Soil burdens of persistent organic pollutants - Their levels, fate and risk. Part I. Variation of concentration ranges according to different soil uses and locations

Detailed soil screening data from the Czech Republic as a typical Central European country are presented here. Determination of a wide selection of organic and inorganic pollutants as well as an assessment of specific soil parameters allowed us to study the soil contamination in relation to the land use and soil properties. While HCHs and HCB were found at highest levels in arable soils, the higher concentrations of PCDDs/Fs, PCBs, PAHs and DDTs were observed in high altitude forest soils. Concentrations of these compounds strongly correlated with the soil organic carbon content. Several possible reasons have been suggested for the observed higher concentrations in mountain forest soils but the impact of each of these influencing factors remains to be identified. An inventory of the soil contamination is needed as a first step in our effort to estimate an extent to which the secondary sources contribute to the enhanced atmospheric levels of POPs.     

Observations on long-term air-soil exchange of organic contaminants

Evidence for long-term changes in the soil composition of selected organic compounds, brought about by exchanges with the atmosphere, is briefly reviewed. In the case of some compounds — such as benzo(a)pyrene and octachlorodibenzo-p-dioxin, soils may be significant long-term environmental sinks for atmospherically-derived material. In other cases — such as phenanthrene and some of the lighter PCBs, de-gassing or volatilisation from soil back to the air can occur under certain conditions. Hence the soil may act as a “short-term” sink, and a potential source to atmosphere. Indeed, for some ‘semi-volatile’ compounds used in large quantities in the past — such as PCBs, soil outgassing may actually be an extremely important source to contemporary air. Furthermore, soil outgassing from areas of former high use may provide an important driving mechanism for continued “global cycling” of a range of semi-volatile organochlorine compounds.     

The global re-cycling of persistent organic pollutants is strongly retarded by soils

'Persistent organic pollutants' (POPs) are semi-volatile, mobile in the environment and bioaccumulate. Their toxicity and propensity for long-range atmospheric transport (LRAT) has led to international bans/restrictions on their use/release. LRAT of POPs may occur by a 'single hop' or repeated temperature-driven air-surface exchange. It has been hypothesised that this will result in global fractionation and distillation-with condensation and accumulation in polar regions. Polychlorinated biphenyls (PCBs)--industrial chemicals banned/restricted in the 1970s--provide a classic illustration of POP behaviour. A latitudinally-segmented global PCB inventory has been produced, which shows that approximately 86% of the 1.3 x 10(6) tonnes produced was used in the temperate industrial zone of the northern hemisphere. A global survey of background surface soils gives evidence for 'fractionation' of PCBs. More significantly, however, very little of the total inventory has 'made the journey' via primary emission and/or air-surface exchange and LRAT out of the heavily populated source regions, in the 70 years since PCBs were first produced. Soils generally occlude PCBs, especially soils with dynamic turnover of C/bioturbation/burial mechanisms. This limits the fraction of PCBs available for repeated air-soil exchange. The forested soils of the northern hemisphere, and other C-rich soils, appear to be playing an important role in 'protecting' the Arctic from the advective supply of POPs. Whilst investigations on POPs in remote environments are important, it is imperative that researchers also seek to better understand their release from sources, persistence in source regions, and the significant loss mechanisms/global sinks of these compounds, if they wish to predict future trends.     

A comparison of POPs bioaccumulation in Eisenia fetida in natural and artificial soils and the effects of aging

The close relationship between soil organic matter and the bioavailability of POPs in soils suggests the possibility of using it for the extrapolation between different soils. The aim of this study was to prove that TOC content is not a single factor affecting the bioavailability of POPs and that TOC based extrapolation might be incorrect, especially when comparing natural and artificial soils. Three natural soils with increasing TOC and three artificial soils with TOC comparable to these natural soils were spiked with phenanthrene, pyrene, lindane, p,p'-DDT, and PCB 153 and studied after 0, 14, 28, and 56 days. At each sampling point, total soil concentration and bioaccumulation in earthworms Eisenia fetida were measured. The results showed different behavior and bioavailability of POPs in natural and artificial soils and apparent effects of aging on these differences. Hence, direct TOC based extrapolation between various soils seems to be limited.     

Vertical distribution of organochlorine pesticides in humus along Alpine altitudinal profiles in relation to ambiental parameters

In forest soils along vertical profiles located in different parts of the Alps, concentrations of persistent organic pollutants (POPs), namely organochlorine pesticides (OCPs) like dichlorodiphenyltrichloroethanes (DDTs), hexachlorobenzene (HCB), hexachlorocyclohexanes (HCH), heptachlor, aldrin, dieldrin and mirex, were measured. Though local characteristics of the sites are influenced by numerous factors like orographic and meteorological parameters, forest stand characteristics and humus parameters, we ascertained a marked vertical increase of concentrations of some organochlorine compounds in the soil. On the basis of climatological values of each site, we found that the contamination increase with altitude can be ascribed to a certain ‘cold condensation effect’. In addition, the perennial atmospheric deposition of POPs is controlled by precipitation. Other key parameters explaining the accumulation of POPs are the soil organic carbon stocks, the turnover times, the re-volatilisation and degradation processes, which vary with altitude.     

Organic marker compounds in surface soils of crop fields from the San Joaquin Valley fugitive dust characterization study

Fugitive dust from the erosion of arid and fallow land, after harvest and during agricultural activities, can at times be the dominant source of airborne particulate matter. In order to assess the source contributions to a given site, chemical mass balance (CMB) modeling is typically used together with source-specific profiles for organic and inorganic constituents. Yet, the mass balance closure can be achieved only if emission profiles for all major sources are considered. While a higher degree of mass balance closure has been achieved by adding individual organic marker compounds to elements, ions, EC, and organic carbon (OC), major source profiles for fugitive dust are not available. Consequently, neither the exposure of the population living near fugitive dust sources from farm land, nor its chemical composition is known. Surface soils from crop fields are enriched in plant detritus from both above and below ground plant parts; therefore, surface soil dust contains natural organic compounds from the crops and soil microbiota. Here, surface soils derived from fields growing cotton, safflower, tomato, almonds, and grapes have been analyzed for more than 180 organic compounds, including natural lipids, saccharides, pesticides, herbicides, and polycyclic aromatic hydrocarbon (PAH). The major result of this study is that selective biogenically derived organic compounds are suitable markers of fugitive dust from major agricultural crop fields in the San Joaquin Valley. Aliphatic homologs exhibit the typical biogenic signatures of epicuticular plant waxes and are therefore indicative of fugitive dust emissions and mechanical abrasion of wax protrusions from leaf surfaces. Saccharides, among which α- and β-glucose, sucrose, and mycose show the highest concentrations in surface soils, have been proposed to be generic markers for fugitive dust from cultivated land. Similarly, steroids are strongly indicative of fugitive dust. Yet, triterpenoids reveal the most pronounced distribution differences for all types of cultivated soils examined here and are by themselves powerful markers for fugitive dust that allow differentiation between the types of crops cultivated. PAHs are also found in some surface soils, as well as persistent pesticides, e.g., DDE, Fosfall, and others.     

Uncertainties and novel prospects in the study of the soil carbon dynamics

Establishment of the Kyoto Protocol has resulted in an effort to look towards living biomass and soils for carbon sequestration. In order for carbon credits to be meaningful, sustained carbon sequestration for decades or longer is required. It has been speculated that improved land management could result in sequestration of a substantial amount of carbon in soils within several decades and therefore can be an important option in reducing atmospheric CO2 concentration. However, evaluation of soil carbon sources and sinks is difficult because the dynamics of soil carbon storage and release is complex and still not well understood. There has been rapid development of quantitative techniques over the past two decades for measuring the component fluxes of the global carbon cycle and for studying the soil carbon cycle. Most significant development in the soil carbon cycle study is the application of accelerator mass spectrometry (AMS) in radiocarbon measurements. This has made it possible to unravel rates of carbon cycling in soils, by studying natural levels of radiocarbon in soil organic matter and soil CO2. Despite the advances in the study of the soil carbon cycle in the recent decades, tremendous uncertainties exist in the sizes and turnover times of soil carbon pools. The uncertainties result from lack of standard methods and incomplete understanding of soil organic carbon dynamics, compounded by natural variability in soil carbon and carbon isotopic content even within the same ecosystem. Many fundamental questions concerning the dynamics of the soil carbon cycle have yet to be answered. This paper reviews and synthesizes the isotopic approaches to the study of the soil carbon cycle. We will focus on uncertainties and limitations associated with these approaches and point out areas where more research is needed to improve our understanding of this important component of the global carbon cycle.     

Modelling the fate of hydrophobic organic contaminants in a boreal forest catchment: A cross disciplinary approach to assessing diffuse pollution to surface waters

The fate of hydrophobic organic compounds (HOCs) in soils and waters in a northern boreal catchment was explored through the development of a chemical fate model in a well-characterised catchment system dominated by two land types: forest and mire. Input was based solely on atmospheric deposition, dominated by accumulation in the winter snowpack. Release from soils was governed by the HOC concentration in soil, the soil organic carbon fraction and soil-water DOC content. The modelled export of selected HOCs in surface waters ranged between 11 and 250 ng day⁻¹ during the snow covered period, compared to 200 and 9600 ng/d during snow-melt; highlighting the importance of the snow pack as a source of these chemicals. The predicted levels of HOCs in surface water were in reasonable agreement to a limited set of measured values, although the model tended to over predict concentrations of HOCs for the forested sub-catchment, by over an order of magnitude in the case of hexachlorobenzene and PCB 180. This possibly reflects both the heterogeneity of the forest soils and the complicated and changing hydrology experienced between the different seasons.     

Developing a multimedia model of chemical dynamics in an urban area.

A multimedia model has been developed to account for the movement of semi-volatile organic compounds (SOCs) in an urban environment. The model, based on a Level III fugacity model of D. Mackay (Multimedia Environmental Models: The Fugacity Approach, Lewis Publishers, Boca Raton, FL, 1991), consists of six compartments: air, surface water, sediment, soil, vegetation, and an organic film that coats impervious surfaces. The latter is a newly identified compartment into which gas-phase SOCs partition and particle-phase SOCs are believed to be efficiently captured (M.L. Diamond, S.E. Gingrich, K. Fertuck, B.E. McCarry, G.A. Stern, B. Billeck, B. Grift, D. Brooker, T.D. Yager, Environ. Sci. Technol., 34 (2000a), 2900-2908). The model, parameterized for downtown Toronto, Ontario, Canada, and run with an illustrative emission rate for selected polycyclic aromatic hydrocarbons and homologues of polychlorinated dibenzodioxins, indicates that the film achieves the highest concentrations among media but that soils are the greatest sink for all but the least hydrophobic chemicals. The film "reflects" the more volatile chemicals into air, facilitates removal to surface waters by wash-off, and provides a surface on which photolytic degradation can occur. As such, the film is a transient sink that increases chemical mobility in urban areas by increasing air concentrations and the cycling of these compounds between air and urban surfaces and increasing water concentrations. Vegetation also accumulates SOCs, a portion of which is transferred to soil that reduces chemical mobility.     

A lysimeter experiment to investigate temporal changes in the availability of pesticide residues for leaching

Leaching of three pesticides (isoproturon, chlorotoluron and triasulfuron) and a tracer (bromide) were determined in four contrasting soils ranging in texture from sandy loam to clay. The compounds were applied to undisturbed columns of soil and four columns for each soil were randomly selected and leached with 24-mm equivalent of water at prescribed time intervals (3, 9, 24, 37 and 57 d after application). A rapid decline in leached loads of isoproturon and chlorotoluron as time from application to irrigation increased was observed in all soils. In contrast, triasulfuron and bromide loads only decreased rapidly in the clay soil. Bromide losses decreased with decreasing clay contents of the soil and therefore with a decrease in structural development. Magnitudes of pesticide losses varied from soil to soil, depending on structural development and the organic carbon content. Pesticide degradation experiments on disturbed and undisturbed soil samples showed that the rapid decline of leached loads with time was faster than could be explained by degradation alone. Five physico-chemical processes are put forward to explain the different patterns of pesticide leached loads observed in the soils: (1) relative extent of preferential flow, (2) sorption capacity of the compounds to the different soils, (3) extent of degradation of the compounds in the soil, (4) variation in sorption kinetics between compounds associated with pesticide diffusion into soil aggregates, and (5) protection of the compounds by a combination of intra-aggregate diffusion and the presence of preferential flow pathways.     

Dioxins, PCBs, and HCB in soil and peat profiles from a pristine boreal catchment

The aim of this study was to explore how atmospherically derived soil pollution is affected by environmental processes at two typical boreal catchment landscape type settings: wetlands and forested areas. Measurements of hydrophobic organic compounds (HOCs) in forest soil and peat from an oligotrophic mire at various depths were performed at a remote boreal catchment in northern Sweden. HOCs in peat were evenly distributed throughout the body of the mire while levels of HOCs in the forest soil increased with increased amount of organic matter. Evaluation of HOC composition by principal component analysis (PCA) showed distinct differences between surface soils and deeper soil and peat samples. This was attributed to vertical transport, degradation and/or shifting sources over time. The calculated net vertical transport differed between surface layers (0.3%) and deeper soils (8.0%), suggesting that vertical transport conditions and processes differ in the deeper layers compared to the surface layers.     

土地处理法处理钻探泥浆的土壤特性研究

论述了中国陆上油田的土壤类型，这些土壤的理化性质以及土壤中的微生物种类。采用土地处理法处理废弃泥浆时，一方面，废弃泥浆中的重金属、有机物对土壤会带来负面的影响，如有机质及阳离子的含量升高，含盐量升高，pH升高，土壤的孔隙度降低等；另一方面，微生物降解有机物而形成腐植质，可以改善土壤性质。并且，通过对土壤的改良，可以解决上述的负面影响。最后得出结论，土地处理法处理废弃泥浆在技术上、经济上、环境上都是可行的。     

Using biochar for remediation of soils contaminated with heavy metals and organic pollutants

Soil contamination with heavy metals and organic pollutants has increasingly become a serious global environmental issue in recent years. Considerable efforts have been made to remediate contaminated soils. Biochar has a large surface area, and high capacity to adsorb heavy metals and organic pollutants. Biochar can potentially be used to reduce the bioavailability and leachability of heavy metals and organic pollutants in soils through adsorption and other physicochemical reactions. Biochar is typically an alkaline material which can increase soil pH and contribute to stabilization of heavy metals. Application of biochar for remediation of contaminated soils may provide a new solution to the soil pollution problem. This paper provides an overview on the impact of biochar on the environmental fate and mobility of heavy metals and organic pollutants in contaminated soils and its implication for remediation of contaminated soils. Further research directions are identified to ensure a safe and sustainable use of biochar as a soil amendment for remediation of contaminated soils.     

Effect of soil properties on bioavailability and extractability of phenanthrene and atrazine sequestered in soil

Sixteen soils with markedly different properties were analyzed to determine their porosity in the range of 7 nm-10 microm, cation-exchange capacity (CEC), surface area and clay mineralogy. The extent of sequestration of phenanthrene and atrazine has been shown to differ markedly among these soils. Correlations were sought between soil characteristics and four methods of measuring sequestration. Simple correlation analysis showed that some but not all measures of phenanthrene and atrazine sequestration were highly correlated with organic C content, nanoporosity or CEC but not other properties of the soils. Multiple linear-regression analysis suggested an interaction of organic C content with soil texture, CEC or surface area in determining the extent of atrazine or phenanthrene sequestration. We conclude that organic C content, CEC and other properties of soil may be useful predictors of sequestration of some compounds.     

Long-range transport and global fractionation of POPs: insights from multimedia modeling studies

The long-range transport of persistent organic pollutants (POPs) is investigated with two multimedia box models of the global system. ChemRange is a purely evaluative, one-dimensional steady-state (level III) model; CliMoChem is a two-dimensional model with different temperatures, land/water ratios and vegetation types in different latitudinal zones. Model results are presented for three case studies: (i) the effect of atmospheric aerosol particles on the long-range transport of POPs, (ii) the effect of oceanic deposition on the long-range transport of different PCB congeners, (iii) the global fractionation of different PCB congeners. The model results for these case studies show: (i) the low atmospheric half-lives estimated for several organochlorine pesticides are likely to be inconsistent with the observed long-range transport of these compounds; (ii) export to the deep sea reduces the potential for long-range transport of highly hydrophobic compounds (but does not remove these chemicals from the biosphere); (iii) there are different meanings of the term global fractionation that refer to different aspects of the fractionation process and need to be distinguished. The case-study results further indicate that the influences of varying environmental conditions on the physicochemical properties and the degradation rate constants of POPs need to be determined.     

The deep-sea as a final global sink of semivolatile persistent organic pollutants? Part II: Organochlorine pesticides in surface and deep-sea dwelling fish of the north and south Atlantic and the Monterey Bay Canyon (California)

The understanding of the global environmental multiphase distribution of persistent organic pollutants (POPs) as a result of the physico-chemical properties of the respective compounds is well established. We have analysed the results of a vertical transport of POPs from surface water to deepwater in terms of the contamination of the biota living in the respective environmental compartments. Samples were taken from the North and the South Atlantic and from the uprising water region of the continental shelf of California (Marine Sanctuary Monterey Bay and its Canyon). The contents of persistent organochlorine pesticides (DDTs, chlordanes, toxaphenes, HCHs, and HCB) in surface-living fish are compared to those in deepwater fish of the same geographic area. The deepwater biota show significantly higher burdens as compared to surface-living species of the same region. There are also indications for recycling processes of POPs of the class of organochlorine pesticides in the biophase of the abyss as well. It can be concluded that the bio- and geophase of the deep-sea may act as an ultimate global sink for persistent semivolatile contaminants in the marine environment like the soil on the continents.     

Layer of organic pine forest soil on top of chlorophenol-contaminated mineral soil enhances contaminant degradation

Chlorophenols, like many other synthetic compounds, are persistent problem in industrial areas. These compounds are easily degraded in certain natural environments where the top soil is organic. Some studies suggest that mineral soil contaminated with organic compounds is rapidly remediated if it is mixed with organic soil. We hypothesized that organic soil with a high degradation capacity even on top of the contaminated mineral soil enhances degradation of recalcitrant chlorophenols in the mineral soil below. We first compared chlorophenol degradation in different soils by spiking pristine and pentachlorophenol-contaminated soils with 2,4,6-trichlorophenol in 10-L buckets. In other experiments, we covered contaminated mineral soil with organic pine forest soil. We also monitored in situ degradation on an old sawmill site where mineral soil was either left intact or covered with organic pine forest soil. 2,4,6-Trichlorophenol was rapidly degraded in organic pine forest soil, but the degradation was slower in other soils. If a thin layer of the pine forest humus was added on top of mineral sawmill soil, the original chlorophenol concentrations (high, ca. 70 μg g^?1, or moderate, ca. 20 μg g^?1) in sawmill soil decreased by >40 % in 24 days. No degradation was noticed if the mineral soil was kept bare or if the covering humus soil layer was sterilized beforehand. Our results suggest that covering mineral soil with an organic soil layer is an efficient way to remediate recalcitrant chlorophenol contamination in mineral soils. The results of the field experiment are promising.     

Reliable QSAR for estimating Koc for persistent organic pollutants: correlation with molecular connectivity indices

Several recent studies have shown that n-octanol/water partition coefficients may not be a good predictor for estimating soil sorption coefficients of persistent organic pollutants (POPs), defined here as chemicals with log Kow greater than 5. Thus, an alternative QSAR model was developed that seems to provide reliable estimates for the soil sorption coefficients of persistent organic pollutants. This model is based on a set of calculated molecular connectivity indices and evaluated soil sorption data for 18 POPs. The chemical's size and shape, quantified by 1chi, 3chiC and 4chiC(v) indices, have a dominant effect on the soil sorption process of POPs. The developed QSAR model was rationalized in terms of potential hydrophobic interactions between persistent organic pollutants and soil organic matrix. Its high predictive power has been verified by an extensive internal and external validation procedure.     

Soil carbon pools and fluxes in urban ecosystems

The transformation of landscapes from non-urban to urban land use has the potential to greatly modify soil carbon (C) pools and fluxes. For urban ecosystems, very little data exists to assess whether urbanization leads to an increase or decrease in soil C pools. We analyzed three data sets to assess the potential for urbanization to affect soil organic C. These included surface (0-10 cm) soil C data from unmanaged forests along an urban-rural gradient, data from "made" soils (1 m depth) from five different cities, and surface (0-15 cm) soil data of several land-use types in the city of Baltimore. Along the urban-rural land-use gradient, we found that soil organic matter concentration in the surface 10 cm varied significantly (P=0.001). In an analysis of variance, the urban forest stands had significantly (P=0.02) higher organic C densities (kg m(-2) to 1 m depth) than the suburban and rural stands. Our analysis of pedon data from five cities showed that the highest soil organic C densities occurred in loamy fill (28.5 kg m(-2)) with the lowest occurring in clean fill and old dredge materials (1.4 and 6.9 kg m(-2), respectively). Soil organic C densities for residential areas (15.5 +/- 1.2 kg m(-2)) were consistent across cities. A comparison of land-use types showed that low density residential and institutional land-uses had 44 and 38% higher organic C densities than the commercial land-use type, respectively. Our analysis shows that as adjacent land-use becomes more urbanized, forest soil C pools can be affected even in stands not directly disturbed by urban land development. Data from several "made" soils suggests that physical disturbances and inputs of various materials by humans can greatly alter the amount C stored in these soils.