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l Appendix to question No. 10 . . DISPOSITICN OF SILVER IODIDE USED AS A SE::['li~:G AGl::l"T.!1 11. Lee rcll~.,;l/. Douglas R. _r.:",ro'1., .:>.r.d f!or,;>lil:t. Klci" ABSTRACT Periodic 5amplin~ of surface 5011 and veRetation in subalpine meado_, spruce, and aspen communities ~as carried out along five transects 1n the target area in an attempt to determine the disposition of silver iodide nucleatin~ agent used in the San Juan snow aug~ mentation project. Samples at toliage, 11tter, and 5011 ~ere taken at several elevations along the tran- sects, when possible during spring and fall~ t~ allow rnonitorin~ of silver content and silver movement 1n the target area. To gain additional information on silver dynamics, laboratory studies of silver move- ment in soil columns, analyses ot silver disposition and movemenc in che vicinity at round-based enera- tor sites, and analyses of Ava! a e data on silver in ~ater and sno~ were completed. The available data sho~ no si l11cant increases 1n silver content were detected on t e target area, an n some cases,s ve levels in samples taken at the end of the monitoring appeared to be lower than in samples taken at the be- RinninR of the program. Silver tends to be retained in surface soil and litter. thou h some movement to subsurface soil was 0 serve. Silver content 0 streamflow did not indicate trends in silver content -~hich could be attributed to the seeding program. lNTRODUCTlON Theoretical considerations of the behavior of silver iodide indicate a low solubility of the compound and an extreme probability of rapid immobili%ation in the soil system. These point to unlikely ecological im- pact. but it was considered necessary to undertake a monitoring program on the target area to determine whether. additions of seeding material could be de- tected, and whether seeded material could be taken up by the major plant communities on the target area. Obiectives * 1. To measure the level of silver in the terrestrial ecosystems of the target area on a periodic basis, in order to determine whether increases of silver can be detected. ~ 2. To evaluate silver disposition surrounding seeding generator sites, to determine possible movement of silver through physical and biological processes. ~ 3. To study silver movement through soils in labora- tory systems, to better understand what influences seeding agent accumulation and movement. ;K~. To analyze areal and temporal trends in silver concentration oi sno~ and gtreamflo~ in the. target area from available non-experimental data. - S",,,,+,'ons O...:Tt-ed- -Co.,cIusfoNS Three vear's I:IonitorinR of silver concentrations in fcliage, litter, and soil of aspen, spruce, and grass ror.mmnities in the San Juan tar~et area of the Uppe~ Colorado River Pilot Pro ect has shown no significant chan~es in these concentrations over the perio 0 sanpling. An analysis of sample numbers required to detect annual additions of 0.002 to 0.006 ppm dry w~ight of silver frOm cloud seeding to spruce litter, indicates that detection of the lower annual addition 11 _~ Steinhoff. H. Ju,-;n Mountains. Collins. ~I Present address: would. require about 200 samples per year for 5 years at the 95 percent confidence level for each compon- ent. If the variance of silver concentration can be reduced frOm 0.08 to 0.02 ppm the pr~sent sampling program should be adequate to detect an annual addi- tion of 0.002 ppm after 4 years. At the Pagoaa Springs generator site No. 25 silver levels in foliage. litter, soil. and grass in spring 1973. ~ere generally slightly lover than in spring 1972, but still somewhat higher than before seeding com- menced in fall 1970. Concentrations shoved a general decline froQ maximum levels at 10-20 m, to background levels at about 200 Q frOm the site. Mean values of silver concentrations on the target area shay that spruce litter (0.18 ppm, ash basis) and foliage (0.17 PP~. ash basis) have significantly higher sUver concentrations than spruce 8011 (0.08 ppm, ash basis). On a dry weight basis, silver con- centration in soil is siRnificantly Rreater than in foliage in all three vegetation types. A seasonaL trend ot si~ver concentrations-rft both soil and vegetation indicated hi her concentra- tions in spring t an in all. ist tes 0 total silver in the forest ecosystems indicate that the top 60 cm of soil contain the bulk of the silver. about 700 tices that in seeded SDOW and 2000 times that present in the folia&e. Foliar washing indicated 5ignltlcant surrace aeposition. Small pilot tests of soil leaching and plant uptake processes indicated strong root adsorption of silver and inhibition of plant uptake. Althou h silver concentrations in .snow were found to be too variable to show statistic. y s go cant differences between sites in the target area and others tn the west and east 01 It. ~ne mean 8~~V~t concentration in snow on the target area is about twice as high as the downwind mean, and three times as hiRh as the upwind mean. Total seeding intensity (i.e.. gAg! burned by all generators pe~ month) was not a good predictor of silver concentration in snow at wolf Creek Pass. No si~n1ficant difference~ in silver concentratiQn of streamflow could be found between 5 streams including 3 on the target area and 2 to the vest (upwind) ot it. Nean annua.l Sl.lver concentration tor the tive streams increased from (0,1 t 0,3) x 10-10 g 1m! in 1971 to (0,6 t 1,1) x lO-lO g Iml in 1973. However, mean metric conversion dischar~e also iccreased from 2.9 m3fsec (103 cfs) in 1971 to 4.5 m3/sec (160 cfs) in 1973. Although a r~13tlonship ohviously exists between discharge and ~ilver concentration ~n individual streams, no general predictive model could be developed between the t~o param~ters. .., rr:a:he::\atica: ::'ll.~del f':{i' the ::-.oVCIJ',el'l.t of silver fro:::r silv~r iodid~ through a.soil column was develo~ec ano tested. It indicates-:hat the silver concentra- ; I tion in the so11 solution. at any given time is best , represented by a kinetic-type reaction, coupled ~ith \; an insignificant equilibrium-type reaction. A high if :tdSOl"p!:ion r.ate constant controls the initial ~reaction, but 10 to 30 percent of input silver con- l~!ntratio~ is likely to move through the pro~ile. \.'. and J. D. lves (Eds.). 1976. Ecological impacts of snolo'pock augmentation in the 5,-;n Colorado. San Juan Ecolo,!:,y Project, Final Report. Colorado State Univ. Publ., Fort '/ United Nations Educational. Sci~ntific. onci Cultural Or~anization. Paris. France. 105 A-8d-l