Effectiveness of wetland remediation on heavy metals in storm water runoff

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1 Effectiveness of wetland remediation on heavy metals in storm water runoff Justin Keller Environmental Sciences Program, Appalachian State University, Boone, NC Abstract Constructed wetlands incorporate many natural processes, such as phytoaccumulation, that are useful for mitigating urbanization impacts on local streams and rivers. The storm water remediation wetland, installed in 2009 off of the Greenway trail in Boone, NC, was designed to remediate storm water runoff from 30 acres of urbanized area in proximity to the South Fork New River. To quantify the effectiveness of the wetlands, water and sediment samples were taken within the wetland itself as well as above, below, and at its point of discharge. These samples were digested with microwave-assistance and analyzed for heavy metal concentrations using an ICP-OES. Results showed varying concentration trends among the seven heavy metals tested (As, Cr, Cu, Fe, Pb, Se, and Zn) with some having major outliers. Remediation seemed most noticeable in the concentration trend lines of As, Cr, Cu, and Se, which showed general downward trends in the water and/or sediment analysis, and reduction of waterborne metals within a short distance of the intake source. Inconsistencies in some of the concentrations could be attributable to influx of metals from resident soils, faults in the design of the wetlands, variable conditions at each site, or limitations in instrument detection. Introduction Phytoremediation is a growing eco-technological practice that involves using plants to extract, sequester, or degrade heavy metals and other pollutants from soils, sediments, groundwater, and sludge [1, 2]. Tolerance levels of contaminants differ among the variety of plant species and, in some cases, certain heavy metals, including Cu2+, Zn2+, Mn2+, Fe2+, Ni2+, and Co2+ are crucial nutrients for plant growth at trace levels [3]. Although there has been extensive research on phytoremediation, limited full-scale application has led to a lack of wide-spread acceptance [1]. Lately however, growing popularity of phytoremediation has risen due to its costeffectiveness compared to standard approaches, its passive/non-intrusive ability to clean up contaminated sites [3], as well as accentuating the aesthetic beauty and naturalization of the space. There are many different methods used in phytoremediation, which are dependent on the species of plant being used in remediation. In general, heavy metals are extracted from the wetland surroundings by hyperaccumulators - plants that can absorb large amounts of metals - and sequestered in the roots and stems of the plant [1, 2]. Presence of the plants also slows the movement of water and allows sedimentation of suspended metals and therefore increased retention of metals in the wetland. These plants can then be harvested and either incinerated into ash, which would be deposited in a landfill, or composted to recycle the metals which can then be sold back to industries for re-use [1]. The focus of this study is to quantify the improvement in the concentrations of certain heavy metals in storm water reaching the South Fork New River and concurrent metal capture in sediment samples due to the recently installed Greenway trail remediation wetlands in Boone, North Carolina. In particular, chemical analysis of the concentrations of arsenic (As), chromium (Cr), copper (Cu), iron (Fe), lead (Pb), selenium (Se), and zinc (Zn) in water and sediment samples will be presented from points along the flow of the watershed from the intake to the outlet. Background concentrations are also measured at a nearby reference stream as well as at the outlet point into the South Fork New River for comparison. Although suppositions about phytoaccumulation or phytoextraction methods are presented, further studies that quantify the actual 36 Journal of Student Research in Environmental Science at Appalachian

2 concentrations of heavy metals accumulated in selected sample plants would complement the results presented. Methods Site Description Data was collected at the storm water remediation wetlands in Boone, NC (Figure 1) which were established in 2009 by the Town of Boone, Watauga County Cooperative Extension and NCSU Biological and Agricultural Engineering, along with funds from the NC Clean Water Management Trust Fund. The wetlands are home to over 50 different plant species, such as swamp butterfly weed, marsh hibiscus, and pickerelweed [4]. The wetlands are intended to mitigate the influx of sediments, heavy metals, chemicals, and bacteria from the storm water runoff from 30 acres of parking lots, roads, and buildings in the Boone Greenway area that are effluent to the South Fork New River. The runoff is drawn to the wetlands by a 2500 gallon underground cistern with a pump - the retention time of the runoff is ~72 hours to maximize treatment - and then is discharged into a nearby stream by a flashboard riser outlet [4, 5]. Figure 2 (pg. 38) shows a picture of the wetlands and the outfall pipe at the input to the wetlands. A total of 8 sites were selected to quantify the concentration profile of each metal at different points of interest (Figure 1). Sampling Water and sediment samples were collected at pre-determined sites within the wetlands as well as above, below, and at its point of discharge (Figure 1). The water samples were collected from each site by submerging acid washed 250 milliliter (ml) plastic containers ~1.0 foot (where possible) below the water surface. Sediment samples were taken using a plastic cup to collect approximately the top 1.0 inch of the sediment surface, corresponding to an estimated 1-2 gram (g) sample. Samples were stored in a labeled Whirl-Pak bag. After collecting each sample, the GPS coordinates for each site were recorded using a Magellan Triton 500 Handheld GPS (471 El Camino Real, Santa Clara, CA). In the lab, the sedi- Figure 1. Layout of the Greenway wetlands with sample site labeling and description. Volume 1, 1st Edition Spring

3 Figure 2. [LEFT] Picture of the wetland from the intake looking downstream (riser outlet is beyond visible range) and [RIGHT] the galvanized steel pipe that delivers water to the wetland. ment samples were frozen at -20 C and the water samples were refrigerated at 4 C until processed [6]. Acid Digestion All plastics and glassware used during the acid digestion process were cleaned with 1% concentrated Alconox cleaning solution, rinsed with deionized water three times, rinsed with 50% concentrated nitric acid (HNO3), followed by a final rinsing with deionized water three times. Sediment samples were freezedried on a lyophilizer between hours and mixed to obtain homogenization. On an analytical balance (Mettler-Toledo AB204-S, Switzerland), two replicates from each field site sample were weighed out with dry weights of 0.50 grams (±0.01 grams). Each replicate was placed into a 60 ml Teflon digestion vessel (CEM, Matthews, NC) with the addition of 10 ml of 70% nitric acid (OmniTrace, EMD). The digestion vessels were capped, organized into a carousel, and placed in a microwave reactor (MARSXpress, CEM, Matthews, NC). The samples were digested following the protocol of EPA method 3051 [7]. The samples were allowed to cool overnight. The following day, the caps were removed and 10 ml of deionized water was added to each digestion vessel. The digested sediment samples were filtered through 9 centimeter (cm) qualitative 413 filter paper (VWR, Cat. No ), held in an acid-washed funnel, and collected in a 50 ml volumetric flask. The volume of each sample was brought up to 50 ml with deionized water. Samples were then transferred to labeled 50 ml centrifuge tubes and taken to an analytical chemistry lab at Appalachian State University for analysis, using an Inductively Coupled Plasma Optical Emission Spectrometer (ICP-OES; Varian 710 ES axial ICP). From the refrigerated 250 ml plastic containers, two 40 ml replicates of each water sample were added to a digestion vessel, along with 10 ml of 70% nitric acid (OmniTrace, EMD). The digestion vessels were sealed with pressure plugs and capped, organized into the digestion carousel, and placed into the microwave reactor. The samples were digested according to the total available metals protocol from EPA method The digested samples were allowed to cool overnight and then filtered, transferred, and analyzed as above. [6]. ICP Calibration and Quality Control In order to calibrate the ICP-OES, six standard solutions of known concentrations were produced from a multi-element stock solution purchased from VWR (Ultra Scientific, Cat No. ULICM-411). Using these standards, the ICP software created a calibration curve for each element by plotting emission intensity versus concentration of the standard. The equation of this curve was then used to calculate the unknown concentrations of each element. To ensure the ICP had been properly calibrated, two types of blanks were added among the samples. The first blank consisted of deionized water, which had been carried through the acid digestion process to identify any possibility of contamination during the process. The second blank was a laboratory-fortified blank (LFB), a blank which was spiked with 2ppm of each element and used for establishing data error. To further assess the quality of the data, duplicates of one or two random samples were also processed. A spiked duplicate of a random sample was also added, which involved adding a known concentration (2 ppm) of a particular 38 Journal of Student Research in Environmental Science at Appalachian

4 Table 1a. Average water concentrations in ppm for each sample site with percent errors (mean error for all sites for each element), where BDL = Below Detection Limit and ADL = Above Detection Limit. Sample Site As Cr Cu Fe Pb Se Zn Site 1 BDL Site Site 3 BDL Site 4 BDL Site 5 BDL BDL Site 6 BDL Site Site Table 1b. Average sediment concentrations in ppm for each sample site with percent errors (mean error for all sites for each element), where BDL = Below Detection Limit and ADL = Above Detection Limit. Sample Site As Cr Cu Fe Pb Se Zn Site Site BDL Site BDL Site Site ADL Site Site Site element to the duplicate. The difference in concentration between the spiked duplicate and the original sample should be nearly equal to the added known concentration, if the ICP has been properly calibrated. Data Results from the ICP analysis for water and sediments were organized into three sets of data: concentration averages for each site (Table 1a and 1b), baseline ratio of concentrations (Table 2a and 2b, pg. 40), and percentage comparisons of inflow, outflow, and baseline (Table 3a and 3b,, pg. 40). The concentration averages were calculated from the two replicates of each site sample. Baseline ratios were derived by dividing each site concentration by its correlating Site 8, which is assumed to be a reference stream of comparable flow rates minimally affected by urbanization. Percentages were also quantified pertaining to heavy metal concentrations discharging from the wetlands compared to intake concentrations. From this, a percent improvement was derived to determine the overall reduction in heavy metal concentrations. Outflow and inflow concentrations were compared to baseline concentrations as a percentage to offer a different perspective for the baseline ratios. Results and Analysis Water Arsenic concentrations were below detection limit (BDL) for many of the sampling sites (Figure 3a, pg. 41). Site 2 registered a concentration value of parts per million (ppm) for As, but it did not differ greatly from the ppm baseline concentration. The concentration for Pb more than doubled from ppm at the intake to ppm below the discharge point. The large spike in Figure 3e (pg. 41) denotes the ppm concentration for Pb, which was nearly 8 times the baseline concentration and followed a BDL Volume 1, 1st Edition Spring

5 Table 2a. Ratios of average water concentrations compared to Site 8 (baseline). Sample Site As Cr Cu Fe Pb Se Zn Site 1 N/A Site Site 3 N/A Site 4 N/A Site 5 N/A N/A Site 6 N/A Site Site 8 (baseline) Table 2b. Ratios of average sediment concentrations compared to Site 8 (baseline). Sample Site As Cr Cu Fe Pb Se Zn Site Site N/A Site N/A Site Site N/A Site Site Site 8 (baseline) Table 3a. Comparison of water concentrations from intake, outlet, and baseline as a percent. Heavy Metal As Cr Cu Fe Pb Se Zn Remaining in Outflow N/A Improvement N/A Outflow Compared to Baseline (Site 8) Inflow Compared to Baseline (Site 8) N/A N/A Table 3b. Comparison of sediment concentrations from intake, outlet, and baseline as a percent. Heavy Metal As Cr Cu Fe Pb Se Zn Remaining in Outflow Improvement Outflow Compared to Baseline (Site 8) Inflow Compared to Baseline (Site 8) Journal of Student Research in Environmental Science at Appalachian

6 Figure 3. Water concentrations in parts per million (ppm) plotted across sample sites. Trend lines are included in each plot with concentrations at baseline site highlighted in red: (a) Arsenic, (b) Chromium, (c) Copper, (d) Iron, (e) Lead, (f) Selenium, and (g) Zinc. concentration at site 5 near the outlet. The concentrations for Cr and Cu varied very little with both having negative improvement percentages (-14% and -1%, respectively). Only Fe, Se, and Zn showed any kind of improvement in concentration reduction with improvement percentages of 62%, 21%, and 46%, respectively. Figure 3g displays the general downward trend in Zn concentration across the wetlands as well as Fe (Figure 3d) being the only heavy metal to have a higher concentration at the baseline (site 8) than at site 7 where water from the creek and the wetlands mixes and flows into the South Fork New River. The concentrations for Fe and Pb at the intake were above the water quality standards of the World Health Organization (WHO) [8]. Lead was the only metal to remain above its standard (0.01 ppm) at the outflow (site 6) and below the discharge (site 7). Sediment As, Cr, Cu, and Se all showed general downward trends in concentration from entering the wetlands at the intake (site 1) to exiting them at the discharge point (site 6) in Figure 4 (pg. 42). In terms of improvement, As, Cr, Cu, and Se concentrations improved by 11%, 49%, 74%, and 75%, respectively (Table 3b). Concentrations for Pb were persistent throughout the wetlands, ranging from ppm to ppm. At the point of discharge, the concentration for Pb was nearly equivalent to the Pb concentration at the baseline (site 8) as shown in Table 1b (pg. 39). As expected, concentrations for Fe were much higher relative to the other metal concentrations. Figure 4e (pg. 42)shows Fe had an overall decrease in concentration by about 12%. In Table 2b, a comparison of concentrations at each site to the baseline revealed that concentrations for As, Cr, Cu, Fe, and Zn at the point of discharge (site 7) were lower than the baseline. Zinc concentrations appeared to be higher at the intake and at the outlet (site 5). The intensity for Zn at the outlet was so large that it was beyond the detection limit of the ICP-OES, thus a true concentration value could not be calculated. The improvement percentage was calculated to be 71% for Zinc. Discussion In general, the data shows for both water and Volume 1, 1st Edition Spring

7 Figure 4. Sediment concentrations in parts per million (ppm) plotted across sample sites. Trend lines included in each plot with concentrations at baseline site highlighted in red: (a) Arsenic, (b) Chromium, (c) Copper, (d) Lead, (e) Iron, (f) Selenium, and (g) Zinc. sediment a reduction in concentrations within the wetland (between sites 1 and 5). Sediment concentrations show slight to noticeable improvement within the watershed due to suspended sediment settling and possible phytoaccumulation. Reduction in concentrations for the water samples was less systematic, likely due to the quiescent flow conditions sampling during rainfall events may provide very different results. Also differences in metal solubility may have influenced water levels of metals such as Se and Pb. In general, sites 6 and 7 showed concentrations consistent with site 5 (within the watershed at the outlet) though mixing occurred with the reference stream at these sites. The sediment concentrations at site 5 were consistent with the concentrations at site 8 (the reference stream) for Chromium, Copper, Iron, Selenium, and Zinc, which suggests that the watershed is at least partially effective at mitigating the heavy metals over long time scales (greater than one rain event). However, it should be noted that during sampling of the baseline (site 8), there were a significant number of tree limbs and other debris partially damming this section of the creek, which slowed the flow rates at site 8. The natural barriers created here could be trapping water and sediment and increasing the abundance of heavy metals in the sediment. Lead was the only metal in this study to have concentrations higher than the WHO water quality standards entering and exiting the wetlands. Consistent concentrations of Pb in the sediment suggest that mitigation of the other heavy metals is due to the combined effect of settling (into the sediments) and phytoaccumulation in the case of Pb, there may be no organisms (plant or animal) present to participate in mitigation through phytoaccumulation. Also, the Pb levels in sediments could be at background levels for soils from the area. The Zn sediment concentration at the outlet was so high that sediment concentrations were unable to be calculated, and so this site was excluded from the scatter plot. It is still unclear why Zn was so abundant near the intake pipe and outlet pipe, but it has been speculated that the Zn within the galvanized layer of both pipes may have leached into the wetlands. If this is the 42 Journal of Student Research in Environmental Science at Appalachian

8 case, the calculated concentration value for Zn at the intake is potentially much greater than the actual value which leads to an over-estimation of the improvement percentage by the wetlands for Zinc. Conclusion The results of this study do carry some evidence that the Greenway wetlands may be effectively remediating As, Cr, Cu, Fe, Se, and potentially Zn (depending on the validity of the Zn concentrations measured at the intake) based on decreased water levels. The high levels of metals found in the sediments near the intake pipe indicate that the wetland is quite good at retaining these metals by capturing sediments and allowing for sedimentation of suspended metals. More research needs to be conducted, looking into alternative methods for lead mitigation to bring concentrations leaving the wetlands below water quality standards. Further studies should include long term monitoring of heavy metal concentrations in water and sediment and plant tissue analysis. Influence from weather patterns should also be taken into account for future sampling. It is expected that during a rain event, runoff entering the wetlands will contain even higher concentrations of metals and other pollutants. in Boone. Watauga County Center. watauga.ces.ncsu.edu/content/constructedstormwaterwetland (accessed Feb, 2011). [5] Ruggiero F (2009) Boone wetland project under way. The Mountain Times. mountaintimes.com/mtweekly/2009/0129/ wetland.php3 (accessed Feb, 2011). [6] Tuberty, SR (2007) Standard operating procedure for elemental analysis of biological, water, and sediment samples. Dept. of Biology, Appalachian State University. [7] U.S. EPA. Test Methods for Evaluating Solid Waste, Physical/Chemical Methods, SW-846, 3rd ed.; U.S. Government Printing Office: Washington, DC, [8] WHO (2004) Guidelines for drinking-water quality. 3rd edition. WHO, Geneva, Switzerland. Acknowledgments Many thanks go to my BIO 3542 Environmental Toxicology teammates Kristi Davis and Jay Scott, to Yosuke Sakamachi, a graduate student in Biology at Appalachian, Dr. Shea Tuberty, my instructor in BIO 3542, and to Wendy Patoprsty of the North Carolina Cooperative Extension. References [1] U. S. Environmental Protection Agency (1999) Phytoremediation Resource Guide. NEPIS. cgi?dockey=10002see.txt (accessed Feb, 2011). [2] Karathanasis AD and Johnson CM (2003) Metal removal potential by three aquatic plants in an acid mine drainage wetland. Mine Water and the Environment 22: [3] Kamal M, Ghaly AE, Mahmoud N, Côté R (2004) Phytoaccumulation of heavy metals by aquatic plants. Environment International 29: [4] North Carolina Cooperative Extension (2009) Constructed Stormwater Wetland Volume 1, 1st Edition Spring