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Comparison of heavy metal accumulation in a natural wetland and constructed wetlands receiving acid mine drainage
Ecological Engineering 16 (2001) 487 – 500
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Comparison of heavy metal accumulation in a natural wetland and constructed wetlands receiving acid mine drainage P.A. Mays a,*, G.S. Edwards b,1 a
Tennessee Valley Authority, 129 Pine Rd., Norris, TN 37828, USA b 101 Scenic Dri6e, Oak Ridge, TN 37830, USA
Received 19 February 1999; received in revised form 4 May 2000; accepted 5 June 2000
Abstract Metal accumulations in sediments and plants of constructed and natural wetlands were compared in two wetlands constructed by the Tennessee Valley Authority (TVA) for the treatment of acid mine drainage and a natural wetland. Load rates and removal efficiencies of most metals were generally greater in the constructed wetlands than in the natural wetland. There were similar sediment and plant metal concentrations between one constructed wetland and the natural wetland and greater metal concentrations in the sediments and plants in the other constructed wetland compared to the natural wetland. Data indicate that Mn, Zn, Cu, Ni, B, and Cr are being accumulated in the plants at all three wetlands, although accumulation of metals by these plants accounts for only a small percentage of the removal of the annual metal load supplied to each wetland. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Constructed wetland; Acid mine drainage; Metals; NPDES; Typha latifolia; Juncus effusus; Scirpus cyperinus
1. Introduction The use of constructed wetlands as biogeochemical systems for the treatment of acid mine drainage has developed rapidly over the last few decades in North America and worldwide (Hammer, 1989; Wieder, 1989; Eger et al., 1994; Hedin et al., 1994; Mitsch and Wise, 1998). Although hundreds of wetlands have been constructed to * Corresponding author. Tel.: +1-865-6321634; fax: +1865-6321493. 1 Fax: +1-865-4813113
treat acid drainage from various mine spoils and refuse and from coal ash disposal areas, treatment effectiveness continues to be both variable and generally unpredictable (Wieder, 1989). While considerable effluent water quality information exists as a result of the US EPA’s National Pollutant Discharge Elimination System (NPDES) requirements for surface waters, influent water chemistry and sediment and plant metal concentrations have not been measured with the same intensity. In addition, since wetlands act as sinks for toxic metals found in acid drainage, accumulation of these elements in constructed wetlands to
0925-8574/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 5 - 8 5 7 4 ( 0 0 ) 0 0 1 1 2 - 9
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levels that would adversely affect the food web is of growing concern. Consequently, a study was conducted to compare metal accumulation in sediments, plants, benthic organisms and fish in constructed wetlands with that occurring in a natural wetland.
2. Methods
2.1. Site descriptions Two aerobic wetlands constructed by the Tennessee Valley Authority (TVA) in the southeastern USA for the treatment of acid mine drainage and a natural wetland were used in the study.
Constructed wetland selection for this study was based primarily on their contrasting treatment efficiencies (Brodie et al., 1989; Brodie, 1993).
2.1.1. Fabius Coal Preparation Plant — Impoundment 1 Impoundment 1 (IMP1) at the Fabius Coal Preparation Plant in Jackson County, AL is a four cell, 0.57 ha wetland (Fig. 1) that was constructed in May 1985 to treat unpermitted acid seepage emanating from the toe of Slurry Lake 2, a fine-coal refuse disposal area. Typical of other constructed wetlands in the TVA program, IMP1’s water depth ranges from 15 to 30 cm, with some deeper and shallower areas to provide for species diversification and for aquatic fauna
Fig. 1. Schematic of Impoundment 1 (IMP1) at the Fabius Coal Preparation Plant in Jackson County, AL. The 0.57 ha wetland contains four cells and was constructed in May 1985.
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489
Fig. 2. Schematic of the Widows Creek Fossil Plant Treatment Wetland (WC) located at Stevenson, AL (Jackson). Constructed in May, 1986 Cells 1 and 2 total 0.36 ha. Cell 3 is not included in this study.
refuge during low flow conditions (Brodie, 1993). IMP1 was planted by hand with Scirpus cyperinus (L.) Kunth, Typha latifolia L., and Juncus effusus L. the following month. Prior to construction, concentrations of Fe and Mn in the seepage of Slurry Lake 2 exceeded NPDES permit limitations (Fe B 3.0 mg l − 1 and Mn B2.0 mg l − 1), and pH was 6.2 (pH 6 – 9). Following construction, these parameters have generally been in compliance in the wetlands discharge. Average influent values, based on data collected from February 1992 through February 1993, were: Fe
(44 mg l − 1), Mn (5.9 mg l − 1), and pH (6.3). Average effluent values for the same time period were: Fe (0.9 mg l − 1), Mn (1.2 mg l − 1), and pH (7.2). Average flow was 45 l min − 1.
2.1.2. Widows Creek Widows Creek (WC) is a three cell, 0.49 ha wetland (Fig. 2) that was constructed along the toe of the coal pile runoff pond and an abandoned coal ash disposal area at Widows Creek Fossil Plant in Jackson County, AL in April 1986. The source of drainage into the wetland is an
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adjacent partially reclaimed coal ash disposal area. To reduce short circuiting and increase residence time, six finger dikes were included in the design of Cell 2. Cells were planted with S. cyperinus, T. latifolia, and J. effusus in May 1986. Because chemical treatment with NaOH is needed in Cell 3 in order to comply with discharge limits, this cell was not included in the current study. Total area for Cells 1 and 2 is 0.36 ha. Average influent values during the study were: Fe (205 mg l − 1), Mn (7.4 mg l − 1), and pH (6.3). Average effluent values were: Fe (6.3 mg l − 1), Mn (3.9 mg l − 1), and pH (3.6). Average flow was 103 l min − 1.
2.1.3. Natural wetland The control wetland used in the study is a 0.2 ha natural wetland (NAT) located in Morgan County, TN (Fig. 3). The sources of drainage into the wetland are surface and subsurface flow. Sub-
surface flow is through subterranean channels which have formed in the silty surface materials and are common within soils of this region (Mays et al., 1991). Dominant plants in the wetland include S. cyperinus, T. latifolia, J. effusus, and Proserpinaca pectinata Lam. Average influent values during the study were as follows: Fe (1.3 mg l − 1), Mn (0.2 mg l − 1), and pH (5.7). Average effluent values were: Fe (1.2 mg l − 1), Mn (0.1 mg l − 1), and pH (6.3). Average flow was 129 l min − 1.
2.1.4. Physiography, geology, and soils Many of the soils surrounding both NAT and IMP1 have several common characteristics, having been formed in similar geologic strata. The Pennsylvanian era caprock that covers most of the Cumberland Plateau consists of alternating layers of sandstone, siltstone, shale, and conglomerate. The physical and chemical properties of
Fig. 3. Schematic of the 0.2 ha natural wetland (NAT) located in Morgan, TN.
P.A. Mays, G.S. Edwards / Ecological Engineering 16 (2001) 487–500 Table 1 Methods used in the analysis of water, sediment, and plant samples Parameter
Description of method
Reference
Emission spectroscopy/ICP Hydride generation/AASd Graphite furnace AAS Onsite measurement/Hydrolab
SM 3120Be SM 3114B SM 1070D DataSonde3 Manual
Dry ash at 500°C, then ICP Double acid extraction, then ICP Hydride generation/AAS Graphite furnace AAS 1:1 H2O Walkley-Black NaOAc Saturation
Isaac and Kerber, 1971 Isaac and Kerber, 1971 ASA 2-1f ASA 2-1 ASA 12-2 ASA 29-3.5 ASA 8-3
As, Se
Dry ash at 500°C, then ICP Hydride generation/AAS
Pb
Graphite furnace AAS
Isaac and Kerber, 1971 Isaac and Kerber, 1971 Isaac and Kerber, 1971
Water Ionsa As, Seb Pbc pH Sediment Total ions Extr. ions As, Se Pb pH OM CEC Plant Ionsa
a Ions (detection limits in mg l−1 or mg g−1): Fe (0.008); Mn (0.002); Al (0.04); (detection limits in mg l−1): Zn (9); Cu (7); Cd (6); B (6). b As and Se detection limits: 0.4 mg l−1 in solution; 0.02 mg g−1 in plant; 2.0 mg kg−1 in sediment. c Pb detection limits: 2 mg l−1 in solution; 0.089 mg g−1 in plant and sediment. d Atomic absorption spectrometry. e Standard Methods for the Examination of Water and Wastewater (Greensberg et al., 1992). f Methods of soil analysis (Page et al., 1982).
soils in this province are closely related to these parent materials. Most soils are extremely to strongly acid throughout the profile, with pHs ranging from 3.8 at the surface to 5.0 in the subsoil. Soils are generally highly leached with low levels of available soil nutrients and weatherable minerals and low buffering capacity. Soils in and around IMP1 have formed in fine-grained sandstone and are fairly shallow (B100 cm) with sandstone outcrops common. Differences in soils at NAT are due primarily to the greater extent of
491
siltstone and shale which are present at the site and are more physical than chemical in nature. The WC constructed wetland is located in the Ridge and Valley physiographic province in northern Alabama. Soils adjacent to this wetland have formed in alluvium from the Tennessee River overlying limestone residuum.
2.2. Field sampling and laboratory analyses Water samples were collected from each wetland on a monthly basis beginning in February 1992. At IMP1, samples were collected near the seep location into Cell 1 and the outlet from each of Cells 1 through 4 (Fig. 1). At WC, sample locations were at the inlet into Cell 1, the inlet into Cell 2, and the outlet from Cell 2 (Fig. 2). At NAT, samples were collected from the inlet and outlet (Fig. 3). At each location in each wetland, two 250 ml grab samples were collected, acidified with HNO3 to pH B2, and stored at 4°C until being analyzed for metal concentrations as described in Table 1. Monthly measurements of pH were also conducted with a DataSonde 3 water quality datalogger (HydroLab Corp. Austin, TX) at each location within each wetland. In addition to monthly water samples, sediment samples (0–5 and 5–10 cm depth) were collected from each wetland at two times during the 1992 growing season. Sampling was conducted in early June (spring), following full shoot expansion, and in early October (fall), just prior to senescence. Sediment samples were collected from a midpoint within each cell for the constructed wetlands and from inlet and outlet locations at NAT. At each location, four sediment samples were collected to a depth of 10 cm. Following collection, samples were subdivided into 0–5 and 5–10 cm sections based on visual observations of rooting zone and degree of soil mottling in an attempt to separate aerobic and anaerobic portions of the sediment profile, and chemical analyses were conducted. Both extractable (double acid) and total metal concentrations were determined as described in Table 1. Extractable forms of these elements are assumed to be readily available for plant uptake and/or cycling within the wetlands ecosystem, and total concentrations represent amounts available
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to benthic organisms (Dunbabin and Bowmer, 1992). Plant samples were collected in conjunction with sediment sampling at each wetland, divided into shoot/leaves and root/rhizome components, and analyzed for metals as described in Table 1. For the spring sample date, four replicates of T. latifolia were collected at each location within each constructed wetland (IMP1 and WC). At NAT, three replicates of S. cyperinus (inlet) and T. latifolia (outlet) were collected. A subsample of roots from species at each wetland was also used to quantify metal concentrations in the oxidized coating on the root surface, as described by Macfie and Crowder (1987). For the fall sample date, additional plant species (J. effusus, S. cyperinus, Sparganium americanum Nutt., and Eleocharis quadrangulata (Michx.) R. & S. from IMP1 and P. pectinata from NAT) were collected for inter-species comparisons of metal concentrations. All other sampling remained the same. Biomass estimates in IMP1 and WC were determined by counting the number of T. latifolia shoots within ten randomly selected 0.5 m2 subplots in each cell. A subsample of T latifolia was harvested to determined shoot/leaves to root/rhizome dry weight ratios. At NAT, an intensive plant survey (5 m × 5 m transects) was conducted in order to determine species distribution and dominance. Five 0.5 m2 subplots were harvested to determine shoot/root dry weight ratios for S. cyperinus and P. pectinata, in addition to T. latifolia shoot counts. All plant materials were oven dried at 65°C for 96 h. These data were used to estimate the total biomass per cell and wetland. During the spring sample period, composite samples of Gambusia holbrooki (mosquito fish), benthic organisms, and Rana sp. (tadpoles) were also collected from each cell in IMP1. Populations of G. holbrooki were also released into NAT. During the fall sample period, fish, benthic organisms and tadpoles were sampled from IMP1 and NAT. No fish, tadpoles, or benthic organisms were present at WC, due to low pH. Due to the lack of replication for fish, tadpoles, and benthic organisms, statistical comparisons could not be conducted. Therefore, bioaccumulation of metals in these organisms will not be addressed in this paper.
2.3. Statistical analyses One-way analysis of variance techniques were used to test for statistical differences among wetlands in effluent water quality (monthly values averaged for the study period). Statistical differences in plant and sediment metal concentration by location within each wetland and by wetland for each sample date (pooled by location) were determined similarly. When significant differences among wetlands occurred, linear contrasts were performed to test for differences between: (1) NAT and IMP1, and (2) NAT and WC (Steel and Torrie, 1980). Analysis of variance was also used to test for differences in metal concentrations among plant species for the fall sample date.
3. Results and discussion
3.1. Water quality Monthly average values for discharge pH, total Fe, and total Mn for IMP1, WC, and NAT are presented in Fig. 4. Impoundment 1 discharge always met monthly NPDES compliance limitations for pH (6–9), Fe ( B 3.0 mg l − 1) and Mn (B2.0 mg l − 1) (although Mn concentrations exceeded 2.0 mg l − 1 during 3 months) and is indicative of its operation since construction in 1985 (Brodie, 1993). Discharge from WC (Cells 1 and 2) consistently exceeded compliance limitations, while discharge from NAT exceeded Fe limitations once and pH limitations twice. Monthly average values for Cr, Cu, Ni and Se were always below the detection limit (DL) in both the influent and effluent for all three wetlands. A summary of metal load, export, and removal for each wetland is given in Table 2. Load and export rates were greatest for WC and least for NAT. Two conditions exist that explain the low treatment efficiency for WC. First, insufficient treatment area exists for the loading rates currently occurring at WC. The treatment area at WC is 0.2 m2 mg Fe − 1 min − 1, whereas successful treatment systems within this region are 0.7 m2 mg Fe − 1 min − 1 (Brodie et al., 1989). Based on average flow rates within both of the
32.4 1.9 0.04 10 10 270 9.9 2 0.07 B0.002 0.02 B9 B6 B6 B0.4 B2
WC Fe Mn Al Zn Cd B As Pb
NAT Fe Mn Al Zn Cd B As Pb 5.42 1.4 0.28 B9 B6 B6 B0.4 B2
474 9.4 1.76 40 40 1500 203 6.1
52.4 7.1 0.04 B9 B6 20 1.46 B2
Max
1.29 0.20 0.10 B9 B6 B6 B0.4 B2
205 7.4 0.29 30 20 1170 100 2.2
44 5.9 0.02 B9 B6 10 0.85 B2
Mean
– – – – –
1.68 0.39 0.06
145 2.3 0.49 10 10 360 56.5 1.75
6.4 0.6 0.01 – – 10 0.29 –
S.D.
– – – – –
123 20 10
8446 305 12.4 1200 800 48 000 4120 90.6
506 67 0.3 – – 100 0.01 –
Loadc
0.01 B0.002 0.04 B9 B6 B6 B0.4 B2
B0.008 B0.002 0.01 B9 B6 190 0.4 2
0.4 0.2 0.02 B9 B6 10 B0.4 B2
Min
Effluent
5.95 0.60 0.30 B9 B6 B6 B0.4 B2
35.3 9.2 1.29 40 B6 40 0.56 2.2
1.3 3.3 0.07 B9 B6 30 B0.4 B2
Max
1.17 0.12 0.10 B9 B6 B6 B0.4 B2
6.3 3.9 0.34 20 B6 560 0.42 1.63
0.9 1.2 0.04 B9 B6 10 B0.4 B2
Mean
– – – – –
1.78 0.18 0.08
10.5 3.4 0.36 10 – 290 0.05 0.86
0.29 1.09 0.02 – – 10 – –
S.D.
– – – – –
111 12 10
260 161 14 800 – 23 000 17 67.2
10 14 0.5 – – 100 – –
Export
– – – – – –
b
a
12 8
8186 144 – 400 800 25 000 4103 23.4
496 53 – – – – 0.01 –
Removal
Cr, Cu, Ni, and Se were always below the detection limit (0.005, 7.0, 0.029, 0.005, and 0.4 mg l−1, respectively) and are not included. Influent and effluent concentrations are in mg l−1 for Fe, Mn, and Al; mg l−1 for B, Zn, Cd, As, and Pb. c Load, export, and removal rates are in mg m−2 day−1 for Fe, Mn, and Al; mg m−2 day−1 for B, Zn, Cd, As, and Pb.
34.3 4.9 0.02 B9 B6 10 0.45 B2
Min
Influentb
IMP1 Fe Mn Al Zn Cd B As Pb
Element
– – – – – –
10 40
97 47 – 33 100 52 99 26
98 79 – – – – 100 –
Efficiency
Table 2 Minimum, maximum, and mean metal concentration of influent and effluent, load, export, and removal at Impoundment 1 (IMP1), Widows Creek (WC), and natural wetland (NAT) during the study perioda
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P.A. Mays, G.S. Edwards / Ecological Engineering 16 (2001) 487–500
constructed wetlands, retention time is least at WC and lower retention time has been observed to reduce treatment effectiveness in other wetland systems (Sobolewski, 1995). In addition, occasionally high water levels caused by beaver activity and recurring infestations of armyworms (Simyra henrici ) may have stressed and reduced the number of T. latifolia, thus reducing the available root surface area needed for oxidation and adsorbtion. Secondly, insufficient alkalinity exists in order to buffer the acidity that is generated by the oxidation of Fe2 + (Faulkner, 1994). This is in contrast to IMP1 where influent alkalinity is much higher due to seepage waters coming into contact with buried road gravel that is a high CaCO3, oolitic limestone (Brodie et al., 1992). These conditions act as an anoxic limestone drain in producing higher alkalinity influent. At NAT, the drainage entering the wetland originates from relatively undisturbed second-growth upland oak-hickory forest. Soils are acid and infertile with only trace amounts of heavy metals (other than Fe and Al). In these highly weathered, highly oxidized soils, significant amounts of iron and aluminum oxides are effective in adsorbing most trace metal cations (Gambrell, 1994). Load rates for IMP1 tended to be greater than those for NAT, while export rates at these wetlands were similar for Mn, but differed for Fe and Al (Table 2). Removal efficiencies based on influent/effluent concentration differences indicate that IMP1 and WC were more effective than NAT at metal removal. Although efficiencies were greater, it should be noted that load rates were less by several orders of magnitude at NAT. Similarities in load and export rates at NAT would indicate a system approaching equilibrium as compared to the two constructed wetlands. Based on the mean values of monthly water samples during the study period, effluent water quality was not significantly different between NAT and IMP1. Contrasting NAT with WC revealed significant differences between the two wetlands for the following concentrations in the effluent: Fe, Mn, Al, B, Zn, and As. In each case, effluent concentrations at WC were greater than those at NAT (Table 2).
3.2. Metal concentrations in sediments At each wetland, extractable metal concentrations in the sediment (both depths) were not significantly correlated with distance from the wetland inlet, and concentrations did not differ significantly between depths and therefore, soil metal concentrations were pooled for depth. When pooled for depth, concentrations of various metals differed by location within each wetland, but most trends were inconsistent and varied by season (Table 3). This inconsistency has been observed in other constructed and natural wetlands within the study area (Sistani et al., 1995). However, Fe concentrations at IMP1 were greatest in Cell 1, while Mn concentrations were greatest in Cell 4 on both sample dates. These data support an earlier study conducted on this wetland in which Faulkner (1994) concluded that the majority of the Fe in the influent was oxidized and precipitated in Cell 1, while Mn removal occurred primarily by co-precipitation in the Cells 3 and 4. Orthogonal contrasts of extractable metal concentrations (pooled for depth and location within each wetland) indicate that Mn and Se concentrations were significantly greater at IMP1 than at NAT during both sample dates, and Cr was significantly greater in the fall. Concentrations of Zn, Cu, and Ni were significantly greater at NAT in the spring, but no differences were found in the fall (Table 4). Sediment pH was also significantly lower and organic matter was significantly greater at NAT than at IMP1 on both sample dates. Much of the inlet area of NAT is shallow and is exposed in times of very low flow. As the sediments begin to dry, creeping rush (Juncus repens) and other nonwetland species often invade from the wetland periphery and cover the entire area. The annual turnover of primarily root materials would certainly increase soil organic matter content within these areas over time. Increased organic acid concentrations and soil oxidation would tend to lower sediment pH within these areas. Manganese, Cu, Pb, B, As, and Se were greater at WC than at NAT in the spring, while Al and Zn were greater at NAT (Table 4). In the fall, Cu, B, As, and Se concentrations were
5.8 5.5 5.3 5.6 4.0 6.3 4.9 5.0
6.0 5.9 6.2 5.6 4.8 4.5 4.9 5.1
Spring IMP1 Cell 1 Cell 2 Cell 3 Cell 4
WC Cell 1 Cell 2
NAT Inlet Outlet
Fall IMP1 Cell 1 Cell 2 Cell 3 Cell 4
WC Cell 1 Cell 2
NAT Inlet Outlet 10.0b 25.4a
25.6 14.3
2.3 1.7 2.2 2.5
11.6b 25.9a
45.5a 3.7b
2.0 1.1 2.1 2.0
OM (%)
5.4b 6.7a
9.9a 7.7b
5.4 6.1 6.6 5.7
5.3b 6.5a
7.6b 13.8a
5.8 5.1 5.9 6.0
CEC (cmol kg−1)
556a 189b
184b 369a
617a 429b 416b 377b
457a 171b
176b 416a
430a 260b 416ab 299b
mg g−1
Fe
52 59
73a 15b
130b 295b 150b 600a
64 68
71b 246a
203b 245ab 93b 402a
Mn
212 255
110b 190a
230b 351a 292ab 365a
230b 315a
82b 97a
426a 339b 387ab 457a
Al
1.6b 4.2a
2.9 2.2
2.2b 1.3b 2.9b 5.7a
1.6b 5.9a
3.1 2.4
1.1b 0.8b 1.8a 1.9a
Zn
1.21 1.05
1.65 1.66
1.20a 1.11a 0.93b 1.33a
0.74b 1.06a
2.01 1.02
0.52 0.45 0.53 0.39
Cu
2.5 1.7
3.2a 1.4b
1.6c 2.1b 2.0b 2.5a
1.2 1.4
2.7a 1.1b
1.1ab 0.9b 1.3a 1.0ab
Pb
0.50b 0.97a
0.92 1.14
0.82b 0.43b 0.95b 1.98a
0.42b 1.29a
0.93 0.50
B0.03 B0.03 B0.03 B0.03
Ni
0.69a 0.37b
0.34 0.50
0.81 0.91 0.79 0.91
B0.005 B0.005
B0.005 B0.005
B0.005 B0.005 B0.005 B0.005
Cr
0.30 0.30
1.61 1.43
0.39 0.37 0.36 0.41
B0.006 B0.006
0.96b 2.08a
B0.006 B0.006 B0.006 B0.006
B
1.5 1.5
4.5b 16.3a
3.4 3.2 2.8 3.1
1.0b 1.9a
58a 22b
3.0 2.4 2.3 2.4
As
0.20 0.27
0.35 0.40
0.25b 0.34b 0.50a 0.51a
0.12 0.18
0.57a 0.23b
0.51 0.42 0.50 0.43
Se
a Data are pooled for depth. Cd was always below the detection limit (0.006) and is not included. OM, organic matter. CEC, cation exchange capacity. Different letters following a given parameter within each wetland indicate significant differences (PB0.05) among locations for each sample date.
pH
Wetland
Table 3 Mean chemical characteristics and metal concentrations by location in the sediment of Impoundment 1 (IMP1), Widows Creek (WC), and natural wetland (NAT) wetlandsa
P.A. Mays, G.S. Edwards / Ecological Engineering 16 (2001) 487–500 495
5.8* 5.0 4.9 6.0* 4.9 5.0
Spring IMP1 WC NAT
Fall IMP1 WC NAT 2.2* 16.6 17.7
1.8* 14.1 18.8
OM (%)
5.9 8.9* 6.1
5.4 9.5* 6.0
CEC (cmol kg−1)
448 217* 372
350 240 314
mg g−1
Fe
277* 59 56
241* 123* 66
Mn
261 126* 233
330 122* 273
Al
2.6 2.3 2.9
1.4* 2.5* 3.7
Zn
1.22 1.52* 1.13
0.49* 1.30* 0.90
Cu
1.8 2.3 2.1
1.0 2.0* 1.3
Pb
0.96 0.90 0.73
B0.03* 0.69 0.85
Ni
0.78* 0.38* 0.53
B0.005 B0.005 B0.005
Cr
0.37 1.58* 0.30
B0.006 1.39* B0.006
B
3.44 7.50* 1.47
3.20 32* 1.43
As
0.35* 0.37* 0.23
0.46* 0.45* 0.15
Se
Data are pooled for depth and location. Cd was always below the detection limit (0.006) and is not included. OM, organic matter. CEC, cation exchange capacity. * Following a given parameter indicates significant differences (PB0.05) between NAT and IMP1 or NAT and WC for each sample date. No statistical comparison is made between IMP1 and WC.
a
pH
Wetland
Table 4 Mean chemical characteristics and extractable metal concentrations in the sediment of Impoundment 1 (IMP1), Widows Creek (WC), and natural wetland (NAT) wetlandsa
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P.A. Mays, G.S. Edwards / Ecological Engineering 16 (2001) 487–500
greater at WC, while Fe and Al concentrations were greater at NAT. There were no significant differences between pH and organic matter at either sample date, but cation exchange capacity was greater at WC during both sample dates. At IMP1 and NAT, extractable metal concentrations in the sediment usually exceeded those of the influent by between two and five orders of magnitude, implying that accumulation in the sediment was not due solely to influent additions to the system. This is particularly noticeable for Cr, Cu, Ni and Se since influent and effluent concentrations of these elements were below detection limits throughout the study period. Rather, plant uptake and decay, plus strong retention of metals by inorganic and organic soil components, are the most probable mechanisms of accumulation of metal ions near the soil surface (Bohn et al., 1985; Dunbabin and Bowmer, 1992).
3.3. Metal concentrations in plants Distance from the wetland inlet also had no significant effect on metal concentrations in plants, and plant metal concentrations were not significantly correlated with sediment or water metal concentrations. The availability of trace metals to aquatic plants is complex and is dependent on specific chemical factors associated with the metal and substrate in which it is found (Taylor and Crowder, 1983; Jackson et al., 1991; Sparling and Lowe, 1997; Zayed et al., 1998). Metal concentrations for roots in this study included metals contained in root coatings, which accounted for 32–93% of the total metal concentration in the roots (Table 5). Consequently, some metal concentrations for the roots are quite high when compared to those of the shoots. This general trend in metal concentration (root \ rhizome \leaf tissue) has been observed in other studies where T. latifolia and J. effusus have been analyzed for heavy metals (Taylor and Crowder, 1983 Shutes et al., 1993). At the spring sample date, Mn, Ni, and Cr concentrations in both roots and shoots of plants at IMP1 were significantly greater than concentrations of these metals in plants at NAT (Table 5). Arsenic concentration in roots at WC was also
497
significantly greater than at NAT. However, Pb concentration in roots and Al, Zn, and Cu concentrations in shoots were significantly greater in plants at NAT than at either constructed wetland at the spring sample date. In the fall, concentrations of Fe and Cd in roots and B in shoots at IMP1 and WC were significantly greater than at NAT. In contrast, root concentrations of Al, Zn, Ni, and Cu and shoot Ni concentration were significantly greater at NAT than at the constructed wetlands. A comparison of metal concentrations in various plant species at IMP1 and NAT at the fall sample date revealed that concentrations of Fe, Cu, Zn, Pb, Cd, Cr, and As were significantly lower in T. latifolia shoots than in other wetland species (J. effusus, S. cyperinus, S. americanum, E. quadrangulata, and P. pectinata). In contrast, P. pectinata, one of the most common species at NAT exhibited significantly greater shoot concentrations of Mn, Al, Cu, Zn, Pb, Ni, and As than all other species. These differences may be closely associated with growth form (emergent vs. submerged, floating species) where several studies have shown that emergent species have lower metal concentrations than submerged or nonrooted, floating species (review by Sparling and Lowe, 1997). Data in the literature indicate great variation in plant metal concentrations achieved in different wetland environments (Jenkins, 1980; Jackson et al., 1991; Bryan and Langston, 1992; Dunbabin and Bowmer, 1992; Zayed et al., 1998). At each wetland in this study, concentrations of Mn, Zn, Cu, Ni, B, and Cr in shoots of T. latifolia (and S. cyperinus at NAT) were greater than the extractable concentrations of these metals in the sediment, indicating that these metals are accumulating in the plant component. Sparling and Lowe (1997) found similar trends for B, Mn, and Zn in control experiments on both rooted and nonrooted species. In addition, accumulation of Mn, Cu, Zn, and Cr is probably greater in other species, especially P. pectinata, based on the interspecies comparison discussed above. In a review provided by Zayed et al. (1998), T. latifolia is shown to be an accumulator of Cd, Cu, Ni, and Pb, while Scirpus sp. also accumulate Cr.
28 660* 27 322* 9121 327 1739* 253
Fall Root IMP1 WC NAT
Shoot IMP1 WC NAT 29
2076* 549 821
2012* 144* 617
1752* 527 751
1786* 121 442
Mn
44
78 122 74
1531* 539* 2582
61* 66* 137
2723 1067* 3358
Al
39
7.5* 12 12
23* 23* 34
16* 16* 38
41 16 34
Zn
32
1.0* 2.5* 1.8
1.2* 4.1 5.4
1.2* 3.6* 6.3
6.5 3.3 6.5
Cu
82
0.6 0.8 0.7
6.1 8.2 9.9
B0.09 1.1 B0.09
4.7* 6.0* 17.9
Pb
92
0.1 0.4* 0.06
6.0* 5.6* 2.2
B0.006 B0.006 B0.006
2.7 2.4 1.7
Cd
80
0.7* 0.3* 1.2
1.5* 0.5* 4.0
6.2* 3.3 2.9
18.1* 10.7 8.5
Ni
72
0.4 0.5 0.7
3.1 2.3* 3.9
12* 6.2 3.3
37* 24 13
Cr
65
6.2* 10.7* 4.5
0.06 0.06 0.06
B0.006 10.0* 4.8
B0.006* 1.1 1.4
B
ND
0.04 1.0* 0.06
3.8 21.1 8.6
0.06 0.07 0.03
3.5 28.8* 3.9
As
ND
0.04 0.06* 0.03
0.09 0.13 0.12
0.05 0.10 0.07
0.17 0.15 0.19
Se
Data are pooled for location. ND, not determined. * Following a given metal concentration for each plant component indicates significant differences (PB0.05) between NAT and IMP1 or NAT and WC for a given sample date. No statistical comparison is made between IMP1 and WC.
a
93
349 381 363
Shoot IMP1 WC NAT
% of metal in root coat
7427 13 077 8820
mg g−1
Fe
Spring Root IMP1 WC NAT
Plant component
Table 5 Mean metal concentrations in plants at Impoundment 1 (IMP1), Widows Creek (WC), and natural wetland (NAT) wetlandsa
498 P.A. Mays, G.S. Edwards / Ecological Engineering 16 (2001) 487–500
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Although some metals are accumulating in the plants at each wetland, the total metal content in the plant component accounted for only a small percentage of the annual metal load supplied to each wetland. For example, Fe and Mn contents in the plants at IMP1 were only 1 and 2%, respectively, of the annual Fe and Mn load received by this wetland. Similar findings have been reported for Ni and Cu removal by plants from mine drainage treated by a natural wetland (Eger and Lapakko, 1988) and for Cd, Cu, and Zn removal from municipal sewage effluent treated by artificial wetlands (Gersberg et al., 1984). Mitsch and Wise (1998) summarize additional studies concerning the role of vegetation in metal retention in wetlands and determine that even in wetlands where luxury consumption is possible (such as IMP1 and WC), the above-water vegetation is storing only 0.07% of the annual inflow of iron. They conclude their summary by suggesting that the critical role of vegetation in metal retention may also include serving as sites for metal precipitation and/or sedimentation.
Acknowledgements Funding for this project was provided by the Tennessee Valley Authority and the Electric Power Research Institute. The authors express their appreciation to C.L. Wylie for assistance with data management and statistical analysis and to J. Scarbrough for technical contributions. We are also grateful to G.A. Brodie, J. Goodrich-Mahoney, F.J. Sikora, F.C. Thornton and N.S. Nicholas for reviewing the manuscript.
References Bryan, G.W., Langston, W.J., 1992. Bioavailability, accumulation and effects of heavy metals in sediments with special reference to United Kingdom estuaries: a review. Environ. Pollut. 76, 89 – 131. Bohn, H.L., McNeal, B.L., O’Connor, G.A., 1985. Soil Chemistry, 2nd Edition. Wiley, New York, pp. 90–94. Brodie, G.A., 1993. Staged, aerobic constructed wetlands to treat acid drainage: case history of Fabius Impoundment 1 and overview of the Tennessee Valley Authority’s program.
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In: Moshiri, G. (Ed.), Constructed Wetlands for Water Quality Improvement. Lewis, Boca Raton, FL, pp. 157 – 165. Brodie, G.A., Hammer, D.A., Tomljanovich, D.A., 1989. Constructed wetlands for treatment of ash pond seepage. In: Hammer, D. (Ed.), Constructed Wetlands for Wastewater Treatment. Lewis, Michigan, pp. 211 – 219. Brodie, G.A., Britt, C.R., Tomaszewski, T.M., Taylor, H.N., 1992. Anoxic limestone drains to enhance performance of aerobic acid drainage treatment wetlands — The Tennessee Valley Authority. Presented at the 1992 West Virginia Surface Mine Drainage Task Force Symposium, Morgantown, West Virginia. Dunbabin, J.S., Bowmer, K.H., 1992. Potential use of constructed wetlands for treatment of industrial wastewaters containing metals. Sci. Total Environ. 111, 151 – 168. Eger, P., Lapakko, K., 1988. Nickel and copper removal from mine drainage by a natural wetland. In 1988 Mine Drainage and Surface Mine Reclamation Conference. U.S. Bureau of Mines, Pittsburgh, PA, pp. 301 – 309. Eger, P., Wagner, J.R., Kassa, Z., Melchert, G.D., 1994. Metal removal in wetland treatment systems. In: 1994 Proc. Int. Land Reclamation and Mine Drainage Conf. and 3rd Int. Conf. on the Abatement of Acidic Drainage, U.S. Bureau of Mines Special Pub. SP 06A – 94, Vol. 1, Pittsburgh, PA, pp. 80 – 88. Faulkner, S.P., 1994. Gambrell, R.P., 1994. Trace and toxic metals in wetlands — a review. J. Environ. Qual. 23, 883 – 891. Gersberg, R.M., Lynn, S.R., Elkins, B.V., Goldman, C.R., 1984. The removal of heavy metals by artificial wetlands. In: Future of Water Reuse, Vol. 2. Proceedings of Water Resource Symposium, San Diego, CA. AWWA Research Foundation, Denver, CO, pp. 639 – 648. Greensberg, A.E., Clesceri, L.S., Eaton, A.D., 1992. Standard Methods for the Examination of Water and Wastewater. American Public Health Association, Washington, D.C. Hammer, D.A. (Ed.), 1989. Constructed Wetlands for Wastewater Treatment. Proc. First Int. Conf. Constructed Wetlands for Wastewater Treatment, Chattanooga, Tennessee, 1988. Lewis, Michigan, 831 pp. Hedin, R.S., Narin, R.W., Kleinmann, R.L.P., 1994. Passive treatment of coal mine drainage. U.S. Dept. of Interior. Bureau of Mines Information Circular 9389. Isaac, R.A., Kerber, J.D., 1971. Atomic absorption and flame photometry: techniques and uses in soil, plant, and water analysis. In: Walsh, L.M. (Ed.), Instrumental Methods for Analysis of Soils and Plant Tissue. Soil Science Society of America, Madison, WI, pp. 17 – 37. Jackson, L.J., Rasmussen, J.B., Peters, R.H., Kalff, J., 1991. Empirical relationships between the element composition of aquatic marophytes and their underlying sediments. Biogeochemistry 12, 71 – 86. Jenkins, D.W., 1980. Biological monitoring of toxic trace metals: Volume 2. Toxic trace metals in plants and animals of the world. U.S. Environmental Protection Agency, Contract 68-03-0443, Report epa-600/3-80-089.
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Mays, P.A., Kelly, J.M., Lietzke, D.A., 1991. Soils of the Camp Branch and Cross Creek Experimental Watersheds. Univ. Tn. Ag. Exp. Sta. Research Report, pp. 91– 21. Macfie, S.M., Crowder, A.A., 1987. Soil factors influencing ferric hydroxide plaque formation on roots of Typha latifolia L. Plant Soil 102, 177–184. Mitsch, W.J., Wise, K.M., 1998. Water quality, fate of metals, and predictive model validation of a constructed wetland treating acid mine drainage. Wat. Res. 32, 1888 – 1900. Page, A.L., Miller, R.H., Keeney, D.R., 1982. Methods of Soil Analysis. Part 2: Chemical and Microbiological Properties, 2nd Edition. ASA/SSSA, Madison, WI. Shutes, R.B., Ellis, J.B., Revitt, D.M., Zhang, T.T., 1993. The use of Typha latifolia for heavy metal pollution control in urban wetlands. In: Moshiri, G. (Ed.), Constructed Wetlands for Water Quality Improvement. Lewis, Boca Raton, FL, pp. 407–414. Sistani, K.R., Mays, D.A., Taylor, R.W., 1995. Biogeochemical characteristics of wetlands developed after strip min-
ing for coal. Commun. Soil Sci. Plant Anal. 26 (19&20), 3221 – 3229. Sobolewski, A., 1995. Metal species indicate the potential of constructed wetlands for long-term treatment of metal mine drainage. Ecol. Eng. 6, 259 – 271. Sparling, D.W., Lowe, T.P., 1997. Metal concentrations in aquatic macrophytes as influenced by soil and acidification. Water Air Soil Pollut. 108, 203 – 221. Steel, R.G.D., Torrie, J.H., 1980. Principles and Procedures of Statistics: A Biometrical Approach. McGraw-Hill, New York. Taylor, G.J., Crowder, A.A., 1983. Uptake and accumulation of heavy metals by Typha latifolia in wetlands of the Sudbury, Ontario Region. Can. J. Bot. 61, 63 – 73. Wieder, R.K., 1989. A survey of constructed wetlands for acid coal mine drainage treatment in the eastern United States. Wetlands 9, 299 – 315. Zayed, A., Gowthaman, S., Terry, N., 1998. Phytoaccumulation of trace elements by wetland plants: 1. Duckweed. J. Environ. Qual. 27, 715 – 721.
.
www.elsevier.com/locate/ecoleng
Comparison of heavy metal accumulation in a natural wetland and constructed wetlands receiving acid mine drainage P.A. Mays a,*, G.S. Edwards b,1 a
Tennessee Valley Authority, 129 Pine Rd., Norris, TN 37828, USA b 101 Scenic Dri6e, Oak Ridge, TN 37830, USA
Received 19 February 1999; received in revised form 4 May 2000; accepted 5 June 2000
Abstract Metal accumulations in sediments and plants of constructed and natural wetlands were compared in two wetlands constructed by the Tennessee Valley Authority (TVA) for the treatment of acid mine drainage and a natural wetland. Load rates and removal efficiencies of most metals were generally greater in the constructed wetlands than in the natural wetland. There were similar sediment and plant metal concentrations between one constructed wetland and the natural wetland and greater metal concentrations in the sediments and plants in the other constructed wetland compared to the natural wetland. Data indicate that Mn, Zn, Cu, Ni, B, and Cr are being accumulated in the plants at all three wetlands, although accumulation of metals by these plants accounts for only a small percentage of the removal of the annual metal load supplied to each wetland. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Constructed wetland; Acid mine drainage; Metals; NPDES; Typha latifolia; Juncus effusus; Scirpus cyperinus
1. Introduction The use of constructed wetlands as biogeochemical systems for the treatment of acid mine drainage has developed rapidly over the last few decades in North America and worldwide (Hammer, 1989; Wieder, 1989; Eger et al., 1994; Hedin et al., 1994; Mitsch and Wise, 1998). Although hundreds of wetlands have been constructed to * Corresponding author. Tel.: +1-865-6321634; fax: +1865-6321493. 1 Fax: +1-865-4813113
treat acid drainage from various mine spoils and refuse and from coal ash disposal areas, treatment effectiveness continues to be both variable and generally unpredictable (Wieder, 1989). While considerable effluent water quality information exists as a result of the US EPA’s National Pollutant Discharge Elimination System (NPDES) requirements for surface waters, influent water chemistry and sediment and plant metal concentrations have not been measured with the same intensity. In addition, since wetlands act as sinks for toxic metals found in acid drainage, accumulation of these elements in constructed wetlands to
0925-8574/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 5 - 8 5 7 4 ( 0 0 ) 0 0 1 1 2 - 9
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levels that would adversely affect the food web is of growing concern. Consequently, a study was conducted to compare metal accumulation in sediments, plants, benthic organisms and fish in constructed wetlands with that occurring in a natural wetland.
2. Methods
2.1. Site descriptions Two aerobic wetlands constructed by the Tennessee Valley Authority (TVA) in the southeastern USA for the treatment of acid mine drainage and a natural wetland were used in the study.
Constructed wetland selection for this study was based primarily on their contrasting treatment efficiencies (Brodie et al., 1989; Brodie, 1993).
2.1.1. Fabius Coal Preparation Plant — Impoundment 1 Impoundment 1 (IMP1) at the Fabius Coal Preparation Plant in Jackson County, AL is a four cell, 0.57 ha wetland (Fig. 1) that was constructed in May 1985 to treat unpermitted acid seepage emanating from the toe of Slurry Lake 2, a fine-coal refuse disposal area. Typical of other constructed wetlands in the TVA program, IMP1’s water depth ranges from 15 to 30 cm, with some deeper and shallower areas to provide for species diversification and for aquatic fauna
Fig. 1. Schematic of Impoundment 1 (IMP1) at the Fabius Coal Preparation Plant in Jackson County, AL. The 0.57 ha wetland contains four cells and was constructed in May 1985.
P.A. Mays, G.S. Edwards / Ecological Engineering 16 (2001) 487–500
489
Fig. 2. Schematic of the Widows Creek Fossil Plant Treatment Wetland (WC) located at Stevenson, AL (Jackson). Constructed in May, 1986 Cells 1 and 2 total 0.36 ha. Cell 3 is not included in this study.
refuge during low flow conditions (Brodie, 1993). IMP1 was planted by hand with Scirpus cyperinus (L.) Kunth, Typha latifolia L., and Juncus effusus L. the following month. Prior to construction, concentrations of Fe and Mn in the seepage of Slurry Lake 2 exceeded NPDES permit limitations (Fe B 3.0 mg l − 1 and Mn B2.0 mg l − 1), and pH was 6.2 (pH 6 – 9). Following construction, these parameters have generally been in compliance in the wetlands discharge. Average influent values, based on data collected from February 1992 through February 1993, were: Fe
(44 mg l − 1), Mn (5.9 mg l − 1), and pH (6.3). Average effluent values for the same time period were: Fe (0.9 mg l − 1), Mn (1.2 mg l − 1), and pH (7.2). Average flow was 45 l min − 1.
2.1.2. Widows Creek Widows Creek (WC) is a three cell, 0.49 ha wetland (Fig. 2) that was constructed along the toe of the coal pile runoff pond and an abandoned coal ash disposal area at Widows Creek Fossil Plant in Jackson County, AL in April 1986. The source of drainage into the wetland is an
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P.A. Mays, G.S. Edwards / Ecological Engineering 16 (2001) 487–500
adjacent partially reclaimed coal ash disposal area. To reduce short circuiting and increase residence time, six finger dikes were included in the design of Cell 2. Cells were planted with S. cyperinus, T. latifolia, and J. effusus in May 1986. Because chemical treatment with NaOH is needed in Cell 3 in order to comply with discharge limits, this cell was not included in the current study. Total area for Cells 1 and 2 is 0.36 ha. Average influent values during the study were: Fe (205 mg l − 1), Mn (7.4 mg l − 1), and pH (6.3). Average effluent values were: Fe (6.3 mg l − 1), Mn (3.9 mg l − 1), and pH (3.6). Average flow was 103 l min − 1.
2.1.3. Natural wetland The control wetland used in the study is a 0.2 ha natural wetland (NAT) located in Morgan County, TN (Fig. 3). The sources of drainage into the wetland are surface and subsurface flow. Sub-
surface flow is through subterranean channels which have formed in the silty surface materials and are common within soils of this region (Mays et al., 1991). Dominant plants in the wetland include S. cyperinus, T. latifolia, J. effusus, and Proserpinaca pectinata Lam. Average influent values during the study were as follows: Fe (1.3 mg l − 1), Mn (0.2 mg l − 1), and pH (5.7). Average effluent values were: Fe (1.2 mg l − 1), Mn (0.1 mg l − 1), and pH (6.3). Average flow was 129 l min − 1.
2.1.4. Physiography, geology, and soils Many of the soils surrounding both NAT and IMP1 have several common characteristics, having been formed in similar geologic strata. The Pennsylvanian era caprock that covers most of the Cumberland Plateau consists of alternating layers of sandstone, siltstone, shale, and conglomerate. The physical and chemical properties of
Fig. 3. Schematic of the 0.2 ha natural wetland (NAT) located in Morgan, TN.
P.A. Mays, G.S. Edwards / Ecological Engineering 16 (2001) 487–500 Table 1 Methods used in the analysis of water, sediment, and plant samples Parameter
Description of method
Reference
Emission spectroscopy/ICP Hydride generation/AASd Graphite furnace AAS Onsite measurement/Hydrolab
SM 3120Be SM 3114B SM 1070D DataSonde3 Manual
Dry ash at 500°C, then ICP Double acid extraction, then ICP Hydride generation/AAS Graphite furnace AAS 1:1 H2O Walkley-Black NaOAc Saturation
Isaac and Kerber, 1971 Isaac and Kerber, 1971 ASA 2-1f ASA 2-1 ASA 12-2 ASA 29-3.5 ASA 8-3
As, Se
Dry ash at 500°C, then ICP Hydride generation/AAS
Pb
Graphite furnace AAS
Isaac and Kerber, 1971 Isaac and Kerber, 1971 Isaac and Kerber, 1971
Water Ionsa As, Seb Pbc pH Sediment Total ions Extr. ions As, Se Pb pH OM CEC Plant Ionsa
a Ions (detection limits in mg l−1 or mg g−1): Fe (0.008); Mn (0.002); Al (0.04); (detection limits in mg l−1): Zn (9); Cu (7); Cd (6); B (6). b As and Se detection limits: 0.4 mg l−1 in solution; 0.02 mg g−1 in plant; 2.0 mg kg−1 in sediment. c Pb detection limits: 2 mg l−1 in solution; 0.089 mg g−1 in plant and sediment. d Atomic absorption spectrometry. e Standard Methods for the Examination of Water and Wastewater (Greensberg et al., 1992). f Methods of soil analysis (Page et al., 1982).
soils in this province are closely related to these parent materials. Most soils are extremely to strongly acid throughout the profile, with pHs ranging from 3.8 at the surface to 5.0 in the subsoil. Soils are generally highly leached with low levels of available soil nutrients and weatherable minerals and low buffering capacity. Soils in and around IMP1 have formed in fine-grained sandstone and are fairly shallow (B100 cm) with sandstone outcrops common. Differences in soils at NAT are due primarily to the greater extent of
491
siltstone and shale which are present at the site and are more physical than chemical in nature. The WC constructed wetland is located in the Ridge and Valley physiographic province in northern Alabama. Soils adjacent to this wetland have formed in alluvium from the Tennessee River overlying limestone residuum.
2.2. Field sampling and laboratory analyses Water samples were collected from each wetland on a monthly basis beginning in February 1992. At IMP1, samples were collected near the seep location into Cell 1 and the outlet from each of Cells 1 through 4 (Fig. 1). At WC, sample locations were at the inlet into Cell 1, the inlet into Cell 2, and the outlet from Cell 2 (Fig. 2). At NAT, samples were collected from the inlet and outlet (Fig. 3). At each location in each wetland, two 250 ml grab samples were collected, acidified with HNO3 to pH B2, and stored at 4°C until being analyzed for metal concentrations as described in Table 1. Monthly measurements of pH were also conducted with a DataSonde 3 water quality datalogger (HydroLab Corp. Austin, TX) at each location within each wetland. In addition to monthly water samples, sediment samples (0–5 and 5–10 cm depth) were collected from each wetland at two times during the 1992 growing season. Sampling was conducted in early June (spring), following full shoot expansion, and in early October (fall), just prior to senescence. Sediment samples were collected from a midpoint within each cell for the constructed wetlands and from inlet and outlet locations at NAT. At each location, four sediment samples were collected to a depth of 10 cm. Following collection, samples were subdivided into 0–5 and 5–10 cm sections based on visual observations of rooting zone and degree of soil mottling in an attempt to separate aerobic and anaerobic portions of the sediment profile, and chemical analyses were conducted. Both extractable (double acid) and total metal concentrations were determined as described in Table 1. Extractable forms of these elements are assumed to be readily available for plant uptake and/or cycling within the wetlands ecosystem, and total concentrations represent amounts available
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to benthic organisms (Dunbabin and Bowmer, 1992). Plant samples were collected in conjunction with sediment sampling at each wetland, divided into shoot/leaves and root/rhizome components, and analyzed for metals as described in Table 1. For the spring sample date, four replicates of T. latifolia were collected at each location within each constructed wetland (IMP1 and WC). At NAT, three replicates of S. cyperinus (inlet) and T. latifolia (outlet) were collected. A subsample of roots from species at each wetland was also used to quantify metal concentrations in the oxidized coating on the root surface, as described by Macfie and Crowder (1987). For the fall sample date, additional plant species (J. effusus, S. cyperinus, Sparganium americanum Nutt., and Eleocharis quadrangulata (Michx.) R. & S. from IMP1 and P. pectinata from NAT) were collected for inter-species comparisons of metal concentrations. All other sampling remained the same. Biomass estimates in IMP1 and WC were determined by counting the number of T. latifolia shoots within ten randomly selected 0.5 m2 subplots in each cell. A subsample of T latifolia was harvested to determined shoot/leaves to root/rhizome dry weight ratios. At NAT, an intensive plant survey (5 m × 5 m transects) was conducted in order to determine species distribution and dominance. Five 0.5 m2 subplots were harvested to determine shoot/root dry weight ratios for S. cyperinus and P. pectinata, in addition to T. latifolia shoot counts. All plant materials were oven dried at 65°C for 96 h. These data were used to estimate the total biomass per cell and wetland. During the spring sample period, composite samples of Gambusia holbrooki (mosquito fish), benthic organisms, and Rana sp. (tadpoles) were also collected from each cell in IMP1. Populations of G. holbrooki were also released into NAT. During the fall sample period, fish, benthic organisms and tadpoles were sampled from IMP1 and NAT. No fish, tadpoles, or benthic organisms were present at WC, due to low pH. Due to the lack of replication for fish, tadpoles, and benthic organisms, statistical comparisons could not be conducted. Therefore, bioaccumulation of metals in these organisms will not be addressed in this paper.
2.3. Statistical analyses One-way analysis of variance techniques were used to test for statistical differences among wetlands in effluent water quality (monthly values averaged for the study period). Statistical differences in plant and sediment metal concentration by location within each wetland and by wetland for each sample date (pooled by location) were determined similarly. When significant differences among wetlands occurred, linear contrasts were performed to test for differences between: (1) NAT and IMP1, and (2) NAT and WC (Steel and Torrie, 1980). Analysis of variance was also used to test for differences in metal concentrations among plant species for the fall sample date.
3. Results and discussion
3.1. Water quality Monthly average values for discharge pH, total Fe, and total Mn for IMP1, WC, and NAT are presented in Fig. 4. Impoundment 1 discharge always met monthly NPDES compliance limitations for pH (6–9), Fe ( B 3.0 mg l − 1) and Mn (B2.0 mg l − 1) (although Mn concentrations exceeded 2.0 mg l − 1 during 3 months) and is indicative of its operation since construction in 1985 (Brodie, 1993). Discharge from WC (Cells 1 and 2) consistently exceeded compliance limitations, while discharge from NAT exceeded Fe limitations once and pH limitations twice. Monthly average values for Cr, Cu, Ni and Se were always below the detection limit (DL) in both the influent and effluent for all three wetlands. A summary of metal load, export, and removal for each wetland is given in Table 2. Load and export rates were greatest for WC and least for NAT. Two conditions exist that explain the low treatment efficiency for WC. First, insufficient treatment area exists for the loading rates currently occurring at WC. The treatment area at WC is 0.2 m2 mg Fe − 1 min − 1, whereas successful treatment systems within this region are 0.7 m2 mg Fe − 1 min − 1 (Brodie et al., 1989). Based on average flow rates within both of the
32.4 1.9 0.04 10 10 270 9.9 2 0.07 B0.002 0.02 B9 B6 B6 B0.4 B2
WC Fe Mn Al Zn Cd B As Pb
NAT Fe Mn Al Zn Cd B As Pb 5.42 1.4 0.28 B9 B6 B6 B0.4 B2
474 9.4 1.76 40 40 1500 203 6.1
52.4 7.1 0.04 B9 B6 20 1.46 B2
Max
1.29 0.20 0.10 B9 B6 B6 B0.4 B2
205 7.4 0.29 30 20 1170 100 2.2
44 5.9 0.02 B9 B6 10 0.85 B2
Mean
– – – – –
1.68 0.39 0.06
145 2.3 0.49 10 10 360 56.5 1.75
6.4 0.6 0.01 – – 10 0.29 –
S.D.
– – – – –
123 20 10
8446 305 12.4 1200 800 48 000 4120 90.6
506 67 0.3 – – 100 0.01 –
Loadc
0.01 B0.002 0.04 B9 B6 B6 B0.4 B2
B0.008 B0.002 0.01 B9 B6 190 0.4 2
0.4 0.2 0.02 B9 B6 10 B0.4 B2
Min
Effluent
5.95 0.60 0.30 B9 B6 B6 B0.4 B2
35.3 9.2 1.29 40 B6 40 0.56 2.2
1.3 3.3 0.07 B9 B6 30 B0.4 B2
Max
1.17 0.12 0.10 B9 B6 B6 B0.4 B2
6.3 3.9 0.34 20 B6 560 0.42 1.63
0.9 1.2 0.04 B9 B6 10 B0.4 B2
Mean
– – – – –
1.78 0.18 0.08
10.5 3.4 0.36 10 – 290 0.05 0.86
0.29 1.09 0.02 – – 10 – –
S.D.
– – – – –
111 12 10
260 161 14 800 – 23 000 17 67.2
10 14 0.5 – – 100 – –
Export
– – – – – –
b
a
12 8
8186 144 – 400 800 25 000 4103 23.4
496 53 – – – – 0.01 –
Removal
Cr, Cu, Ni, and Se were always below the detection limit (0.005, 7.0, 0.029, 0.005, and 0.4 mg l−1, respectively) and are not included. Influent and effluent concentrations are in mg l−1 for Fe, Mn, and Al; mg l−1 for B, Zn, Cd, As, and Pb. c Load, export, and removal rates are in mg m−2 day−1 for Fe, Mn, and Al; mg m−2 day−1 for B, Zn, Cd, As, and Pb.
34.3 4.9 0.02 B9 B6 10 0.45 B2
Min
Influentb
IMP1 Fe Mn Al Zn Cd B As Pb
Element
– – – – – –
10 40
97 47 – 33 100 52 99 26
98 79 – – – – 100 –
Efficiency
Table 2 Minimum, maximum, and mean metal concentration of influent and effluent, load, export, and removal at Impoundment 1 (IMP1), Widows Creek (WC), and natural wetland (NAT) during the study perioda
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P.A. Mays, G.S. Edwards / Ecological Engineering 16 (2001) 487–500
constructed wetlands, retention time is least at WC and lower retention time has been observed to reduce treatment effectiveness in other wetland systems (Sobolewski, 1995). In addition, occasionally high water levels caused by beaver activity and recurring infestations of armyworms (Simyra henrici ) may have stressed and reduced the number of T. latifolia, thus reducing the available root surface area needed for oxidation and adsorbtion. Secondly, insufficient alkalinity exists in order to buffer the acidity that is generated by the oxidation of Fe2 + (Faulkner, 1994). This is in contrast to IMP1 where influent alkalinity is much higher due to seepage waters coming into contact with buried road gravel that is a high CaCO3, oolitic limestone (Brodie et al., 1992). These conditions act as an anoxic limestone drain in producing higher alkalinity influent. At NAT, the drainage entering the wetland originates from relatively undisturbed second-growth upland oak-hickory forest. Soils are acid and infertile with only trace amounts of heavy metals (other than Fe and Al). In these highly weathered, highly oxidized soils, significant amounts of iron and aluminum oxides are effective in adsorbing most trace metal cations (Gambrell, 1994). Load rates for IMP1 tended to be greater than those for NAT, while export rates at these wetlands were similar for Mn, but differed for Fe and Al (Table 2). Removal efficiencies based on influent/effluent concentration differences indicate that IMP1 and WC were more effective than NAT at metal removal. Although efficiencies were greater, it should be noted that load rates were less by several orders of magnitude at NAT. Similarities in load and export rates at NAT would indicate a system approaching equilibrium as compared to the two constructed wetlands. Based on the mean values of monthly water samples during the study period, effluent water quality was not significantly different between NAT and IMP1. Contrasting NAT with WC revealed significant differences between the two wetlands for the following concentrations in the effluent: Fe, Mn, Al, B, Zn, and As. In each case, effluent concentrations at WC were greater than those at NAT (Table 2).
3.2. Metal concentrations in sediments At each wetland, extractable metal concentrations in the sediment (both depths) were not significantly correlated with distance from the wetland inlet, and concentrations did not differ significantly between depths and therefore, soil metal concentrations were pooled for depth. When pooled for depth, concentrations of various metals differed by location within each wetland, but most trends were inconsistent and varied by season (Table 3). This inconsistency has been observed in other constructed and natural wetlands within the study area (Sistani et al., 1995). However, Fe concentrations at IMP1 were greatest in Cell 1, while Mn concentrations were greatest in Cell 4 on both sample dates. These data support an earlier study conducted on this wetland in which Faulkner (1994) concluded that the majority of the Fe in the influent was oxidized and precipitated in Cell 1, while Mn removal occurred primarily by co-precipitation in the Cells 3 and 4. Orthogonal contrasts of extractable metal concentrations (pooled for depth and location within each wetland) indicate that Mn and Se concentrations were significantly greater at IMP1 than at NAT during both sample dates, and Cr was significantly greater in the fall. Concentrations of Zn, Cu, and Ni were significantly greater at NAT in the spring, but no differences were found in the fall (Table 4). Sediment pH was also significantly lower and organic matter was significantly greater at NAT than at IMP1 on both sample dates. Much of the inlet area of NAT is shallow and is exposed in times of very low flow. As the sediments begin to dry, creeping rush (Juncus repens) and other nonwetland species often invade from the wetland periphery and cover the entire area. The annual turnover of primarily root materials would certainly increase soil organic matter content within these areas over time. Increased organic acid concentrations and soil oxidation would tend to lower sediment pH within these areas. Manganese, Cu, Pb, B, As, and Se were greater at WC than at NAT in the spring, while Al and Zn were greater at NAT (Table 4). In the fall, Cu, B, As, and Se concentrations were
5.8 5.5 5.3 5.6 4.0 6.3 4.9 5.0
6.0 5.9 6.2 5.6 4.8 4.5 4.9 5.1
Spring IMP1 Cell 1 Cell 2 Cell 3 Cell 4
WC Cell 1 Cell 2
NAT Inlet Outlet
Fall IMP1 Cell 1 Cell 2 Cell 3 Cell 4
WC Cell 1 Cell 2
NAT Inlet Outlet 10.0b 25.4a
25.6 14.3
2.3 1.7 2.2 2.5
11.6b 25.9a
45.5a 3.7b
2.0 1.1 2.1 2.0
OM (%)
5.4b 6.7a
9.9a 7.7b
5.4 6.1 6.6 5.7
5.3b 6.5a
7.6b 13.8a
5.8 5.1 5.9 6.0
CEC (cmol kg−1)
556a 189b
184b 369a
617a 429b 416b 377b
457a 171b
176b 416a
430a 260b 416ab 299b
mg g−1
Fe
52 59
73a 15b
130b 295b 150b 600a
64 68
71b 246a
203b 245ab 93b 402a
Mn
212 255
110b 190a
230b 351a 292ab 365a
230b 315a
82b 97a
426a 339b 387ab 457a
Al
1.6b 4.2a
2.9 2.2
2.2b 1.3b 2.9b 5.7a
1.6b 5.9a
3.1 2.4
1.1b 0.8b 1.8a 1.9a
Zn
1.21 1.05
1.65 1.66
1.20a 1.11a 0.93b 1.33a
0.74b 1.06a
2.01 1.02
0.52 0.45 0.53 0.39
Cu
2.5 1.7
3.2a 1.4b
1.6c 2.1b 2.0b 2.5a
1.2 1.4
2.7a 1.1b
1.1ab 0.9b 1.3a 1.0ab
Pb
0.50b 0.97a
0.92 1.14
0.82b 0.43b 0.95b 1.98a
0.42b 1.29a
0.93 0.50
B0.03 B0.03 B0.03 B0.03
Ni
0.69a 0.37b
0.34 0.50
0.81 0.91 0.79 0.91
B0.005 B0.005
B0.005 B0.005
B0.005 B0.005 B0.005 B0.005
Cr
0.30 0.30
1.61 1.43
0.39 0.37 0.36 0.41
B0.006 B0.006
0.96b 2.08a
B0.006 B0.006 B0.006 B0.006
B
1.5 1.5
4.5b 16.3a
3.4 3.2 2.8 3.1
1.0b 1.9a
58a 22b
3.0 2.4 2.3 2.4
As
0.20 0.27
0.35 0.40
0.25b 0.34b 0.50a 0.51a
0.12 0.18
0.57a 0.23b
0.51 0.42 0.50 0.43
Se
a Data are pooled for depth. Cd was always below the detection limit (0.006) and is not included. OM, organic matter. CEC, cation exchange capacity. Different letters following a given parameter within each wetland indicate significant differences (PB0.05) among locations for each sample date.
pH
Wetland
Table 3 Mean chemical characteristics and metal concentrations by location in the sediment of Impoundment 1 (IMP1), Widows Creek (WC), and natural wetland (NAT) wetlandsa
P.A. Mays, G.S. Edwards / Ecological Engineering 16 (2001) 487–500 495
5.8* 5.0 4.9 6.0* 4.9 5.0
Spring IMP1 WC NAT
Fall IMP1 WC NAT 2.2* 16.6 17.7
1.8* 14.1 18.8
OM (%)
5.9 8.9* 6.1
5.4 9.5* 6.0
CEC (cmol kg−1)
448 217* 372
350 240 314
mg g−1
Fe
277* 59 56
241* 123* 66
Mn
261 126* 233
330 122* 273
Al
2.6 2.3 2.9
1.4* 2.5* 3.7
Zn
1.22 1.52* 1.13
0.49* 1.30* 0.90
Cu
1.8 2.3 2.1
1.0 2.0* 1.3
Pb
0.96 0.90 0.73
B0.03* 0.69 0.85
Ni
0.78* 0.38* 0.53
B0.005 B0.005 B0.005
Cr
0.37 1.58* 0.30
B0.006 1.39* B0.006
B
3.44 7.50* 1.47
3.20 32* 1.43
As
0.35* 0.37* 0.23
0.46* 0.45* 0.15
Se
Data are pooled for depth and location. Cd was always below the detection limit (0.006) and is not included. OM, organic matter. CEC, cation exchange capacity. * Following a given parameter indicates significant differences (PB0.05) between NAT and IMP1 or NAT and WC for each sample date. No statistical comparison is made between IMP1 and WC.
a
pH
Wetland
Table 4 Mean chemical characteristics and extractable metal concentrations in the sediment of Impoundment 1 (IMP1), Widows Creek (WC), and natural wetland (NAT) wetlandsa
496 P.A. Mays, G.S. Edwards / Ecological Engineering 16 (2001) 487–500
P.A. Mays, G.S. Edwards / Ecological Engineering 16 (2001) 487–500
greater at WC, while Fe and Al concentrations were greater at NAT. There were no significant differences between pH and organic matter at either sample date, but cation exchange capacity was greater at WC during both sample dates. At IMP1 and NAT, extractable metal concentrations in the sediment usually exceeded those of the influent by between two and five orders of magnitude, implying that accumulation in the sediment was not due solely to influent additions to the system. This is particularly noticeable for Cr, Cu, Ni and Se since influent and effluent concentrations of these elements were below detection limits throughout the study period. Rather, plant uptake and decay, plus strong retention of metals by inorganic and organic soil components, are the most probable mechanisms of accumulation of metal ions near the soil surface (Bohn et al., 1985; Dunbabin and Bowmer, 1992).
3.3. Metal concentrations in plants Distance from the wetland inlet also had no significant effect on metal concentrations in plants, and plant metal concentrations were not significantly correlated with sediment or water metal concentrations. The availability of trace metals to aquatic plants is complex and is dependent on specific chemical factors associated with the metal and substrate in which it is found (Taylor and Crowder, 1983; Jackson et al., 1991; Sparling and Lowe, 1997; Zayed et al., 1998). Metal concentrations for roots in this study included metals contained in root coatings, which accounted for 32–93% of the total metal concentration in the roots (Table 5). Consequently, some metal concentrations for the roots are quite high when compared to those of the shoots. This general trend in metal concentration (root \ rhizome \leaf tissue) has been observed in other studies where T. latifolia and J. effusus have been analyzed for heavy metals (Taylor and Crowder, 1983 Shutes et al., 1993). At the spring sample date, Mn, Ni, and Cr concentrations in both roots and shoots of plants at IMP1 were significantly greater than concentrations of these metals in plants at NAT (Table 5). Arsenic concentration in roots at WC was also
497
significantly greater than at NAT. However, Pb concentration in roots and Al, Zn, and Cu concentrations in shoots were significantly greater in plants at NAT than at either constructed wetland at the spring sample date. In the fall, concentrations of Fe and Cd in roots and B in shoots at IMP1 and WC were significantly greater than at NAT. In contrast, root concentrations of Al, Zn, Ni, and Cu and shoot Ni concentration were significantly greater at NAT than at the constructed wetlands. A comparison of metal concentrations in various plant species at IMP1 and NAT at the fall sample date revealed that concentrations of Fe, Cu, Zn, Pb, Cd, Cr, and As were significantly lower in T. latifolia shoots than in other wetland species (J. effusus, S. cyperinus, S. americanum, E. quadrangulata, and P. pectinata). In contrast, P. pectinata, one of the most common species at NAT exhibited significantly greater shoot concentrations of Mn, Al, Cu, Zn, Pb, Ni, and As than all other species. These differences may be closely associated with growth form (emergent vs. submerged, floating species) where several studies have shown that emergent species have lower metal concentrations than submerged or nonrooted, floating species (review by Sparling and Lowe, 1997). Data in the literature indicate great variation in plant metal concentrations achieved in different wetland environments (Jenkins, 1980; Jackson et al., 1991; Bryan and Langston, 1992; Dunbabin and Bowmer, 1992; Zayed et al., 1998). At each wetland in this study, concentrations of Mn, Zn, Cu, Ni, B, and Cr in shoots of T. latifolia (and S. cyperinus at NAT) were greater than the extractable concentrations of these metals in the sediment, indicating that these metals are accumulating in the plant component. Sparling and Lowe (1997) found similar trends for B, Mn, and Zn in control experiments on both rooted and nonrooted species. In addition, accumulation of Mn, Cu, Zn, and Cr is probably greater in other species, especially P. pectinata, based on the interspecies comparison discussed above. In a review provided by Zayed et al. (1998), T. latifolia is shown to be an accumulator of Cd, Cu, Ni, and Pb, while Scirpus sp. also accumulate Cr.
28 660* 27 322* 9121 327 1739* 253
Fall Root IMP1 WC NAT
Shoot IMP1 WC NAT 29
2076* 549 821
2012* 144* 617
1752* 527 751
1786* 121 442
Mn
44
78 122 74
1531* 539* 2582
61* 66* 137
2723 1067* 3358
Al
39
7.5* 12 12
23* 23* 34
16* 16* 38
41 16 34
Zn
32
1.0* 2.5* 1.8
1.2* 4.1 5.4
1.2* 3.6* 6.3
6.5 3.3 6.5
Cu
82
0.6 0.8 0.7
6.1 8.2 9.9
B0.09 1.1 B0.09
4.7* 6.0* 17.9
Pb
92
0.1 0.4* 0.06
6.0* 5.6* 2.2
B0.006 B0.006 B0.006
2.7 2.4 1.7
Cd
80
0.7* 0.3* 1.2
1.5* 0.5* 4.0
6.2* 3.3 2.9
18.1* 10.7 8.5
Ni
72
0.4 0.5 0.7
3.1 2.3* 3.9
12* 6.2 3.3
37* 24 13
Cr
65
6.2* 10.7* 4.5
0.06 0.06 0.06
B0.006 10.0* 4.8
B0.006* 1.1 1.4
B
ND
0.04 1.0* 0.06
3.8 21.1 8.6
0.06 0.07 0.03
3.5 28.8* 3.9
As
ND
0.04 0.06* 0.03
0.09 0.13 0.12
0.05 0.10 0.07
0.17 0.15 0.19
Se
Data are pooled for location. ND, not determined. * Following a given metal concentration for each plant component indicates significant differences (PB0.05) between NAT and IMP1 or NAT and WC for a given sample date. No statistical comparison is made between IMP1 and WC.
a
93
349 381 363
Shoot IMP1 WC NAT
% of metal in root coat
7427 13 077 8820
mg g−1
Fe
Spring Root IMP1 WC NAT
Plant component
Table 5 Mean metal concentrations in plants at Impoundment 1 (IMP1), Widows Creek (WC), and natural wetland (NAT) wetlandsa
498 P.A. Mays, G.S. Edwards / Ecological Engineering 16 (2001) 487–500
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Although some metals are accumulating in the plants at each wetland, the total metal content in the plant component accounted for only a small percentage of the annual metal load supplied to each wetland. For example, Fe and Mn contents in the plants at IMP1 were only 1 and 2%, respectively, of the annual Fe and Mn load received by this wetland. Similar findings have been reported for Ni and Cu removal by plants from mine drainage treated by a natural wetland (Eger and Lapakko, 1988) and for Cd, Cu, and Zn removal from municipal sewage effluent treated by artificial wetlands (Gersberg et al., 1984). Mitsch and Wise (1998) summarize additional studies concerning the role of vegetation in metal retention in wetlands and determine that even in wetlands where luxury consumption is possible (such as IMP1 and WC), the above-water vegetation is storing only 0.07% of the annual inflow of iron. They conclude their summary by suggesting that the critical role of vegetation in metal retention may also include serving as sites for metal precipitation and/or sedimentation.
Acknowledgements Funding for this project was provided by the Tennessee Valley Authority and the Electric Power Research Institute. The authors express their appreciation to C.L. Wylie for assistance with data management and statistical analysis and to J. Scarbrough for technical contributions. We are also grateful to G.A. Brodie, J. Goodrich-Mahoney, F.J. Sikora, F.C. Thornton and N.S. Nicholas for reviewing the manuscript.
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.