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

Abstract

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.

Efficiency of a constructed wetland for retention of sediment-associated phosphorus

“…45% in a study of a Swedish catchment (Persson, 2001). A recent study of sediments in constructed wetlands confirmed this, as the clay content correlated well with the potentially bioavailable P (Maynard et al, 2009). Hence, wetland retention of clay particles is of prime importance for downstream water bodies prone to eutrophication.…”

Capturing Spatial Variability of Biogeochemical Mass Exchanges and Reaction Rates in Wetland Water and Soil through Model Compartmentalization

“…3). The materials presented in this section are adapted from Maynard (2009) and Maynard et al (2011), who conducted research examining biogeochemical cycling and retention of carbon and nutrients in this wetland. The 4.4-ha wetland was restored in 1993 from an agricultural field with the intent of providing wildlife habitat and improving water quality of agricultural runoff.…”

“…The compartmental model was applied to data collected from a restored wetland receiving agricultural runoff on the west side of the San Joaquin River in California's Central Valley during the 2007 growing season (May-September; Maynard 2009;Maynard et al 2011). Due to close vicinity of inflow and outflow structures in the northern end, a large stagnant zone was created in the southern portion of the wetland, which constitutes more than 50% of the wetland area (Maynard 2009).…”

“…Due to close vicinity of inflow and outflow structures in the northern end, a large stagnant zone was created in the southern portion of the wetland, which constitutes more than 50% of the wetland area (Maynard 2009). Through detailed sensitivity and mass balance analyses, the most important processes engaging nitrogen and carbon constituents along the activity gradient line (i.e., active to passive transition zone) were investigated.…”

Engineering the global ecosystem

“…Focusing on the fundamental sustainability concepts and effective and sustainable approach to engineering design would include planning for ecosystem services, such as integrating engineered wetlands into the agricultural landscape to provide nitrogen processing and other ecosystem services. When ecosystem services are incorporated into the engineering design of larger projects, experience has demonstrated tangible benefits to engineered systems (Sommer et al 2001;Schemel et al 2004;Fleming-Singer and Horne 2006;Maynard et al 2009;Miller and Fujii 2010;Stringfellow et al 2010).…”