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Phosphorus is an essential component of modern agriculture. Long-term land application of phosphorous-enriched fertilizers and animal manure leads to phosphorus accumulation in soil that may become susceptible to mobilization via erosion, surface runoff and subsurface leaching. Globally, highly water-soluble phosphorus fertilizers used in agriculture have contributed to eutrophication and hypoxia in surface waters. This paper provides an overview of the literature relevant to the advances in phosphorous management strategies and surface water quality problems in the U.S. Over the past several decades, significant advances have been made to control phosphorus discharge into surface water bodies of the U.S. However, the current use of phosphorus remains inefficient at various stages of its life cycle, and phosphorus continues to remain a widespread problem in many water bodies, including the Gulf of Mexico and Lake Erie. In particular, the Midwestern Corn Belt region of the U.S. is a hotspot of phosphorous fertilization that has resulted in a net positive soil phosphorous balance. The runoff of phosphorous has resulted in dense blooms of toxic, odor-causing phytoplankton that deteriorate water quality. In the past, considerable attention was focused on improving the water quality of freshwater bodies and estuaries by reducing inputs of phosphorus alone. However, new research suggests that strategies controlling the two main nutrients, phosphorus and nitrogen, are more effective in the management of eutrophication. There is no specific solution to solving phosphorus pollution of water resources; however, sustainable management of phosphorus requires an integrated approach combining at least a reduction in consumption levels, source management, more specific regime-based nutrient criteria, routine soil fertility evaluation and recommendations, transport management, as well as the development of extensive phosphorus recovery and recycling programs.

Arsenic (As) and antimony (Sb) concentrations and speciation were determined along flow paths in three groundwater flow systems, the Carrizo Sand aquifer in southeastern Texas, the Upper Floridan aquifer in south-central Florida, and the Aquia aquifer of coastal Maryland, and subsequently compared and contrasted. Previously reported hydrogeochemical parameters for all three aquifer were used to demonstrate how changes in oxidation-reduction conditions and solution chemistry along the flow paths in each of the aquifers affected the concentrations of As and Sb. Total Sb concentrations (Sb T ) of groundwaters from the Carrizo Sand aquifer range from 16 to 198 pmol kg -1 ; in the Upper Floridan aquifer, Sb T concentrations range from 8.1 to 1,462 pmol kg -1 ; and for the Aquia aquifer, Sb T concentrations range between 23 and 512 pmol kg -1 . In each aquifer, As and Sb (except for the Carrizo Sand aquifer) concentrations are highest in the regions where Fe(III) reduction predominates and lower where SO 4 reduction buffers redox conditions. Groundwater data and sequential analysis of the aquifer sediments indicate that reductive dissolution of Fe(III) oxides/oxyhydroxides and subsequent release of sorbed As and Sb are the principal mechanism by which these metalloids are mobilized. Increases in pH along the flow path in the Carrizo Sand and Aquia aquifer also likely promote desorption of As and Sb from mineral surfaces, whereas pyrite oxidation mobilizes As and Sb within oxic groundwaters from the recharge zone of the Upper Floridan aquifer. Both metalloids are subsequently removed from solution by readsorption and/or coprecipitation onto Fe(III) Electronic supplementary material The online version of this article (oxides/oxyhydroxides and mixed Fe(II)/Fe(III) oxides, clay minerals, and pyrite. Speciation modeling using measured and computed Eh values predicts that Sb(III) predominate in Carrizo Sand and Upper Floridan aquifer groundwaters, occurring as the Sb(OH) 3 0 species in solution. In oxic groundwaters from the recharge zones of these aquifers, the speciation model suggests that Sb(V) occurs as the negatively charged Sb(OH) 6 -species, whereas in sufidic groundwaters from both aquifers, the thioantimonite species, HSb 2 S 4 -and Sb 2 S 4 2-, are predicted to be important dissolved forms of Sb. The measured As and Sb speciation in the Aquia aquifer indicates that As(III) and Sb(III) predominate. Comparison of the speciation model results based on measured Eh values, and those computed with the Fe(II)/Fe(III), S(-II)/SO 4 , As(III)/As(V), and Sb(III)/Sb(V) couples, to the analytically determined As and Sb speciation suggests that the Fe(II)/Fe(III), S(-II)/SO 4 couples exert more control on the in situ redox condition of these groundwaters than either metalloid redox couple.