Evaluation of the Possible Sources and Controlling ... - ACS Publications

Page 1 of 29

Environmental Science & Technology

Evaluation of the possible sources and controlling factors of toxic metals/metalloids in the Florida Everglades and their potential risk of exposure Yanbin Li†，┴*, Zhiwei Duan‡，§, Guangliang Liu‡，§, Peter Kalla∆, Daniel Scheidt∆, Yong Cai‡，§* †

Key Laboratory of Marine Chemistry Theory and Technology, Ministry of

Education/Qingdao Collaborative Innovation Center of Marine Science and Technology, Ocean University of China, Qingdao, 266100, China ‡

Department of Chemistry & Biochemistry, Florida International University, Miami,

Florida, 33199, USA §

Southeast Environmental Research Center, Florida International University, Miami,

Florida, 33199, USA ∆

US Environmental Protection Agency, Region 4, Science and Ecosystem Support

Division, Athens, GA , 30605, USA ┴

College of Chemistry and Chemical Engineering, Ocean University of China,

Qingdao 266100, China

*Corresponding Author: Yanbin Li Tel: +86-0532-66786355 Fax: +86-0532--66782301 Email address: [email protected] Yong Cai Tel: 305-348-6210 Fax: 305-348-3772

1

ACS Paragon Plus Environment


Email address: [email protected]

TOC (Table of contents graphics)

2


Page 2 of 29

Page 3 of 29


ABSTRACT The Florida Everglades is an environmentally sensitive wetland ecosystem with a number of threatened and endangered fauna species susceptible to the deterioration of water quality. Several potential toxic metal sources exist in the Everglades, including farming, atmospheric deposition, and human activities in urban areas, causing concerns of potential metal exposure risks. However, little is known about the pollution status of toxic metals/metalloids of potential concern, except for Hg. In this study, 8 toxic metals/metalloids (Cd, Cr, Pb, Ni, Cu, Zn, As, Hg) in Everglades soils were investigated in both dry and wet seasons. Lead, Cr, As, Cu, Cd and Ni were identified to be above Florida SQGs (sediment quality guidelines) at a number of sampling sites, particularly for Pb, which had a similar level of potential risk to organisms as Hg. In addition, a method was developed for quantitative source identification and controlling factor elucidation of toxic metals/metalloids by introducing an index, enrichment factor (EF), in the conventional multiple regression analysis. EFs represent the effects of anthropogenic sources on metals/metalloids in soils. Multiple regression analysis showed that Cr and Ni were mainly controlled by anthropogenic loading, while soil characteristics, in particular natural organic matter (NOM), played a more important role for Hg, As, Cd, and Zn. NOM may control the distribution of these toxic metals/metalloids by affecting their mobility in soils. For Cu and Pb, the effects of EFs and environmental factors are comparable, suggesting the combined effects of loading and soil characteristics. This study is the first comprehensive research with a vast amount of sampling sites on the distribution and potential risks of toxic metals/metalloids in the Everglades. The finding suggests that, in addition to Hg, other metals/metalloids could potentially be an environmental problem as well in this wetland ecosystem.

3



INTRODUCTION The Florida Everglades is a subtropical wetland, located in the southern Florida, and it is one of the largest freshwater marshes in the world.1 With the fast development of economy in surrounding areas, the Everglades has been found to face increasing environmental problems, e.g., eutrophication (especially from phosphorus (P))2,

3

and mercury (Hg) contamination.4-7 The Everglades provides habitat for a

variety of threatened and endangered species of fauna, such as Florida panthers and alligators, as well as hundreds of thousands of wading birds. Water quality deterioration would pose a great threat to animals, humans, and even greatly alter the ecosystem. Metals/metalloids in aquatic environments originate from a variety of natural and anthropogenic sources.8 Several potential sources of metals/metalloids exist in the Everglades, including farming in the northern Everglades Agricultural Area (EAA), atmospheric deposition, road traffic, and human activities within the Everglades and surrounding urban areas, e.g., landfills, wastewater management, tourism (airboat), and airports. These potential sources could discharge a large amount of toxic metals/metalloids

8-13

into the Everglades, both historically and currently,

causing concerns of metal pollution in this ecosystem. In addition, as an environmentally sensitive wetland ecosystem with a number of habitats susceptible to the ecosystem alteration,14 even a small increase in the concentration of toxic metals/metalloids may result in severe impact on this ecosystem. In addition to extensive Hg research, a few studies15-18 have been conducted to study the pollution of some toxic metals/metalloids in the Everglades. However, these previous studies investigated only very limited portions of the Everglades, most of which were located in the canals of the Everglades and areas close to the Florida Bay. Findings from these limited data underscore the possibility that metal pollution, in

4


Page 4 of 29

Page 5 of 29


addition to Hg, could be a problem in this ecosystem. For example, toxic metals in sediment at 10 of 32 investigated sites of the south Florida freshwater canals were found to exceed the Florida Sediment Quality Assessment Guideline (SQAG).15 The deterioration of Everglades water quality resulting from Hg and P has been well documented in previous studies.4-6, 19-21 The concern for their adverse impact on this ecosystem led in part to the launch of the Comprehensive Everglades Restoration Plan (CERP), the largest environmental restoration project in the world, in 2000. However, little attention is paid to other metal pollutants that could also cause the impairment of Everglades water and the missing of this information would restrict the effect of CERP on improving the water quality. In consideration of the existence of a variety of potential sources and detection of elevated toxic metals/metalloids in some Everglades sediment, toxic metals/metalloids, in addition to Hg, could also be the pollutants of concern in this ecosystem. A more comprehensive study should therefore be conducted to investigate the status of metal pollution and evaluate their potential risks in the Everglades. Sediment is known to be the sink for many contaminants and it has been commonly used to evaluate the risk of metal pollution and identify the source of metals in aquatic environment.22-24 A common approach for conducting sediment risk assessment is to compare chemical data in sediment with Sediment Quality Guidelines (SQGs).25, 26 Identification of the major sources for toxic metals is critical, but usually difficult, due to the fact that 1) it is hard to differentiate the metals originating from natural and anthropogenic sources, and 2) quantitative measurement of toxic metal loading flux from all sources are not feasible. The most commonly used method for identifying the major source of toxic metals is to classify metals into several groups according to the spatial distribution of their concentrations using various statistical

5



methods, e.g., PCA (Principal Component Analysis) and CA (Cluster Analysis).23, 25, 27-29

Metals in the same group were considered to originate from similar sources.

Metals in the same group with the essential metals (e.g, Fe, Al, K) were commonly identified to be mainly from the natural sources, while the others were thought to be from anthropogenic sources.25 Although such method is often used with some success in source identification, it should be noted that soil metal pollution is due to not only natural and anthropogenic factors, but also sediment characteristics that can modulate the fate of these pollutants in soil. These sediment characteristics include natural organic matter (NOM), pH, oxidation/reduction potential (Eh), etc.8 These environmental factors can also determine the variation in the distribution of different metals. A feasible method for source identification should be able to quantify or estimate the contributions of anthropogenic loading and environmental factors to toxic metal levels. The major hypotheses of this study was that 1) pollution from certain metals/metalloids may pose a risk to wildlife in the Everglades and 2) the distribution of these pollutants are determined by both sources (natural/anthropogenic) and soil characteristics that can modulate the fate of these pollutants in soil. To test these hypotheses, 8 commonly concerned toxic metals/metalloids (cadmium (Cd), chromium (Cr), lead (Pb), nickel (Ni), copper (Cu), zinc (Zn), arsenic (As), and Hg) in Everglades soils were investigated both in dry and wet seasons. These metals/metalloids include not only the ones that are toxic and have no biological role (Pb, Hg, Cd, Cr, As, Ni), but also some micronutrients (Zn, Cu), which are toxic at high concentrations. The possible risk of these metals/metalloids was evaluated by comparing metal concentrations with the Florida SQGs. Conventional regression method was improved for quantitatively evaluating contributions of anthropogenic

6


Page 6 of 29

Page 7 of 29


loading and environmental factors to toxic metal distributions. The major improvement was the inclusion of an index, i.e. enrichment factor, representing the anthropogenic loading, in multiple regressions of metals/metalloids against possible controlling factors. Finally, the major sources and controlling factors for these toxic metals/metalloids in Everglades soils were identified and discussed.

MATERIALS AND METHODS Collection of Samples Sampling covered the entire freshwater Everglades, including Arthur R. Marshall Loxahatchee National Wildlife Refuge (LNWR), water conservation area 2 (WCA-2), WCA-3, and Everglades National Park (ENP). Although covered by water, the term ‘soil’, rather than sediment, is generally used in the literature for most areas of the Everglades except the canals. Soil samples were collected at 226 sites during the sampling campaign for the Regional Environmental Monitoring and Assessment Program Project (REMAP), Phase III in 2005.30 Surface soil samples of the top 10 cm were collected with a 3-inch diameter clean polycarbonate coring tube in both dry and wet seasons (May and November 2005, respectively) (Fig. 1a). Samples were preserved in refrigerator at 4 ºC before sample processing and analysis. The tracemetal clean techniques were followed during sample collection, shipment, and analysis.6 Determination of Metals/metalloids and other Ancillary Parameters Soil samples were dried according to EPA method 200.8 and digested using a method modified from EPA method 3050 B. Details for these procedures can be found in the Supporting Information. Concentrations of metals/metalloids in the digested solution were analyzed using an ELAN DRC-E inductively coupled plasma-mass

7



spectrometry system (ICP-MS).

Page 8 of 29

Standard quality assurance and control procedures

were followed during analysis of metals/metalloids in samples (see Supporting Information for the details).6 Wet soil samples, rather than oven dried samples were homogenized and analyzed for mercury to avoid the loss of Hg0 during heating. Mercury data used here has been published previously.6 They were also included in this study for the purpose of comparing with the other toxic metals/metalloids. Sources and controlling factors of Hg in the Everglades were well-documented in previous studies6, 7, 31, 32 and thus it is also useful for testing the reliability of the proposed method by including Hg in the analysis. Detailed analytical procedures for Hg and other ancillary parameters (dissolved organic carbon (DOC), PO4-, Cl-, and SO42- in porewater; pH, Eh, total organic carbon content (TOC), total nitrogen content, mineral content, ash free dry weight and water content of soil) can be found elsewhere.30 Calculation of Enrichment Factors Enrichment factor (EF) is a normalization technique widely used to estimate the relative contribution of metals/metalloids derived from natural sources and anthropogenic activities.23,

25, 33

EF was defined as the relative ratio of the

concentrations of target metal to a reference metal which is assumed to be hardly affected by anthropogenic sources (e.g., Al , Fe, Ca, Mg) in soil vs. that in the background. Due to the lack of background values in most studied areas, earth’s average crust or shale content was usually adopted as background value.

23, 25, 33

The

ratio of target metal to Al in shales 34 was used as the background value in this study. An EF value of 0.8), only TOC was included in the multiple regression to avoid the collinearity. In order to obtain approximate normally distributed residuals with constant variance,35 the logarithm was used for Cr, Ni, Cu, Cd while square root was used for Mn, As, Zn, Pb, and Hg. The magnitudes of the standardized coefficient (β) can be used to approximately evaluate the relative importance of biogeochemical parameters. Variance inflation factor (VIF) was calculated using SPSS to detect the possible existence of collinearity among the input variables.

RESULTS and DISCUSSION Distribution and Risk Assessment of Cd, Cr, Pb, Ni, Cu, Zn, As, and Hg in Florida Everglades Soils Concentrations of the 8 toxic metals/metalloids in Everglades soils are shown in Table 1. Most metals/metalloids (Cd, Ni, Cu, Zn, and As) exhibited a similar 9



concentration in dry and wet seasons, as illustrated by the Mann-Whitney test (p>0.1) and their small variations of their median values between the two seasons (Table 1). Chromium and Pb were much higher (pCd>Hg>As), indicating that these 4 toxic metals/metalloids could be severely (10≤EF

Evaluation of the Possible Sources and Controlling ... - ACS Publications

Recommend Documents