Publication Date


Date of Final Oral Examination (Defense)


Type of Culminating Activity


Degree Title

Master of Science in Hydrologic Sciences



Major Advisor

Shawn Benner, Ph.D.


Matthew J. Kohn, Ph.D.


Hans P. Marshall, Ph.D.


James P. McNamara, Ph.D.


Glaciogenic cloud seeding increases the fraction of super cooled liquid water precipitating from a given storm. Orographic clouds tend to be inefficient at higher cloud temperatures due to the lack of active natural ice nuclei. Adding artificial ice nuclei active at temperatures greater than -12oC (where most natural ice nuclei are inactive) may result in an increase in snow precipitation, especially in orographic clouds. Silver iodide (AgI) is typically the artificial nucleating agent for winter orographic cloud seeding. Recent estimates suggest the addition of AgI to orographic storm clouds enhance precipitation by 3 - 15%. However, the National Research Council stated “the areas affected by AgI remains an open question”.

In this study, we seek to understand how well AgI is delivered to regions intended for cloud seeding in the central mountains of Idaho. To accomplish this, we develop and validate methods to detect sub-part-per-trillion silver concentrations in snow. These methods were specific to an ICP-MS laboratory not housed in a Class 100 Clean room. Unique laboratory layout and protocols are employed to reduce laboratory contamination potential. Using clean field methods, we sample a series of snow profiles within the target area of active cloud seeding. The results demonstrate the ability of these new methods to reproduce distinct elevated Ag concentrations over a small scale (0.25 km2) and at the basin scale (2,400 km2). A localized enrichment factor highlighted silver enrichments likely from AgI rather than from other local sources. This enrichment factor can delineate a seeding signature at sites far downwind from AgI sources, where Ag concentrations are only 2 parts per trillion above background levels. The localized enrichment factors consistently correspond to known cloud seeding events.

After developing reliable trace chemical snow methods in the 2015 water year, the 2016 water year applied these methods to assess Idaho Power’s overall AgI targeting in the Payette Basin. Improper targeting is regarded by some as the biggest obstacle to achieving statistically significant estimates of silver iodide (AgI) impacts on precipitation. To better understand AgI targeting, we (1) assessed AgI targeting effectiveness spatially for aerial and ground-based seeding, (2) quantified temporal variability of AgI targeting using real-time snow collection methods, and (3) determined the maximum distance from AgI sources at which seeding signatures in snow exist. We addressed these issues by analyzing more than 4,000 snow samples. Sample collection took place in the target zone and up to 180 km downwind of AgI sources using both real-time and traditional snow pit methods. We found silver enrichments in 90% of cases involving ground generators seeding, but in only 11% aircraft-only seeding events. We also assessed, for the first time, the maximum spatial extent of AgI enrichments (AgI > 3 ppt and an Enrichment Factor > 1) in snow using ultra-clean methods. All sites sampled beyond 80 km (n = 13) of the seeding source lacked detectable AgI signatures in snow.

We developed methods during the 2015 and 2016 water years to detect sub-ppt silver concentrations and validation of areas impacted by AgI. Next, we wanted to understand whether the AgI in snowpack would cause adverse environmental impacts. Based on the 2.8 ppb silver concentrations within measured within 5 m of ground generators, we concluded AgI is unlikely to harm known fauna. The toxicity of silver depends primarily on concentration, speciation, and bioavailability. The silver ion (Ag+) is a bioavailable and the most toxic form of silver known. Silver iodide is not soluble nor bioavailable, and secondary EPA standards are four orders of magnitude higher than concentrations found in all seeded snow samples. The silver ion is typically the dominant species in laboratory toxicity studies quantifying the toxicity of silver (where silver nitrate is used, a solution not found in natural environments). Modern cloud seeding programs disperse extremely small amounts of AgI annually (< 25 kg) over large areas (> 2,000 km2). Environmental sampling indicated no adverse effects on wildlife, nor silver accumulating at detectable levels above background in soils, streams, or aquatic species in seeded areas.


Available for download on Wednesday, August 21, 2019