WMOD00467 - Laserfiche WebLink

Home Browse Search

: -If '-I t. ,{. \~ ~I .." LS </ - FEBRUARY 1970 VOL. 6, NO.1 WATER RESOURCES RESEARCH Ecological Effects of Silver Iodide and Other Weather Modification Agents: A Review c.:. CHARLES F. COOPER AND WILLIAM C. JOLLY University of Michigan, School of Natural Resources, Ann Arbor, Michigan 4-8104- Abstract. The silver ion is among the most toxic of heavy metal ions, particularly to microorganisms and to fish. The ease with which Ag forms insoluble compounds, however, reduces its importance as an environmental contaminant. Ag is not likely to concentrate to harmful levels through either terrestrial or aquatic food chains. There is some possibility that Ag from cloud seeding will retard growth of algae, fungi, bacteria, and fish in fresh water; additional laboratory investigations are needed. Inhibition of aquatic microorganisms would interfere with the cycle that returns essential nutrients to the water. Ag in air and water should be regularly monitored. Iodine in AgI poses no environmental danger. Organic seeding agents currently being proposed are not likely to be toxic in either the short or the long run, but dispersal of proprietary organic compounds should be prohibited unleEs their composition is fully revealed. Most currently proposed methods for inten- tional modification of weather depend on artifi- cial nucleating agents to alter microphysical processes within clouds. Silver iodide is the most widely used nucleating agent, although certain organic compounds hold promise for the future. Experience with hard pesticides and recent indications of environmental contamination by mercury point up the need to consider physical and biological concentration mechanisms, de- gradability, and effects on nonhuman parts of ecological systems before deliberately introduc- ing a new material into the environment. So far, only human health aspects of silver iodide have been evaluated in the literature; there have been no examinations of potential ecolog- ical effects of cloud seeding agents. According to Douglas [1968a, b], properly adjusted silver iodide generators supply so little Ag and I to air and rainfall that the health hazard to the general public is 'nonexistent'. This conclusion seems correct as far as it goes, but Douglas [1968a] specifically stated that 'Reconcentra- tion through biological or botanical [sic] pro- cesses is not considered.' It is just the possibility of biological concentration that must be assessed before it is established that AgI or other arti- ficial nucleating agents may be safely introduced into the environment. This paper reviews the 1,,; existing literature bearing on this topic, with particular reference to cloud seeding aimed at increasing precipitation. It is an outgrowth of a broader survey of ecological aspects of weather modification [Cooper and Jolly, 1969]. SILVER IN RAIN AND SNOW FROM SEEDED CLOUDS The amount of AgI used in seeding operations depends on cloud characteristics, purpose of seeding (i.e., hail suppression, lightning sup- pression, precipitation enhancement), intensity of seeding effort, generating device used, sup- porting fuel burned, ambient temperature, burn- ing rate, and experience of operator. Delivery rates vary widely (Table 1). Typically, one generator is installed for each 100 square miles of target area. Silver concentrations in rainfall from both seeded and unseeded storms are generally near the present limit of detectability. Weather modi- fication researchers have encouraged develop- ment of sensitive analytic techniques for mea- surement of silver, since silver concentrations in precipitation indicate the efficacy of seeding methods. Silver concentrations of 10-1< g ml-1 can be determined fairly reliably in otherwise pure water [Fletcher and Millar, 1968]. Silver in precipitation from unseeded storms has been measured at levels up to 20 X 10-1. g ml-1; concentrations in precipitation from seeded 88