Environ. Sci. Technol. 2001, 35, 2334-2340
Capping Efficiency for Metal-Contaminated Marine Sediment under Conditions of Submarine Groundwater Discharge CHUNHUA LIU,* JENNIFER A. JAY,† RAVEENDRA IKA,‡ JAMES P. SHINE,† AND TIMOTHY E. FORD† Environmental Science and Engineering Program, Department of Environmental Health, Harvard School of Public Health, 655 Huntington Avenue, Boston, Massachusetts 02115
Theoretical estimations and laboratory studies suggest that capping can effectively retard contaminant transport from sediments under undisturbed conditions. However, contaminated near-shore areas, commonly selected as capping sites, are frequently subjected to submarine groundwater discharge (SGD). Column experiments were set up in the laboratory to simulate metal transport through sediment and capping material in the presence and absence of SGD. In the absence of SGD, capping enhanced Mo flux and initial Mn flux while having no effect in retarding Fe flux, presumably due to altered redox conditions. This effect was more pronounced in the presence of SGD (4.7 × 10-4 m/h specific discharge). Capping enhanced Cd flux and initial fluxes of Ni, Cu, and Zn under conditions of simulated SGD, which may be caused by co-transport with Mn and Fe and oxidation of sulfide. Capping retarded Cr and Pb fluxes and steady-state Ni, Cu, Zn, and Fe fluxes in the presence of simulated SGD. However, capping efficiency decreased relative to that with no SGD. Elevated Mn concentration was detected at the capping surface with simulated SGD. Results indicate that advective flow may lead to significantly higher metal fluxes than those under undisturbed conditions.
Introduction The sediments in many rivers, lakes, and coastal areas are highly contaminated by pollutants (e.g., nutrients, hydrophobic organic compounds, and heavy metals) as a result of past industrial, municipal, and agricultural pollution. Oneeighth to one-fourth of all Superfund National Priority List sites require contaminated sediment remediation (1). The contaminants can subsequently migrate from sediments and adversely affect aquatic ecosystems long after active sources of pollution have been eliminated. For example, an EPA study of the Great Lakes found that more than 90% of the ongoing polychlorinated biphenyl (PCB) contamination in Green Bay sport fish came from contaminated bottom sediments (2). One remediation option receiving considerable attention for application to sediment in low hydrodynamic energy environments is subaqueous containment, termed capping
(3). There are two kinds of capping: in situ capping (ISC) and ex-situ capping (ESC). ISC is a form of in-place containment, referring to placement of a covering or cap over an in-situ deposit of contaminated sediment (3). ESC is a two-phase process. First, dredged contaminated sediment (usually fine-grained silt and clay) is shipped to the capping site. After deposition this layer is overlain by a sufficiently thick layer of clean rock, silt, silty sand, sand, or geotextile fabric to isolate the contaminated sediment from the surrounding aquatic ecosystem (4). The effectiveness of capping depends on transformation and transport processes. Transformation processes may significantly change contaminant partitioning between sediment and pore water. Transport processes can be divided into two categories: molecular diffusion and advective flow. Theoretical estimations suggest that capping is highly efficient in retarding contaminant transport if molecular diffusion is the major transport mechanism (4, 5). Laboratory and field studies suggest that sufficiently thick caps have been effective in retarding chemical migration (6-8, and others). Tides, storms, waves, bioturbation, or anthropogenic activities may cause intermittent advective flow. Submarine groundwater discharge (SGD), groundwater discharge into marine water through marine sediments, will cause continuous advective flow through the sediment and is far more significant and widespread than had been previously thought (9). The near-shore portions of lakes and streams in the midwestern portions of the U.S. commonly function as groundwater discharge sites (3). Groundwater flows toward the coastal area, where it is deflected upward by salinity density gradients. Consequently, most groundwater discharge from unconfined aquifers is restricted largely to the upper portion of the aquifer near the shoreline and the flow rate is thought to be much faster than the mean rate of groundwater movement throughout the entire aquifer (10). Not only does submarine groundwater infiltrate the sediment-water interface in coastal areas, but marine water itself can also be circulated through sediments in the continental shelf ecosystems. The magnitude of this cycling process is enormous, equal to about one-fourth of the evaporation, precipitation, and drainage back through river systems (10). Specific discharge rates of submarine flow reported by various field studies range from 1.08 × 10-6 to 0.3 m/h (9-19). A significant influence of advective flow on contaminant porewater concentration profiles in lake sediment has been observed at specific discharges less than 1.14 × 10-8 m/h (20). Contaminated nearshore areas are often candidate sites for capping. However, the effectiveness of nearshore capping may be compromised by the presence of SGD. It is recommended by the U.S. EPA that both advective and diffusive processes should be considered in determining the necessary design for isolation (3). However, often, neither monitoring nor evaluation of hydraulic condition is conducted for capping projects. In addition, no laboratory and field studies have been conducted to evaluate capping efficiency under conditions of advective flow. The goal of this laboratory study was to investigate capping efficiency for heavy-metalcontaminated sediments under controlled conditions of both advective and diffusive flows.
Materials and Methods * Corresponding author phone: (617)395-5576; fax: (617)395-5001; e-mail:
[email protected]. Present address: Gradient Corporation, 238 Main Street, Cambridge, MA 02142. † Harvard School of Public Health. ‡ Envitec Corporation. 2334
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Material Collection. Sediment was collected from a heavymetal-contaminated nearshore site in New Bedford Harbor, MA in October 1998. Sediment was stored at room temperature (∼20 °C) for less than one week before large particles 10.1021/es0015702 CCC: $20.00
2001 American Chemical Society Published on Web 04/06/2001
TABLE 1. Characterization of Simulated Groundwater Used in the Experiment
this study RIAa
Ni
Cr
Cu
0.72 NA
ND NA
22.1 NDb
metal concentration (ppb) Zn Mo Cd 7.72 29
0.24 NA
0 NA
Pb
Mn
Fe
dissolved O2 (ppm)
pH
OC (ppm)
0.34 NA
246 270
525 130
1 segment, to produce the cut-point, slope, and 95% confidence interval of each segment (23). The regression slope normalized to the sediment surface area in the column (0.0227 m2) provided the flow-normalized flux value (nmol m-2 day-1). The efficiency of capping was assessed by comparison of fluxes and total metal release through time. For columns with simulated groundwater inflow, cumulative dissolved metal release at a certain time point, t, was calculated by summing the metal release prior to t, derived from dissolved metal concentration and volume of effluent water collected. Metal input from groundwater was not subtracted from the metal release. The rationale for this decision is that metal concentrations in the groundwaters in this study were within the range of metal concentrations in natural groundwaters (24) and that the bottom sand may adsorb most metals from groundwater. The flow rate during the experiment was reasonably uniform and close for the two columns ((4.75 ( 0.04) × 10-4 and (4.74 ( 0.02) × 10-4 m/h specific discharge for the uncapped and capped column, respectively). Thus, the curve of cumulative metal release (mol) vs time (day) was plotted for each metal and flux was calculated using the method discussed above. The efficiency of capping was evaluated with regard to fluxes, total metal release through time, and capping efficiency in the absence of groundwater inflow.
Results Metal Release from Columns with No Simulated SGD. For most metals an initial high flux was followed by a decreased flux, which is assumed to reach steady-state. Shown in Table 2 are the steady-state metal fluxes followed by 95% confidence intervals. Initial higher fluxes of Ni, Cr, Cu, and Fe occurred within 2 weeks after initiating the experiment. Negative fluxes (e.g., Mn, Fe, and Pb fluxes from both capped and uncapped columns) suggest metal in the overlying water is adsorbed by the solid phase (capping material or sediment). Capping retarded Ni, Cr, Cu, Zn, Cd, Pb, and Mn fluxes from contaminated sediment while increasing Mo flux and the initial Mn flux. Capping appeared to have no significant effect in retarding Fe flux. 2336
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Metal Release from Columns with Simulated SGD. As in the case of no SGD, for most metals an initial high flux was followed by a decreased flux, which is assumed to reach steady-state (Figure 1). For Ni, Cu, Zn, and Cd, initial high fluxes were followed by decreased fluxes around the point when sediment pore water breakthrough occurred, suggesting that high initial fluxes were mainly caused by migration of original pore water. The data suggest that capping enhanced initial fluxes of Ni, Cu, Zn, Cd, Mn, and Fe. Capping enhanced steady-state Mo, Cd, and Mn fluxes; while the increase of Mn flux is not statistically significant (Table 2). Capping decreased steady-state fluxes for all other metals. Comparison of fluxes in the presence and absence of groundwater discharge suggests that SGD significantly increased fluxes for almost all metals from both capped and uncapped sediment. EH Distribution in Sediment and Capping Layers. Oxic and anoxic environments were formed in both capped and uncapped columns (Figure 2). Although the oxic/anoxic boundary in the uncapped column was mainly at the watersediment interface, it extended throughout the whole capping layer in the capped column. In addition, part of the capped sediment had lower EH than the uncapped sediment. Dissolved oxygen in the overlying water was uniformly 4-5 ppm for capped columns and decreased from 4-5 ppm at the surface to 0.5 ppm at the bottom for uncapped columns in both the absence and presence of SGD. Surface Mn Accumulation. Brown material was observed at the capping surface in the column with simulated SGD (Figure 3), which started within 1 week after initiating the experiment. Significant elevation in Mn concentrations was detected in this brown material (Table 3). No significant elevation of other metals was detected.
Discussion Consolidation Effect. Disturbance of sediment and exposure of anoxic sediment to an oxic environment may increase metal mobility and cause high pore-water concentrations in the sediment (25). In the presence of SGD, groundwater inflow transported the pore water through the sediment and capping material and caused an initial high metal flux. In the absence of simulated groundwater flow, the consolidation process may transport the pore water and cause high initial fluxes from uncapped sediment to the overlying water (e.g, Ni, Cr, Cu, Zn, and Fe). Some level of consolidation will naturally occur at any capping site as underlying sediment is compressed and capping material settles in place. Studies have shown that most consolidation occurs during the first 1-2 weeks following deposition of dredged material (26, 27). An estimation according to water content change in the sediment before and after the experiment suggests that about 240 and
FIGURE 1. Cumulative metal release vs time for columns with simulated groundwater inflow. 330 mL of sediment pore water was expelled by consolidation for uncapped and capped columns, respectively. For capped columns, this amount of water can penetrate a 30-cm capping layer, which is larger than the experimental 15-cm capping layer. However, this effect may be successfully mitigated by the retardation effect of capping materials.
Capping Efficiency. Capping retarded release of Ni, Cr, Cu, Zn, Cd, Pb, and Fe from contaminated sediment in the absence of simulated groundwater inflow. This agrees with previous studies showing 90% reduction of Zn flux by capping under undisturbed conditions (6). Capping effectively retarded Cr and Pb transport even under conditions of SGD, VOL. 35, NO. 11, 2001 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 2. EH Distribution along columns with simulated groundwater inflow. although the effects were less dramatic than those in the absence of SGD. Cr and Pb bind strongly to fulvic acids (28) and only a very small fraction is thought to be Mn-Fe bound in sediments (e.g., ref 29). Co-transport with Mn is thought to be less significant for Cr and Pb release than for other metals. For example, metal elution with dissolution of Mn and Fe was observed for other metals but not for Pb in studies of diverse soil types (30). In addition, capped sediment may decrease dissolved Pb concentration in the sediment pore water. At pH 8, the solubility of Pb increases dramatically as p increases from -4 to -3 (31). Average p values in this experiment for all the columns were approximately -4 (EH ) -237 mV) and pH was approximately 8, which means the experimental conditions were within the range in which Pb solubility is sensitive to redox change. Uncapped columns had a p of approximately -4 while capped columns had a p of approximately -6 (EH ) -350 mV). Thus, capping lowered p values in part of the sediment, which may cause a decrease in soluble Pb concentration in the sediment pore water and explain lower Pb fluxes from capped columns. However, capping appeared to have no significant effect on, or may even enhance, initial Mn flux and steady-state fluxes of Mo and Fe both in the absence and presence of SGD. Capping may act to lower redox potential, in turn favoring transformation of Mn(IV) and Fe(III) to more soluble Mn(II) and Fe(II). Previous studies (6, 32) have observed
increased concentrations of soluble Mn(II) and Fe(II) at the expense of Mn(IV) and Fe(III) with sediment depth. However, whether capping will enhance Mn and Fe release relies not only on pore water concentrations, but also on the path from the sediment to the overlying water, and other environmental factors such as organic and inorganic ligand concentrations. In anoxic sediments, Mo occurs mainly in its lower valance state (IV) as a sulfide complex (33). However, all the Mo species in the lower oxidation states are progressively unstable and tend to revert to Mo(VI). The critical EH value for Mo(VI)/ Mo(IV) transformation is around -231 mV at pH 8 (34). MoO42- is soluble and the predominant species at pH 7-8 (35); while Mo(IV) has low solubility compared to Mo(VI). For both capped and uncapped columns, the EH values detected in the bottom sediment were less than the critical EH, while the EH values measured in the upper part of the sediment being greater than the critical EH. In addition, EH values for the whole capping depth were greater than the critical EH value. Sediment particles containing Mo may be transported by groundwater and stay in an environment appropriate for oxidation longer in capped columns, enhancing Mo release. Capping under conditions of SGD enhanced Cd transport relative to the uncapped case. In an anoxic environment, Cd is mainly bound to sulfide, and oxidation of sulfide may enhance Cd release. Previous studies (25, 36) suggested that small changes from reduced to oxidized conditions enhance Cd release. Changes of redox conditions caused by the addition of a capping layer therefore may enhance Cd flux to the overlying water. Co-transport with Mn may also contribute to the higher Cd flux from the capped column. Fate and transport of Ni and Zn in aquatic environments are highly related to manganese/iron oxides. Fate and transport of Cu are related to both manganese oxides and fulvic acids (28). When Mn is highly released under conditions of SGD, Mn-bound metals may be co-transported and released, thus causing high initial metal fluxes and higher total metal release over long time periods. Although only elevated Mn concentrations were measured in surface material, other metals may not have been detected at elevated concentrations because of the difficulty in separating this material from underlying capping material. High initial fluxes
FIGURE 3. Surface deposition of Mn (left: capped column with simulated SGD; right: capped column with no simulated SGD).
TABLE 3. Metal Concentrations in the Original Sand and in the Solid Material Accumulated at the Capping Surface in the Column with Simulated Groundwater Inflowa metal concentration (µg/g dry sediment) Ni
Cr
Cu
Zn
Mo
Cd
Pb
Mn
Fe
original sand 3.37 ( 0.55 4.44 ( 0.07 1.67 ( 0.09 27.2 ( 1.1 0.42 ( 0.03 0.054 ( 0.0007 13.16 ( 0.55 148 ( 5.9 5297 ( 229 surface brown 2.49 ( 0.30 2.8 ( 2.0 2.08 ( 0.39 25.1 ( 3.1 0.36 ( 0.05 0.044 ( 0.011 14.5 ( 2.8 554 ( 61 5990 ( 966 material a
The number following ( gives the 95% confidence interval.
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of the above metals may also be caused by oxidation of sulfide in the oxic and oxic-anoxic boundary area in capped columns. After initial fluxes, capping may have some effect in retarding fluxes of these metals. However, the efficiency was significantly decreased when compared with undisturbed conditions. Capping may be a good option for Cr and Pb in the presence and absence of SGD. However, it may not be a satisfactory remediation option for redox-sensitive metals such as Mo, Mn, and Fe in the presence or absence of SGD. Other metals that are likely to be co-transported with Mn and Fe (e.g., Ni, Cu, Cd, and Zn) may not be retarded under conditions of SGD, although capping may decrease their release under undisturbed conditions. Effect of Simulated Groundwater Inflow. Advective transport of metals due to simulated SGD is highly significant in our experiment. Movement of sediment into capping layers by SGD was observed in the laboratory, physically decreasing the capping depth. Because Mn2+ is not stable under oxic conditions, large amounts of Mn2+ transported by SGD to the overlying water can rapidly accumulate at the capping surface as MnO2. MnO2 can then sorb other metals (37). Though no significant elevation in concentrations of other metals was observed in this material, accumulation of other heavy metals from sediment or other sources at the capping surface may occur over longer time periods. Ecological Significance. The magnitude, and thus, ecological significance, of metal fluxes to the aquatic system is highly dependent on hydrology and background metal concentrations in the system. To begin to examine the significance of our flux data, we compared steady-state sediment-water fluxes from columns with simulated SGD with metal inventories in the overlying water column quoted from previous research from our laboratory (21). The ratio of inventory over flux was greater than 7 yr for Ni, Cu, and Zn. However, estimated fluxes could supply the total mass of Pb and Cd in this aquatic system in as short as 50 days and 2.6 yr, respectively, without capping, and 150 days and 1.6 yr, respectively, with capping under conditions of SGD. We conclude that greater emphasis on groundwater hydrology is critical before selection of near-shore sites for capping or disposal of contaminated sediment. Even in the absence of SGD, an advective flux may still be observed due to consolidation, waves, tides, bioturbation, and anthropogenic activities, etc. Thus caution needs to be taken when designing a capping project in order to prevent release of toxic metals to the overlying water.
Acknowledgments This publication was made possible by Grant 2 P42 ES-05947 from the National Institute of Environmental Health Sciences, NIH, with funding provided by U.S. EPA. Its contents are solely the responsibility of the authors and do not necessarily represent the official views of the NIEHS, NIH, or EPA. The authors would like to thank MIT Sea Grant for financial support. The authors also express their thanks to Nick Lupoli and Robert Weker for laboratory support and Paul Catalano for assistance with statistical analysis.
Supporting Information Available Submarine groundwater discharge rates from various field studies, quality control data, simultaneously extracted metal content of sediment, and figures showing salt release and movement of sediment into the capping layer in the column with simulated groundwater inflow. This material is available free of charge via the Internet at http://pubs.acs.org.
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Received for review August 9, 2000. Revised manuscript received January 22, 2001. Accepted January 24, 2001. ES0015702
