Molecular Markers in Environmental Geochemistry - American

Chapter 14

Downloaded by STANFORD UNIV GREEN LIBR on October 4, 2012 | http://pubs.acs.org Publication Date: July 1, 1997 | doi: 10.1021/bk-1997-0671.ch014

Linear Alkylbenzenesulfonates and Polychlorinated Biphenyls as Indicators of Accumulation, Biodegradation, and Transport Processes in Sewage Farm Soils Thorsten Reemtsma, Irena Savric, Claudia Hartig, and Martin Jekel Department of Water Quality Control, Technical University of Berlin, Secretariate K F 4, Strasse des 17. Juni 135, D-10623 Berlin, Germany A comparative study was conducted on the concentrations and compositions of polychlorinated biphenyls (PCB) and linear alkyl benzenesulfonates (LAS) in two former sewage farms to obtain insights into the history of operation and post-depositional processes having affected the sewage-derived organics accumulated in the soils over the last decades. Total concentrations in the surface soils are in the range of 1 - 6 μg g for PCB and 0.1 - 4 μg g for LAS. Differences in the intensity of sewage discharge between the two sites are recorded in the total contents of PCB and LAS, while different qualities of wastewater treatment prior to disposal are visible in the PCB composition and the LAS homologs distribution of the surface soils. Alterations in the homolog composition of LAS in depth profiles reflect desorption and sorption processes during downward transport through the unsaturated zone, while the largely unaltered isomeric composition might suggest that LAS were not degraded in the soil column. -1

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Sewage farms were installed as an alternative to wastewater treatment plants by the City of Berlin in the second half of the 19th century. The infiltration basins were flooded with settled wastewater up to six times a year (i), and the water itself as well as the organic matter and dissolved nutrients of the wastewater were used for tillage. Sewage farming reached its largest expansion in the early 1920s with up to 110 x 10 m of farm land and an average amount of 700,000 m sewage d" treated at the farms. Since the 1920s the sewage farms were successively closed down and only a very few sewage farms are still in use. During theirtimeof operation the sewage farm soils accumulated nutrients and heavy metals (2) as well as organic wastewater constituents such as polycyclic aromatic hydrocarbons (PAH) and polychlorinated biphenyls (PCB) (5) and large 6

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© 1997 American Chemical Society

In Molecular Markers in Environmental Geochemistry; Eganhouse, R.; ACS Symposium Series; American Chemical Society: Washington, DC, 1997.

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MOLECULAR MARKERS TS ENVIRONMENTAL GEOCHEMISTRY

amounts of unknown extractable organohalogens (4). Due to the high infiltration rates of some meters per year applied over a period of approximately a hundred years, organic wastewater constituents originally deposited in the surface soils might have entered deeper soil layers and the surficial aquifer. Breakthrough of wastewater constituents into the surficial groundwater has been investigated in rapid infiltration systems by field (5-6) and column studies (7). However, detailed knowledge of the organic xenobiotics deposited in the sewage farms as well as of their long term dynamics and controlling factors is very limited. We selected two classes of substances, PCB and linear alkylbenzenesulfonates (LAS), to obtain information on the processes detenmning die fate of organic wastewater constituents in the sewage farms. Based on differences in their biogeochemical profiles, we used these compound classes to provide complementary information on sewage sources, and on post-burial migration and degradation processes in the sewage farm soils. While PCBs are among the most intensively investigated xenobiotic compounds (8), present knowledge on the biogeochemistry of LAS in soils is still fragmentary. Although several investigations on the behaviour of LAS in sludge amended soils (9-13) and in groundwater influenced by secondary treated wastewater (14-16) or septic tanks (17-20) have been performed, LAS have apparently not been used as molecular markers for transport and degradation processes in soils. Materials and Methods Sampling Sites. The sampling sites are displayed in Figure 1. Both sewage farms were established in the late 19th century and were used for the treatment of raw wastewater. However, the history of operation at these sites differed significantly. Site I (SF-I, Karolinenhohe) was primarily used for the treatment of municipial wastewater, with average infiltration rates below 4 m yr . The application of raw wastewater ceased in the 1960s, after which secondary effluent of a sewage treatment plant was discharged. Municipal and industrial wastewater was treated on site II (SF-n, Buch). Situated outside of the City boundaries, this farm was operated by the water authorities of East Berlin (German Democratic Republic) after World War n. Sewage discharge was intensified in the 1960s, and the sewage farm was closed in 1985. Soils from both sites are loamy, fine to medium grain sands with low organic carbon (< 0.1 %) and clay (< 1 %, often < 0.5 %) contents in the C-horizons. The Yah-horizons (0 - 20 cm; humic layer) exhibit organic carbon contents of 2 5%. The water table is 6 m and 9 m below land surface at sites n and I, respectively. 1

Sampling. Soil samples were collected from each of the sewage farms in spring of 1994 (SF-I; SF-II/1 - n/3); samples for additional LAS analyses were taken in spring 1996 at site II (SF-II/4). All samples were scraped from the vertical walls of freshly excavated holes after careful removal of the surficial material. Sludge aggregates could be isolated from the surface soil of sewage farm n. The soils were sieved over 5 mm (removal of small stones and plant litter) and stored frozen until analyses.


14.

REEMTSMA ET AL.

LASs & PCBs as Indicators in Sewage Farm Sods

215


Frozen samples were put in an oven at 45°C and dried within 3 - 4 days until they reached constant weight. They were then homogenized in a stainless steel disk vibratory mill, and an aliquot was subjected to Soxhlet extraction. PCB analyses. Sample preparation, work-up and analysis followed the procedure of Nolte et. al. (27). Briefly, the soils were Soxhlet-extracted with hexane/toluene (9/1) overnight, and the extracts were cleaned by shaking with concentrated sulfuric acid and by passing the extracts through silica columns (500 mg Bakerbond SPE; Baker, Philipsburg, USA). A PE 8420 gas chromatograph with electron capture detection (GC-ECD) and equipped with an AS 8300 autosampler (all from PerkinElmer, Uberlingen, Germany) was employed for PCB analyses. Separation was performed on a CP-SIL 8cb column 50 m x 0.25 mm i.d. (Chrompack, Frankfurt, Germany) with helium as carrier gas (170 kPa, column head pressure). Injector temperature was 250°C, detector temperature 350°C.; the column was held at 120°C for 1 min, heated at 20°C min to 180°C and at 1°C min" to 240°C.; after 3 rnin of isothermal operation the column was heated at 10°C min" to 280°C and was finally kept constant for 10 min. 1 fd of sample was injected in the splitless mode. About 100 congeners could be identified based on coelution with individual reference congeners (obtained from Promochem, Wesel, Germany). Quantitation of all detected peaks was based on relative response factors determined by a two point calibration with two technical PCB mixtures (Clophen A30, Clophen A60; Promochem, Wesel, Germany) of known relative composition (22). Recovery from spiked samples was between 75% and 90% for all but three detected peaks, and the standard deviation of 24 replicates was 5-10% (27). Detection limits rangefrom0.2 to 1 ng g' depending on the relative response factors of the individual congeners. Blank analyses were performed with every series of PCB analyses, and their PCB contents were below the detection limit. 1

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LAS notation. All LAS used in detergent formulations are l-alkyl-4-sulfonyl benzenes. The length of the alkyl chain and the position of the sulfophenyl-substituent in the alkyl chain, however, vary from C10 to C13 and from the 2- to die mid-chain position in recent detergent formulations. An individual LAS compound is, therefore, clearly defined by the position of the sulfophenyl substituent at the alkyl chain and by the alkyl chain length: 3-LAS12 denotes dodecyl-3-benzenesulfonate. LAS analyses. Extraction and clean-up were adopted from the procedure developed by Reiser for fluvial sediments (23; see also Reiser and Giger, this volume). Briefly, the dried sample was Soxhlet-extracted with methanol, and octyl-l-phenylsulfonate (1-LAS8) was added as internal standard. Nonionic and cationic compounds were removed by passing the methanolic extract through a strong anion exchange cartridge, from which LAS were eluted with 1% (v/v) concentrated HC1 in methanol. LAS in the dried eluates were derivatized by a two step procedure: i) formation of sulfonic acid chlorides by thionylchloride and ii) formation of trifluoroethylesters by the reaction with trifluoroethanol in triethylamine. The derivatives were then separated from more polar impurities by aluminium oxide (Alox N) chromatography on a Pasteur-pipette rninicolumn.


216

MOLECULAR MARKERS IN ENVIRONMENTAL GEOCHEMISTRY

LAS trifluoroethylesters were analyzed with a GC-quadrupole-MS system (MD 800; Fisons, Manchester, Great Britain) with 5 pi of the extract splitless injected at an injector temperature of 220°C. Separation was performed on a SPBS 30 m x 0.25 mm i.D. column (Supelco, Bellefonte, USA) with helium as carrier gas at 80 kPa column head pressure; the column temperature was held at 80°C for 1 min, then heated at 10°C min to 200°C and at 6°C min to 300°C, followed by 10 min isothermal operation. The mass spectrometer was operated in the electronimpact mode (ET), and the trifluoroethylesters of LAS were detected by monitoring their molecular ions. Since the elution times of the isomers of two adjacent homologs overlap, two traces were recorded simultaneously at a scan rate of 0.7 sec . All isomers from LAS 10 to LAS 13 (total of 20 components) were quantified on a mass basis assuming identical relative response factors. The average recovery of LAS from soil samples spiked with 0.22 fig g' total LAS was 94% (85% - 112%). The standard deviation of triplicate samples was 8%. The detection limit of individual LAS components (S/N > 10) is in the range of 0.1 ng injected onto the column, corresponding to a concentration of 0.1 - 0.2 ng gr of an individual component and 2 - 4 ng g total LAS detectable from 20 g of sample. A blank sample was run in each analytical sequence. If care is taken to avoid contamination by LAS during extraction and work-up, blank values below the detection limit are obtainable. 1

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Results and Discussion Factors Detennining the Organic Matter of Sewage Farm Soils. The major processes detennining the fate of organic substances in the sewage farms are displayed in Figure 2. The amount of organics deposited in the farms is dependent on the amount of wastewater discharged and the kind of wastewater treatment applied prior to the sewage disposal. The primary removal process of organic compounds in the sewage farms is mineralization in the soil surface layer, which is accompanied by humification. Less degradable or persistent substances accumulate in the surface soil. A part of the organics deposited at the soil surface might be removed by wind erosion, volatilization and photolysis; the importance of these processes depends on the degree of soil covering by plants and may, therefore, strongly vary in time and space. Dissolved and colloidal substances delivered with the wastewater or desorbed from the soil surface layer penetrate the unsaturated zone of the soil column, where further microbial degradation and multiple sorption/desorption processes determine the amounts remaining in the soil column or entering the surficial groundwater. Total Contents of LAS and PCB. Total PCB and LAS contents of the surface soils are given in Table I. The level of contamination is generally one order of magnitude higher in sewage farm n than in sewage farm I. This is consistent with prolonged and more extensive discharge of untreated wastewater on sewage farm n. Sewage farm soils are known to exhibit strong spatial variations in their inorganic and organic load, with contaminant contents decreasing from the location of wastewater discharge onto the infiltration basins (Figure 2, left) towards the opposite side (26).



14.

REEMTSMA ET AL.

LASs & PCBs as Indicators in Sewage Farm Soils

Figure 1. Sewage farms of the City of Berlin (around 1979) and the two sampling sites.

Figure 2. Major processes governing the organic matter pool of sewage farms.


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Table I. Contents of PCB and LAS in sewage farm surface soils


depth (cm)

LAS Gtgg" )

surface soil I

0 - 12

PCB 0*gg') 0.9

surface soil n

0 - 10

3-6

0.4-4

sludge aggregates n

0 - 10

14-86

10-11

0.1-20*

2000 - 12000

sewage sludges a

1

0.1

s

b

Data from ) Ref. 24, 25 ) Ref. 11, 24. The elevated concentrations of PCB and LAS in the sludge aggregates isolated from the soil surface layers of sewage farm n reflect the importance of suspended wastewater particles for the input of these contaminants. While the PCB-contents of the soils are in the range of those reported for sewage sludges during the 1980s (24-25), the LAS contents are significantly lower and are comparable to sewage sludge amended soils (9, 12). This reflects the purification potential of the surface layers towards easily degradable wastewater constituents such as LAS. It is notable that LAS were determined at all. Half lives of LAS in sludge amended, well aerated soils have been calculated to range from 3 - 3 0 days (9; 12). The nine years between the end of sewage application (1985) on sewage farm II and the time of our sampling, thus, correspond to at least one hundred half lives. Assuming first order kinetics, LAS should be completely degraded after this space of time, regardless of their initial concentration. This is, however, not the case. Either due to decreasing concentrations or due to the increasing age of the contamination even easily degradable substances such as LAS are being preserved in the soil system. Binding onto the soil matrix has been suggested to be an important mechanism of LAS preservation in soils (10). The depth profiles of the total contents of LAS and PCB are displayed in Figure 3. PCB are detectable down to 40 cm depths (B-horizons, Figure 3a), but the concentrations decrease drastically. At 40 cm depth total PCB contents are between one and two orders of magnitude lower (0.01 - 1 fig g' ) than in the uppermost layer (1 - 6 /xg g" ). The depth profiles of LAS (Figure 3b) show a similar pattern with decreases between one and two orders of magnitude. In all but one case, concentra tions of total LAS at 60 cm are close to or below 0.1 fig g" . These depth profiles indicate that wastewater constituents deposited in the surface layers of the sewage farms have been transported to deeper soil layers. In the case of LAS, elevated contents at depths of 30 - 40 cm in sewage farm E (0.1 - 1 fig g ) compared to sewage farm I (0.02 fig g") correspond to higher surface soil contents of LAS and to elevated discharge rates. 1

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PCB Composition in Surface Soils. The PCB composition in the surface layers of the two farms as detenriined by gas chromatography differs strongly (Figure 4): only late eluting highly chlorinated biphenyls are found in sewage farm I soil, while



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REEMTSMA E T A L .


Figure 3. Total contaminants contents in the sewage farm profiles; (top): PCB G*g g )> (bottom): LAS (pg g" ); depth 0 cm refers to sludge aggregates. _1

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mixtures with a wide range of chlorination are found in the surface soils of farm H. Quantitative data are given in Table n. The PCB distribution appears to reflect the quality of wastewater treatment prior to the sewage disposal onto the sewage farm land. Sewage farm I soil resembles the Clophen A60 distribution (Table U) with an additional minor contribution of lower chlorinated PCBs. Untreated wastewater was discharged onto sewage farm I until the mid 1960s together with the highly chlorin ated PCBs such as Clophen A60, which have been used in open systems until 1972 in Western Germany (27). The secondary treated wastewater discharged since the mid 1960s did not transport appreciable amounts of PCB, since these compounds are effectively ehminated during wastewater treatment. EUrnination rates of up to 99% by activated sludge treatment are reported for PCBs (28-30) with sorption onto the activated sludge being the major removal process (29). Therefore, the replacement of highly chlorinated PCBs (Clophen A60) by low chlorinated mixtures (Clophen A30) in the early 1970s (27) is not visible in the PCB pattern of sewage farm I. The distribution in sewage farm n, however, reflects input from high (Clophen A60) to low chlorinated (Clophen A30) PCB mixtures (Table II). At sewage farm II untreated wastewater was discharged over the whole period of operation (until 1985), and the soil PCB distributions are, thus, likely to record the qualitative changes in PCB use and discharge. Unfortunately, data on the use of PCB and its restriction in the former German Democratic Republic are not available. The data presented here, however, suggest that highly chlorinated PCBs have been replaced by less chlorin ated mixtures in the German Democratic Republic, too. This view is supported by comparing the PCB composition of the surface soils of sewage farm n and the sludge aggregates isolated from it (Table II). The latter are representative of the latest particulate organic matter deposited in the sewage farm prior to its shut-down in 1985. The proportions of di- to tetrachlorinated biphenyls in the sludge aggregates is significantly higher (73 %) than in the surrounding soil (54 %), suggesting that the most recently discharged PCBs were less chlorinated than the PCBs previously accumulated in the soil surface layers. Alternatively, the higher percentage of less chlorinated PCBs in the sludge aggregates might be due to their shorter residence times in the soils. Table n. PCB distribution (%) according to the degree of chlorination in the sewage farm surface soils and in Clophen A30, Clophen A60 chlorine cont.

2

3

4

5

6

7

8

9

soil I

5

3

6

12

42

27

5

< 1

soil IP

2

20

32

20

20

5

1

< 1

sludge II*

5

29

39

16

9

3

< 1

< 1

Clophen A30*

20

48

25

6

1

-

-

-

Clophen A60"

-

-

1

17

52

30

6

< 1

b

• average of three samples; datafromRef. 22.



14.

REEMTSMA ET AL.


PCB Composition in the Depth Profiles. The octanol-water partition coefficients of PCBs (log A^,) range from 4.5 for monochloro- up to 8.1 for octachlorobiphenyls (31) and are inversely related to their water solubilities (52). Correspond ingly, laboratory experiments have shown a distinct inverse relation between the degree of chlorination and mobility in sediments (33) and soil (34). An additional retardation of the more hydrophobic congeners might be attributed to the partly irreversible nature of their sorption onto soil material (35). Therefore, downward transport of PCBs via the dissolved phase in soils should be recorded in a relative enrichment of the more soluble less chlorinated congeners. Transport of PCBs as colloids or sorbed onto particles might not alter their composi tion or would even result in a relative increase of the more hydrophobic congeners, since those are suggested to form more stable aggregates. Colloidal transport, also regarded as a partitioning of hydrophobic compounds into dissolved organic matter (DOM), has gained much attention (36) and it has been shown that DOM can sub stantially enhance the mobility of PCBs in soil columns (37). In other cases, however, aggregate formation of DOM with hydrophobic contaminants might decrease the mobility of the latter (38). It can, thus, hardly be predicted which mechanisms underlie the contaminant transport at a certain site. Changes in the homolog or isomeric composition of a class of compounds with depth might, however, allow one to identify the mechanisms that have promoted the transport of the respective compounds. In the sewage farms less chlorinated biphenyls are relatively enriched with increasing depth as displayed in Figure 5; this corresponds to the transport of PCBs by the aqueous phase. However, even hexachlorinated PCBs are detected at depths of 50 cm, suggesting that PCB have also been displaced as colloids or on particles. It has to be noted, that the most water soluble fraction of PCBs is also most susceptible to aerobic degradation (39), although the extent of PCB degradation in the field can vary substantially (40), and might even be negligible for aged contam inations (41). The less chlorinated PCB are also preferentially lost by volatilization (42). In fact, volatilization has been shown to be a major removal process for PCBs (Aroclor 1254) in sludge amended soils (43). Accordingly, one might look upon this PCB pattern (Figure 5) as an impoverishment of less chlorinated biphenyls in the soil surface, rather than as a relative enrichment with increasing depth. It is a feature common to most biogenic and xenobiotic compound classes that the isomers or homologs of highest water solubility are also most susceptible to biodeg radation and volatilization. This substantially limits their use as molecular markers in many environmental systems. Beside PCB this applies especially to PAHs (44). Use of LAS as Molecular Markers. The biogeochemical profile of LAS is gov erned by the biodegradability and sorption behaviour of its homologs and isomers. Biodegradation of LAS is initiated by the co-oxidation of the alkyl chain, and the rate of primary degradation of homologs and isomers has been shown to be related to the distance between the co-end of the alkyl chain and the xenobiotic sulphophenyl moiety ("distance principle , 45). The rate of complete degradation, however, is reported to be independent of structural influences (46), indicating that the rate deterrnining step of mineralization is the ring opening rather than the initial oxidation H


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PCB congener No. 1

Figure 4. Contents of individual PCBs (ng g ) (in the order of elution from GC-column) in the surface soils of sewage farm I (bottom) and sewage farm n (top); every fifth congener is denoted.

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Molecular Markers in Environmental Geochemistry - American

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