Published June 24, 2014
Journal of Environmental Quality
TECHNICAL REPORTS WETLANDS AND AQUATIC PROCESSES
Phosphorus Removal in a Surface-Flow Constructed Wetland Treating Agricultural Runoff Marc W. Beutel,* Matthew R. Morgan, Jonathan J. Erlenmeyer, and Elaine S. Brouillard
P
hosphorus (P) is a critical limiting nutrient in surface
Agricultural runoff is a leading source of phosphorus (P) pollution to lakes and streams. The objective of this study was to evaluate P removal dynamics in a constructed treatment wetland (CTW) treating agricultural irrigation return flows. The CTW included a sedimentation basin (SB) followed by two surface-flow wetlands in parallel. Typical retention times and total P (TP) loading were 1.4 d and 50 to 110 g m-2 yr-1 P, respectively, for the SB and 5 to 6 d and 4 to 10 g m-2 yr-1 P, respectively, for wetlands. On the basis of this multiyear study, concentration removal efficiency in the SB averaged 21% for TP and 32% for reactive phosphorus (RP). Concentration removal efficiency in wetlands averaged 37 and 43% for TP and 22 and 33% for RP. Areal first-order removal rates for TP averaged 22 and 31 m yr-1 in wetlands. Total P removal in wetlands exhibited a strong seasonal pattern, with minimum removal in the summer when high temperatures likely enhanced P release from decaying plant biomass. The performance of the CTW was stochastic, with removal unpredictably poorer in some years in part as a result of muskrat bioturbation and plant harvesting. In years before muskrat impacts, concentration removal efficiencies in wetlands were 50% for TP and 65% for RP.
waters (Schindler, 2012), and agricultural nonpointsource runoff is a leading source of P pollution to lakes and streams (USEPA, 2005). Constructed treatment wetlands (CTWs) are an economical and sustainable ecotechnology to treat P in wastewaters and nonpoint sources (Mitsch et al., 2000; Horne and Fleming-Singer, 2005; Kadlec and Wallace, 2009). They also provide important ancillary benefits, including wildlife habitat and human recreational and educational opportunities, which is especially important considering the dramatic loss of wetlands over the past century (Mitsch and Gosselink, 2000). The biogeochemical P cycle within wetland ecosystems involves numerous transformations, including sorption to soil surfaces, precipitation of P-rich minerals, uptake by plants and microbiota, and accretion of soil and peat (Kadlec, 2005; Vymazal, 2007; Kadlec and Wallace, 2009). Because P removal by sorption to wetland sediment and P uptake during new plant growth are significant only in the first few years of CTW operation, the key long-term P removal mechanisms in surface-flow wetlands are burial of inorganic P associated with particulates and organic P from residual plant biomass. Estimates for sustainable levels of P removal in surface-flow CTWs polishing dilute wastewaters range from 0.5 to 5 g m-2 yr-1 P (Mitsch et al., 2000; Vymazal, 2007; Kadlec and Wallace, 2009). The P cycle in wetlands is complex also because of asynchronous seasonal patterns of P uptake and release. In what Kadlec and Wallace (2009) refer to as the “flywheel effect,” high rates of P uptake and storage occur in the spring when vegetation growth is substantial, but much of this P is released back into the wetland water column via decay of leaf litter in the summer. An estimated 10 to 20% of P taken up by emergent wetland plants in surface-flow CTWs is permanently lost through burial (Kadlec, 2005). Another complication of the wetland P cycle is the contrasting characteristics of aboveground and belowground vegetation biomass (Kadlec, 2005; Vymazal, 2007). Aboveground biomass accounts for half of the total biomass of a typical emergent macrophyte. This biomass grows
Copyright © American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America. 5585 Guilford Rd., Madison, WI 53711 USA. All rights reserved. No part of this periodical may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher.
M.W. Beutel and J.J. Erlenmeyer, Dep. of Civil and Environmental Engineering, Washington State Univ., 101 Sloan Hall, Pullman, WA 99164; M.R. Morgan and E.S. Brouillard, Roza-Sunnyside Board of Joint Control, 120 S. 11th St., Sunnyside, WA 98944. Assigned to Antonio Delgado.
J. Environ. Qual. 43:1071–1080 (2014) doi:10.2134/jeq2013.11.0463 Received 21 Nov. 2013. *Corresponding author (
[email protected]).
Abbreviations: ARR, areal removal rate; CRE, concentration removal efficiency; CTW, constructed treatment wetland; RP, reactive phosphorus; RSBOJC, RozaSunnyside Board of Joint Control; SB, sedimentation basin; TP, total phosphorus; TSS, total suspended solids.
1071
and dies throughout the season, thereby releasing much of its sequestered P back into wetland waters. In contrast, plant roots live for multiple years, and, being more intimate with the sediment, P in root biomass has a greater probability of being buried. The belowground and aboveground components are critically linked to P cycling. In the fall, macrophytes transfer P from the leaves to roots. This P is stored over the winter and is used to support spring growth the following season. The dynamic exchange of P between aboveground and belowground biomass and the exchange between biomass, water, and sediment in the wetland control both the seasonal pattern and the magnitude of P removal in CTWs. The objective of this study was to document P removal in the Yakima CTW based on a detailed evaluation of a multiyear water quality database. The vegetated surface-flow CTW was located in the semiarid Yakima Basin in central Washington and treats dilute agricultural irrigation runoff (Beutel et al., 2009). This study was unique for a number of reasons. There are limited case studies of surface-flow CTWs treating dilute agricultural irrigation source waters. In contrast to studies that evaluate inflow and outflow quality in a single natural treatment system, this study evaluated the performance of a sedimentation basin (SB) as well as duplicate wetlands following the SB. In addition, the loading to the system was fairly constant, allowing for a comprehensive evaluation of annual and seasonal removal rates. Finally, the study evaluated two P species: (i) total P (TP), which is the typical parameter of interest in treatment wetland studies, and (ii) reactive P (RP), which is the more bioavailable fraction of TP. The paper focuses on three key themes: (i) quantifying TP and RP removal rates, including the annual areal first-order removal rate constants using the P-k-C* model proposed by Kadlec and Wallace (2009); (ii) evaluating the seasonal dynamics of P removal in the wetlands; and (iii) exploring the stochastic nature of CTW performance.
Materials and Methods Description of the Study Site In 2003 the Roza-Sunnyside Board of Joint Control (RSBOJC) began operation of a 1.6-ha natural treatment system to treat pollutants in agricultural irrigation runoff near Sunnyside, Washington in the lower Yakima Basin (46.5° N, 120.0° W). The climate is semiarid with hot summers ( July mean air temperature, 23°C) and cold winters (December mean air temperature, 0°C). Average annual precipitation in the area is 30%, whereas the fraction of RP was around 50%. Other studies have observed higher TP removal when TP was dominated by particulate P, mainly because this form of P is effectively trapped via sedimentation in CTWs (Kovacic et al., 2006; Braskerud, 2002). Values of CRE for RP in the SB were 1074
variable (32 ± 15%; n = 4) (Table 2) but on average were higher than for TP (21 ± 14%; n = 7) (Table 1). Relative to other years, removal of RP in the SB was particularly low in 2007 (13.1%) when TP concentration in inflow was dominated by RP (76%) (Table 2; Fig. 6). This suggests that the SB was less effective when inflow was dominated by RP, much of which may have been in the dissolved phase and therefore less susceptible to deposition in the SB. This factor—the dominance of inflow with RP that is resistant to deposition—may account for the relatively low TP CRE values observed in the period 2004 to 2006 (Table 1).
Phosphorus Removal in Wetlands Concentration of TP in outflow from wetlands generally ranged between 0.04 to 0.1 mg L-1 P (Fig. 2), and TP in wetland Journal of Environmental Quality
Fig. 3. Total suspended solids (TSS) concentration versus total P concentration for inflow to the Yakima constructed treatment wetland. Line is linear regression for TSS concentrations above 20 mg L−1.
outflow showed a seasonal pattern, with elevated levels in summer months (>0.1 mg L-1 P) and low levels in the spring and fall (
