A METHOD FOR RAPID INSITU EXTRACTION AND LABORATORY DETERMINATION OF TH, PB, AND RA ISOTOPES FROM LARGE VOLUMES OF SEAWATER

An in situ pump and six channel extraction system, operating typically at 35 1 min-1 , can not only filter seawater samples but also extract dissolved Th, Pb and Ra isotopes from large volumes of seawater at six different depths on a single lowering. By gamma counting the ash residues of three cartridges (one particle extractor and two nuclide extractors), we can determine Th-234 and Pb-210 concentrations in the particulate and dissolved phase. Extraction efficiencies for Th-234 from the dissolved phase average 93 +/- 5% and for Pb-210 average greater-than-or-equal-to 90%. At the same time we can determine radium isotopic ratios precisely, but, at present radium concentrations can only be established by the determination of Ra-226 in a 30-l cohort sample. This system can be deployed easily from ships having an A-frame. It eliminates the problems of transporting and processing large volumes of samples and involves relatively little manual labor and analytical time. We have tested and used this device repeatedly in the Gulf of Mexico. Our preliminary results agree well with values of Th-134 and Pb-210 concentrations and Ra-228/Ra-226 activity ratios reported in the literature for the Gulf of Mexico.