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Methods Development

Objectives

The objective of the soil-sampling portion of this task was to familiarize project staff with the best available approaches for sample selection and sample collection. This was accomplished through colleague interactions with experts on this topic, application of in-house expertise on statistically valid sampling approaches, and acquisition of appropriate soil sampling equipment.

The ultimate goal of our selective-extraction research was a geochemical toolkit that can be tailored to meet the needs of particular studies. Several of our pilot tasks were in agricultural areas, and the impact of soil chemistry on plant physiology is of interest. For plants, selective extractions have been used to address both element deficiency and toxicity concerns. The chelating agent diethylenetriaminepentaacetic acid (DTPA) is a widely used extractant to determine bioavailability of elements to plants. However, this test has been found to be problematic when soil pH is less than about six. Neutral salt extractants (e.g., calcium chloride) have been successfully used to predict plant uptake of Cd from soils. Bioavailability to humans was another focus since soil can be inhaled or ingested. Human impacts were of concern in our tasks dealing with soils in the Great Valley, where dust derived from these soils migrates widely in the southwestern U.S. Human exposure was also a concern in our task in the northeastern U.S. where coal fly ash with elevated levels of potentially toxic elements such as arsenic and lead may be accumulating in soils of the region. Consequently, extractions that simulate gastric or pulmonary fluids were evaluated for this application. Selective extraction data, along with mineralogical characterization, on the very small size fractions of soil (less than 50 micrometers) also provided information on the amount and bioavailability of potentially toxic elements that may be present in airborne dusts. This provided linkages with the USGS projects currently studying the health effects of dust.

The objectives of the hydrologic portion of this task were three-fold. The first was to develop expertise in studying the geochemical evolution of soil waters under a variety of soil types and geographic settings. The second was to establish linkages with Water mission area scientists in our study areas to assure that we achieved the maximum impact of our respective research. All the sites for proposed Landscape Geochemistry tasks coincide either totally or partially with NAWQA study units. The third objective was to develop research protocols that identify important natural processes that link hydrologic and geochemical landscapes.

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