Comparison of U.S. Environmental Protection Agency (EPA) Soil Extraction Method 3051 with USGS Perchloric Acid Soil Extraction Method
Reclamation laboratories commonly run EPA method 3051 extraction, but interpretation of this method for toxic element evaluations in return flows is lacking. The primary question this research would address is:
* What trace element levels in the Method 3051 extraction procedure can be used as threshold levels indicating further soluble element testing is required?
Secondary questions this research will attempt to answer are:
* Are the elements not extracted by the 3051 method so biologically inactive that they can be discounted in trace element return flow analysis?
* How does the 3051 extraction procedure recovery for each element relate to the total element recoveries associated with the U.S. Geological Survey (USGS) perchloric acid digestion procedure?
Need and Benefit
Reclamation's soil and water quality experts are often called upon to evaluate the potential for toxic return flows originating in potential new projects as well as operating projects. The first step in the analysis has been to determine the total trace element concentration in irrigated or potentially irrigated soils of the project. The perchloric acid digestion procedure has traditionally been the first step in the screening process to determine potential toxicity hazards. This test is supported by an extensive data base of Western United States soils. Using this database, one can determine if the elements are present in uncommonly high, common, or uncommonly low concentrations when compared to other western soils. This digestion has been done by the USGS since Reclamation laboratories generally do not have the capability.
Reclamation project managers often prefer to use in- house laboratories or local contract laboratories for trace element testing. These labs provide Reclamation Soil Scientists with method 3051 data that are very hard to interpret. The test is more of a severe extraction than a true digestion procedure. Analysts report that depending on the nature of the soil sample solid undigested residue is present after the extraction. In the limited data collected to date (19 samples), comparing the digestion procedures has been inconclusive. The element recovery for different elements in different matrices is quite variable. For example, selenium recoveries in different soils have varied from 1 to 100 percent.
Needless to say our confidence in using the USGS data base of total element concentrations for method 3051 interpretive purposes is low. Therefore, we usually select a conservatively low threshold for the 3051 method. This research would permit a sound scientific basis for element hazard potential in return flows when the 3051 method is used.
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