Baseline Concentrations of 15 Trace Elements in Florida Surface Soils

The objective of this study was to establish baseline concentrations for 15 potentially toxic elements (Ag, As, Ba, Be, Cd, Cr, Cu, Hg, Mn, Mo, Ni, Pb, Sb, Se, and Zn) based on 448 representative Florida surface soils using microwave assisted HNOrHCI-HF digestion. Baseline concentrations of those elements were (mg kg-I): Ag 0.072.50, As 0.02-7.01, Ba 1.67-112, Be 0.04-4.15, Cd 6-0.33, Cr 0.89-80.7, Cu 0.22-21.9, Hg 0.00075--0.0396, Mo 0.13-6.76, Ni 1.70-48.5, Pb 0.6942.0, Sb 0.06-0.79, Se 0.01-1.11, and Zn 0.89-29.6, respectively. Upper baseline values for most elements corresponded with these reported in literature, except Ba, Hg, Mn, Sb, and Zn, which were 3 to 23 times lower. Soil properties, including pH, organic carbon (OC), particle size, cation-exchange capacity (CEC), available water, extractable base, extractable acidity, total Ca, Mg, P, K, Fe, and AI concentrations, were related to metal concentrations using factorial analysis. Eight factors were identified (total Fe and AI, CEC, pH, clay, OC, total Ni and Mo, total Sb and Pb, and total Hg) and accounted for 87% of the total variance, suggesting that metal concentrations were primarily controUed by soil compositions. Multiple regression of elemental concentrations against total Fe, total AI, clay, OC, CEC, and pH was significant for aU elements. Partial correlation coefficients indicated that total Fe and/or AI explained most of the variance for Mn, Ni, Ba, Be, Hg, As, Cd, Cr, Cu, Mo, Pb, and Zn concentrations. Most of the variance in Se was related to clay, whereas those of Ag and Sb related to clay and total AI. T HE PRESENCE of potentially toxic metals in landapplied waste materials is of public concern. Federal and state regulations list Ag, As, Ba, Be, Cd, Cr, Cu, Hg, Mn, Mo, Ni, Pb, Sb, Se, and Zn as potentially toxic elements (Florida Department of Environmental Protection, 1995; U.S. Environmental Protection Agency, 1996). The U.S. Environmental Protection Agency (1996) has established risk-based soil screening levels as a reference for site-specific cleanup for trace metals. However, no federal regulation specifies the maximum metal concentrations in non-hazardous wastes for land Soil and Water Sciences Dep., Univ. of Florida, Gainesville, FL 326110290. Approved for publication as the Florida Agricultural Experiment Station Journal Series No. R-06229. Received 19 Mar. 1998. *Corresponding author ([email protected]). Published in J. Environ. Qual. 28:1173-1181 (1999). application, except in the case of sewage sludge (U.S. Environmental Pt;otection Agency, 1995). Natural background concentrations of trace elements in soils where these materials are to be applied can be used as a reference (Kabata-Pendias and Pendias, 1992). Unless a reliable database on concentrations of trace metals in soils is available, inaccurate or unrealistically low mandatory guideline levels may be set by regulators (Davies, 1992; McGrath, 1986; Pierce et aI., 1982). Thus, it is important to establish background concentrations of trace metals for soils occurring within a region, and to document systematic variation in concentrations according to soil classes and properties. Background measurement represents natural elemental concentrations in soils without human influence (Kabata-Pendias et aI., 1992; Gough, 1993). This measurement depicts an idealized situation. Due to longrange transport of contaminants, truly pristine ecosystems may no longer exist, making establishing background concentrations a difficult task. For example, background levels for Pb are commonly elevated due to long-term usage of Pb-based gasoline and paint. Thus, it is almost impossible to find a surface soil sample completely free of Pb contamination (Fergusson, 1990). The term geochemical baseline concentration is often used to express an expected range of element concentrations around a mean in a normal sample medium. It is not generally a true background concentration and is defined as 95% of the expected range of background concentration (Kabata-Pendias et aI., 1992; Dudka, 1993; Gough, 1993). Based on lognormal distribution theory, the expected range can be expressed as the average of logarithms ::!:: 2 standard deviations (Dudka et aI., 1995). Since it is becoming more and more difficult to determine background levels of certain elements, the baseline values have been recognized as the only means to establish reliable worldwide elemental concentraAbbreviations: AM, arithmetic mean; ASD, arithmetic standard deviation; CEC, cation exchange capacity; FCSSP, the Florida Cooperative Soil Survey Program; GM, geometric mean; GSD, geometric standard deviation; OC, organic carbon. 1174 J. ENVIRON. QUAL., VOL. 28, JULY-AUGUST 1999 conducted jointly by the University of Florida Soil and Water Science Department and the USDA Natural Resources Conservation Service. Soil horizons were delineated and sampled using USDA soil survey conventions and procedures (Soil Survey Division Staff, 1993) as guidelines. Based on the mean coefficient of variations from a previous study (Ma et aI., 1997), a minimum of 214 soil samples are required to establish a statistically valid database for Florida soils (with 95% confidence level and 20% accepted error). In the present study, a total of 448 archived soil samples were selected to assure both taxonomic and geographic representation. The overall taxonomic representation was achieved by weighting the number of samples for each soil order by their estimated areal occurrences in Florida. The total mapped area is 11 265 532 ha and covers as much as 80% of the total land area of Florida. Most of the mapped areas in Florida are represented in the current study. Seven soil orders were identified from 51 counties in Florida and their approximate coverages are: Spodosols (28%), Entisols (22%), Ultisols (19%), Alfisols (14%), Histosols (10%), Mollisols (4%), and Inceptisols (3%). Based on the areal occurrence of each soil order, the samples included surface horizons from 122 Spadosols, 107 Entisols, 90 Ultisols, 60 Alfisols, 39 Histosols, 17 Mollisols, and 13 Inceptisols. Physical, chemical, and mineralogical analyses were previously determined through the FCSSP, include taxonomic class, morphological information and mineralogy (Sadek et aI., 1990). A statistical summary of selected properties for the 448 soils samples used in this study is presented in Table 1. MATERIAL AND MEmODS Sample Preparation and Trace Element Analysis All soil samples were air dried, ground, and passed through a 6O-mesh sieve. The screened samples were stored in polyethylene containers before analysis. Approximately 1 g of soil sample was weighed into a 120-mL teflon pressure digestion vessel; 9 mL of concentrated HN03, 4 mL of concentrated HF, and 1 mL of concentrated HCI were then added. Samples and reagents were well mixed, sealed, and digested in a CEM MDS-2000 digestion microwave oven (CEM, Matthews, NC) for 20 min at 120 psi. After cooling, 2 g of boric acid were added to the digested solution to neutralize excess HF. For Sample Selection and Characterization Histosols rich in organic matter, only 0.5 g of sample was used Soils used in this study were sampled and characterized as and 1.0 mL of HzOz was added prior to digestion. The fmal part of the Florida Cooperative Soil Survey Program (FCSSP) volume of the digested solution was 100 mL after filtration Table 1. Statistical summary of selected properties for the 448 soil samples used in this study. tions in natural materials (Gough et aI., 1988; KabataPendias and Pendias, 1992). Baseline concentrations of many elements can be obtained for soils of the USA (Shacklette and Boerngen, 1984; Gough et aI., 1988, 1994; Ames and Prych, 1995), China (Wei et aI., 1990), Great Britain (McGrath, 1986), and other European countries (Dudka, 1993). Researchers pointed out that baseline concentrations were a better measure of the variation in trace element concentrations than the observed ranges (i.e., ranges of background concentrations) since the distorting effects of a few high values were minimized by log-transformation of the data (Dudka et aI., 1995). They recommended the use of baseline concentrations as alternative criteria for assessing possible trace element contamination in soils (Gough et aI., 1994), or the use of the upper limit of the baseline concentration range to assess the background concentration with an acceptable degree of confidence (Dudka et aI., 1995). Unfortunately, existing data on baseline concentrations of trace elements in Florida soils are inadequate for determining the issue of how clean is clean for cleaning up contaminated soils and how dirty is dirty for land application of waste materials. Since only 40 soil samples were used in a previous study (Ma et aI., 1997), a larger soil sample pool and more systematic sampling strategy is necessary to establish a comprehensive database for baseline concentrations of potentially toxic elements in Florida soils. The present investigation was conducted to (i) establish baseline concentrations of the 15 potentially toxic trace elements in 448 representative Florida surface soil horizons; and (ii) investigate relationship among elements and between soil properties and elemental concentrations. Results of this research can be used as a reference in assessing anthropogenic vs. natural levels of trace elements in Florida soils. Cation exchange capacity pH-H2O pH-KO