependent on surface clogging materials and variable flooding techniques, given their occurrence during the investigation. A clogging layer at the northern end of RB-I was found to reduce infiltration rates by up to 95% while shallow, high rate, pulsed flooding increased recharge volumes by 6% over constant inflows of equal length. A simple model of observed field data was also constructed with Visual MODFLOW to clarify subsurface flow along and through the low permeability interface. Perched mounding was found to cause lateral spreading; however, vertical flow was dominant between the basin floor and regional aquifer. Sensitivity analyses of the model suggest that local perched mounding was sensitive to hydraulic conductivity, recharge rates, and to a lesser degree specific yield.