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<br />Appendix to question No. 3-d
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<br />D!SPUSITlCN vT SiI.V:i::R ronIDr.:: l">~:e \.~ A SE::r.~:G r.GE:;T1J
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<br />.-\BSTRACT
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<br />
<br />Periodic s~mplin~ of surface soil and vegetation in
<br />subalpine meado~, spruce, and aspen communities was
<br />carried out along five transects in the target area
<br />in an attenpt to determine the disposition of silver
<br />todide nucleatin~ agent used in the San Juan snow aug~
<br />mentation project. Samples of foliage. litter. and
<br />soit ~ere taken at several elevations along the tran-
<br />sects, vhen possible during spring and fall, CD allow
<br />monitori.ng of silver content and silver movement in
<br />the tar~et area. To gain a_c!.ditional information on
<br />silver dynamics. laboratory studies of silver move-
<br />ment in soil col~~s, analyses ot silver disposition
<br />and movement in the vicinity ot ground-based genera-
<br />tOr. sites, and analyses of available data on silver
<br />in water and snO~ were completed. The available data
<br />show no signiticant increases 1n silver content were
<br />detected on the target area, and 1n some cases, s1lvel
<br />levels in samples taken at the end of the monitoring
<br />a eared to be lower than in ~amples taken at the be-
<br />ginning 0 t e program. ver ten s to e reta ne
<br />in surface soil and litter. though some movement to
<br />subsurface soil was observed. Silver content of
<br />streamflow did not 1ndicate trends in silver content
<br />which could be attributed to the seeding program.
<br />
<br />INTRODUCTION
<br />
<br />Theoretical considerations of the behavior of silver
<br />iodide indicate a low solubility of the compound and
<br />an extreme probability of rapid immobilization in the
<br />soil system. These point to unlikely ecological im-
<br />pact, but it was considered necessary to undertake a
<br />monitoring program on the target area to determine
<br />whethe~ additionS of seeding material could be de-
<br />tected, ~nd whether seeded material could be taken up
<br />by the major plant communities on the target area.
<br />
<br />Objectives
<br />
<br />'* L To ~easure the level of silver in the terrestriaJ
<br />ecosystems of the target area on a period it basis, in
<br />order to determine whether increases of silver can be
<br />detected.
<br />~ 2. To evaluate silver disposition surrounding seeding
<br />generator sites, to determine possible movement of
<br />silver through physical and biological processes.
<br />~ 3. To study silver movement through soils in labora-
<br />tory systems, to better understand what influences
<br />seeding agent accumulation and movement.
<br />~ 4. To analyze areal and temporal trends in silver
<br />concentration oi snow and streamflow in the target
<br />are~ from available non-experimental data.
<br />- 5",,",,+"O"S D...:1't"ed-
<br />CO~CLUSIONS
<br />
<br />Thre8 vear's monitorinR of silver concentrations in
<br />Cclia~e, litter, and soil of aspen, spruce, and grass
<br />rnr:rrnll~~ties in the San Juan tar~et area ot the,~~pt:r.
<br />Colorado River Pilot Pro ect has shown no signl!lcant
<br />cnsnRes ir. these concentrations over the period 0
<br />s.:lmpling. An analysis of sample numbers requir~d to
<br />detect annual addItions of 0.002 to 0.006 ?pm dry
<br />weight of silver from cloud seeding to spruce li:ter,
<br />indicates that detection of the lower annual add~tion
<br />
<br />,,-1,:1:1
<br />
<br />~ould require about 200 samples per year for 5 years
<br />at the 95 percent confidence level for each compon-
<br />ent. If the variance of silver concentration can be
<br />reduced from 0.08 to 0.02 ppm the pr~sent sampling
<br />program should be adequate to detect an annual addi-
<br />tion of Q.002 ppm after 4 years. At the Pagosa
<br />Springs generator site No. 25 silver levels in
<br />foliage. litter, soil, aod grass in spring 1973,
<br />were generally slightly lower than in spring 1972,
<br />but still somewhat higher than before seeding com-
<br />menced in fall 1970. Concentrations showed a general
<br />decline from maximum levels at 10-20 00, to background
<br />levels at about 200 m from the site.
<br />
<br />Mean values of silver concentrations on the target
<br />area show that spruce litter (0.18 ppm, ash basis)
<br />and foliage (0.17 ppm, ash basis) have significantly
<br />higher silver concentratiotls than spruce soil (0.08
<br />ppm, ash basis). On a dry weight basis, silver con-
<br />centration in soil is significantlY Rreater than 1n
<br />foliage 1n all three vegetation types. A
<br />seasonal trend ot silver concentrations-rn
<br />both soil aod vegetation indicated higher concentra-
<br />tions in s?riog tban io fall. estLmates of total
<br />silver in the forest ecosystems indicate that the top
<br />60 cm of soil contain the bulk of the silver, about
<br />700 ti~es that in seeded snow and 2000 times that
<br />present in the foliase. Foliar washing indicated
<br />gignltlcant surrace aeposition. Small pilot tests
<br />of soil leaching and plant uptake processes indicated
<br />strong root adsorptio:1 of silver and inhibition of
<br />plant uptake.
<br />
<br />Althou h silver concentrations in snow were fouod to
<br />be too variable to show statistical y s gn cant
<br />differences between sites id the target area and
<br />others to the west and east or It, tne mean B1Lver
<br />concentration in snow on the target area is about
<br />twice as hi~h as the downwi~d mean, and three times
<br />as high as the upwind mean. Total seeding intensiLY
<br />(i.e., g AgI burned by all generators per month)
<br />was not a good predictor of silver concentration in
<br />sno~ at wolf Creek Pass. No slRnificant dlfference9
<br />In silver concentration of streamflow could be found
<br />between 5 streams including 3 on the' tar~et area and
<br />2 to the west (upwind) ot it. Nean annual SHyer
<br />concentration tor the tive streams increased from
<br />(0.1 I 0,3) x 10-10 g /ml in 1971 to (0.6 l 1.1) x
<br />10-10 g Iml in 1973. However, mean metric conversion
<br />dischar~e also increased from 2.9 ~J/sec (101 cfs) in
<br />1971 to 4.5 m3/sec (160 cis) in 1973. Although a
<br />r~latidnship obvio~sly exists between discharge and
<br />silver conce~tration in individual streams, no
<br />~cn~r~l predictive model could be developed between
<br />th~ twO ra~ar.~teLs.
<br />
<br />!~.,:\::ht!::l.1::i<.:,;~ '!\'.',~~l f-::'t t.he =.OVi..'met".:: vI: ,:.lvec frO;;1
<br />:-.ilv~r icdide [hcr-,ugh a soil column ',,'as J.evet(l'~'e'~
<br />dnd tested. It lndicates. :hat the silver concentra-
<br />tior. in the soil solution at any given time is best
<br />rep~esented by a kinetic-type reaction, coupled with
<br />~n insignificant equilibri~-type reaction. A high
<br />'~dsorp~ion ra:e congtant controls the initial
<br />reaction. but 10 to 30 percent of input silver con-
<br />~entration is ~ikely to move through the profile.
<br />
<br />:/_~~ Stdnhoff. H. '..'. and J. D. Ive~ (:::ds.). 1976, Ecob",j,cA.l in;actOo
<br />Juan }h..nmt.3ins. Col.orado. San J\\;:;n E..c.olohJ' P~0)t;:C::, Final R.~"t'\n,-.
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