Greenhouse Experimental Methods Towards in-situ Burial and Restoration of Contaminated Sites in Submerged Wetlands

As a result of commercial and industrial activit ies conducted in the absence of environmental regulations and enforcement in the past, sediments contaminated by organic compounds, heavy metals, and other potentially toxic chemicals have accumulated in many of the world’s deepwater and wet land environments. These sediment-borne contaminants can eventually become incorporated into aquatic food webs and adversely affect ecological receptors like benthic organisms and fish, and u ltimately pose a risk to human health. This laboratory research tested a commercial product AquaBlok (patented, composite-aggregate technology comprised of a solid core, an outer layer of clay material, and polymers) as an in-situ capping technology that could be used to remediate and/or manage contaminated sediments in the New Jersey Hackensack Meadowlands, a superfund site. In a greenhouse setting, tubs containing representative Meadowland marsh soil and water were capped with AquaBlok. This research not only examined the potential use of this product as an in-situ capping material and possible substrate for flora co lonization, but also examined the improvements of the same patented, clay mineral-based composite aggregate technology (SubmerSeed) as an alternative to tradit ional means of wetland p lant propagation. At the end of a two-year period, both the sediment/cap and vegetation plant tissues were examined for metallic contaminants (including Cd, Cr, Cu, Pb, Hg, and Zn). Overall, capping provided a less contaminated substrate. Results indicated that AquaBlok cap alone did not allow contaminants in the sediment below to breakthrough. Nevertheless, vegetation colonization was restricted to a limited number of p lant species. Furthermore, plants growing in AquaBlok were less robust with lower dry weight and s maller root system than plants growing in uncapped sediments despite the fact that their tissue contained smaller amounts of metallic contaminants. The improvements of the clay mineral-based composite aggregate technology (SubmerSeeds) as an alternative to trad itional means of plant propagation worked very well in successfully delivering aquatic plant seeds into permanently inundated conditions.


Introduction
As a resu lt o f co mmercial and indust rial act iv it ies conducted in the absence of environ mental regulations and enforcement in the past, sediments contaminated by organic co mpounds, heavy metals, and other potent ially to xic chemicals have accumu lated in large quantities in the New Jersey Hackensack Meadowlands [1]. The contamination of marin e an d fres h water s ed imen ts with o rg an ic and inorganic pollutants has become a world wide problem with implications for hu man and ecological health [2]. The need for remediat ion or management of contaminated sediments has become increasingly evident Contaminated sediments can be managed in various ways: They can be removed by dredging and treated ex-situ, as appropriate, p rior to disposal. The sediments can also be managed in-situ by capping, or treatment by biolog ical o r chemical means. Am ong these techniques, treatment of contaminated sediment sites with in-situ caps has become an established practice that can provide advantages over other alternative methods [3].
Advantages of in-situ capping relat ive to sediment re mov al include minimal environmental and habitat disturbance, minimal sediment exposure and handling, minimal release of volatile organics and elimination of the need for disposal facilit ies, resulting in lower remediation costs. In addition, in-situ sediment caps are relatively easy to construct, repair or replace as part of ongoing operation and maintenance.
Clean sand has traditionally been emp loyed as capping material and remains a large co mponent of many field-scale capping applications. Sand-based caps have the potential to delay contaminant breakthrough when diffusive transport dominates [4][5], but eventual contaminant breakthrough remains a source of concern. Additionally, traditional sand caps are less effective at sites where groundwater seepage or mobile contaminants are present [4].
Recent research studies have focused on in-situ sequestra tion [6][7], in-situ transformation [8][9] and the development of active caps that incorporate reactive and/or absorptive constituents designed to reduce contaminant and bioavailabi lity [6], [10][11][12][13][14]. Ideally, active caps eliminate the risk of contaminant breakthrough into the overlying water column, and can potentially be implemented as alternative sediment.
The objectives of this laboratory research were to: 1) Test the ability of a patented "active cap" i.e. AquaBlok (AB) to serve as physical barrier between contaminated sediments and overlying biological receptors. 2) Evaluate the ability of plants to colonize AB as an alternative to sediment. 3) Determine if adding organic matter (2% peat) improved AB's suitability as a substrate. 4) Evaluate the effectiveness of AB pellet (Sub merSeeds) as an alternative tool for plant propagation in permanently inundated waters.

Why AquaBl ok TM
AquaBlok (AB) [15] is a co mmercial patented, clay mineral-based composite aggregate technology comprised of a solid core (typically stone aggregate), an outer layer of clay material (bentonite) and polymers. Bentonite, which is well known and widely used throughout the environmental industry, comprises the primary clay material for typical freshwater product formulations, however, other clay or quasi clay-sized materials such as organic matter or plant seeds (Co mposite Seeding Technology "Sub merSeeds TM ") can also be incorporated into product formu lations as needed. Emp irical and preliminary laboratory data fro m the manufacturing industry indicate that AB [16] not only serves as a physical barrier between sediments and overlying biological receptors but it can also serve as a benthic substrate for flora and fauna colonization [17].
Based on a series of EPA reports [18][19], AB has distinct advantages over sand for sediment capping with respect to meet ing targeted remedial capping functions. It offers physical and chemical blocking o f contaminant pathways to overlying receptor organisms and minimizes adverse impacts to wetland hydrology. Granular materials (e.g., sand) can serve as appropriate and adequate capping materials at many sediment remed iation sites [20]. Granular materials are typically inexpensive, often locally available, relatively easy to place and can provide substrate for marsh biota. However, typical granular materials are also relat ively easily eroded and permeable to the d iffusive and adverting movement of dissolved sediment-borne contaminants [21]. Additionally, sand-based remedial caps are often relat ively thick (i.e. on the order of feet rather than inches) in order to adequately overcome these performance limitations. Relatively thick caps may adversely affect waterway navigation at some deepwater sites or adversely impact the hydrology and vegetative commun ities in wetland ecosystems [22][23]. Finer-grained materials (e.g., clays) are typically mo re cohesive, less permeable, and more reactive than sands. A relatively thin clay-based cap (i.e. on the order of inches rather than feet) can provide a better capping remedy that is less disruptive of wetland quality and function. Additionally, clay has a higher cation exchange capacity than sand making it more effect ive in t rapping contaminants such as heavy metals.
The addition of p lants seeds into their formu lation -Co mposite Seeding Technology "SubmerSeeds TM " creates an attractive alternative to trad itional means of plant propagation in wetland/aquatic settings, especially when confronted with the challenges of establishing a favourable vegetative commun ity in permanently inundated conditions.

Experimental Design
To study the effectiveness of AquaBlok (A B) and AquaBlok amended with 2% peat moss (ABPM) as in-situ "active" capping material and benthic substrate for wetland biota in a greenhouse setting, a total of six 100 gallon tubs containing Hackensack Meadowlands soil and water were used during a two year period. Wetland vegetation seeds of 28 co mmon regional species (Table 1) were incorporated into the AquaBlok formu lat ion to produce SubmerSeed TM . Replica sediment samples fro m d ifferent areas representing the marsh were collected fro m the New Jersey Hackensack Meadowlands Marsh. Pre-capping sediments were characterized by measuring % mo isture and % total organic carbon (TOC) (ASTM-D2974 [24]), grain size (ASTM-D422 [25]) and heavy metals of concern (HMOC) (SW 846 Method 7000A [26]).
Each experimental tubs received one of three t reatments in duplicate: Treat ment 1 (Soil) consisting of Hackensack Meadowlands Marsh sediment (sufficient to fill the tub to the 22" mark). Treat ment 2 (AB ) consisting of Hackensack Meadowlands Marsh sediment (sufficient to fill the tub to the Contaminated Sites in Submerged Wetlands 13" mark) and 150 lb of AB (8" hydrated). Treat ment 3 (ABPM) consisting of Hackensack Meadowlands Marsh sediment (sufficient to fill the tub to the 13" mark) and 150 lb of ABPM (8" hydrated). All tubs were filled with Hackensack Meadowlands Marsh water to the rim mark. Marsh sediments and water were allo wed to settle on the tubs for t wo weeks before capping. Once the cap had co mp letely hydrated to 8"-9", appro ximately one week, wetland plant species were sown into the tubes in the form of 1 ½ lb of SubmerSeeds as per the manufacture reco mmendations.
At the end of each growing season, plants were harvested and characterized based on their numbers, size, dry weight, and root system. Plant tissues as well as sediments in the tubs were mon itored for HM OC using the SW 846 Method 7000A [26]. Tables 2 and 3 represent the Meadowlands sediments characterizat ions including percent moisture, ash content (Ash), total organic matter (TOC) and grain sizes. The average percentage moisture of the sediment was 91.1%, ash content varied fro m 11.9 % to 15.2%, and the average total organic matter content was 86.31% (Tab le 2). The grain size distribution included 50.59% of sand (coarse, mediu m, fine and very fine), 33.48% silt, 13.92% clay and 2.19% of pebbles and granules (Tab le 3).Heavy metals of concern present in sediment during the initial collection are presented on Table 4. Based on the Ontario Aquatic Sediment Criterion [27] concentrations for all init ial HMOC were above LEL (Lo west Effects Limit) with Cu (281.66 mg/Kg) and Pb (392.86 mg/Kg) concentrations being above SEL (Severe Effects Limit). During the first growing season, wetland vegetation grew better in uncapped sediments when compared to AquaBlok capped sediments. Zizania aquatica, Alisma subcordatum and Typha angustifolia germinated and developed better in ABPM when co mpared to AB alone. In addition to the above-mentioned species, Phragmites australis fro m the local seed/rhizo me banks equally co lonized both capped and uncapped sediments (Figure 1) Observations of wetland vegetation during the second growing season revealed that plant germination and growth rates in ABPM declined considerably when compared to the AB and Soil treat ments (Figure 1).  Table 5).

Initial pre-capping analysis
The average plant dry weight growing in soil, (control) was 3.8g/plant for Zizania aquatica, 5.4g/plant for Alisma subcordatum, 12.9g/plant for Typha angustifolia and 158.5gr/plant for Phragmites australis. Plants growing in AquaBlok (AB and A BPM) have produced smaller plants with lower dry weight when co mpared to those growing in soil ( Figure 2 and Table 5); 1.2g/plant and 1.33g/plant for Zizania aquatic, 2.1g/plant and 1.7g/plant for Alisma subcordatum, and 10.1 g m/plant for Typha angustifolia.   While growing within the AquaBlok, roots appeared attached to the aggregate core of the SubmerSeeds and continued to be heavily covered by the clay; this being more noticeable in the AB than in A BPM (Figure 4).

Heavy Metals of Concern
Heavy metal of concern concentrations in sediment shows treatment related effects ( Figure 5). Total metal concentrati ons in AquaBlok (A B and ABPM) treated tubs declined significantly after capping fro m 1496.83 mg/ Kg to 330.70 mg/Kg in AB and 315.18 mg/Kg in ABPM. In uncapped tubs (soil), sediment concentrations of Cu (450.67 mg/Kg ) and Pb (460.63 mg/Kg) were consistently above their SEL and Hg (1.21 mg/Kg) and N (45.72 mg/ Kg) were above LEL when compared to capped sediment tubs. Cd and Hg was above LEL in all tubes, uncapped soil had twice as much Cd and five times more Hg than capped ones. Uncapped sediments had 5 to 6 times mo re Cr and Cu than capped sediments. Ni quantities above ELE in uncapped sediments were five and one half times h igher than capped sediments. Pb levels above SEL in uncapped sediments were also five and one half times higher than capped sediments. Figure 5. Amounts of heavy metals of concern variation in sediments at the different experimental treatments (Soil, AB and ABPM) before (initial) and after two vegetation growing seasons Table 6 summarizes the amount of heavy metals of concern concentrated in sediments and plant material g rown within each of the treatments during two growing seasons.
Cad Vegetation growing at the different sediment treat ments (soil, A B and ABPM ) showed great variation in the amounts of HM OC in their t issues ( Figure 6).  6. Amounts of heavy metals of concern (HMOC) in plant tissues from representatives growing in the different experimental sediment treatments (Soil, AB and ABPM) Most HMOC tended to accumulated in h igher amounts in plants' underground structures ( Figure 6). When growing in the control marsh soil Zizania aquatic accumulated 653.7 mg/kg of heavy metals excluding Fe, were 84% of that amount was found in their root tissues with exception of Hg, which was found to be concentrated twice as much in the leaves and stems than the roots. While gro wing in A B and ABPM, the total accu mulat ion of HM OC were 542.76 mg/ kg and 447.77 mg/ kg respectively excluding Fe, were 74% in AB and 69% in ABPM were found in the underground root tissue with exception of Hg wh ich was not detected on roots growing in AB.
Alisma Subcordatum when growing in marsh soil has a total excluding Fe of 1230.31 mg/kg of heavy metals in their tissues were 68% was found in their roots with exception of Hg where mo re than 80% of its total amount was found in the aboveground (leaves and stems) portion of the plants. When growing in AB and ABPM, the total accumulation of HM OC was 343.43 mg/kg and 372.56 mg/kg and 58% o f these metals were found in the belo wground structures of the plant. As in the plants growing in marsh soil, Hg concentrates mostly in the aboveground portion of the plants.
Typha angustifolia has accumulated 840.08 mg/ kg of heavy metals excluding Fe wh ile growing in marsh soil. Fro m th is total, more than 78% was found in their belowground structures. Most heavy metals were found in higher quantities approximately 4 t imes more in the roots than the leaves with exception of Hg and Cr, wh ich had similar amounts throughout the plant. When growing in AB and ABPM, very similar results were observed.
Phragmites australis like the other plants accumulated higher quantities (5 t imes mo re) of heavy metals in their underground portions (rhizo mes and roots). Fro m its 510.6 mg/kg of total heavy metals excluding Fe, 84% was found in tissues bellow the substrate surface with exception of Hg.

Conclusions
Overall, capping provided a less contaminated substrate, 1496.83 mg/ Kg total contaminant of concern versus 330.70 mg/Kg in A B and 315.18 mg/Kg in A BPM. The concentrations of metals in the cap itself were much lower than in the sediments they covered. Co mparison of collection dates showed no significant increase in heavy metals in the cap over a two-year period indicated that the heavy metals below the cap were not breaking through it. A mending the AquaBlok with peat moss (ABPM ) d id not significantly affect metal concentrations or plant gro wth. The init ial germination and growth of plants in ABPM was better when compared to A B alone, but it quickly declined after the first growing season. In general, plants growing in A B and ABPM as an alternative sediment source were less robust than plants growing in uncapped sediments control (soil) despite the fact that they have smaller amounts of heavy metals in their tissues.
Most of the plants growing in the experimental tubs (soil, AB and ABPM) concentrated higher amounts of heavy metals into their roots and/or underground portion of their stems, between 2.5 and 5 times mo re than the amounts concentrated in their aerial stems and leave. This is consistent with other research findings such as the works of Raskin et al. [28], Sawidis et al. [29], Bennett et al. [30], and Reboreda and Caçador [31]. In all, the reduction of heavy metals provided by the capping material d id not increase the growth or the health of vegetation in contaminated environment.
AquaBlok has proven to be an active barrier between contaminants and the biota. Nevertheless, it is not a good substrate for plant colonization despite the addition of organic matter (2% peat moss) to its formu lation. Plants growing in AquaBlo k capped sediments were less robust with lower dry weight and smaller root systems. The improvements of the clay mineral-based composite aggregate technology (SubmerSeeds) as an alternative to traditional means of plant propagation worked very well in successfully delivering aquatic plant seeds into permanently inundated conditions.