Fiber Optic Distributed Temperature Monitoring in Mini-Anaheim Basin, Orange County, California

Artificial recharge basins are used to replenish aquifers and supply most of the drinking water for northern and central Orange County, California. Surface water is spread across basins or trenches and, given the right conditions, water percolates downward and recharges the aquifer. Once the recharge basin is operating, percolation rates may decrease over time due to clogging from sediment and/or biofouling. The purpose of this project is to test a method to measure spatial distribution of percolation rate in a recharge basin using heat as a tracer of water infiltration.  Temperature gradients have been measured using fiber optic cables buried at two depths below the basin and along the bed of the basin. Fiber Optic Distributed Temperature Sensing (FODTS) made hourly temperature measurements over a period of months. Temperature measurements provided the travel time (time lag) between the surface and two depths below the basin using cross correlation wavelet analysis. The time lag data were used to estimate specific discharge and hydraulic conductivity across the basin.  Specific discharge fluctuation agreed with estimates from water balance on the basin. Specific discharge and hydraulic conductivity were also calculated using independent methods to verify the accuracy of the method. Specific discharge was calculated using a one-dimensional heat transport equation (Stallman’s equation), and hydraulic conductivity was calculated using grain size analysis (Terzaghi’s equation). The results indicated that the time lag method does accurately measure specific discharge and hydraulic conductivity, and locations of low water flux in the basin were identified. The locations can potentially be remediated by removal, thus improving the basin’s percolation rates.