Jeq50243 1072..1077

External loading of phosphorus (P) from agricultural surface discharge (tailwater) is the main cause of excessive algae growth and the eutrophication of the Salton Sea, California. Continuous polyacrylamide (PAM) applications to agricultural irrigation water inflows were evaluated as a means of reducing sediment and P in tailwater. Zero (control) and 1 mg L PAM (PAM1) treatments were compared at 17 Imperial Valley field sites. Five and 10 mg L PAM treatments (PAM5, PAM10) were conducted at one site. The particulate phosphorus (Pp) fraction was determined as the difference between total phosphorus (Pt) and the soluble phosphorus (Ps) fraction. We observed 73, 82, and 98% turbidity reduction with PAM1, PAM5, and PAM10 treatments. Although eight field sites had control tailwater sediment concentrations above the New River total maximum daily loads (TMDL), all but one were made compliant during their paired PAM1 treatments. While PAM1 and PAM10 reduced tail water Pp by 31 and 78%, none of the treatments tested reduced Ps. This may have been caused by high irrigation water Na concentrations which would reduce Ca adsorption and Ca–phosphate bridging on the PAM. The PAM1 treatments resulted in,0.5 mg L 21 drain water polyacrylamide concentrations 1.6 km downstream of PAM addition, while PAM5 and PAM10 treatments produced .2 mg L 21 drain water polyacrylamide concentrations. We concluded that, although PAM practically eliminates Imperial Valley tailwater sediment loads, it does not effectively reduce tailwater Ps, the P fraction most responsible for the eutrophication of the Salton Sea. EXTERNAL LOADING of phosphorus (P) is the main cause of excessive algae growth and the eutrophication of the Salton Sea, a 980-km saline lake located in California near the U.S.–Mexico border. Schroeder et al. (2002) stated that “the Salton Sea is highly eutrophic and that P is by far the limiting nutrient relative to N” noting that there is 26 times more nitrogen (N) than P in the Salton Sea relative to ocean water. Most of the external P load to the sea comes from agricultural surface discharge (tailwater) from the Imperial Valley. Due to fertilizer application, P is one of the few constituents in Imperial Valley soils that has increased over the last 50 yr (De Clerck et al., 2003). The Alamo and New Rivers deliver roughly 80% of the sea’s inflow of 1.6 km yr. Data from Holdren and Montaño (2002) indicate that two-thirds of the rivers’ total P load to the sea is soluble phosphorus (Ps) which is the most bioavailable fraction for algae growth. The other third is particulate phosphorus (Pp). It bears to reason that reductions in suspended solids and P in agricultural discharges should eventually abate the Sea’s eutrophic cycles. With the existing sediment total maximum daily loads (TMDLs) and impending nutrient TMDLs for the New and Alamo Rivers, there is growing interest in on-field best management practices (BMPs) that are inexpensive and simple to implement. In addition to the typical BMPs that slow runoff flow rates, anionic polyacrylamide polymers (PAMs) added to irrigation water at rates of 1 to 10 mg L can provide numerous benefits (Sojka et al., 2005). Each year in the United States, 1 to 10 mg L PAM solutions are applied to 400000 irrigated hectares for tailwater nutrient and sediment removal (Orts et al., 2000; Entry and Sojka, 2003). Removing sediment particles from tailwater has the added benefit of removing adsorbedpesticides (Agassi et al., 1995; Singh et al., 1996). Another benefit is bacteria removal: PAM, PAM1 CaO, and PAM1Al2(SO4)3 treatments decreased enteric bacteria in animal wastewater surface flows and in leachate from soil columns (Sojka and Entry, 2000; Entry and Sojka, 2000; Entry et al., 2002, 2003). Polyacrylamide treatments can also decrease the spread of weed seeds in irrigated furrows by 62 to 90% (Sojka et al., 2003). Furthermore, anionic PAM treatments improve soil structure and increase net irrigation water infiltration (Shainberg et al., 1990; Lentz and Sojka, 1994; Sojka et al., 1998). Finally, the toxicity effects of anionic PAM to mammals and fish are small to none (Barvenik, 1994). However, there is little information published on PAM’s potential to remove Ps and Pp while furrow irrigating heavy-textured, salt-affected soils like those found in the Imperial Valley. In this study we tested the efficacy of PAM-treated irrigation water to decrease suspended solids, Pp, and Ps on Imperial Valley agricultural fields. MATERIALS AND METHODS The Imperial County Farm Bureau (ICFB) identified growers willing to volunteer agricultural field sites located throughout the Imperial Valley. Given free PAM treatments and an assurance of confidentiality, growers allowed us to conduct controlled experiments to evaluate the effects of PAM amended irrigation water on turbidity, total suspended solids (TSS), Pt and Ps. Throughout the study, Colorado River water was used to furrow irrigate fields with bare, tilled soils (Table 1). Soils at the 17 selected field sites were silty clays and silty clay loams (hyperthermic Typic Torrifluvents) with high shrink–swell potential (Zimmerman, 1980). Field slopes C.C. Goodson and C. Amrhein, Department of Environmental Sciences, University of California, Riverside, CA 92521. G. Schwartz, Kent SeaTech, P.O. Box 757, Mecca, CA. Received 15 June 2005. *Corresponding author ([email protected]). Published in J. Environ. Qual. 35:1072–1077 (2006). Technical Reports: Surface Water Quality doi:10.2134/jeq2005.0243 a ASA, CSSA, SSSA 677 S. Segoe Rd., Madison, WI 53711 USA Abbreviations: DOM, dissolved organic matter; OPP, liquid orthophosphate phosphorus fertilizer; PAM, water-soluble anionic polyacrylamide powder; Pp, particulate phosphorus; Ps, soluble phosphorus; Pt, total phosphorus; TMDL, total maximum daily load; TSS, total suspended solids. R e p ro d u c e d fr o m J o u rn a l o f E n v ir o n m e n ta l Q u a lit y . P u b lis h e d b y A S A , C S S A , a n d S S S A . A ll c o p y ri g h ts re s e rv e d . 1072 Published online May 31, 2006