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TECHNICAL REPORTS

Wetlands and Aquatic Processes

Bioavailability and Fate of Phosphorus in Constructed Wetlands Receiving Agricultural Runoff in the San Joaquin Valley, California

Dep. of Land, Air and Water Resources, Univ. of California, Davis, One Shields Ave, Davis, CA 95618-8627

^* Corresponding author (jjmaynard{at}ucdavis.edu).

Received for publication February 15, 2008. Elevated nutrient concentrations in agricultural runoff contribute to seasonal eutrophication and hypoxia in the lower portion of the San Joaquin River, California. Interception and filtration of agricultural runoff by constructed wetlands may improve water quality of return flows ultimately destined for major water bodies. This study evaluated the efficacy of two small flow-through wetlands (2.3 and 7.3 ha; hydraulic residence time = 11 and 31 h) for attenuating various forms of P from irrigation tailwaters during the 2005 irrigation season (May to September). Our goal was to examine transformations and removal efficiencies for bioavailable P in constructed wetlands. Inflow and outflow water volumes were monitored continuously and weekly water samples were collected to measure total P (TP), dissolved-reactive P (DRP), and bioavailable P (BAP). Suspended sediment was characterized and fractionated into five operationally-defined P fractions (i.e., NH₄Cl, bicarbonate-dithionite, NaOH, HCl, residual) to evaluate particulate P (PP) transformations. DRP was the major source of BAP with the particulate fraction contributing from 11 to 26%. On a seasonal basis, wetlands removed 55 to 65% of PP, 61 to 63% of DRP, 57 to 62% of BAP, and 88 to 91% of TSS. Sequential fractionation indicated that the bioavailable fraction of PP was largely associated with clay-sized particles that remain in suspension, while less labile P forms preferentially settle with coarser sediment. Thus, removal of potentially bioavailable PP is dependent on factors that promote particle settling and allow for the removal of colloids. This study suggests that treatment of tailwaters in small, flow-through wetlands can effectively remove BAP. Wetland design and management strategies that enhance sedimentation of colloids can improve BAP retention efficiency.

Abbreviations: AER, anion exchange resin • BAP, bioavailable phosphorus • BAP-PP, readily bioavailable particulate phosphorus • BD, bicarbonate-dithionite extraction • DRP, dissolved reactive phosphorus • HCl, hydrochloric acid extraction • NaOH, sodium hydroxide extraction • NH₄Cl, ammonium chloride extraction • NRP, non-reactive phosphorus • PAM, polyacrylamide • PBAP, potentially bioavailable phosphorus • PBAP-PP, potentially bioavailable particulate phosphorus • PP, particulate phosphorus • TDP, total dissolved phosphorus • TP, total phosphorus • TSS, total suspended solids • RP, reactive phosphorus

This article has been cited by other articles:

				J. J. Maynard, A. T. O'Geen, and R. A. Dahlgren Spatial Relationships of Phosphorus Sorption in a Seasonally Saturated Constructed Wetland Soil Soil Sci. Soc. Am. J., August 19, 2009; 73(5): 1741 - 1753. [Abstract] [Full Text] [PDF]