Lead-210 in Southern California Groundwaters

南加州地下水之鉛－210含量與分佈

鐘玉嘉(Yu-Chia Chung) ； Harmon Craig

《Terrestrial, Atmospheric and Oceanic Sciences》 4卷2期 (1993/06) Pp. 237-242

並列摘要

As part of a geochemical monitoring program for earthquake prediction studies in Southern California, both radon and helium in ground waters were measured monthly at the network sites from 1974 to 1985. Along with this monitoring program, lead-210 and radium-226 were also measured at most of the network sites, including those in the Palmdale area, for their spatial variation and correlation with radon during the first few years. These measurements show that both the (superscript 210)Pb and (superscript 226)Ra activities at the same site are comparable, but they are only about 10^(-4) times the radon activities. The extremely high activities of radon relative to those of (superscript 226)Ra suggest that radon diffuses into the circulating ground waters from the ambient rocks. The low activities of (superscript 210)Pb relative to those of radon imply that either (superscript 210)Pb produced by radon decay in the ground waters is removed rapidly by adsorption onto fractured rock surfaces or radon is injected into the ground waters only at shallow depths with a very short residence time. An apparent model age of the groundwater since the injection of (superscript 222)Rn can be calculated from the (superscript 210)Pb/(superscript 222)Rn activity ratio assuming no (superscript 210)Pb present in the groundwater when (superscript 222)Rn was injected. The calculated model ages, ranging from 3 hours to 9 days, are indeed very short compared to any estimate of groundwater circulation times. If(superscript 210)Pb is removed from the circulating water by particulate scavenging and/or adsorption onto the fractured rock surfaces in contact with the water, then a typical residence time for (superscript 210)Pb in the water can also be calculated based on the (superscript 210)Pb/(superscript 222)Rn activity ratio. This calculated residence time for (superscript 210)Pb is quite comparable to the apparent model age of the groundwater since the injection of radon. However, the extremely low (superscript 210)Pb/(superscript 222)Rn activity ratios are more likely due to rapid removal of (superscript 210)Pb from the waters by adsorption onto the fractured rock surfaces or particulate matter.