New Mexico Geological Society Annual Spring Meeting — Abstracts

On the afternoon of January 7, 2008, an approximately 1300 km² region of Grant County, New Mexico, stretching northeast from White Signal, NM to Gila Cliff Dwellings, was subject to rainfall of milky-white color. Concerned residents, who collected samples from cisterns, rain gauges, and roof runoff, contacted area universities and state agencies to arrange for chemical analyses. Initial speculation targeted everything from mine tailings to Pacific Rim volcanic ash before wind back trajectories and MODIS satellite images indicated a possible regional source: Willcox Playa sediments. Analysis of major ions indicated elevated levels of calcium, sodium, sulfate and chloride, as would be expected in playa-type deposits. Ratification came from a National Atmospheric Deposition Program site at the Gila Cliff Dwellings which determined the calcium content of the rain to be within the top 1.0% of all data gathered by the network in a typical 5 year period. On February 5, 2008, the New Mexico Environment Department concluded that the event samples had similar chemistry to surface water samples obtained from the Lordsburg Playas in the early 1990’s and reassured the public regarding its possible toxicity. Further research will include trace element analysis of dissolved constituents in the rain water, analysis of the suspended (non-dissolved) materials and recently acquired dust samples. We present an overview of this event, the meteorological characteristics of the “milky rain” storm, and the geochemistry of the materials.