Strontium-90 deposition in New York City.

Measurements of strontium-90 deposited in New York City over the past 12 years make for broader understanding of the fallout phenomenon. The data indicate a stratospheric half-residence time of 8 to 10 months. The seasonal oscillation of strontium-90 fallout is very symmetrical and consistent from year to year and completely independent of the timing and magnitude of nuclear tests. The predicted fallout of strontium-90 in 1970 is less than 1 percent of that during the peak year 1963.

Strontium-90 Fallout: Comparison of Rates over Ocean and Land.

Measurements of strontium-90 in waters residing on the Bahama Banks for periods between 12 and 180 days suggest that fallout rates onto these waters are substantially the same as the average rates at all fallout collection stations between 20 degrees and 30 degrees north latitude. If the amount of preciptation on the Bahama Banks is reasonably representative of the oceanic areas in this latitude band, our results support the conclusion that the amount of strontium-90 deposited per unit area of ocean surface is within a factor of 2 of that on land.

Strontium-90: concentrations in surface waters of the Atlantic Ocean.

From the large body of analyses of strontium-90 in surface waters of the Atlantic Ocean, annual average concentrations (from 10 degrees N to 70 degrees N) have been compared to those predicted. The data indicate higher fall-out over ocean than over land and confirm the rapid rates of down-mixing shown by most studies of subsurface strontium-90.

Threat to the New York City water supply--plutonium.

The mayor of the City of New York received an anonymous letter on April 1st 1985 threatening to contaminate the water supply with plutonium unless all criminal charges against Mr Bernhard Goetz, the suspect in a dramatic subway shooting incident, were dismissed by April 11th 1985. Local and Federal authorities were called upon to evaluate the credibility of the threat and to institute a "round the clock" monitoring program by New York City personnel. The Environmental Measurements Laboratory, EML, was requested by the City to analyse a composite, large volume (approximately 175 litres) drinking water sample collected by City personnel on April 16th 1985. The concentration measured was 21 fCi/l which was a factor of 100 greater than previously observed results in our data base, and the mass isotopic content of the plutonium was very unusual. Additional samples were collected one to three months later at various distribution points in the water supply system. The plutonium concentrations were much lower and comparable to EML's earlier data. Mass isotopic analysis of these samples provided more reasonable compositions but with high uncertainties due to very low plutonium concentration. Recent measurements of large volume samples, approximately 1000 litres, collected in the Fall of 1985 from the New York City and New Jersey water supplies showed identical plutonium concentrations of 0.05 fCi/l. Mass isotopic analyses indicated similar 240Pu/239Pu ratios which were slightly lower than global fallout estimates. Due to our inability to confirm the elevated plutonium concentration value for the composite sample of April 16th 1985, it is impossible to conclude whether the threat to contaminate the New York City water supply was actually carried out or whether the sample was contaminated prior to receipt at EML.