Following the recommendations of SCOPE in 1978, a ten year study of the methodology for background level monitoring has been conducted using a lake remote from human activities as a sampling “vessel”. The whole study has consisted of: 1. Selection of suitable lakes, 2. Selection of indicator pollutants, 3. Methods for ultra-trace analysis, 4. Examination of the representativeness of lake water samples, 5. Reading the history of pollution contained in the sediments, and 6. Elucidation of changes in loading sources of pollutants. The last subject is not yet completed. This paper focuses mainly on results for subjects 4 and 5. The characteristics of the remote Lake Mashu are discussed from the viewpoint of the effective background monitoring systems with high reliability. The representativeness of the water samples is assured by the hydrological characteristics of the lake and the homogeneity of lake water (caused by mixing). The mechanisms and critical conditions for thorough mixing are estimated. Records of pollution contained in the bottom sediments have been analyzed in conjunction with age data for the sediment layers. Sufficient fundamental information has been accumulated with the results of this study to propose an effective monitoring program for Lake Mashu to be continued for more than 100 years. It was concluded that an international network for monitoring of lakes is advisable.

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