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-~...... -- _. ~ , Appendix to question No. 12 --. 'j I I i " DISPOSITION OF SILVER IODIDE USED AS A SEE~lNG AGtNT1/ Ii. Leo Tel.lcr:Il. Dougla!'> R. [uTll(:ron. and fJoooltl ,\. Kl~1n A ma~hemacica~ model {crT the movement of silver fro~ silver iodide through 8'so11 column vas develo~ed I and tested. It Indicates~ tho.t the silver concentra- I tion in the soU solution at an)' given 'time 18 best \i represented by a kinetic-type reaction, coupled with '~' an insignificant equilibrium-type reaction. A high adsorption rnte constant controls the initial , I-reaction. but 10 to 30 percent of input ailver con- :,L~ent'ratio.n 1s likely to move through the profile. -I ' ~. and J. D. Ives (Eds.). 1976. Ecological impacts of ~nowpack augmen~ation in the San Colorado. San JUSn Ecology Project. Final Report. Colorado State Univ. Publ.. Fort ABSTRACT Periodic samplin~ of surface soil and vegetation in subalpine meadow, spruee. and aspen communities was carried ou~ along five transecta in the target area in an attempt to determine the disposition of silver iodide nucleattnR a ent used in the San Juan snow au ~ mentation project. amp es 0 0 aRe. tter. an ! soil were taken at several elevations along the tran-~ sects. when possible during spring and fall, to allow' monitorin~ of silver content and silver movement in the target area. To gatn additional info~tion on silver dynamics, laboratory studies of silver move- ment-in soil columns, analyses ot silver disposition and movement in the vicinit ot round-based enera- tor aites, and analyses of available data on silver in water and snow were completed. The available data show no slgnitlcant lncreasea 1n .ilver content were detected on the' target area, and in some cases, 81~Ve levels in aam lea taken at the end of the monitorina ~ared to be lover than in aamplea taken at the be- Rinning 0 thO program. ver ten s to reta ne surface soil and litter. thou2h acme movement to subsurfae.e sarr waa 0 aerved. -~irver content aT streamflow did not indicate trends in silver content which could be attributed to the seed1ng program. , LNTRDDUCTION Theoretical consideratioDS of the behavior of silver iodide.indicate a low 80lubility of the compound and an extreme probability of rapid immobilization in the soil system. These point to unlikely ecological im- pact, but it was considered necessary to undertake s monitoring program on the target area to determine whethee additions of seeding material could be de- tected. and wh~ther seeded material could be taken up by the major plant communities on the target area. Ob1ectives * 1. To measure the level of silver in the terrestrial ecosystems of the target area on a periodic basis, in or~~r to determine whether increases of silver can be detect~. I ~ 2. To evaluate silver disposition surrounding seeding' generator sites. to determine possible movement of silver through ph)'sical and biological processes. ~ 3. To study silver movement through soils in labora- tory systems. to better understand what influences seeding agent accumulation and movement. ~ 4. To analyze areal and temporal trends in sUver concentration oi snow and streamflow in the target area from available non-experimental data. - Sec+,'o"S O...:-rreJ- -co.~CL(jSIONS ,/ Three vearls monitorinK of silver concentrations in folia:e, litter. and soil of aspen, spruce, and grass rnmmu_ir.1e!J in the San Juan tarRet area of the IPpe~ Colorado River Pilot Pro act has shown no signi icant chanRes in these concentrations over t e per 0 0 sampling. An analysis of sample numbers required ~o detect annual additions of 0.002 to 0.006 ppm dry v~ight of silver from cloud aeeding to spruce litter, indicates that detection of the lower annual addition .!I .!!! Steinhoff. ". Juan Mountains. Collins. ~I Present add~e'a: would,require about ZOO 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~aent sampling program should be adequate to detect an annual addi- tion of 0.002 ppm after 4 )'ears. At the Pagoss Springs generator site No. 25 silver levels in foliage, litter, soil, and grass in spring 1973. were generally slightly lower than in spring 1972, but still somewhat higher than before seeding com- menced in fall 1970. Concentrations showed a general decline from maximum levels at 10-20 m. to background levels at about 200 m from the aite. Mean values of silver concentrations on the target area show that spruce litter (0,18 ppm, ash basis) and foliage (0.17 ppm, ash basis) have significantly higher silver concentrations than spruce 8011 (0.08 ppm, ash basis). On a dry' weight basis. slIver con~ centration in soil is s1KnificantlY Ereater than In foliage in all three vegetation typ~. A seasona~ trend ot s1~ver concentr.~ns-rn both solI and va atation indicated hi her concentra~ tion. n spr ns t an n a . tat tes 0 tot silver in the- foreat ec08 .t~ ladlc.te that the top cm 0 so conta n t e u 0 t e s vert a out 700 time. that in seeded anow aDd. 2000 time. that present in the foliaae. Foliar waShing'indicated sign1tlcant suruce aeposition~ StDal1" pilot tests of soil leaching and plant uptake'processea indicated strong root adsorption of silver and inhibition of plant uptake. , ~.'!;. . Althou h silver concentration. in snow were found to be too variable to 8 ow stat t . y. go cant differences between sitea in the' target- area and others to the weat and east OL 1~, tne mean-B1~vel concentration in snow on the target area 1a about twice as high as the downwind ..an. and three times as high as the upwind mean. Total seeding intensity (i.e., g Agl burned by aLl generators pe~-DOnth) was not a good predictor of silver concentration in snow at Wolf Creek Pass. No alEn1flcant differences in silver concentration of streamflow could be found between 5 streams including 3 on the target area and 2 to the \lest (up.,lnd) ot it. ,.Iean annua.L s1J.ver concentration tor the t1ve streams increased from (0.1 to,)) x 10-10 g 1m! in 1971~o (0,6 r 1.1) x 10-10 g 1m! in 1973. However, mean metric conversion dlschar~e also increased from 2.9 m3/acc (103 cfa) In 1971 ~o 4..5 m3/sec (160 cfs) in 1973. Although a relationship Qbviously exIsts between discharge and silver concentration in indivIdual streams, no general predictive model could be developed between the tvo parameters. Un!ted Nations Educational. Scientific. and Cultural Orr,anitation. Paris. france. t05 A-Bd-I