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<br />2.3 Silver Concentrations in Seeded and Nonseeded Snowfalls <br /> <br />Review of the above projects provided information on the background or natural <br />(nonseeded) concentrations of silver to be expected in mountain snowfalls in the western United <br />States. Past project results give a basis for estimation of silver concentration enhancements to be <br />expected with successful T &D of the AgI seeding agent over target areas. Because of budget <br />limitations it was possible for the present study to obtain only three Colorado snowpack samples <br />in areas believed to be unseeded. In view of this small sample, prior background sampling from <br />other states should also be considered in estimating typical background concentrations in <br />Colorado. <br /> <br />Warburton and Young (1968) reported that seeded snowfall had silver concentrations <br />above the then current level of detection of about 10 ppt. Natural precipitation was found to <br />contain lower silver concentrations; that is, less than 10 ppt. <br /> <br />Warburton et a1. (1995a) established background levels of 2 ppt in both the Lake Tahoe- <br />Truckee River and Lake Almanor watersheds, with a standard deviation of 1 ppt. Fewer than 5 <br />percent of background samples contained silver at concentrations of at least 4 ppt, which they used <br />as their "threshold" value. Snow samples with concentrations higher than 4 ppt had a 9S percent <br />confidence of containing seeding silver. The authors referred to several thousands of snow samples <br />in the same region from 1967 through 1992, which showed the mean background concentration of <br />silver had remained steady near 2 ppt. <br /> <br />In the nonseeded central Utah samples reported by Super and Huggins (1992) all but one <br />possibly contaminated "outlier" sample had silver concentrations between 2-10 ppt, with the <br />distribution peaking at 5 ppt. The highest value observed during seeding was 26 ppt with all other <br />samples below 20 ppt, and usually below 10 ppt, so little could be said about values expected from <br />appropriate seeding. <br /> <br />Previously cited evidence from the Bridger Range of southwestern Montana showed <br />background concentrations below the then existing detection limit near 10 ppt <br /> <br />Based on the California, Utah and Montana sampling, silver-in-snow concentrations less <br />than 4-5 ppt are highly likely to be unaffected by Agl seeding, and levels below about 10 ppt are <br />likely to be unaffected. Even at 10 ppt it is unlikely that seeding contributed meaningful snowfall <br />amounts. <br /> <br />The BRE indicated that successful T &D from high altitude generators, with Agl outputs <br />near 30 g h-l, could be expected to produce seasonal concentrations between 30-100 ppt on the <br />target area about 5-10 miles downwind of the generators. Since less than half of the BRE storm <br />days were seeded, similar operational seeding would likely produce target area values sometimes <br />approaching 200 ppt, but usually in the 60-150 ppt range, based on the 14 seasonal samples. <br /> <br />12 <br /> <br />