Lead-210 in southern California Groundwaters

Abstract

As part of a geochemical monitoring program for earthquake prediction studies in Southern California, both radon and helium in groundwaters 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 210Pb and 226Ra 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 226Ra suggest that radon diffuses into the circulating groundwaters from the ambient rocks. The low activities of 210Pb relative to those of radon imply that either 210Pb produced by radon decay in the groundwaters is removed rapidly by adsorption onto fractured rock surfaces or radon is injected into the groundwaters only at shallow depths with a very short residence time.
An apparent model age of the goundwater since the injection of 222Rn can be calculated from the 210Pb/ 22Rn activity ratio assuming no 210Pb present in the groundwater when 222Rn 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 210Pb is removed fr m the circulating water by particulate scavenging an or adsorption onto the fractured rock surfaces in contact with the water, then a typical residence time r 210Pb in the water can also be calculated based on the 210Pb/222Rn activity ratio. This calculated residence time for 210Pb is quite comparable to the apparent model age of the groundwater since the injection of radon. However, the extremely low 210Pb/222Rn activity ratios are more likely due to rapid removal of 210Pb from the waters by adsorption onto the fractured rock surfaces or particulate matter.

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