|Deverel, Steven - HYDROFOCUS, INC|
|Fujii, Roger - U.S. GEOLOGICAL SURVEY|
Submitted to: Book Chapter
Publication Type: Book / Chapter
Publication Acceptance Date: December 1, 2011
Publication Date: January 1, 2012
Citation: Deverel, S.J., Goldberg, S.R., Fujii, R. 2012. Chemistry of trace elements in soils and groundwater. In: Wallender, W.W. and Tanji, K.K., editors. ASCE Manual and Reports on Engineering Practice No. 71. Agricultural Salinity Assessment and Management (2nd Edition). ASCE, Reston, VA. Chapter 4. p. 89-137. Technical Abstract: We present information about sources of processes that affect trace elements in soils and groundwater; precipitation and dissolution, surface interactions and absorption and oxidation-reduction reactions. For each element or group of elements, we provide a review of mode of occurrence, sources and relevant biogeochemical processes affecting mobility and plant health and trace-element uptake. Lastly, we present a discussion of considerations of evaluation of pollution potential in soils. Salient general conclusions are as follows. Because of their tendency to form oxy-anions, the transition metals, metalloids and non-metals (arsenic, boron, chromium selenium, vanadium) are frequently mobile in soils and groundwater. Mobility depends primarily on redox potential and pH and secondarily on surface interactions. Depending on geologic sources, high concentrations of these elements are often observed in arid and semi-arid areas because of the typically oxidized and alkaline conditions. The formally-named heavy metals (cadmium, copper, lead, nickel and zinc) are generally unlikely to be mobile in soils due to their strong potential to adsorb to soil minerals, soil organic matter and precipitate as carbonates and phosphates. There is typically little potential for leaching to groundwater at near neutral pH and above. Long-term application of sewage sludge, manures and pesticides can cause high soil concentrations that may affect plant productivity and affect the soil microbial community. Low pH, formation of soluble organic complexes and preferential flow can result in increased mobility and leaching to deeper soil horizons. Mercury is an important contaminant in many locations throughout the world. It is subject to a complex series of biogeochemical processes which affect its mobility and toxicity. Evaluation of the pollution potential of soils requires evaluation of multiple factors and criteria. These include: 1) spatial variability of soil trace element concentrations and the processes affecting concentrations; 2) background concentrations unaffected by anthropogenic activity; and 3) the forms of constituents of concern. Considerations are as follows. To effectively compare soil trace element concentrations with background levels requires collection of sufficient numbers of samples to effectively represent measures of central tendency and variance which necessitates analysis of spatial variability relative to possible sources. Use of multivariate statistical methods such as factor and principle component analysis can provide insight into geochemical associations and processes affecting distribution and mobility and provide a framework for analysing large amounts of trace-element data. Geographic Information Systems can be an effective tool for regional and subregional assessment of the distribution, and mobility of soil and groundwater trace element concentrations and the potential for movement of trace elements to groundwater. Soil extractants can provide useful information about how trace elements are partitioned in soils especially relative to plant uptake. However, for some elements such as arsenic, these selective extraction techniques are ineffective. Trace-element adsorption in soils is a key factor for understanding mobility of many trace elements and can be quantified in different ways. These include surface complexation, constant capacitance, diffuse layer and triple layer models which have been applied successfully in a variety of systems and to numerous trace elements.