RECHARGE ESTIMATES AND HYDROLOGIC PROPERTIES OF THE YESO FORMATION IN THE SOUTHERN SACRAMENTO MOUNTAINS DERIVED FROM HYDROGRAPH ANALYSIS

We installed continuous water level recorders in several unused wells completed in the Yeso Formation as part of the southern Sacramento Mountains hydrogeology study. Well SM-49 is in the center of a 280 acre drainage basin at 9070 feet elevation. We recorded data at 15-minute (initially) and one-hour intervals from June 2006 until August 2009. The hydrograph from well SM-49 has numerous water level spikes that can be correlated to precipitation events (summer thunderstorms) recorded by nearby weather stations. Based on these spikes we performed analyses to estimate hydrologic properties of the Yeso Formation and to estimate recharge.

We adapted the method of Shevenell (1996) and Powers and Shevenell (2000) developed for carbonate karst aquifers to the mixed clastic – fractured carbonate aquifer of the Yeso Formation. They identified distinct, sequential, decreasing slopes in hydrograph recessions after precipitation-induced water level peaks as corresponding to conduits, fractures and matrix in a karst aquifer. We identified analogous decreasing slopes in the SM-49 hydrograph as corresponding to fractured limestone beds, sandstone intervals, and limestone matrix and/or siltstone and mudstone beds. These rock types have been observed in well logs and in Yeso outcrops in the study area. The method involves calculating ratios of recession slopes and equating these to ratios of specific yields for the different aquifer divisions. If one specific yield is known, the others can be calculated. The method yields one transmissivity value, which must be assumed constant for the whole aquifer. Using a range of aquifer thicknesses based on water table rise and saturated thickness we determined hydraulic conductivities consistent with those derived from traditional aquifer testing methods.

We estimated recharge to the aquifer from the SM-49 hydrograph using the water-table fluctuation method. Here the change in water table elevation due to a precipitation event is assumed to be due to recharge water arriving at the water table. Using the specific yields calculated from this hydrograph for the Yeso Formation we estimated that 5-10% of the precipitation from summer storms enters the groundwater system as recharge. Much of the water from these short, high-intensity events is probably lost by runoff and evapotranspiration.