OCR Text |
Show Normal background levels of silver in naturally occurring snow are on the order of 1 x lO*"-^ g/ ml, but can vary significantly, depending on the particular locality sampled. In seeding precipitation concentrations of 10""^ or 10" g/ ml are typical ( l0~ 9 g/ ml = .001 part per million). Silver iodide is soluble in cold water to the extent of 3 x 10~° g/ ml. It has an ion product constant of 0.32 x lO- 1^ at 13° C. These values indicate that silver iodide is difficult to dissolve and does not ionize readily. For comparison, the values of the two preceding quantities for silver nitrate, a typical active salt of silver, are 1.2 g/ ml and 86 percent ionization, respectively. From this data, theoretical considerations indicate that silver iodide would be unlikely to have a significant ecological impact. Studies being conducted through the Bureau's San Juan and Medicine Bow Ecology Projects tend to confirm that silver iodide is a rather innocuous substance. No change was found in the silver content of soil, litter, or the foliage of spruce, aspen, or grass following the first season of cloud seeding in the Colorado River Basin Pilot Project, with which the ecology project is associated. No change was found following the second seeding season. Laboratory studies showed that silver is held in litter and the first 1 or 2 centimeters of soil at the concentrations found in seeding precipitation. At unusually high concentrations of l6 parts per million ( ppm), movement of silver into lower levels of the soil was observed. Laboratory studies of the effect of silver on a common soil organism ( Arthrobacter spp) were also performed. A concentration of 1,000 ppm of silver iodide on soil had no effect on respiration. Readily soluble silver nitrate at 1.0 ppm showed easily discernible inhibition of respiration. A solution. of silver iodide generator product was applied to soil samples at concentrations of 3.5, 1.2, 0.13, 0.0U, and 0.012 ppm with no adverse effects. At 3.6 and 1.2 ppm, increased growth was observed. It has been suggested that acetone components in the generator product were used as growth substrates. 510 |