Published online 3 January 2006
Published in J Environ Qual 35:93-100 (2006)
DOI: 10.2134/jeq2005.0221
© 2006 American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America
677 S. Segoe Rd., Madison, WI 53711 USA
TECHNICAL REPORTS
Organic Compounds in the Environment
Temporal and Spatial Distributions of Sediment Total Organic Carbon in an Estuary River
Y. Ouyanga,*,
J. E. Zhangb and
L.-T. Ouc
a Department of Water Resources, St. Johns River Water Management District, P.O. Box 1429, Palatka, FL 32178-1429
b Department of Ecology, College of Agriculture, South China Agricultural University, Wushan Road, Tianhe, Guangzhou, China
c Soil and Water Science Department, University of Florida, Gainesville, FL 32611-0290
* Corresponding author (youyang{at}sjrwmd.com)
Received for publication June 3, 2005.
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ABSTRACT
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Understanding temporal and spatial distributions of naturally occurring total organic carbon (TOC) in sediments is critical because TOC is an important feature of surface water quality. This study investigated temporal and spatial distributions of sediment TOC and its relationships to sediment contaminants in the Cedar and Ortega Rivers, Florida, USA, using three-dimensional kriging analysis and field measurement. Analysis of field data showed that large temporal changes in sediment TOC concentrations occurred in the rivers, which reflected changes in the characteristics and magnitude of inputs into the rivers during approximately the last 100 yr. The average concentration of TOC in sediments from the Cedar and Ortega Rivers was 12.7% with a maximum of 22.6% and a minimum of 2.3%. In general, more TOC accumulated at the upper 1.0 m of the sediment in the southern part of the Ortega River although the TOC sedimentation varied with locations and depths. In contrast, high concentrations of sediment contaminants, that is, total polycyclic aromatic hydrocarbons (PAHs) and polychlorinated biphenyls (PCBs), were found in sediments from the Cedar River. There was no correlation between TOC and PAHs or PCBs in these river sediments. This finding is in contradiction to some other studies which reported that the sorption of hydrocarbons is highly related to the organic matter content of sediments. This discrepancy occurred because of the differences in TOC and hydrocarbon source input locations. It was found that more TOC loaded into the southern part of the Ortega River, while almost all of the hydrocarbons entered into the Cedar River. This study suggested that the locations of their input sources as well as the land use patterns should also be considered when relating hydrocarbons to sediment TOC.
Abbreviations: PAH, polycyclic aromatic hydrocarbon • PCB, polychlorinated biphenyl • SJRWMD, St. Johns River Water Management District • TOC, total organic carbon
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INTRODUCTION
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NATURALLY occurring total organic carbon (TOC) in river sediments is a key component in a number of chemical, physical, and biological processes. It contributes significantly to acidity of natural waters through organic acids (Eshleman and Hemond, 1985; Kerekes et al., 1986), biological activity through light absorption and carbon metabolism, and water chemistry through the complexation and mobilization of metals and organic pollutants. By forming organic complexes, TOC can influence nutrient availability and control the solubility and toxicity of contaminants (Moore, 1989).
In general, TOC consists of dissolved organic carbon (DOC) and particulate organic carbon (POC). Dissolved organic C is known to be a strong complexing agent for many toxic metals such as iron, copper, aluminum, zinc, and mercury. Dissolved organic C can also increase the weathering rate of minerals and increase the solubility and thus the mobility and transport of many metals and organic contaminants (Drever, 1988). It has been reported that POC is a feature of some rivers and its seaward plume (Trefry et al., 1992). The flow of POC can act as a carrier to transport contaminants along rivers. Decomposition of POC associated with contaminants in water columns and sediments plays an important role in river water quality.
The primary source of naturally occurring TOC is plant material. Under natural conditions the leaves, stems, and roots of trees, shrubs, grasses, and other native plants supply large quantities of organic residues to aquatic systems. As these residues are decomposed and digested by soil organisms, they become organic matter at the soil surface and migrate to the underlying horizons by infiltration or load into rivers. Animals are usually considered secondary sources of TOC. As they attack the original plant tissues, they contribute waste products and leave their own bodies as their life cycles are consummated. Certain forms of animal life, especially the earthworms, centipedes, and ants, also play an important role in the translocation of plant residues (Brady, 1984).
Natural processes and human activities have resulted in elevated content of TOC in soils, sediments, and streams. These include diverse inputs from throughfall, stemflow, inappropriate animal waste applications and disposals, forest clear cuttings, agricultural practices, and changes in land uses (Moore and Jackson, 1989). In recent years several studies have been devoted to measuring sediment TOC contents in rivers and lakes. Feng et al. (1998) reported that TOC contents in sediments of the Hudson River in New York, USA, vary from 1.06 to 2.73%. Mueller et al. (1982) reported that approximately half of the TOC input into the Hudson River was due to wastewaters. These authors further argued that the variations of TOC contents could also be caused by differences in particle grain size and lability of carbon with respect to microbial degradation. Jia and Peng (2003) found that the sediment TOC contents in Pearl River estuary of southern China range from 0.61 to 1.54%, which is very close to that of the Hudson River. In contrast, a study on Lake Apopka of central Florida, USA, by Silliman and Schelske (2003) showed that the sediment TOC content ranges from 33 to 37%, and originated mainly from the primary producer community. These studies have provided good insights into the TOC contents in salt and freshwater sediments. However, the temporal and spatial distributions of TOC in estuary river systems associated with their impacts on sediment contaminants are still poorly understood. Recently, a field project was performed to sample and analyze TOC contents from sediments in the Cedar and Ortega Rivers in Florida, USA (Durell et al., 2004). Although this project provided useful field data on sediment TOC contents in the rivers, the temporal and spatial distributions of TOC and their impacts on hydrocarbons have not yet been thoroughly investigated (Ouyang et al., 2002).
The goal of this study was to investigate the temporal and spatial distributions of TOC in sediments from the Cedar and Ortega Rivers using kriging analysis and to determine the relationships between TOC and sediment contaminants using field data. The specific objectives were to (i) examine temporal variations of TOC with sediment depth, (ii) evaluate spatial trends of TOC in the river sediments, and (iii) determine relationships of TOC with sediment hydrocarbons. The hydrocarbons used in this study were total polycyclic aromatic hydrocarbons (PAHs) and polychlorinated biphenyls (PCBs).
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MATERIALS AND METHODS
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Study Site and Sample Collection and Analysis
The Cedar and Ortega Rivers are the estuarine environments located in the St. Johns River Water Management District (SJRWMD) in Florida, USA (Fig. 1
). It has been reported that about 4.1 to 5.6 m3 s–1 of domestic and industrial wastewater, mainly from the City of Jacksonville, Florida, is discharged into the Cedar and Ortega Rivers and enters the St. Johns River (Ouyang et al., 2002). The land uses around the river area include agriculture (1.1%), barren lands (0.4%), rangeland (3.8%), transportation (4.8%), upland forest (8.3%), urban and built up (66%), water (5.3%), and wetlands (10.1%). Several industrial complexes consisting of petrochemical, electrical, and plastic industries are located in the area. Recent studies found that this area was contaminated with hydrocarbons, pesticides, and heavy metals (Durell et al., 2004; Ouyang et al., 2002). Variable sampling depth intervals, ranging from 0 to 1.88 m, were used to collect grab and core sediment samples from 58 locations along the rivers during the period between February 1998 and February 1999 by the SJRWMD staff (Fig. 1). A Global Position System was employed to identify each location following the USEPA's Estuarine Monitoring and Assessment Program random sampling protocol (Summers et al., 1991).
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Fig. 1. Location of the study area showing the Cedar and Ortega Rivers, Florida, USA, and sampling points (•). The selected locations are used to plot total organic carbon (TOC) variations with sediment depth.
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Surface sediment grab samples were obtained using stainless steel petite Ponar dredges. Subsurface undisturbed sediment samples were collected using piston-core samplers. After collections, the samples were stored at 4°C and shipped on ice to Battelle Marine Sciences Laboratory located in Sequim, Washington, USA, for analyses. An elaborate description of sampling and sample analysis procedures can be found in Durell et al. (2004).
Following Method 9060 (USEPA SW-846), the sediment samples were dried at 70°C, ground to a powder, and then treated with 10% hydrochloric acid. After effervescing was completed, more HCl was added. This process of incremental addition of acid continued until introduction of an additional aliquot caused no effervescing. After acid treatment, the sample was dried at 70°C and placed in a desiccator to cool. A 5- to 30-mg aliquot of the ground dry sediment sample was weighed to the nearest milligram and placed in a carbon-free crucible. Total organic carbon was determined using a high-temperature furnace to combust the material to carbon dioxide in an oxygen atmosphere. From the reaction chamber the sample combustion gases were carried through a Balston (Haverhill, MA) water vapor filter to two reaction filters. The first filter contained magnesium perchlorate, which removes any remaining water vapor. The second filter contained acid dichromate on Silocel and manganese dioxide; the manganese dioxide absorbs any sulfur oxides present and the acid dichromate oxidizes and removes NOx products that would otherwise interfere with the analysis. The gases then passed to a coulometer that measured the CO2 by coulometric titration. Detailed descriptions of sample analytical procedures can be found in Durell et al. (2004).
Kriging Analysis
Spatial distribution of TOC in sediments from the Cedar and Ortega Rivers was determined by three-dimensional kriging estimation using the ISATIS model (Bleines et al., 2000) and field data with variable depths obtained by SJRWMD (Durell et al., 2004). The ISATIS model is a geostatistical tool that includes an extensive range of geostatistical methods combined with an efficient data management system. A detailed discussion of the kriging technique is published elsewhere (Cooper and Istok, 1988; American Society of Chemical Engineers, 1989; Rouhani et al., 1996; Goovaerts, 1999; Triantafilis et al., 2001).
The kriging steps used in this study were: preliminary data analysis, data structural analysis, and kriging estimation, which were similar to our previously study on spatial distribution of DDT [1,1,1-trichloro-2,2-bis(p-chlorophenyl) ethane] in river sediments (Ouyang et al., 2003). A histogram plot of the data showed that TOC was normally distributed and therefore a transformation of the data was not performed. Table 1 lists the kriging parameters, while Fig. 2
shows the experimental variogram and variogram model used in this study.
A cross validation shows that the kriging standardized error for TOC is –0.06 g kg–1, which indicates the adequacy of the model and of the parameters used in the kriging estimations. Cross validation is a general procedure that checks the compatibility between a set of data and their structural model (American Society for Testing and Materials, 1996). It is a simple way to compare various assumptions either about the model (e.g., the type of variogram and its parameters, the size of the kriging neighborhood) or about the data (e.g., values that do not fit their neighborhood like outliers or the point-wise anomalies). A cross validation is robust when its standardized error lies between –2.5 and 2.5 (Bleines et al., 2000).
Three-dimensional kriging estimations of TOC distribution were performed using the ISATIS model. The kriging domain used in this study was 5000 x 6000 x 2 m, which encompassed the entire Cedar and Ortega Rivers. The kriging grid intervals were 
x = 100 m, y = 100 m, and z = 0.1 m. Table 1 lists the major kriging parameters used in this study. In this study, the kriging neighborhood search radius used was about 200 m since the maximum river width is in this range, which was much less than the modeled variogram range (1250 m). As discussed above, the range is the maximum distance over which the data exhibit spatial correlation. The cross validation standardized error of –0.06 g kg–1 further confirmed that the kriging estimations were acceptable.
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RESULTS AND DISCUSSION
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Temporal Distribution of Total Organic Carbon
Temporal variations in TOC concentrations from sediments for three time periods at four selected locations in Cedar and Ortega Rivers are shown in Fig. 3
. This figure was constructed based on our field TOC data and the sediment accumulation rate obtained at the same location by Alexander et al. (1993). These authors found that the sediment accumulation rate in this river segment during the last 70 yr is 1.1 cm yr–1. By dividing a sediment depth with the sedimentation rate, one can determine the time when TOC accumulates. For example, if the sediment depth is 50 cm, the time for the sediment accumulation is about 46 yr (i.e., 50 cm divided by 1.1 cm yr–1 = approximately 46 yr). The geological positions for those four selected locations are given in Fig. 1.
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Fig. 3. Concentration of total organic carbon (TOC) in sediments from three time periods at four selected locations in Cedar and Ortega Rivers. The selected locations can be identified from Fig. 1.
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Large temporal changes in sediment TOC concentrations were observed in the rivers, which reflected changes in characteristics and magnitude of inputs into the rivers during approximately the last 100 yr. In general, a decline in TOC contents was evident at the selected Locations 2, 3, and 4 (i.e., the middle and northern parts of the Ortega River and the Cedar River) in the mid 1990s as compared to 1971 or early 1980s. For example, the concentration of TOC was 151 g kg–1 in 1995 but 164 g kg–1 in 1971 at the selected Location 2; 97 g kg–1 in 1995 but 109 g kg–1 in 1983 at the selected Location 3; and 120 g kg–1 in 1994 but 134 g kg–1 in 1971 at the selected Location 4. In contrast, there were no changes in TOC content (140 g kg–1) from 1979 to 1995 at the selected Location 1 (i.e., the southern Ortega River). Results imply that the TOC loadings into the middle and northern parts of the Ortega River and the Cedar River were reduced and into the southern Ortega River were stable during the last 10 to 20 yr. The decline in TOC content in sediments was attributed to the increasing residential and commercial land uses, whereas a stable TOC content in sediments from the southern Ortega River was due to fewer land use changes and more areas of hardwood forested wetlands during the last two decades.
The concentrations of TOC decreased from 181 g kg–1 in 1923 to 140 g kg–1 in 1979 at the selected Location 1 (the southern Ortega River) although the exact reasons for such decrease remain unknown. However, the opposite result was obtained for the rest of the three selected locations at about the same time interval. That is, the concentrations of TOC increased from early 1900s to 1971 or early 1980s. This indicates that inputs of TOC into the rivers were at the peaks during the period of 1970 to 1980 from the Cedar River and the middle and northern Ortega River watersheds. This could be the result of increasing urban buildup with more forest cuttings and loadings into the rivers during that timeframe.
The average concentration of TOC in sediments from the entire Cedar and Ortega Rivers was 12.7% with a maximum of 22.6% and a minimum of 2.3%. This concentration was higher than that (1.06 to 2.73%) in the Hudson River, New York, USA (Feng et al., 1998), but was less than that (33 to 37%) in Lake Apopka of central Florida (Silliman and Schelske, 2003). It is apparent that the Cedar and Ortega Rivers watershed is a complex environment where both natural and anthropogenic processes influence the characteristics and distributions of sediments.
Spatial Distribution of Total Organic Carbon
Spatial distribution of TOC in sediments from the Cedar and Ortega Rivers at sediment depths of 0.1, 0.5, 1.0, and 1.5 m, estimated by kriging analysis, is shown in Fig. 4
. This figure shows that more TOC accumulated in the southern part of the Ortega River at the upper 1.0 m of the sediments. For instance, the TOC content was about 200 g kg–1 in the southern part of the Ortega River at x = 431.5 km and y = 3347 km at the depth of 0.5 m, but it was only 100 g kg–1 in the northern part of the Ortega River at x = 432 km and y = 3350 km at the same depth. The former was twice as large as the latter. Results suggested that a high TOC loading rate occurred in the southern part of the river. This could be a result of different land uses. While the land uses in the Cedar River and in the northern end of the Ortega River are mostly residential and commercial lands, there is about 6.1% more hardwood forested wetlands on the southern part of the Ortega River. Although the specific sources of TOC remain unknown historically, residential and commercial lands normally contribute less TOC sources than the hardwood forested wetlands.
Figure 4 further reveals that TOC content in the Cedar River sediments at the sediment depth of 1.0 m was higher than that of the other three depths (i.e., 0.1, 0.5, and 1.5 m). For example, the TOC content was about 150 g kg–1 at the depth of 1.0 m at x = 429.5 km and y = 3350 km in the Cedar River area, but it was about 100 g kg–1 at the depths of 0.1, 0.5, and 1.5 m at the same location. However, a relatively uniform distribution of TOC contents was found at z = 1.5 m for the entire Cedar and Ortega Rivers (Fig. 4). Result suggested that inputs of TOC from the watershed into the rivers varied with locations and sediment depths.
Little effort has been devoted to investigating three-dimensional distribution of TOC contents in the river sediments. Figure 5 shows the spatial distribution of TOC in three-dimensional view, constructed using kriging estimate results. This diagram further confirms our findings that high TOC content was located at the southern part of the Ortega River and near z = 1.0 m of the Cedar River. These findings are useful in evaluating relationships between TOC and sediment contaminants in the area.
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Fig. 5. Three-dimensional view of total organic carbon (TOC) distribution in the Cedar and Ortega River sediments.
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Input Sources of Total Organic Carbon and Hydrocarbons
A plot of TOC, total PAHs, and PCBs against the x axis (easting) of the rivers is given in Fig. 6
. Data for PAHs and PCBs were obtained from the same geological locations as those for TOC (Durell, 2004). Only the top 50 cm TOC and contaminant data were used for constructing Fig. 6 since most of the anthropogenic contaminants were input into the rivers during the last 60 yr (Alexander et al., 1993). Figure 6 shows that while high sediment TOC content was mainly located in the Ortega River area, the opposite was true for total PAHs and PCBs. That is, the high total PAH and PCB contents were found in the Cedar River area. A possible explanation of this phenomenon is that, unlike the cases of total PAHs or PCBs with their likely input sources from the Cedar River area where more industrial plants and waste facilities resided, the naturally occurring TOC was heavily loaded into the sediments from the Ortega River area, especially the southern part of the river. This TOC input is consistent with the land uses. While the land uses in the Cedar River and in the middle and northern parts of the Ortega River are mostly residential and commercial lands, there is about 6.1% more hardwood forest and hardwood forested wetland in the southern part of the Ortega River. Apparently, the input sources of the naturally occurring TOC and the hydrocarbons did not come from the same locations.
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Fig. 6. Sediment total organic carbon (TOC) and hydrocarbon concentrations as a function of distance in the x direction.
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Relationships of Total Organic Carbon and Hydrocarbons
Hydrocarbons entering the river environment, due to their hydrophobicity, rapidly become associated with sediments (Readman et al., 1984; Viguri et al., 2002). Sediment retention capacity is normally related to physicochemical properties such as grain size and organic carbon (Grant and Middleton, 1998). Several studies demonstrated that the sorption of hydrocarbons is highly related to organic matter of the sediments (Karickoff, 1981; Boehm and Farrington, 1984; Feng et al., 1998). A plot of the TOC contents against the total PAH and PCB concentrations in the river sediments reveals that high hydrocarbon contents were found within TOC contents, ranging from 100 to 150 g kg–1 (Fig. 7
). No linear correlations were found between TOC and hydrocarbons (i.e., total PAHs and PCBs). This finding was in contradiction to other studies as mentioned above. The discrepancy between our study and other studies could be the differences in TOC and hydrocarbon input source locations in the Cedar and Ortega Rivers. It was found that more TOC was loaded into the river from the southern part of the Ortega River watershed, whereas almost all of the hydrocarbons came from the Cedar River watershed. Results suggested that when relating hydrocarbons to sediment TOC, the locations of their input sources and the patterns of the land use also need to be considered.
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Fig. 7. Contaminant concentrations as a function of total organic carbon (TOC) contents at the top 50 cm sediment depth.
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CONCLUSIONS
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Temporal variations of TOC with sediment depth in the Cedar and Ortega Rivers were evaluated using field measurements along with the sedimentation rate obtained at the same location by Alexander et al. (1993). Large temporal changes in sediment TOC concentrations were observed in the rivers, which reflected changes in characteristics and magnitude of inputs into the rivers during approximately the last 100 yr. The average concentration of TOC in sediments from the entire Cedar and Ortega Rivers was 12.7% with a maximum of 22.6% and a minimum of 2.3%. This concentration was higher than that in the Hudson River in New York, USA (Feng et al., 1998), but was less than that in Lake Apopka of central Florida (Silliman and Schelske, 2003). It is apparent that the Cedar and Ortega Rivers watershed is a complex environment where both natural and anthropogenic processes influence the characteristics and distributions of sediments.
Three-dimensional kriging analysis was performed to characterize spatial distribution of TOC in sediments from the Cedar and Ortega Rivers, using the ISATIS model and field measurements. The analysis procedures included preliminary data analysis, data structure analysis, and kriging estimation. Kriged results showed that more TOC accumulated at the upper 1.0 m of the sediment in the southern part of the Ortega River although the TOC sedimentation varied with locations and depths.
No linear correlations were observed between TOC and hydrocarbons (i.e., total PAHs and PCBs). This finding was in contradiction to other studies and occurred because of the differences in TOC and hydrocarbon input source locations. It was found that more TOC was loaded into the southern part of the Ortega River, while almost all of the hydrocarbons entered into the Cedar River.
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NOTES
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Florida Agric. Exp. Stn. Journal Series no. R-11043
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TOP
ABSTRACT
INTRODUCTION
