The removal of phosphorus (P) from the effluent of Onsite Wastewater Systems (OWS) is an important issue due to the high concentrations of phosphorus in the effluent and the role of phosphorus in the eutrophication of surface water.  As OWS effluent infiltrates into the subsurface (i.e., via septic-tank “leach fields”), phosphorus is removed either through sorption to soil, geochemical precipitation, or both.  Sorption is often considered, while precipitation is often neglected.  However, thermodynamic models show that precipitation of P in wastewater effluent is likely based on typical solute content.   Sorption and precipitation work together to decrease dissolved P concentrations.  Typically, the two processes are not studied simultaneously; thus, a clear understanding does not exist regarding which process is more important for a particular site.  Removal via sorption involves dissolved P components becoming attached to soil surfaces and would decrease aqueous P content as well as delay P travel times.  Precipitation removes P from the aqueous solution by creating immobile phosphorus minerals. The precipitation reaction may be rapid (equilibrium model) or relatively slow requiring kinetic formulations.  The goal of this project is to test various conceptual models for P transport using data from 1-D unsaturated column experiments and the geochemical transport model PHREEQC. Various conceptual models will be tested including: (1) sorption reactions only, (2) precipitation reactions only, (3) combinations of sorption and precipitation, (4) equilibrium and kinetic precipitation for cases 2 and 3 above.  The goodness of fit of the model results to experimental data will be used to assess whether the conceptual models are feasible or infeasible, and to choose the best conceptual model.  This research should lend insight into the fundamental mechanisms controlling removal processes of phosphorus from OWS effluent.