Mechanisms of Phosphorus Removal by Phosphorus Sorbing Materials.

Stormwater filters are a structural best management practice designed to reduce dissolved P losses from runoff. Various industrial byproducts are suitable for use as P sorbing materials (PSMs) for the treatment of drainage water; P sorption by PSMs varies with material physical and chemical properties. Previously, P removal capacity by PSMs was estimated using chemical extractions. We determined the speciation of P when reacted with various PSMs using X-ray absorption near edge structure (XANES) spectroscopy. Twelve PSMs were reacted with P solution in the laboratory under batch or flow-through conditions. In addition, three slag materials were collected from working stormwater filtration structures. Phosphorus K-edge XANES spectra were collected on each reacted PSM and compared with spectra of 22 known P standards using linear combination fitting in Athena. We found evidence of formation of a variety of Ca-, Al-, and/or Fe-phosphate minerals and sorbed phases on the reacted PSMs, with the exact speciation influenced by the chemical properties of the original unreacted PSMs. We grouped PSMs into three general categories based on the dominant P removal mechanism: (i) Fe- and Al-mediated removal [i.e., adsorption of P to Fe- or Al-(hydro-)oxide minerals and/or precipitation of Fe- or Al-phosphate minerals]; (ii) Ca-mediated removal (i.e., precipitation of Ca-phosphate mineral); and (iii) both mechanisms. We recommend the use of Fe/Al sorbing PSMs for use in stormwater filtration structures where stormwater retention time is limited because reaction of P with Fe or Al generally occurs more quickly than Ca-P precipitation.

Assessing Coastal Plain Risk Indices for Subsurface Phosphorus Loss.

Phosphorus (P) Index evaluations are critical to advancing nutrient management planning in the United States. However, most assessments until now have focused on the risks of P losses in surface runoff. In artificially drained agroecosystems of the Atlantic Coastal Plain, subsurface flow is the predominant mode of P transport, but its representation in most P Indices is often inadequate. We explored methods to evaluate the subsurface P risk routines of five P Indices from Delaware, Maryland (two), Virginia, and North Carolina using available water quality and soils datasets. Relationships between subsurface P risk scores and published dissolved P loads in leachate (Delaware, Maryland, and North Carolina) and ditch drainage (Maryland) were directionally correct and often statistically significant, yet the brevity of the observation periods (weeks to several years) and the limited number of sampling locations precluded a more robust assessment of each P Index. Given the paucity of measured P loss data, we then showed that soil water extractable P concentrations at depths corresponding with the seasonal high water table (WEP) could serve as a realistic proxy for subsurface P losses in ditch drainage. The associations between WEP and subsurface P risk ratings reasonably mirrored those obtained with sparser water quality data. As such, WEP is seen as a valuable metric that offers interim insight into the directionality of subsurface P risk scores when water quality data are inaccessible. In the long term, improved monitoring and modeling of subsurface P losses clearly should enhance the rigor of future P Index appraisals.