Article pubs.acs.org/est
Natural Wetlands Are Efficient at Providing Long-Term Metal Remediation of Freshwater Systems Polluted by Acid Mine Drainage Andrew P. Dean,*,† Sarah Lynch,† Paul Rowland,† Benjamin D. Toft,† Jon K. Pittman,†,‡ and Keith N. White†,‡ †
Faculty of Life Sciences, University of Manchester, Michael Smith Building, Oxford Road, Manchester M13 9PT, U.K. The Centre for the Genetics of Ecosystem Services, University of Manchester, Michael Smith Building, Oxford Road, Manchester M13 9PT, U.K.
‡
S Supporting Information *
ABSTRACT: This study describes the first long-term (14-year) evaluation of the efficacy of an established (>100 years) natural wetland to remediate highly acidic mine drainage (AMD). Although natural wetlands are highly valued for their biodiversity, this study demonstrates that they also provide important ecosystem service functions through their ability to consistently and reliably improve water quality by mitigating AMD. The Afon Goch river flows from Parys Mountain copper mine via a natural wetland, and was the major source of Zn and Cu contamination to the Irish Sea. Prior to 2003 the wetland received severe acidic metal contamination and retained a large proportion of the contamination (55, 64, and 37% in dissolved Fe, Zn, and Cu) leading to a greatly reduced metal flow to the Irish Sea. Reduced wetland loadings midway through the sampling period led to a reduction of metals by 83−94% and a pH increase from 2.7 to 5.5, resulting in long-term improvements in the downstream benthic invertebrate community. High root metal accumulation by the dominant wetland plant species and the association of acidophilic bacteria in the wetland rhizosphere indicate that multiple interacting processes provide an efficient and self-sustaining system to remediate AMD.
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INTRODUCTION Acid mine drainage (AMD) occurs when metal sulfides, most commonly pyrite (FeS2), are oxidized in the presence of water releasing Fe and other toxic trace metals in the exposed mine rock and generating acidity.1,2 Mixing of AMD with natural waters of rivers and streams can impact significantly on the chemistry and biology of these ecosystems,3 reducing the value of the water for agricultural, recreational, or industrial uses, rendering it unsafe for human consumption and having a negative effect on the biota. AMD is a global environmental problem as it is generated from abandoned mines worldwide.4 In the United States alone it has been estimated that there are around 500,000 abandoned mines, with AMD affecting 25,000 km of streams.5 The UK has an even longer history of metal mining leading to significant pollution problems as a result of AMD.6 A number of strategies for mitigating AMD have been proposed and evaluated,7−11 including constructed wetlands, anoxic limestone drains, and addition of chemicals to increase pH and precipitate dissolved metals. The use of wetlands to treat polluted mine waters arose from the recognition in the 1970s to 1980s that natural Sphagnum bog wetland systems could decrease the toxic characteristics of polluted mine waters through natural biological and chemical reactions that remove metal contaminants.12−14 This recognition led to the develop© XXXX American Chemical Society
ment of appropriated natural wetlands, in which natural wetlands are adapted for more efficient metal retention,15 and constructed wetlands designed to mimic natural processes for AMD remediation.16,17 Wetlands are a particularly attractive remediation strategy due to low operating cost and potential wider biodiversity benefits18,19 and are now an established technology.20 However, since the earlier studies, little attention has been paid to natural wetland systems. Such wetlands may be highly effective in remediating AMD having adapted to the low pH and high metal environment of an AMD-polluted system over a long period of time.21,22 However, there has been no evaluation of natural wetland remediation performance over a long time period and there is still a poor understanding of the biological composition and diversity of a natural AMD wetland. This study has made use of an AMD-polluted river system (the southern Afon Goch) in Anglesey, North Wales, UK to examine the remediation potential of a small natural wetland that has existed since at least the 1890s. The river receives mine drainage waters from Parys Mountain, a former Cu mine, which was continuously active from the mid 18th century until mining Received: March 22, 2013 Revised: September 26, 2013 Accepted: October 2, 2013
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source, the river flows through a natural wetland of ∼0.1 km2. A steady flow of minewater at a rate of ∼0.01 m3 s−1 entered the river at the Mona adit but in 2003 drainage management was implemented to reduce flood risk.24 This resulted in the minewater that previously drained to the southern Afon Goch entering another stream, the northern Afon Goch. Contamination of the southern Afon Goch now occurs only from spoil leachate runoff. This has provided the opportunity to evaluate the system before and after drainage diversion, and quantify the mechanisms and indicators of recovery of the southern river from long-term AMD. A previous study of the southern Afon Goch prior to drainage diversion found the river to be highly contaminated with Fe (259 mg L−1), Cu (60 mg L−1), and Zn (42 mg L−1) and highly acidic (pH 0.05). However, drainage management did result in a diversion of 0.01 m3 s−1 from the river,24 which is likely to be highly significant under low flow conditions. Therefore we examined the river during two occasions in 2010 following a very low and very high rainfall period, causing low and high river flow rate, respectively (SI Figure S1). When mass flow of metals before and after the wetland were quantified, there was a significant (p < 0.05) decrease in dissolved Fe, Zn, and Cu mass flow after the wetland (site S2 compared to site S1) on both rainfall occasions, despite total mass flow being much higher at both sites during high flow rate (SI Figure S1). There was also no significant alteration over the 14-year period in mean flow rate at site S5 (SI Figure S2). Thus, reductions in dissolved metal concentrations after the wetland were not due to differences in water flow at the source. There were no significant changes in particulate metal concentrations among sites, but pre- and post-2003 particulate Fe and Cu concentrations were significantly reduced at sites S4 and S5 to tributary levels (SI Figure S3). The concentrations of sedimentary metals did not vary significantly among the three sampling periods (SI Figure S3) but concentrations of Fe and Cu were typically higher at the upper sites (S1 and S2), but at tributary levels at site S5. Particulate and sedimentary Zn showed little change among sites or between the sampling periods. The improvements in water quality were reflected in the benthic invertebrate fauna with the poor water quality prediversion reflected in the low number of taxa at all sites (Figure 2F). These were restricted to acid-tolerant taxa, with site S1 largely restricted to Chironomidae and Sialidae, while at sites S2−S5 Corixidae and Tubificidae were additionally present. Following 2003 the number of invertebrate families improved slightly though they were still largely restricted to AMD-tolerant species. However, later sampling (2008−11) found that the invertebrate fauna was much more diverse, with a number of pollution-intolerant taxa, such as Perlodidae, Ephemeridae, and Gammaridae observed at the lower sites after the wetland (SI Table S2). Effect of the Wetland on Water Quality. Although there was no change in pH across the wetland before 2003, following drainage diversion (2004−2010) the wetland was responsible for a significant increase in pH (p < 0.05) at site S2 (Figure 2D). For conductivity there was a significant decrease (p < 0.05) across the wetland area, which during 2004−2010 had fallen to tributary levels (Figure 2E). Likewise the wetland also resulted in significant changes in dissolved metal concentration. In 1997−2003 Fe, Zn and Cu concentrations were significantly reduced (p < 0.05) after the wetland (Figure 2A−C). Post-2003 the wetland was responsible for significant decreases in both Cu
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RESULTS Effect of Drainage Diversion on Water Quality in the Southern Afon Goch. Variations in the dissolved metals, pH, and conductivity for the three sampling periods are shown in Figure 2. During 1997−2003 pH at source (site S1) was very low (pH ∼2.5) and remained low at site S2. pH values then increased steadily through the downstream sites, with a mean of 5.3 at the final site (S5), though still below the value (6.2) at the non-AMD polluted tributary (site ST). Immediately following mine drainage diversion (2003), no change in pH was observed at source. However, at site S2 a small significant increase in pH was observed, with a further increase to pH 6.6 at site S3, where the pH was comparable to the tributary. By 2009/10, pH at site S1 was still unchanged, however, it had risen to 5.6 at site S2, and was comparable to the tributary at S3−S5. Comparison of pH values pre- and postdrainage diversion (2004−2010) show that with the exception of site S1, diversion resulted in significantly higher (p < 0.05) pH at all sites. Conductivity also showed significant changes along the river and between time periods. During 1997−2003 conductivity at the source was ∼2250 μS. There was a significant decrease at site S2, however, conductivity remained well above tributary levels at site S3, and it was not until site S4 that conductivity fell to tributary levels (
and Zn; however, as stated above, Fe showed a delayed response to drainage change, and inclusion of 2004 data shows that the wetland has a nonsignificant effect on Fe concentrations; however, statistically significant reductions in Fe are evident if concentrations from 2005 onward are considered. During 1997−2003 the decrease in dissolved Fe, Zn, and Cu at site S2, together with a total wetland area estimated by site surveys of 9.9 ha suggested a mean metal retention of 2.15 g m−2 d−1 for Fe, 0.21 g m−2 d−1 for Zn, and 0.17 g m−2 d−1 for Cu. Calculations of metal retention based on mass flow data collected in 2010 (SI Figure S1 and Table S3) show that estimated retention efficiencies vary under different flow regimes but are nevertheless equivalent to those determined prior to drainage diversion. Afon Goch as it Flows Through the Wetland. Data from the Afon Goch clearly demonstrated improvement of water quality downstream from the natural wetland, therefore metal behavior within the wetland was determined. Sampling at multiple sites through the wetland (W1−W4) was performed in 2011 (Figure 3). Little change was observed between site S1
Figure 3. Dissolved Fe, Zn, and Cu, pH, and conductivity at sites in the southern Afon Goch as it flows through the natural wetland in 2011. All values are means and error bars show the maximum and minimum recorded values (for dissolved metals) and standard error of the mean (for pH and conductivity).
and the first site W1 as the Afon Goch enters the wetland, indicating little influence of the river channel. However, a further 300 m into the wetland (site W2), pH had risen to 3.5, conductivity had fallen by 50%, and dissolved Fe, Zn, and Cu concentrations reduced by 76%, 43%, and 47%, respectively. At 700 m into the wetland at site W3, pH had risen to 5.5 and the concentrations of dissolved Fe, Zn, and Cu had fallen by 92%, 83%, and 94%, respectively, and were at concentrations approaching those seen in the unpolluted tributary. Most Zn and Cu was in a dissolved form at all sites indicating rapid deposition of particulate metal. Most (>95%) Fe was in dissolved form at sites S1 and W1, but at sites W2 to S2, the dissolved Fe contribution ranged from 40% to 79% of the total Fe load. River sediment Fe ranged between 43 and 230 mg g−1; Zn between 200 and 5200 μg g−1, and Cu between 70 and 7000 μg g−1, and showed no consistent pattern. Computational speciation modeling of Fe, Zn, and Cu was performed for pH 2−7 using water chemistry parameters from site S1 (SI Figure S4). Al speciation prediction was performed for comparison as speciation of this metal in response to pH is well understood.27 The expected distribution of multiple Al species at pH >5 and a low proportion of free Al3+ was
Figure 4. Concentrations of Fe, Zn, and Cu in sediment surrounding roots (A) and in roots and shoots (B) of J. ef f usus plants collected across the Afon Goch wetland and from a control non-AMD polluted wetland site (n = 4−18). Metal concentrations in plants at individual sites within the wetland are shown in SI Figure S6. Boxes show the 25th and 75th percentiles, the colored lines within the boxes show the median values, and the black lines show the mean values. Whisker bars show the 10th and 90th percentiles, and black circles show all outlying data points. For comparisons between AMD and control wetland samples, data points that do not share lowercase letters are significantly different (p < 0.05) as determined by Mann−Whitney U test.
0.05) in plant or rhizosphere sediment metal concentration among wetland sites (SI Figure S6). These values were compared to control samples taken from a nearby non-AMDpolluted reservoir wetland site. Concentrations of Fe and Cu in the shoots, roots, and sediment were significantly higher at all sites than at the control site (p < 0.05). The accumulation of Fe into the roots of J. ef fusus was particularly high, with a mean concentration of 88.7 mg Fe g−1 from the four wetland sites compared to a mean concentration of 19.8 mg Fe g−1 in the D
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Figure 5. Phylogenetic analysis of 16S rRNA sequences obtained from bacteria in sediment samples collected from sites W2 and S2, and a nonpolluted control site (C). Sequences from unknown bacterial strains were compared with selected species including known mineral-converting acidophilic bacteria (bold). Information about each clone and GenBank accession numbers are given in SI Tables S5 and S6. Bootstrap percentage values are indicated at branch nodes. The scale bar indicates evolutionary distance.
waste,18,19 and have been used as the inspiration for passive treatment of AMD by appropriated natural wetlands and constructed wetland systems.15−17 A constructed wetland should mimic natural systems in terms of biological composition and function, and ideally should be more efficient. We aimed to evaluate a natural wetland system for AMD remediation efficiency, stability, and biological composition, in order to evaluate the long-term potential of wetland AMD remediation and to aid our understanding of wetland remediation mechanisms. Natural wetlands have been suggested to function efficiently as long-term metal sinks over centuries rather than decades.22 Yet a single year-long study of a 4.4-ha natural willow and sedge wetland in Colorado which had received AMD for most of the 20th century, concluded that the wetland was losing metal retention efficiency over time and was therefore unlikely to act as an infinite metal sink.21 Here we have performed a longerterm (14-year) evaluation of an AMD natural wetland. During the 14 years of study, the Afon Goch water quality was found to consistently improve with regard to reductions in dissolved Fe, Cu, and Zn downstream from the natural wetland. Furthermore, this decrease in metal concentration is clearly due to the wetland acting as a sink, rather than just due to a change in flow regime, as both concentration and mass flow of metals showed substantial decreases across the wetland. The wetland caused metal retention regardless of the flow rate into the wetland, although the rate of metal removal by the wetland ranged widely under low and high flow rate conditions: 4−1204 mg s−1 for Fe, 17−800 mg s−1 for Zn, and 3−153 mg s−1 for Cu. It must be noted that these metal removal rates were calculated from a limited set of data during a single year in the summer. However, the collection of data during two extremes in rainfall and thus flow rate extremes provides an indication of the large differences in metal retention that can occur during a season. The metal retention efficiencies estimated from these values (SI Table S3) were equivalent to those calculated over
control plants. This root Fe concentration was substantial compared to the mean sediment concentration of 162.3 mg Fe g−1 surrounding the plants. Most of the Fe accumulated within root tissue, as determined by EDTA washing, which reduced root Fe concentration by only 10.2%. To evaluate the diversity of microorganisms within the wetland, including acidophilic mineral-converting bacteria, DNA samples were collected from soil sediment within the wetland (site W2), after the wetland (site S2) where there were a few wetland plants, and at the nonpolluted site. 16S rRNA amplification and phylogenetic analysis revealed the presence of a wide diversity of bacterial species (Figure 5). A variety of soil bacteria, predominantly of the order Sphingobacteriales, were present at all three sites (SI Table S5); however, sequences were found at site W2 and S2 that group with acidophilic bacteria including Acidobacterium, sulfate-reducing bacteria (SRB) including Desulfovibrio, and oxidizing bacteria including Sulf urimonas (Figure 5). In contrast, there was no evidence of acidophilic species from the control site. A large number of 16S rRNA sequences identified from sites W2 and S2 were from eukaryotic chloroplasts. All were from eukaryotic algal species; two from site W2 grouped with acidtolerant Euglena (SI Figure S7), and many sequences from sites W2 and S2 were similar to diatom sequences (SI Table S5). All of these W2 and S2 sequences grouped with 16S sequences from known acid-tolerant diatoms including Navicula, Nitzschia, and Synedra,28 whereas the three sequences from the control site grouped with the acid-sensitive diatom Cymbella (SI Figure S7). Within the wetland stream, the diversity of benthic invertebrates was poor and similar to that of S1 but with Polycentropodidae and Asellidae also present (SI Table S2).
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DISCUSSION Natural wetlands have long been recognized as a means of remediating pollution, particularly from sewage and mine E
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The Afon Goch wetland was dominated by E. angustifolium, P. australis, and J. ef f usus, and highly elevated concentrations of Fe, Cu, and Zn were found in J. ef f usus tissues. Metal storage was mostly in the roots, as has been found for other wetland species.30−32 The metal concentrations observed in the wetland vegetation were much higher than those seen in other wetland plants contaminated by AMD despite similar loading concentrations.21,33 For example, in the Colorado natural wetland exposed to ∼20−30 mg Fe L−1 inflow, the willow and sedge plants accumulated only 0.3 mg Fe g−1 compared to 87 mg Fe g−1 in the Afon Goch J. eff usus roots. A direct role of plants in AMD remediation has been questioned.4 Although bacterial activity in the wetland is clearly critical, this data is indicative of the role that plant metal accumulation is playing in this wetland. Storage of metals mainly in root rather than shoot tissue will reduce release back into the environment through die-back of aerial tissues and will reduce transfer of metals into food chains by herbivory. Nevertheless, based on an approximate calculation of the total metal concentration in the dominant plant species (J. eff usus) within the wetland (data not shown), the overall role of the wetland plants for metal remediation by bulk extraction is a minor component of total metal load entering the wetland system compared to accumulation of metals in the sediment, as has been previously observed in other systems.20 Indeed, very high concentration of Fe, Zn, and Cu deposits in the organic sediment surrounding the wetland plant roots were measured, including >150 mg Fe g−1. The aerobic conditions around the roots will cause metal oxidation and the formation of metal hydroxides, while the increase in pH within the wetland will also promote precipitation and complexation with organic ligands in the sediment. A combination of plant and microbial activities is likely to generate alkalinity which controls pH of the water. Wetland plants such as cottongrass have been shown to increase external pH when exposed to metals, possibly caused in part by release of anionic organic acids.34 However, SRB will play a major role in generating net alkalinity and yield insoluble metal sulphides.4 SRB activity plays a major role in many constructed wetland systems4,10,18 and has been observed in natural AMD wetlands.35,36 Here we found evidence of a complex microbial community structure present within the wetland, including bacteria of unknown species which cluster with known acidophilic bacteria. Clustering of DNA clones with known SRB, oxidizing bacteria, and heterotrophic bacteria indicate that a range of activities is present in the wetland for the transformation of metallic and sulfide compounds. This is similar to the situation seen previously in natural and constructed wetlands.10,36 Further analysis will be required to confirm the activities and functions of the bacteria present in the wetland, while analysis of deeper sediment may yield further anaerobic SRB activities. In addition to quantifying efficiency of AMD remediation, it is essential that improvement is validated by assessment of chemical and biological quality of the downstream water course. Although a number of studies have detailed the negative impact of AMD on river biota, only a few have looked at biotic recovery following remediation.37 In this study, downstream (sites S3−S5) benthic invertebrate diversity showed a 3-fold improvement over a 7-year period following 2003, although it has not yet reached the diversity present in the non-AMD polluted tributary. Previous assessments of AMD recovery using benthic invertebrates showed a similar improvement;
the 1997−2003 period. This indicates that when sampled both pre- and postdrainage diversion, the wetland was acting as a net sink for all dissolved metals, but as has been shown for other natural wetlands,21 a wetland could act as a source for export of metals, such as during the winter, although seasonal changes were not studied here. It must also be acknowledged that this study was unable to perform a full mass balance analysis of metal retention by the wetland during all years due to limitations in data collection. The wetland also caused metal retention regardless of the concentration of metal inflow, but the efficiency of metal retention and the effectiveness for removing acidity were dependent on influent metal concentration. Mine drainage management in 2003, when the mine water table was lowered and much of the drainage diverted into the northern Afon Goch, allowed us the opportunity to compare two different wetland inflow regimes. In the years after 2003, dissolved Fe concentration at source was moderate and the efficiency of the wetland was very high (97% decrease of 19 mg L−1 Fe input in 2011). When concentration of Fe inflow into the river was substantially higher before 2003 (to almost 500 mg L−1) the wetland was still able to remove on average 55% of dissolved Fe with an estimated retention rate of 2.15 g Fe m−2 d−1, which was much better than some other wetland remediation systems with similar pH and Fe loading.29 However, any change in pH during this high Fe loading period was minor. This is consistent with previous observations of poor wetland function for treating acidic waters when Fe levels are high,16 but we also observed that even under moderately high Fe inflow, there was a significant rise in pH at site S2: up to pH 3.1 when inflow was 51 mg Fe L−1 (2005) and up to pH 5.3 when inflow was 26 mg Fe L−1 (2010). The 9.9-ha Afon Goch wetland appears to be of suitable size for efficient remediation of the high acidic and high Fe drainage based on previous calculations that estimate the need for 500 m2 of surface flow wetland to remove 1 kg Fe d−1 at pH 3.17 For example, a loading of 142.6 kg Fe d−1 as measured in 2010 would require a wetland of 7.1 ha. However, the metal reduction and pH improvement occurred within just 700 m of the AMD entering the wetland, which is an area of only 3.8 ha, indicating that this wetland is particularly effective for its size. We suggest that the mechanisms operating in this natural wetland are efficient due to the longevity and stability of the wetland. Although the size and species composition of the wetland was not directly quantified during this study, we did not observe significant changes in the wetland vegetation or any change in the wetland area during these 14 years. Furthermore, cartographic evidence suggests that the Afon Goch wetland is in excess of 100 years old, during which time AMD loading would have been continuous, potentially leading to a plant and microbial community that is fully adapted to AMD tolerance and removal. The importance of adaptation to mine drainage is unclear. One study has suggested that tolerance to high Zn by J. eff usus and E. angustifolium is innate, as Zn tolerance was observed in both species regardless of whether they derived from a mine site or an uncontaminated site.30 However, Fe toxicity experiments by P. australis derived from an uncontaminated site found that it was sensitive to high concentrations of Fe.31 Further studies are needed to examine potential adaptation of AMD wetland plants and to validate whether adapted plants from a long-established natural wetland such as this one are more suitable as plant stock for constructed systems than plants derived from less-polluted locations. F
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(4) Johnson, D. B.; Hallberg, K. B. Acid mine drainage remediation options: A review. Sci. Total Environ. 2005, 338 (1−2), 3−14. (5) DeGraff, J. V. Addressing the toxic legacy of abandoned mines on public land in the western United States. Rev. Eng. Geol. 2007, 17, 1−8. (6) Mayes, W. M.; Potter, H. A. B.; Jarvis, A. P. Inventory of aquatic contaminant flux arising from historical metal mining in England and Wales. Sci. Total Environ. 2010, 408 (17), 3576−3583. (7) Cravotta, C. A. Abandoned mine drainage in the Swatara Creek Basin, Southern Anthracite Coalfield, Pennsylvania, USA: 2. Performance of treatment systems. Mine Water Environ. 2010, 29 (3), 200− 216. (8) Hedin, R.; Weaver, T.; Wolfe, N.; Weaver, K. Passive treatment of acidic coal mine drainage: The Anna S mine passive treatment complex. Mine Water Environ. 2010, 29 (3), 165−175. (9) Skousen, J.; Ziemkiewicz, P. Performance of 116 passive treatment systems for acid mine drainage. In Proceedings of National Meeting of the American Society of Mining and Reclamation, Breckinridge, USA; 2005; pp 1100−1133. (10) Hallberg, K. B.; Johnson, D. B. Microbiology of a wetland ecosystem constructed to remediate mine drainage from a heavy metal mine. Sci. Total Environ. 2005, 338 (1−2), 53−66. (11) Mitsch, W. J.; Wise, K. M. Water quality, fate of metals, and predictive model validation of a constructed wetland treating acid mine drainage. Water Res. 1998, 32 (6), 1888−1900. (12) Huntsman, B. E.; Solch, J. G.; Porter, M. D. Utilization of Sphagnum species dominated bog for coal acid mine drainage abatement. In 91st Annual Meeting; Geological Society of America: Toronto, Ontario, Canada, 1978; p 322. (13) Wieder, R. K.; Lang, G. E. Modification of acid mine drainage in a freshwater wetland. In Proceedings of the Symposium on Wetlands of the Unglaciated Appalachian Region; McDonald, B. R., Ed.; West Virginia University: Morgantown, WV, 1982. (14) Wieder, R. K.; Lang, G. E. Fe, Al, Mn, and S chemistry of Sphagnum peat in four peatlands with different metal and sulfur input. Water Air Soil Pollut. 1986, 29 (3), 309−320. (15) Younger, P. L.; Large, A. R. G.; Jarvis, A. P. The creation of floodplain wetlands to passively treat polluted minewaters. In Hydrology in a Changing Environment. Proceedings of the International Symposium organised by the British Hydrological Society; Wheater, H., Kirby, C., Eds.; Exeter, UK, 1998; Vol I, pp 495−515. (16) Hedin, R. S.; Narin, R. W.; Kleinmann, R. L. P. Passive Treatment of Coal Mine Drainage; Circular 9389; U.S. Bureau of Mines: Washington, DC, 1994. (17) Kleinmann, R. L. P. Acid mine water treatment using engineered wetlands. In Proceedings of the International Mine Water Association Conference 1990, Lisbon, Portugal, 1990; pp 269−276. (18) Mayes, W. M.; Batty, L. C.; Younger, P. L.; Jarvis, A. P.; Koiv, M.; Vohla, C.; Mander, U. Wetland treatment at extremes of pH: A review. Sci. Total Environ. 2009, 407 (13), 3944−3957. (19) Moreno-Mateos, D.; Comin, F. A. Integrating objectives and scales for planning and implementing wetland restoration and creation in agricultural landscapes. J. Environ. Manage. 2010, 91 (11), 2087− 2095. (20) Younger, P. L.; Banwart, S. A.; Hedin, R. S. Mine Water: Hydrology, Pollution, Remediation; Kluwer Academic Publishers: Dordrecht, 2002. (21) August, E. E.; McKnight, D. M.; Hrncir, D. C.; Garhart, K. S. Seasonal variability of metals transport through a wetland impacted by mine drainage in the Rocky Mountains. Environ. Sci. Technol. 2002, 36 (17), 3779−3786. (22) Beining, B. A.; Otte, M. L. Retention of metals originating from an abandoned lead-zinc mine by a wetland at Glendalough, Co Wicklow. Biol. Environ. 1996, 96B (2), 117−126. (23) Johnston, D.; Potter, H.; Jones, C.; Rolley, S.; Watson, I.; Pritchard, J. Abandoned Mines and the Water Environment. In Environment Agency Science Report SC030136/SR41UK, 2008. (24) Younger, P. L.; Potter, H. A. B. Parys in Springtime: Hazard Management and Steps Towards Remediation of the UKs Most
however, those systems were less severely impacted than the Afon Goch, with a pre- and post- remediation pH of 5.5 and 7.5 respectively, and lower metal concentrations.37,38 This study on the Afon Goch therefore shows that even in severely polluted and acidic systems the benthic fauna can show relatively rapid and substantial recovery once efficient remediation measures are implemented. We have demonstrated that the Afon Goch natural wetland is highly efficient for providing long-term remediation of AMD by a combination of metal sequestration, metal sedimentation, and alkalinization, promoted by a community of plant and microbial species that are likely to have adapted to the AMD conditions. The existence of this wetland for at least a century indicates a self-sustaining system which should allow continued remediation over many decades. Further study of AMD remediation mechanisms by natural wetlands will aid our understanding of the processes operating in these systems. Wetlands are a declining resource worldwide and such losses are a threat to biodiversity, but as demonstrated here wetlands additionally provide important ecosystem service functions through their ability to consistently and reliably improve water quality by mitigating pollution damage.19 Wetlands, including constructed wetlands, that provide mine pollution remediation characteristics while maintaining or enhancing biodiversity will be of significant benefit.
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ASSOCIATED CONTENT
S Supporting Information *
Detailed Materials and Methods, site locations (Table S1), observed benthic invertebrate families (Table S2), metal removal rates (Table S3), rRNA sequence information (Table S5−S6), metal mass flow during low and high flow rate (Figure S1), mean river flow rate (Figure S2), metal particulate and sediment concentrations (Figure S3), metal speciation modeling (Figure S4, Table S4), metal content in wetland plants (Figure S5−S6), phylogenetic analysis of eukaryotic micro-organisms (Figure S7). This information is available free of charge via the Internet at http://pubs.acs.org/.
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AUTHOR INFORMATION
Corresponding Author
*Phone: +44 161 275 1710; fax: +44 161 275 5082; e-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS 'We especially thank Amanda Bamford for assistance with field work. We thank Ian Willey, Stephanie Towers, and Anna McMahon for sampling work, and Paul Lythgoe (School of Earth, Atmospheric and Environmental Sciences) for ICP-AES analysis. We thank Environment Agency Wales for flow rate data and the Meteorological Office for rainfall data.
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REFERENCES
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Environmental Science & Technology
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dx.doi.org/10.1021/es4025904 | Environ. Sci. Technol. XXXX, XXX, XXX−XXX
