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<br />All samples reached DRl without an) apparent melt and were kept frozen lhere unlil <br />analyses. ^ high resolution. magnctic ~"Ctor Inductively Coupled Plasma Mass Spectrometer <br />(ICP.~1S) was uSt.-d in the anal)1ical method. The instrument is a Thermo-Finnigan E1emem2 <br />and is considered the state-of-the.art instrument tor elemental analyses according to Dr. Joc <br />\kConncll (pen;onal communication). director of the DRI laboratory \\hich performed the <br />analyses. Dr. McConnell provided the analyses results on June 6. 2003. and stated that. "From an <br />anal)1ical perspective. these numbers arc quile solid." <br /> <br />4.0 Results of Sihrer-in-Snow Samplin~ <br /> <br />The results of th< DR! analyses of silver-in-snow conccmmlions arc given in Table ::: <br />along \\ith UPS readings of north latitude. \\t;:st longitudc and elevation for each location <br />sampled. Sample dates and the standard deviation for each sample are also listed. Each snow <br />sample was subjected to multiple tests at the DRllahoratory to provide the standard deviations <br /> <br />Table 2 shows that t\\O of the three control station.", had Ag concentrations of 4.6 and 4.9 <br />ppl. Thn..--c of the ten targ.et locations had slightly lowcr valw..'S, between 2.6-3.1 ppt. This quite <br />limited set of five observations suggcsts a mean background value near 4 ppt.. similar to values <br />tound in other \tol."Stem states as discussed above. Howe....er. it should be understood that <br />background concentrations vary somewhat in spacc and time. n.'Sulting in a ran~e of natural A,g <br />levels. <br /> <br />Warburton et al. (1995a) had access to thousands of samples O\"er many years used to <br />establish a mean background of 2 ppt for Sierra :-.Jevada snov.faJls. which lhey noted to be. <br />"cxtremely low in this region." Their large sample allowed them to cakulah.' [hat only 5 percent <br />of the natural snowtall had silver le....e1s above 4 ppt which they defincd as thc "threshold" value. <br />This value was used to separate snow considered to have "seeding silver" from nonscl.-dcd <br />snowfalls. with the realization that such low Ag concentrations were likely associated with little <br />if any :x,""\.--t.ling impact. <br /> <br />Long (1984) reported a similarly detinl.-d but higher threshold ....alue of 11.6 ppt lor <br />nonsecdl.-d samplcs from the Tushar Mountains and Se\;er Plateau of sou them Utah. Except for an <br />obvious outlier. 13 samples from non$(.--cdcd periods ranged from 2-10 ppt on the Wasatch Range <br />of northern Utah according to Super and Huggins (19Q2). They also reported that onl)' 3 of 25 <br />(12 percent) approximately bi-weekly samples. including both scl-ded and nonst."Cded periods. <br />cxceeded 7 ppt on the Wa..o;.atch Plateau of central Utah. Thc combined Utah observations <br />suggest that Ag conccntrations should exceed about 10 ppl before the pre~nce of "SCtXling <br />sih'cr" is pn.'Sumed. It is sp.:culak-d that the higher Utah values may be related to more transport of <br />\...ind-blov.n dust (rom silver-rich soils. It is further speculated that thc L.tah "threshold" values near <br />10 ppt may be more typical of Colorado snov.13.1ls than the 4 ppt levels of the SiCl'nl Nevada. <br />Sampling of the nonseeded Colorado snowpack was too limited to pcnnit precise calculation ofthc <br />5 percent significance level for background Ag concentrations (threshold le\d). Howe....er. the <br />o....erall discussion of Section .2 strongly suggests that levels less than 10 ppt an: unlikely lo be <br />associated with much snowfall augmcntation. and only three targl.'1 samples in Table 2 exceed that <br />value. <br /> <br />1- <br />