SOURCE TO SINK TRACING OF PHOSPHORUS CONTAMINATION PATHWAYS IN SURFACE WATER AND GROUNDWATER OF WESTERN WISCONSIN

This investigation involves a comprehensive source to sink assessment of phosphorus (P) transport pathways in the hydrologic system of western Wisconsin. Excess P loading in surface water causes eutrophication events that result in degradation of surface water quality. Understanding the source and mobility of P is becoming urgent as 60% of the 225 bodies of water on the WDNR 2016 Impaired Water list exceed the total P criteria. The study area encompasses the northeastern upper Mississippi River watershed. This area has seen a dramatic increase in both silica sand mining (300% since 2011) and CAFOs (600% since 2000) making it vital to determine the potential impact that these growing industries have on surface water and groundwater quality.

A multidisciplinary approach integrating stratigraphy, geochemistry, sequential extraction, surface water and groundwater chemistry, and hydrologic flow characterization will be used to assess the spatial and temporal distribution of P, and to constrain potential natural and anthropomorphic sources. Ongoing chemical analyses have documented differences in concentrations of P in geology, surface water, and groundwater. Whole rock geochemistry indicates that bedrock P₂O₅ values range from 0.25-2.5%. Surface water concentrations commonly exceed the Wisconsin surface water limit of 100 ppb, while groundwater concentrations are far higher (10 to >1000 ppb). These high concentrations suggest that P is mobile and concentrating in groundwater reservoirs.

The source of excess P in groundwater is a matter of debate. Sequential extraction is utilized to determine the natural flux of P from bedrock to groundwater. Sequential extraction is a five-stage iterated dissolution process that mimics natural conditions, including; A) easily exchangeable, B) carbonate bound, C) Iron-Manganese oxide bound, D) Sulfide and Organic bound, and E) Residual Mineral Phase. Preliminary results show that P is being held in the Fe-Mg oxide and residual mineral stages, with no appreciable P in the easily exchangeable, carbonate, or sulfide and organic stages. This suggests that P is liberated from the source rock in anaerobic environments or when subjected to highly acidic conditions, which are unusual in natural settings, suggesting that bedrock is not the cause of elevated P in western Wisconsin.