DATING RECENT RESERVOIR SEDIMENTS1

Radioactive fallout l”%s can be used to date sediment deposited in lakes and reservoirs since 1959. Within small regional watersheds, fallout 13YZs is assulncd to be uniformly distributed on the surface soil and is tightly adsorbed to surface soil particles. Some of these soil particles naturally labeled with 137Cs then move through the sedimentation cycle and can be used to date sediment profiles. Two distinct periods of erosion and sedimenta-tion ( 1959-1960 and 1963-1964) can be associated with periods of maximum atmospheric fallout. Data from other reservoirs illustrate the utility of this method. On a quantitative basis sediment is the largest pollutant in our waterways ( Grant 1971), with artificial reservoirs filling at an estimated rate of a million acre-feet (10 ” X 1,234 m3) per year (Glymph and Storey 1967). The rate and time of deposi-tion of sediment within a reservoir are in-fluenced by many watershed and reservoir paramctcrs, making it difficult to predict sediment rates in a particular reservoir ( Dcndy 1968). Geologic sediments have been dated by radiocarbon (Watts 1971) and by other natural radionuclides (Faul 1954). Recent sediments have been dated by fallout radionuclides ( Krishnaswamy et al. 1971). Present methods of determining recent sedimentation rates involve the sur-vey and subscqucnt resurvey of rcscrvoirs (Task Committee 1970). We describe a method for dating recent sediments and computing recent rates?f sedimentation using fallout ln7Cs concentrations measured in the deposited sediment. With the advent of the nuclear age, a number of radionuclides were introduced into the environment, Yearly fallout rcc-ords for one of these, 137Cs, show a maxi-mum fallout in 1963 with another distinc-tive peak in 1959; a small increase from the minimum in 1967 is shown in the re