Published in J. Environ. Qual. 32:2421-2427 (2003).
© ASA, CSSA, SSSA
677 S. Segoe Rd., Madison, WI 53711 USA
TECHNICAL REPORTS
Wetlands and Aquatic Processes
Effects of Static vs. Tidal Hydrology on Pollutant Transformation in Wetland Sediments
W. James Catallo*,a and
Thomas Junkb
a Lab. for Ecological Chemistry, CBS Dep., School of Veterinary Medicine, Louisiana State Univ., Baton Rouge, LA 70803
b Dep. of Chemistry, Univ. of Louisiana-Monroe, Monroe, LA
* Corresponding author (jcatallo{at}mail.vetmed.lsu.edu).
Received for publication May 7, 2002.
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ABSTRACT
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This work addressed effects of hydrology on biogeochemical processes relevant to pollutant chemical transformation in wetland sediments. Microcosms were designed to impose three hydrologic conditions on salt marsh sediments: (i) drained–oxidized redox potenial (Eh); (ii) flooded–reduced Eh and, (iii) diurnal tide–oscillating Eh. The test chemicals were N- and/or S-heterocycles (NSHs) including quinoxaline (1,4-benzodiazine), 2-methylquinoxaline(2-methyl-1,4-benzodiazine), 2,3-dimethylquinoxalinen (2,3-dimethyl-1,4,benzodiazine), phenazine (2,3,5,6-dibenzo-1,4-diazine), acridine (2,3,5,6-dibenzopyridine), dibenzothiophene (2,3,5-dibenzothiophene), phenothiazine (dibenzo-1,4-thiazine), and phenanthridine (2,3-benzoisoquinoline). Biogeochemical processes reflected the hydrologic conditions in ways analogous to field settings, e.g., Eh characteristics were drastically different: static (flooded and drained) systems had reduced (µ = -428 mV ± 57) and oxidized (µ = +73 mV ± 32) values, respectively, with no evidence of periodic variation while the tidal systems exhibited regularly oscillating Eh (amplitudes 40–250 mV). Sediment trace gases also corresponded to the Eh, with the major species detected being CO2 and H2O (drained, tidal) vs. CO2 + H2O + sulfides + methane (flooded). The NSH transformation rates were different in each hydrologic regime and decreased as follows: tidal 
drained >> flooded. These results indicated that there were subtle differences in NSH processing in drained and tidal systems, but both of these systems transformed NSHs faster and to lower levels than flooded sediments. These data suggest that in situ remediation options that preserve wetland integrity and tidal hydrology can be as or more effective than static conditions that obtain in approaches such as impoundment and excavation–upland placement.
Abbreviations: Ahs, aromatic hydrocarbons • DCM, dichloromethane (methylene chloride) • Eh, redox potential • FTIR, fourier transform infrared spectrometry • GC-MS, gas chromatography–mass spectrometry • NSHs, N- and S- heterocycles • SCE, saturated calomel–KCl reference electrode
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INTRODUCTION
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HYDROLOGY is a major determinant of biogeochemical processes that govern transport and transformation of many chemicals in wetland sediment systems. Biogeochemical refers to a group of coupled biological (e.g., microbial, plant) and physicochemical (e.g., flood-drain events from tides and floods) processes that to a large extent determine the productivity, habitat quality, and regional–global significance of wetland and other ecosystems (Catallo et al., 1999; Odum, 1983). It has been well established that the degree and duration of sediment saturation as well as intrinsic microbial, mineralogical, and chemical variables determine the electrochemical status (i.e., redox potential, or Eh) of sediments (Ponnamperuma, 1972; DeLaune et al., 1997; Guo et al., 1997; reviewed in Catallo, 1999). It has also been shown repeatedly that the Eh is a "master variable" that greatly influences the disposition and transformation of sediment constituents including C, H, O, N, P, S, Fe, Mn, and a range of natural and anthropogenic organic chemicals (Catallo et al., 1999; Guo et al., 1997; Mulbah et al., 2000; DeLaune et al., 1997, 2000; Gambrell, 1994).
Previous work showed that degradation and transformation rates and pathways of pollutant aromatic hydrocarbons (AHs) and N-, O-, and S-heterocycles differed significantly in marine sediments of different particle sizes and under oxidized vs. reduced conditions (Catallo, 1996a). In these experiments, AH and NSH transformation was evaluated in controlled Eh/pH reactors containing sediment slurries maintained as oxidized, well-drained, moderately reducing–anoxic, and highly reducing–methanogenic. Measured AH and NSH degradation rates generally were: oxidized moderately reducing >> methanogenic.
Apart from the differential NSH and AH transformation rates observed in these experiments, spectral analyses of time series showed that the Eh measured at replicate Pt electrodes responded quickly and significantly to changing chemical conditions and physical interventions (Catallo, 1999). It was clear that the interaction of tides with sediments should be considered as a significant variable (i.e., a treatment) in further studies of biogeochemical processing of contaminants in coastal wetlands. The dynamic nature of sediment Eh, particularly with respect to diurnal and semidiurnal variations, has not been widely studied (Catallo, 1999).
The aims of the current work were to evaluate the effects of simulated diurnal tides (vs. static flooded or drained conditions) on biogeochemical signals in salt marsh sediment microcosms and evaluate these conditions with respect to NSH compound transformation. The NSHs are found widely in sediments near urban centers, ports–harbors, industrial–energy activity, airports, and landfills and are of concern because many of them are toxic, mutagenic, and/or carcinogenic in exposed organisms (Warshawsky, 1992; Turov et al., 1987).
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MATERIALS AND METHODS
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Hydrodynamic Microcosms
These systems were 76-L (20-gallon) glass enclosures containing sediment columns (Fig. 1)
. With appropriate containment and plumbing this basic design easily scales by up to 100x, and can be modified for other flow regimes (e.g., semidiurnal or longer period water fluctuation). Surface (10 cm) sediments were collected from streamside and shallow channel areas of a natural salt marsh in Terrebonne, LA. The sediments were collected with shovels and placed in covered barrels along with estuarine water collected in situ. During collection, care was taken to avoid unnecessary impacts on the natural system: plants were not collected, and excavated sediments were replaced with sediments dredged from a proximal waterway. The bulk sediment samples were returned to the laboratory and immediately mixed with sand and commercial potting soil (5% each, on a wt. basis). The measured dry weight bulk density of these experimental sediments were 0.48 to 0.50 g/mL, which was somewhat higher than average for salt marshes (i.e., typically 0.40 g/mL) (Hatton et al., 1983). The sediments then were added to a height of 19 cm in cylinders (24 cm tall, 12 cm in diam.) made of 1.2 cm diameter plastic mesh. These were positioned in the center of the glass enclosures with no physical contact between sediments and solid surfaces, except at the bases. The enclosures were covered (not sealed) with plexiglass containing ports for gas sampling and electrode leads. A diurnal tide was simulated by pumping artificial seawater (15 g/L) into and out of the system at rates needed to provide a smooth flood-drain regime once daily (i.e., a diurnal tide: one high and one low per 24 h). The pumps were timer-controlled. The seawater-holding reservoirs were aerated continuously. At "high tide," the sediment columns were covered with approximately 8 cm of water. At "low tide" about 5 cm of water remained in the aquaria; thus, there was about 14 cm of "tide range" per sediment column. Two Eh electrodes (below) were positioned in the top 1 to 5 cm of sediment (surface) and two additional electrodes were placed deep within the column, near the continually flooded zone at the bottom of the sediment cylinders (i.e., about 15 cm below the sediment surface). Water in each of the microcosms and reservoirs was changed weekly. The used water from each enclosure–reservoir was extracted and analyzed (below) to detect and compensate for any wash out of NSH compounds.
Redox potential (Eh) data were collected using Pt wire-SCE reference electrode cells connected to a custom multichannel logger constructed in-house. The platinum electrodes were constructed by welding Cu wire to 0.5-cm lengths of 12 gauge Pt wire. The junction and Cu lead were enclosed in glass tubes of varying lengths, and sealed using a blowtorch (silicone sealant was used to seal the top ends). Hence, a 0.5-cm length of Pt wire was exposed to the sediments while junctions and leads were enclosed in sealed glass. Each electrode was calibrated before use with a 1% quinhydrone solution. Sampling was performed at rates between 1 Hz and 1/h, depending on the application (in general, the rates were 1–2/h).
Trace gases were analyzed by direct sampling of headspace gases under vacuum to a 10-m cell connected to a Buck Scientific FTIR spectrometer with gold optics and 4 cm-1 resolution. Compounds were identified using authentic gas standards (Scott) and published digital spectral libraries. Digital background subtractions were performed using spectra of normal laboratory air sampled immediately before sampling the microcosms, and handled identically. Thus, CO2 and water vapor signals in the experimental spectra represent excesses of these materials in the microcosm headspaces vs. the laboratory air.
Synthesis of Deuterated NSH Isotopic Dilution Standards
To quantify minor or subtle differences in transformation of NSH target compounds (Fig. 2)
between hydrologic treatments, it is desirable to have stable isotope-labeled (e.g., deuterated) standards for each target compound of interest. These standards are added to sediments before extraction, and serve as internal "monitors" of extraction and analytical efficiencies for the target compounds. As these deuterated standards have properties virtually identical to the target compounds, their losses in the various steps of extraction and analysis are the same as the compounds under study. With appropriate controls and other procedures (e.g., tuning, method blanks, calibration with respect to linear range, appropriate signal/noise requirements), this allows for truly quantitative analyses (i.e., small percent differences can be measured), rather than semiquantitative data (i.e., order-of-magnitude differences) provided by many analytical approaches (Catallo, 1996b). Unfortunately, commercial availability of many deuterated NSHs is limited or nonexistent. As a result, methods were developed to completely label the target compounds with deuterium either using de novo or postsynthetic approaches (Junk and Catallo, 1996, 1998), the latter typically using supercritical deuterium oxide (D2O), or "heavy water." A schematic example of this strategy is given below:
Dibenzothiophene-d8, phenothiazine-d8, and phenanthridine-d9 were prepared by base-catalyzed supercritical isotope exchange following Junk and Catallo (1996)(1997). In general, 30 mL (iv) Hastelloy-C22 autoclaves were charged with the 0.3 g of the respective NSH substrate, 15 mL deuterium oxide, and 0.1 mL 40% sodium deuteroxide (i.e., NaOD in D2O) solution (Aldrich). Exchange was achieved by heating to 400°C for 6 h. Compounds were collected by filtration and purified by flash chromatography over 200 mesh silica gel using hexane as mobile phase for dibenzothiophene and phenanthridine, and DCM for phenothiazine. Yields for dibenzothiophene and phenanthridine were >75%, as reported for a range of other AHs and NSHs (Junk and Catallo, 1996, 1998), and phenothiazine was 86%. No attempts were made to preclude the facile back-exchange of the N–H proton of phenothiazine during workup under ambient (open-air) conditions because it would be exposed to water during the sample extraction anyway.
Quinoxaline-d6, 2-methylquinoxaline-d8, and 2,3-dimethylquinoxaline-d10 also required custom syntheses to provide yields and quantities sufficient for the current work. These have been detailed elsewhere (Junk et al., 1997).
Attempts to prepare phenazine-d8 and acridine-d9 by (i) base-catalyzed supercritical isotope exchange at 400°C for 6 h following Junk and Catallo (1996)(1997), (ii) acid catalyzed isotope exchange at 220°C for 2 d, and (iii) palladium catalyzed exchange at 250°C for 2 d all resulted in extensive substrate decomposition. All compounds were subsequently prepared by base-catalyzed near-critical exchange at 300°C for 4 h. Exchange was performed by heating 300 mg substrate, 15 mL D2O, and 0.1 mL 40% NaOD in an autoclave. The crude products were extracted with dichloromethane and the solvent evaporated and deuterated quinoxaline and 2,3-dimethylquinoxaline were purified via microdistillation. Phenazine and acridine were crystallized from methanol. Further purification was achieved by flash chromatography (200 mesh silica gel, hexane/DCM 10:1 v/v). Yields ranged from 45 to 55%.
NSH Transformation Studies
The microcosms for the two static hydrologic treatments (drained and flooded) were established identically with the tidal systems. Three sediment columns were equilibrated under drained, flooded, and tidal conditions as described above for 2 wk. On a weekly basis throughout the equilibration and experimental periods, the water was changed in the flooded and tidal systems, while the drained system was flooded briefly (1 h) and then redrained. After the 2-wk equilibration, sediments were collected from the surface third of the enclosures from each hydrologic type. Protiated (hydrogenated) NSH target compounds in acetone were added to the sediments with mechanical mixing (2 h) to provide uniform levels of the individual contaminants between 200 and 400 mg/kg (ppm) on a weight basis. Care was taken to maintain the biogeochemical condition of the sediments (oxidized vs. reduced) during mixing by purging the system and flooding the headspace of the mixing enclosure with air or Ar, respectively. The sediments then were reintroduced to their respective microcosms in fresh column enclosures. The three hydrologic regimes were initiated and "time zero" samples were collected within 8 h of placement using a glass corer. Subsequent samples were taken at intervals (1–4 wk) depending on estimators of microbial activity and previous recovery of NSH target analytes. After collection, the sediment–water samples (ca. 10 g) were weighed and amended with deuterated isotopic dilution standards for each NSH target analyte at the levels near the beginning sediment concentrations. The samples then were Soxhlet extracted with DCM for 48 h, with the extract subsequently dried (Na2SO4) and concentrated under N2. The residual sediments in the extraction thimbles were dried and weighed. The sample extracts then were subjected to GC-MS in the full scan mode. The NSH target analyte concentrations in the extract were determined vs. the isotopic dilution standard by ratios of corresponding peak integrals. These extract concentrations were corrected for concentration factor and dry weight of sediment in the original sample. Reservoir and water samples also were collected and extracted–analyzed for the NSH analytes and transformation products at each experimental time point.
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RESULTS AND DISCUSSION
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Microcosm Processes
The biogeochemical behavior of sediment columns in the hydrodynamic microcoms was similar to that observed in natural settings (Catallo, 1999), and in accord with theory (Odum, 1983). Redox potential time series and sediment trace gas signatures from the different treatments were analogous to those encountered in field settings. With respect to the redox potential, the continuously flooded and drained systems had reduced (µ = -428 mV ± 57) and oxidized (µ = +73 mV ± 32) Eh values, respectively, with no evidence of daily or longer periodic variation. The routine (weekly) wetting–draining of the drained system, and water changing in the flooded system did not affect these trends: apparently the pre-equilibration of sediments before addition of the NSH analytes afforded a degree of redox potential poising (i.e., electrochemical buffering) that was not affected by these maintenance interventions. The tidal systems, however, exhibited oscillating Eh values, with significant amplitudes (40– 250 mV; Fig. 3)
in surface sediments. Analysis of these signals using the wavelet transform (Mallat, 1998) confirmed the presence of strong diurnal Eh variations, with a mean value period of 23.78 ± 2.10 h. It has been shown that these diurnal signals were (i) reproducible in the tide microcosms, (ii) found in the corresponding larger (1140 L [300 gallon]) tidal mesocosms containing the plant smooth cordgrass (Spartina alterniflora Loisel.) and, (iii) observable in tidal field sites vs. impounded areas not receiving tidal input that showed no diurnal Eh fluctuations (Catallo, 1999). Thermal and light cycles in the laboratory had no discernable effect on the Eh time signature: variation of tide stage throughout the day and at different times of the year (the experiments were conducted in an annex not equipped with climate control) did not affect the periodicity of the tide-driven Eh signal. It can be seen from Fig. 4
that the trace gases leaving the different treatments also reflected biogeochemical status of the system as determined by its prevailing hydrology. As the trace gases detected in the drained and tidal systems were virtually identical, only data from the tidal vs. flooded system are compared in Fig. 4. Clearly, the tidal (and drained) systems showed mainly the products of aerobic respiratory processes in situ (i.e., CO2 and water), while the flooded system showed evidence obligate anaerobic respiration (i.e., sulfides and methane). These trace gas patterns did not vary within treatment type over the course of the experiments.
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Fig. 4. Typical sediment trace gas spectra observed in the hydrologic treatments. Tidal and drained systems were similar throughout the runs.
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As an exercise, the effect of sampling frequency on the detection and accurate isolation of the 24-h waveform in the Eh data was examined by simulating the time series derived from a 16-h sampling frequency. This was chosen because many laboratory and most field studies acquire Eh data with rates on this order or less (e.g., 1 sample/day and longer) (Catallo, 1999). The results were striking: sampling at 1/16 h eliminated the detection of the 24-h waveform in the Eh time series, and subsequent wavelet analysis gave the (erroneous) result of a waveform close to 48 h in period. Thus, in diurnal and semidiurnal tidal systems, Eh sampling frequencies must be rapid enough to detect significant changes occurring inside of several hours. For a semidiurnal system (two highs and lows a day), this sampling frequency should be hourly (i.e., 1/h) or greater (Brockwell and Davis, 1991).
The Eh traces shown in Fig. 3 are characteristic of the tidal systems in three main respects: (i) the sinusoidal Eh oscillations continued for as long as the tides were applied, but ceased immediately when static conditions ensued; (ii) the waveforms are asymmetric, exhibiting hysteresis with respect to oxidation and reduction; and (iii) electrodes at the different depths were out of phase (as expected), with deeper electrodes showing smaller amplitudes (probably reflecting the effects of compaction). The hysteresis of measured Eh (Item 2, above) suggests that the Pt electrode-SCE cell is quasireversible with respect to oxidation and reduction in sediments (i.e., it is patently non-Nernstian) and this almost certainly reflects compositional as well as bulk phase chemical variables in situ. Unpacking these factors and adequately describing the physics involved is far beyond the scope of this communication, and might well be impossible given the current level of theory on heterogeneous electrochemical systems. In spite of this problem, the electrodes in the tidal systems remained functional throughout the duration of the experiments, and rarely had to be replaced. The electrodes in the static oxidized and reduced microcosms, however, were replaced more frequently because of passivation of active surface by oxides and sulfides (respectively). It would seem that dynamic Eh conditions reduced the level of passivation of the working electrodes, and provided for conditions most favorable for obtaining accurate potential values vs. the more static, well-poised conditions in the other treatments. "Reconditioning" of solid electrode surfaces by alternating oxidation–reduction cycles (i.e., cyclic removal of surface metal oxides and sulfides by alternating redox processes) has not been reported, and represents an area of further research.
NSH Transformation
Transformation of the NSH analytes in the sediment columns was different in each of the hydrologic regimes, with the rate of transformation generally decreasing in the following order: tidal drained >> flooded (Table 1; Fig. 5–7)
. For convenience, consider the time in weeks required for each NSH analyte to be degraded to <25 mg/kg (ppm), which represents approximately a 10-fold concentration reduction. Except for one sample showing a concentration spike (Week 17), the oxidized conditions in the drained systems (Fig. 5) allowed for reduction of NSH concentrations to <25 mg/kg (ppm) in 18 wk. Similarly under tidal conditions (Fig. 7), all NSHs except for dibenzothiophene were transformed so that recoveries were <25 mg/kg (ppm). Under flooded/reducing conditions (Fig. 6), only quinoxaline, 2-methylquinoxaline, 2,3-dimethylquinoxaline, and phenothiazine were degraded to recovery levels <25 mg/kg (ppm) within 20 wk. The sediment concentrations of the other NSHs remained static, with recovery levels remaining relatively constant between 100 and 170 mg/kg (ppm) for acridine, phenanthridine, dibenzothiophene, and phenazine between Weeks 7 and 20. Thus, in terms of NSH transformation rate, the flooded–reducing system was slower in every case vs. both drained and tidal treatments. Conservatively construed, the drained and tidal systems were comparable in terms of NSH transformation over the experimental period. On the other hand, as the data in Table 1 and Fig. 5 and 7 indicate, the transformation of quinoxaline, 2-methylquinoxaline, 2,3-dimethylquinoxaline, phenanthridine, and phenothiazine was more rapid under tidal vs. drained conditions. The drained system showed clearly faster transformation rates vs. tidal in only two cases: acridine and dibenzothiophene. Interestingly, the three quinoxaline compounds and phenothiazine were consistently more labile across all three hydrologic treatments. It is clear from this that, except for quinoxaline, 2-methylquinoxaline, and perhaps phenothiazine, there was a redox effect on transformation rate for the NSH compounds studied here.
Even if one adopts a conservative interpretation, i.e., that there is no difference between drained and tidal systems in terms of NSH degradation, it is obvious that there are large differences between tidal and flooded. Thus, in contaminated systems, the option of excavation followed by upland (aerobic) placement of sediments does not offer a clear advantage in terms of remediation of the contamination itself. On the other hand, impoundment and subsequent extended flooding of the contaminated sediments clearly affords a large disadvantage in this regard: not only is the ecosystem compromised by this kind of intervention, but the residence times of the toxic contaminants are increased. Data presented here and elsewhere (Catallo, 1996a, 1999) suggest that this result is generalizable to a range of toxic hydrocarbons and N-, O-, and S-heterocycles. It is important to note that the waste issue to be remediated needs to be completely understood: while tidal conditions causing oscillating Eh may enhance desired transformation of target pollutants, other materials occurring in complex mixtures may not be affected, or may undergo undesirable reactions resulting in increased toxicity, mobility, and/or availability (Guo et al., 1997). For example, in cases of Hg contamination, tidal conditions might promote increased organic matter mineralization and thereby stimulate the release of Hg ions from sediment organic matter stores to the water. This in turn could increase the production and availability of methylmercury and related toxic species in the system (see Richardson, 1999; Gambrell, 1994).
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CONCLUSIONS
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These transformation studies indicated that tidal flushing optimized the transformation rates of NSH compounds vs. static hydrologic conditions. This means that in situ or "passive" remediation of organic chemical pollution in coastal wetlands could benefit from design features that accommodate prevailing hydroperiods, including tides and seasonal events. For most settings where NSH and AH contamination is present, the well-drained–oxidized approach is not an option unless complete excavation of the system is envisaged. Obviously in this case we are no longer dealing with a "wetland" sediment or ecosystem.
In an abstract sense, the use of hydrologic intervention for simultaneous chemical remediation and ecological recovery of polluted systems seems promising in light of this and related work (Catallo, 1999), particularly when the rates of so-called "passive" remediation are considered. Such an approach would involve, at the very least, attempts to optimize the tidal volume of the wetland without compromising its integrity, e.g., through increased erosion of exposed streamside sediments. Obviously, this kind of undertaking in a real wetland would involve an integrated set of engineering interventions that encompass hydrologic, sedimentologic, and plant–system variables in a progressive sense (Odum, 1983). The same is true of the use of marginally contaminated dredge materials for coastal habitat restoration projects. The authors are aware of no actual cases in which this has been successfully attempted or contemplated. Much further ecological study is called for in mesocosms and other controlled settings where variables and causal relationships can be identified and ranked with respect to holistic endpoints including, but not limited to, pollutant transformation.
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ACKNOWLEDGMENTS
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The authors are indebted to R.S. Murray, Dr. R.D. DeLaune, Dr. W.H. Patrick, Dr. S.A. Barker, and Dr. J.L. Aravena of Louisiana State University, and Dr. P. Roscigno (USDOI/MMS, New Orleans LA). Work was supported by multiyear grants from the U.S. Department of Interior, Mineral Management Service through the Louisiana State Univeristy Coastal Marine Institute, the Louisiana Oil Spill Research and Development Program (Oil Spill Coordinator, Office of the Governor), and private-sector donations.
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REFERENCES
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ABSTRACT
INTRODUCTION
