RESUMEN
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 yr P, respectively, for the SB and 5 to 6 d and 4 to 10 g m yr 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 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.
RESUMEN
Constructed treatment wetlands can be an effective and sustainable method to remove pathogens that pose health risks from agricultural runoff. This study evaluated the removal of fecal coliform (FC) from agricultural runoff in a lightly loaded surface-flow treatment wetland prior to discharge to the Yakima River, Washington State, USA. The 1.6 ha system consisted of a sedimentation basin (1.4 d hydraulic retention time) followed by two wetlands (5-6 d hydraulic retention time). FC in inflow ranged from 100 to 1,000 cfu/100 mL. Mean annual FC log-removal in the sedimentation basin was 0.66 ± 0.17 (mean plus/minus standard deviation; n = 7). However, there was a comparable production of FC within the two wetlands where annual log-removal averaged -0.71 ± 0.39 in the north wetland and -0.57 ± 0.17 in the south wetland. FC removal in the sedimentation basin weakly correlated with turbidity removal (R(2) = 0.13, p < 0.01, n = 61), suggesting that settling was an important FC loss mechanism. FC removal in the wetlands negatively correlated with temperature (R(2) = 0.27-0.33, p < 0.01, n = 26) indicating that survival and/or reproduction was an important FC production mechanism. Muskrat colonization in the wetlands in 2007 and 2008 corresponded with a marked increase in FC in wetland outflow. Results suggest that, regardless of the presence of muskrats, sedimentation basins alone are more effective than a combined sedimentation basin-wetland system in removing FC from dilute agricultural runoff.