THE FEASIBILITY OF RECHARGE RATE DETERMINATIONS USING THE STEADY-STATE CENTRIFUGE METHOD

The establishment of steady unsaturated flow in a centrifuge permits a ccurate measurement of small values of hydraulic conductivity (K). Thi s method can provide a recharge determination if it is applied to an u nsaturated core sample from a depth at which gravity alone drives the flow. A K value determined at the in situ water content indicates the long-term average recharge rate at a point. Tests of this approach hav e been made at two sites. Unsaturated K values were measured easily fo r sandy core samples from a site in the San Joaquin Valley of Californ ia. The results indicate that a better knowledge of the matric pressur e profiles is required before a recharge rate can be determined. Fine- textured cores from a site in southeastern Washington required new dev elopments of apparatus and procedures, especially for making centrifug e measurements with minimal compaction of the samples. Measured K valu es led to preliminary recharge rate determinations that are reasonable considering the known hydrology and topography of the site.