Effects of Long-Term Over-Application of P on Management and Sustainability of Groundwater Resources

Conventional wisdom in North America has been that P applied to land by agronomic and wastewater practices is immobilized by soil and poses little threat to groundwater resources.  This has led to many policies and programs that focus on preventing soil erosion as a means of controlling P loss and using land application of wastewater instead of advanced P removal from effluents.  Further, because there is no drinking water standard for P, P in groundwater is largely ignored in most of the U.S..  Several related issues and research efforts are now bringing these ideas into question and serve as early warning to likely serious future problems. 

Agricultural practices in the low countries of northern Europe, have been applying P at greater than agronomic rates for several centuries.  Downward transport of P past the agronomic soil profile and dense drainage networks, result in widespread elevated concentrations of P in shallow groundwater and rapid transport of P to surface water and development of serious eutrophication problems in the estuaries that provide much of the region’s fish.  In response, these countries have created very strict P management policies and remediation requirements.  Some areas of the US Atlantic Coastal Plain have similar land use and geology and are beginning to experience similar problems.  Federal response to P management of agricultural systems has been fragmented and often contradictory. 

Research in the northeast US has found that P readily leaches and moves with groundwater at concentrations in the mg/L range.  Transport distances of hundreds of meters are observed.  Though P transport is retarded by sorptive processes, the long-term bleed out of P will leave a legacy of contamination not unlike those observed at sites with metals and radionuclide problems.  National guidance on agricultural and wastewater practices needs to be modified to account for these experiences.