Reduced Artificial Recharge Rates beneath a Spreading Basin Associated with Biogenic Gases

Sand Hollow Reservoir, located in southwestern Utah, is managed for both surface-water storage and artificial recharge to the underlying Navajo Sandstone aquifer. During the five years since its inception (2002 through 2007), there has been a decrease in monthly artificial recharge rates from about 0.1 to 0.001 m/d, likely related to a combination of a decreasing hydraulic gradient and clogging processes such as siltation, biofilm development, or trapped gases. Superimposed on this trend is an autumn increase in recharge rates, followed by a winter decrease. Possible explanations include either a decrease in physical clogging associated with biofilm reduction or a decline in biogenic gas generation as water temperature and algal growth diminish in the cooler fall season; this dissipation of trapped gas bubbles would increase the effective vertical permeability of the underlying sediments. The subsequent decline in recharge rates during the winter may be caused by the increased viscosity related to cooler water temperatures.

Recent dissolved-gas samples from temporary piezometers installed in the unconsolidated sediments beneath the reservoir indicate the presence of large amounts of biogenic gases (CH4 and CO2); conversely, these samples all show reduced concentrations of O2. Preliminary total dissolved-gas pressure measurements in the reservoir during the spring season (prior to the warm summer season) indicate that there was no gas phase present beneath water depths of about 5 m. Furthermore, total dissolved-gas pressures, quantities of biogenic gases, and the CH4:C02 ratio varied considerably at different piezometers. This suggests that biofilm growth and biogenic gas generation may be dependent on a variety of parameters such as water temperature, water depth, sediment coarseness, and nutrient availability. Additional sampling is planned to evaluate the temporal and spatial variability of biogenic gas generation and implications for permeability reduction during artificial recharge.