GEOCHEMICAL ESTIMATES OF PALEORECHARGE IN THE PASCO BASIN - EVALUATION OF THE CHLORIDE MASS-BALANCE TECHNIQUE

The Pasco Basin in southeastern Washington State provides a unique hyd rogeologic setting for evaluating the chloride mass balance technique for estimating recharge. This basin was affected by late Pleistocene c atastrophic floods when glacial darns in western Montana and northern Idaho were breached. It is estimated that multiple Missoula floods occ urred between similar to 13,000 and 15,000 years B.P. and reached a hi gh water elevation of similar to 350 m. These floods removed accumulat ed chloride from the sediment profile, effectively resetting the chlor ide mass balance clock at the beginning of the Holocene. The rate of c hloride accumulation go in the sediments was determined by two methods and compared. The first method measured go by dividing the calculated natural fallout of Cl-36 by a measured ratio of Cl-36/Cl in the pore water, while the second method used the total mass of chloride in the profile divided by the length of time that atmospheric chloride had ac cumulated since the last flood. Although the two methods are based on different approaches, they showed close agreement. In laboratory studi es the sediment to water ratio for chloride extraction was sensitive t o the grain size of the sediments; low extraction ratios in silt loam sediments led to significant underestimation of pore water chloride co ncentration. Br/Cl ratios were useful for distinguishing nonatmospheri c (e.g., rock) sources of chloride. Field studies showed little spatia l variability in estimated recharge at a given site within the basin b ut showed significant topographic control on recharge rates in this se miarid environment. An extension of the conventional chloride mass bal ance model was used to evaluate chloride profiles under transient, tim e-varying annual precipitation conditions. This model was inverted to determine the paleorecharge history for a given soil chloride profile, and the parameters of the root extraction model required to estimate paleoprecipitation.