Comparision between background concentration of arsenic in urban and non urban areas of florida

Comparision between background concentration of arsenic in urban and non urban areas of florida

Advances in Environmental Research 8 (2003) 137–146 1093-0191/03/$ - see front matter ᮊ 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S1093-0191(02)00138-7 Comparison between background concentrations of arsenic in urban and non-urban areas of Florida Tait Chirenje *, Lena Q. Ma , Ming Chen , Edward J. Zillioux a, aa b Soil and Water Science Department, University of Florida, Gainesville, FL 32611, USA a Florida Power and Light, 700 Universe Boulevard, Juno Beach, FL 33408, USA b Received 10 April 2002; received in revised form 5 November 2002; accepted 17 November 2002 Abstract Arsenic contamination is of great environmental concern due to its toxic effects as a carcinogen. Knowledge of arsenic background concentrations is important for land application of wastes and for making remediation decisions. The soil clean-up target level for arsenic in Florida (0.8 and 3.7 mg kg for residential and commercial areas, y1 respectively) lies within the range of both background and analytical quantification limits. The objective of this study was to compare arsenic distribution in urban and non-urban areas of Florida. Approximately 440 urban and 448 non- urban Florida soil samples were compared. For urban areas, soil samples were collected from three land-use classes (residential, commercial and public land) in two cities, Gainesville and Miami. For the non-urban areas, samples were collected from relatively undisturbed non-inhabited areas. Arsenic concentrations varied greatly in Gainesville, ranging from 0.21 to approximately 660 mg kg with a geometric mean (GM) of 0.40 mg kg , which were lower y1 y1 than Miami samples (ranging from 0.32 to 112 mg kg ; GMs2.81 mg kg ). Arsenic background concentrations y1 y1 in urban soils were significantly greater and showed greater variation than those from relatively undisturbed non- urban soils (GMs0.27 mg kg ) in general. y1 ᮊ 2003 Elsevier Science Ltd. All rights reserved. Keywords: Background concentration; Natural and anthropogenic; Arsenic; Florida 1. Introduction Arsenic occurs naturally in a wide range of minerals in soils. This, coupled with the once widespread use of arsenic pigments, insecticides, herbicides, and industrial wastes, makes it a common trace constituent of most soils. In fact, arsenic is the 20th most abundant element Abbreviations: 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; SCTL, soil clean-up target level. *Corresponding author. Tel.: q1-352-392-1951; fax: q1- 352-392-3902. E-mail address: tchirenj@ufl.edu (T. Chirenje). in the earth’s crust and is a major constituent of )245 different minerals with sulfur deposits being the most common culprits (Woolson, 1983). Arsenic concentra- tions are variable even in virgin components of the environment including soils, sediments, bodies of water, animals, and plants. Since arsenic is a known human carcinogen, its distribution and behavior in soils needs to be docu- mented to better understand its human exposure. The United States Environmental Protection Agency (USE- PA) has set the levels of arsenic allowed in oral intake, drinking water and breathing air at 0.0003 mg kg d , 0.050 mg l and 0.0043 mg m , respec- y1 y1 y1 y3 tively, (USEPA, 1998). The World Health Organization (WHO) has, in fact, recommended lowering the primary drinking water standard to 0.010 mg l . y1 138 T. Chirenje et al. / Advances in Environmental Research 8 (2003) 137–146 Arsenic was widely used in Florida during the early part of the 20th century as an insecticide to control disease-carrying ticks on cattle. Arsenic was also used, along with copper and chromium as a wood preservative (CCA, Grant and Dobbs, 1977). The most common present day uses of arsenic compounds include pesti- cides, wood preservatives and as growth promoters for poultry and pigs (O’Neill, 1990). Mining activities, smelters and fuel combustion also contribute significant amounts of arsenic to the environment. Arsenic distribution in Florida soils is likely to encompass at least three populations of concentrations, which may or may not be easily distinguishable. These include (1) natural background, (2) a diffuse anthro- pogenic influence, or ‘anthropogenic background,’ and (3) localized point sources. The relative proportion of each population varies between urban and non-urban areas. Therefore, knowing the distribution of arsenic in these three populations in both urban and non-urban soils aids our understanding of the impacts of human activity on natural concentrations of arsenic in soils (O’Neill, 1990). Significant land-use changes have occurred over the decades due to the migration of people to Florida in search of warmer climate and better economic oppor- tunities. Currently, 11% of the total land area in Florida (total area 14 258 000 ha) is considered urbanized (Nizeyimana et al., 2001) and this urbanization trend continues to increase. This is relatively greater than the national urbanized area of 3%. Unlike natural areas, arsenic concentrations in urban soils vary considerably over short intervals. Urban soils are complex and heterogeneous in their structure and composition (Craul, 1985; Davies et al., 1987). Human activity is the predominant active agent in the modifi- cation of these soils (Barrett, 1987). A fitting definition of an urban soil is, a soil material having a non- agricultural, usually manmade surface layer more than 50 cm thick, that has been produced by mixing or filling of the land surface in urban and suburban areas (Craul, 1985). There is a greater probability of historic anthropogenic contamination, vertical mixing during development, use of fill from different geologic areas, deposition andyor contributions from the use of pesti- cides or amendments from other sources in urban areas than non-urban areas (Craul, 1985; Thornton, 1987). Intensive human activity significantly alters the original native soils, making it difficult to describe urban soils using typical soil classification schemes. Arsenic concentrations in relatively undisturbed areas can still be attributed to purely geological factors with a few exceptions where non-point sources due to agri- cultural use of arsenic-containing pesticidesyherbicides and aerial deposition are significant. It may still be reasonable to consider the arsenic concentrations in these soils as the true natural arsenic background con- centrations. Areas that have had significant human activity (urban soils in general) are likely to exhibit what we may call ‘anthropogenic background concen- trations’ of arsenic. Background concentrations of arsenic in relatively undisturbed Florida soils are established and they vary from 0.01 to 61.1 mg kg , with a geometric mean y1 (GM) of 0.27 mg kg (Chen et al., 1999). Typical soil y1 arsenic concentrations range between 0.1 and 40 mg kg worldwide, with an arithmetic mean (AM) y1 concentration of 5–6 mg kg (Kabata-Pendias and y1 Pendias, 1992). A survey of soils in the US indicated that arsenic levels for undisturbed soils ranged from -0.1 to 97 mg kg with a GM arsenic concentration y1 of 5.2 mg kg (Shacklette and Boerngen, 1984). y1 This investigation was conducted to (i) compare arsenic background concentrations in urban and non- urban soils in Florida, and (ii) investigate the relation- ship between arsenic background concentrations and the extent of human activity and other soil properties. A medium-sized city (Gainesville) and a relatively large city (Miami, in terms of population and level of devel- opment) were used to represent urban areas. 2. Methodology Three different sets of samples (i) urban soils col- lected from a medium-sized city, Gainesville (popula- tion, 96 000; size, 93 km ), (ii) urban soils collected 2 from a relatively large city, Miami (population, 370 000; size, 91 km ), and (iii) natural soils from relatively 2 undisturbed non-urban soils, were used. 2.1. Soils from undisturbed areas The non-urban soils used in this study were sampled and characterized as a part of the Florida Cooperative Soil Survey Program conducted jointly by the University of Florida Soil and Water Science Department and the United States Department of Agriculture–Natural Resources Conservation Service (USDA–NRCS). Dur- ing sampling, great care was taken to select sites without known sources of anthropogenic contamination. Soil horizons were delineated and sampled using USDA guidelines (Soil Survey Division Staff, 1993). Based on the mean coefficient of variation from a previous study (Ma et al., 1997), a minimum of 214 soil samples were required to establish a statistically valid database for Florida soils (with 95% confidence level and 20% accepted variability between samples). However, 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 was 11 265 530 ha and covered 139T. Chirenje et al. / Advances in Environmental Research 8 (2003) 137–146 approximately 80% of Florida’s total land area. Seven soil orders were identified from 51 to 67 counties and their approximate coverage was: Spodosols (28%), Enti- sols (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 Spodosols, 107 Entisols, 90 Ultisols, 60 Alfisols, 39 Histosols, 17 Mollisols, and 13 Inceptisols. 2.2. Soils from urban areas (Gainesville and Miami) The Gainesville study was done as a pilot test to develop a comprehensive sampling protocol for other cities. The number of samples collected was based on soil heterogeneity and determined using the following Eq. (1): 2 wz x| Ns S=t yR (1) Ž. a y~ where N is the number of samples, S is the estimated standard deviation of the AM of all single values (in this case, S was calculated from the 25 samples collected from the University of Florida campus in Gainesville), t is the Student t value for a given confidence interval a (1.96 for the 95% confidence interval) and R is the accepted variability in mean estimation (usually 10– 20% depending on the scale and budget of project).A value of 20% was used and the minimum number of samples needed for Gainesville was determined to be 130. Three land-uses were selected for sampling. These were residential, commercial and public land. These were chosen because they cover the largest area in most urban settings. Differentiating the samples from these three land-use classes enabled us to test for differences among them. The number of categories selected from these three land-uses depends on the depth of detail required in the final sample. Five categories were chosen from the three land-uses in Gainesville (i.e. residential right-of-way, residential yards, public buildings, public parks and commercial areas). Forty surface samples (0–20 cm depth) were collected in May 2000 from each category, resulting in a total of 200 samples. One out of every 5 samples taken from each category was duplicated (for compari- son of reproducibility), bringing the total number of samples to 240. However, at least three cores were taken and composited at each of the remaining sites. The sites for sample collection were randomly selected within each category of land-use using a set of strict exclusion criteria to avoid any potentially contaminated areas. Chirenje et al. (2001) discuss both the randomi- zation process and the exclusion criteria in detail. Based on the pilot study, no significant difference was observed in arsenic concentrations between soils in residential-yard and residential-right-of-way, thus the latter was used to represent residential soil, reducing land-use categories to four for all subsequent studies. It was also later determined that the focus of such back- ground studies should produce a good estimate of the overall concentration distribution in each stratum with- out primarily focusing on the central tendency of each stratum. Therefore the precision target would be set on an upper percentile of the concentration distribution. Conover (1980) described a method for calculating the minimum number of samples needed for a given per- centile of a distribution to be exceeded by the maximum observed sample value with a given confidence level. For example, the sample size needed to assure exceed- ence of the upper 95th percentile with 95% confidence is 59. Based on this, 60 samples (0–10 cm depth) were collected in January, 2001 from four land-use categories in the Miami study (residential areas, commercial areas, public parks and public buildings). The change in depth was instituted after the revision of the sampling protocol and depths of 0–10, 10–30 and 30–60 cm were subse- quently sampled in Miami and other cities that followed. However, results from the top 10 cm only are discussed in this publication. These changes are discussed in detail by Chirenje et al. (2001). 2.3. Sample preparation and trace element analysis All soil samples were air dried, ground, and passed through a 2-mm sieve. The screened samples were stored in sealed polyethylene containers before analysis. The non-urban soils were digested using USEPA Meth- od 3051a whereas for the urban soils, USEPA Method 3051 was used. A simpler protocol, USEPA Method 3051, was instituted after the non-urban soils study, therefore the new method was used for the urban soils study. The soils were digested in a microwave digester using USEPA Method 3051 (or 3051a), which is com- parable to USEPA Method 3050, the hotplate digestion method (USEPA, 1996). In summary, 0.5–2 g of soil samples were weighed into 120-ml Teflon tubes and digested in 9 ml of concentrated HNO for Method 3 3051 (or 9 ml of concentrated HNO plus 3 ml of 3 concentrated HCl for Method 3051a) in a CEM MDS- 2000 microwave digester (Matthews, NC). For Histo- sols rich in organic matter, only 0.5 g of sample was used and 1.0 ml of H O was added prior to digestion. 22 The resulting solution was filtered through a Whatman No. 42 filter paper and made up to 100 ml. Arsenic concentrations in the digests (or digested samples) were determined on a SIMAA 6000 graphite furnace atomic absorption spectrophotometer (GFAAS, Perkin-Elmer, Norwalk, CT) using USEPA method 7060A (USEPA, 1995). 140 T. Chirenje et al. / Advances in Environmental Research 8 (2003) 137–146 Table 1 Summary statistics for soil arsenic concentrations in different land-uses in Gainesville and Miami (all concentrations in mg kg ) y1 Urban soils Non-urban soils Residential Commercial Public parks Public buildings Combined South Florida c North Florida c Miami ࠻ of samples 58 60 60 59 237 65 158 AM a 5.37 2.56 0.52 3.46 4.00 2.71 0.85 Median 3.47 2.11 3.29 2.39 2.60 0.24 0.20 GM b 3.72 1.93 3.49 2.49 2.80 0.44 0.21 GSD b 2.25 1.99 2.13 2.24 2.24 7.37 4.09 Gainesville ࠻ of samples 79 39 38 40 196 65 158 AM 0.68 1.19 0.52 0.57 0.73 2.71 0.85 Median 0.52 0.52 0.35 0.48 0.50 0.24 0.20 GM 0.46 0.63 0.23 0.34 0.40 0.44 0.21 GSD b 2.27 0.88 2.58 1.33 1.57 7.37 4.09 AM, arithmetic mean. a GM, geometric mean; GSD, geometric standard deviation. b South Florida includes Miami and North Florida includes Gainesville. c In addition, soil properties that have been shown to affect arsenic concentrations (pH, clay content, total organic carbon (OC), and total Fe and Al) were meas- ured using internationally accepted standard procedures (Page et al., 1982). The concentrations of Fe and Al were determined using a Thermo-Jerroll Ash 61E Induc- tively Coupled Plasma Atomic Emission Spectropho- tometer (ICP-AES, Spectro, Fitchburg, MA). 2.4. Data analyses All element concentrations are presented on a dry matter basis. Both AM and GM were used to describe the central tendency of the data. Baseline concentrations of arsenic were calculated using GMyGSD and 2 GM=GSD (upper baseline limit (UBL)) of the sam- 2 ples, which include 97.5% of the sample population (Dudka et al., 1995). Chen et al. (1999) provide details on definition and calculation of baseline concentrations. All statistical analyses were performed using SAS ᭨ (SAS Institute, 2000). The generalized linear model was used in preference to the analysis of variance procedure to account for the unequal number of samples within each class and quantile–quantile (QQ) plots were used to eliminate outliers from our dataset. These outliers represented samples with abnormally high arsenic concentrations that could not be attributed to the background levels. However, outliers were not eliminat- ed when distribution graphs were plotted. The Shapiro- Wilks test was used to test for normality. Because the distribution of arsenic concentrations was not normal (data not shown), the data were log-transformed before analysis to meet the assumption of normality required for the regression model. Spatial analyses were done using Spatial Analyst tools in Arcview Geographical Information Systems ᭨ software (ESRI, Redlands, CA). Pathfinder (Trimble, ᭨ Sunnyvale, CA) was used to geoprocess the Global Positioning System unit-logged positions and transform them into forms that could be read by Arcview . These ᭨ images were used to assess spatial distribution, and graphically display the analytical results from the study on a digital map (not shown). 3. Results and discussion It is important to note that most Florida soils are very sandy. This leads to low retention of trace elements in general, with important implications on regulatory con- centrations for many trace elements. Furthermore, the populations in this study only approached the normal distribution after log-transformation. Therefore, the 95% upper confidence limit (UCL) of the mean was calcu- lated using the H-statistic from Eq. (2): 22 0.5 wx UCL sexp(x q0.5s qs =H y ny1 )(2) 1ya y1ya where x is the AM of the log-transformed data, s is y the standard deviation of the log-transformed data, n is the number of samples, H and H are the H-statistic 1yaa from tables provided by Land (1975) for the UCL. The UCL depends on x , n and the chosen confidence limit y (Gilbert, 1987). Therefore, the calculated UBL, dis- cussed previously, was also based on the GM. 3.1. Comparison of soil arsenic concentrations between urban and non-urban areas Table 1 summarizes the mean concentrations and other relevant descriptive statistics for soil arsenic con- 141T. Chirenje et al. / Advances in Environmental Research 8 (2003) 137–146 Fig. 1. Soil arsenic concentration (raw data) distribution in (a) Gainesville (ns200), (b) Miami (ns240), and non-urban areas (ns448) in Florida. Table 2 The UCL, 95th percentile and percentage of soil samples with arsenic concentrations exceeding the SCTL (residential and com- mercial) in different areas in Florida Gainesville North Florida Miami South Florida AM 0.73 0.85 4.00 2.71 UCL a as0.05 0.99 2.14 4.30 11.6 UBL b as0.05 2.30 3.32 14.3 22.1 %)0.8 (mg kg ) c y1 29.4 16.5 94.6 43.8 %)3.7 (mg kg ) d y1 4.00 5.06 32.5 14.1 UCL : upper confidence limit of the mean at as0.05. a as0.05 UBL : upper baseline limit at as0.05. b as0.05 0.8 mg kg : the Florida SCTL for residential areas. c y1 3.7 mg kg : the Florida SCTL for commercial areas. d y1 centrations in non-urban areas surrounding the two cities and land-use categories analyzed within the two urban areas. The distributions of arsenic concentrations in the three separate classes, with the exception of values greater than 60 mg kg , are shown in Fig. 1. For the y1 non-urban soils, samples from South Florida (ns65) and North Florida (ns158) were used to compare with Miami and Gainesville samples, respectively. Arsenic concentrations from the urban areas of Miami and Gainesville were significantly greater than those from non-urban soils (as0.05) in the same regions (GM s0.44 vs. GM s2.80 and South Florida Miami GM s0.21 vs. GM s0.40 mg kg ; y1 North Florida Gainesville Table 1). As discussed earlier, non-urban soils have lesser anthropogenic disturbances than urban areas as they are not exposed to the same activities that often lead to increases in concentrations of trace elements in urban soils. In general, the differences in the distribution of arsenic in urban areas can be attributed to land-use, while those in non-urban areas can be attributed to soil forming factors. Based on the GM, the upper baseline limit (UBL , 95% of all data fall below this value) and as0.05 the 95% upper confidence level (UCL) of the GM for both urban and non-urban soils were calculated (Table 2). The combined UBL for all the land-use cate- as0.05 gories for Miami (14.3 mg kg ) was more than 6 y1 times greater than for Gainesville (2.3 mg kg ; Table y1 2). Both the UCL and UBL are dependent on the variation of the data set, hence these results demonstrate the greater variation in urban areas than non-urban areas. The UCL is not a very reliable measure of the confidence level of the mean for background studies because it is highly dependent on the number of sam- ples, approaching the mean as the number of samples increases. Table 1 demonstrates this point for both Gainesville and Miami. The UCL is generally useful for site-specific measurements of arsenic concentrations. Comparison of properties of soils from Gainesville with soils collected from non-urban areas close to the city and on the same parent material did not show any significant difference, except for pH. This is discussed in more detail in a later section. There was a significant difference between urban soils from Miami and non- urban soils from the surrounding areas. The non-urban areas surrounding Miami had significantly greater arsen- 142 T. Chirenje et al. / Advances in Environmental Research 8 (2003) 137–146 Table 3 Comparison of pH, OC and siltqclay content between urban and non-urban soils Non-urban Gainesville Miami pH 5.04 6.31 7.23 Siltqclay (%) a 10.6 9.30 28.0 OC (%) b 4.50 1.40 5.70 Siltqclay: represents the sum of silt and clay as a a percentage. Organic carbon. b ic background concentrations than both the non-urban and urban areas in Gainesville (Fig. 1; Table 1). These differences can be attributed to the different soils, which are a manifestation of different parent materials in these two regions. The comparison between public parks in each city and non-urban soils in areas surrounding the same city provided the best results because public parks in most urban areas have very little human disturbance. However, it must be noted that most of the parks in Miami had significant fill in them, unlike parks in Gainesville. Another comparison was also made between ‘dis- turbed’ and ‘undisturbed’ non-urban soils, where dis- turbed soils represented areas that had significant anthropogenic influence e.g. farmland, managed plan- tations etc. There was no significant difference between undisturbed and disturbed non-urban soils (GMs0.25 and 0.29 mg kg , respectively). This can be explained y1 by the fact that, although disturbed non-urban soils have some anthropogenic influence, these activities do not directly lead to contamination by point sources, as is the case in most urban areas. There were significant interactions between cities and land-use categories, hence comparisons of the combined land-use categories from the two cities from non-urban areas were not possible. Nonetheless, all four land-use classes in Miami had significantly greater arsenic con- centrations than the corresponding land-use classes in Gainesville (Table 1). In fact, the land-use category with the lowest arsenic concentration in Miami (public parks) had significantly greater arsenic concentrations than the land-use category with the highest arsenic concentration in Gainesville (commercial areas). Approximately a third of all samples collected in Miami had arsenic concentrations greater than the Florida soil clean-up target level (SCTL) for commercial areas, 3.7 mg kg . Gainesville, on the other hand, had approxi- y1 mately 29% samples above the Florida SCTL for resi- dential areas and only 4% were above the SCTL of 3.7 mg kg for commercial areas. Corresponding propor- y1 tions of samples falling above the SCTL for residential and commercial areas in both North and South Florida non-urban areas were lower than those of Gainesville and Miami, respectively (with the notable exception of North Florida for the commercial SCTL, Table 2). The differences in the arsenic concentrations between Gainesville and Miami soils can be explained by several factors. First, the depth of sampling for the top layer of soil was different between the two cities. The sampling depth for the analyzed samples in Gainesville was 0– 20 cm while that in Miami was 0–10 cm. This has important implications on the observed concentrations because arsenic concentrations generally decrease with depth in the top 30 cm of soil. However, we can still compare results from these two cities because a smaller subsample (ns30) that was reanalyzed in Miami showed a difference in arsenic concentration of less than 30% between 0–10 and 10–20 cm depths. This difference is relatively smaller in magnitude than the difference between the two cities (Gainesville and Miami). Comparisons of arsenic concentrations between the depths of 0–10 and 10–20 cm in Daytona Beach (ns64) also showed very small differences, possibly due the to the extensive mixing in the top 50 cm in urban soils (data not shown). Secondly, Gainesville soils have greater sand (quartz) content than Miami soils (91 vs. 72%, Table 3) which is expected to facilitate greater arsenic leaching or loss with runoff. The presence of significant amounts of carbonate in South Florida soils, 30–94% CaCO (Li, 2001) would 3 also help retain trace elements and hence such soils are expected to show greater accumulation of anthropogen- ically-added trace elements such as arsenic. The high background concentrations of soil arsenic observed in the urban areas in Florida are supported by observations in studies from other parts of the US and in other countries (Murphy and Aucott, 1998; Tiller, 1992; Tripathi et al., 1997). For example, Folkes and Kuehster (2001) observed very high baseline concentra- tions of arsenic in the suburban areas of Denver, Colorado (residential areas had GM ;6 mg kg while y1 urban areas in general had GM ;7mgkg ). However, y1 the rural background concentrations of arsenic in Colo- rado were also significantly greater than those of Florida soils (GMs3.7 vs. 0.4 mg kg , respectively). This y1 difference may be attributed to geologic factors, e.g. Colorado soils are derived from parent materials with higher concentrations of arsenic than parent materials from which Florida soils are derived. In New Jersey, Murphy and Aucott (1998) attributed the high arsenic concentrations in residential areas to historical land-use and former heavily sprayed orchards. The importance of historical land-use was also demon- strated by Tiller (1992) in a similar background study in Australian urban areas. Tiller (1992) avoided areas whose historical land-uses increase their probability of being contaminated. In spite of these efforts, arsenic concentration ranges of -1–8 mg kg were observed. y1 The relative contributions of both natural and anthro- 143T. Chirenje et al. / Advances in Environmental Research 8 (2003) 137–146 Fig. 2. QQ plots for (a) Gainesville (ns200), (b) Miami (ns 240), and (c) non-urban areas (ns448), for transformed data. pogenic activities in the distributions of soil arsenic concentrations were investigated in detail by Bak et al. (1997). Not surprisingly, they concluded that sludge application contributed the greatest amount of arsenic to the soil annually. This has important ramifications because land spreading of sludge is a common practice in many urban areas worldwide, and the regulations governing these applications often have loopholes that can be exploited by many unscrupulous waste managers. 3.2. Soil arsenic distribution characteristics The complexity of urban soils often leads to distinct patterns in arsenic distribution. The distinction between natural background, anthropogenic background, and contaminated arsenic concentrations is more discernible in urban areas than in non-urban areas (Fig. 2). There is a greater possibility of finding contaminated areas in urban environments due to greater human disturbance than in non-urban areas (Fig. 2a). On the other hand, non-urban areas are likely to exhibit mostly natural background concentrations of trace elements. The most critical shortcoming of these distribution plots is that not all soils that have high concentrations of arsenic have been exposed to contamination. Some soils naturally have high arsenic concentrations from their parent material. The determination of pollution can only be done if the parent material is known or if the historical land-use of the sites in question suggests contamination. Furthermore, some sites with sandy soils (e.g. most Gainesville sites) may be exposed to contam- ination, but the arsenic is not retained in the soil long enough to be picked up in studies like the current one. In such cases, the low concentration observed is not necessarily the natural background. Such a determina- tion can only be made if the groundwater at all sites is analyzed. However, analyzing groundwater may not provide the clues needed if enough time elapses between the pollution and sampling events. Censoring data on both ends (non-detects and outli- ers) can also have a significant impact on the shape and nature of the distributions. The plots of the Gainesville data demonstrate this point. Lower end censoring (non- detects) may yield a set of ‘equal’ concentrations leading to clumping on the lower (left tail) end of the curve. Furthermore, if the data are also censored at the high end before plotting the distributions, the ‘contam- inated sites’ disappear from the distribution. Nonethe- less, the slope of the curves gives us a clear indication of the variation in each sample stratum. 3.3. Correlation between soil arsenic concentrations with soil properties Correlation is widely used in trace element analyses (Bradford et al., 1996; Dudka et al., 1995; Lee et al., 1997) because of its ability to quantify how one factor changes in response to the other. Correlation analyses between elemental concentrations and soil properties (total Fe, total Al, pH, clay, OC, and cation exchange capacity (CEC)) of both the urban and non-urban soils were conducted in this study. The correlation between pH and arsenic concentrations in urban areas was both very low statistically insignificant. There was higher correlation between clay content and arsenic concentration in the non-urban soils than urban soils (Table 4, significant at as0.05). This is consistent with previously published data by Ma et al. (1997). They reported that arsenic concentrations were strongly correlated with clay content in 40 Florida surface soils. Higher correlation was also reported between clay content and concentrations of arsenic in Canadian (Mermut et al., 1996), Polish (Dudka, 1993) and Dutch soils (Forstner, 1995; Edelman and de Bruin, 1986) suggesting that clay content is important in controlling the level and distribution of trace metal concentrations in soils. A study conducted in both urban and non-urban areas in Denmark and Holland showed 144 T. Chirenje et al. / Advances in Environmental Research 8 (2003) 137–146 Table 4 Correlation coefficients of arsenic concentrations with soil a properties in urban and non-urban areas Element pH Clay OC Total Fe Total Al Non-urban 0.14 0.33* 0.58* 0.66* 0.60* Gainesville 0.10 0.01 y0.05 y0.04 0.02 Miami 0.09 0.04 y0.08 -0.09 y0.06 Correlations coefficients denoted with ‘*’ are significant at a as0.05. low correlation coefficients for soil texture although clay soils consistently had higher arsenic concentration than sandy soils in the non-urban areas (Bak et al., 1997). The investigation concluded that arsenic concen- trations in studied areas were more sensitive to soil factors (e.g. clay content) than anthropogenic activities. Anthropogenic activities in urban areas, especially the use of fill, tend to interfere with the relationship between soil forming factors and trace element concentrations. In contrast to urban soils, there was significant cor- relation between OC and arsenic concentrations in non- urban areas (Table 4). Humic substances in organic soils (peat) can serve as strong reducing and complexing agents and influence the processes controlling mobili- zation of many toxic elements including arsenic (Gough et al., 1996). Similar to the results from the non-urban soils, other researchers have reported strong positive correlation between trace element concentrations and OC and the siltqclay content of the soil (Aloupi and Angelidis, 2001; Chirenje, 2000; Wilcke et al., 1998). There was significant (as0.05) correlation between arsenic concentrations and total Fe and Al concentra- tions in non-urban soils (Table 4). Both Fe and Al react with the arsenate to form stable, immobile compounds in the soil, and oxides and hydroxides of both elements also provide reactive surfaces on which arsenic can be adsorbed. However, the same trend was not observed in urban soils, possibly due to the increased use of fill. Dudka (1993) found good correlation between concen- trations of arsenic and concentrations of Al and Fe in surface soils of Poland. He concluded that levels of most elements were mainly controlled by the minerals (Fe and Al oxides) present in the soils (Dudka, 1992). Total Fe and Al concentrations (2300 and 2200 mg kg ) in Florida soil are 16–32 times lower than y1 the average concentrations reported for other soils (38 000 and 71 000 mg kg ; Lindsay, 1979). Nonethe- y1 less, total Fe and Al, even at such low concentrations, are significant in controlling metal concentrations in Florida soils. Multiple regression of concentrations of trace elemen- ts against clay, OC, pH, CEC, and total concentrations of Al and Fe supported the relationships of trace elements with important soil properties (data not shown). Regressions of log-transformed concentrations of arsenic against six soil variables explained between 9 and 65% of the total variance. However, no such correlation was observed in urban areas. In the non- urban soils, partial correlation analyses confirmed that total Fe and total Al were the two major variables controlling concentrations and distributions of arsenic in Florida surface soils as demonstrated previously using simple correlation analysis. 4. Conclusions This study compared the distribution of arsenic in soils from urban and non-urban areas. In general, arsenic concentrations in urban areas were higher than those in non-urban areas. Arsenic concentrations varied signifi- cantly with land-use in Miami but only parks had lower arsenic concentration than the other land-uses in Gaines- ville. Soil arsenic concentrations in non-urban areas showed significant correlation with natural soil proper- ties (clay content, OC, and total Fe and Al) because they are exposed to relatively lower disturbance than urban soils. Knowledge of classical pedology can easily be employed to predict arsenic distribution in these areas. On the other hand, land-use categories can serve as good indicators of arsenic distribution in urban areas. More research is needed to better understand the tem- poral variation of arsenic in different compartments in both urban and non-urban areas so that better decisions can be made about land application of waste and remediation of possibly contaminated soils. 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