Information compiled in 1985 while assessing the environmental impact of radionuclides previously discharged with aqueous releases from the Rancho Seco Nuclear Power Generating Plant is presented. In October 1984, the quantities of gamma-emitting radionuclides in water discharged to Clay Creek from the plant were reduced below operationally defined detection limits for liquid effluents. However, radionuclides previously discharged persist in the downstream environment and are found in many aquatic dietary components. /sup 134/Cs and /sup 137/Cs are the primary gamma-emitting radionuclides detected in the edible flesh of different fish, crayfish, and frogs. Coefficients for exponential equations are generated, from a least square analysis, that relate the change in concentration of /sup 137/Cs in fish to distance downstream and time between March and October 1985. Concentrations of /sup 137/Cs in surface creek sediments also decreased in the downstream direction much in the same manner as concentrations decreased in fish. However, there was no significant difference in the radiocesium concentrations in surface sediements collected from comparable locations during both 1984 and 1985.

As anionic species, both Tc-99 and I-129 are expected to be mobile in the soils beneath the burial ground. These two isotopes were analyzed in groundwater collected from research wells screened in the tritium plume leaving the burial ground. The maximum observed concentrations of 22 pCi/L of Tc-99 and 12 pCi/L of I-129 in the plume wells are above background levels and confirm the mobility of these species. The earlier measurements included in this report have been reported before.1,2 The data indicate that the I-129 concentrations may be increasing with time. Because of the long time (greater than one year) involved in obtaining results for these ultra low-level analyses, this work included analysis of only a few wells inside the burial ground. This limited selection of wells does not permit a mapping of the groundwater concentration isopleths.