Sandia National Laboratories recently designed a sampling and analysis strategy to implement several techniques for quantifying the recharge rate through thick unsaturated, unconsolidated alluvium. Several methods are employed to estimate the recharge rate, including a soil-physics based (i.e., Darcy's law) method, a chloride mass balance technique, a stable-isotope method, bomb-pulse tritium method, and 36Cl techniques. Each of these techniques yields an independent estimate of the recharge rate which may be used to define background percolation rates of conservative solutes, to approximate the upper boundary flux condition for numerical flow and transport modeling of the site, or as calibration criteria in flow and transport modeling.
The soil-physics based approach is an indirect means of estimating the recharge rate through the application of Darcy's law with measured hydraulic characteristic data. The chloride mass balance method assumes piston displacement to estimate the recharge rate utilizing cumulative chloride versus water content data. The stable-isotope method is a tracer technique used to estimate the transport of the isotopic species deuterium (D or 2H) and 18O in the water molecule (H218O and HD16O), which in turn allows for the determination of the recharge rate. The tritium and 36Cl versus depth profiles can be used to estimate soil-water movement rates and soil-water age.