BEDROCK INFLUENCE ON SOLUBLE PHOSPHORUS IN WEST-CENTRAL WISCONSIN SURFACE WATERS

The Red Cedar Watershed (RCW) is a 4,900 km² catchment in west-central Wisconsin. Agriculture and forestland are the dominant land uses within the watershed, while only 6% is urban/developed. Total phosphorus (TP) loads within surfaces waters of the RCW are often above the EPA recommended limit of 0.1 mg-P/L. The Wisconsin Department of Natural Resources (WI-DNR) lists more than 14 tributaries and impoundments within the RCW as impaired waterways.

A detailed survey in early spring of 2015 sampled baseflow in streams within and adjacent to the RCW. Of the 55 different sampling locations, 23 of these locations had TP above 0.1 mg-P/L. 22 of these sample locations were on the eastern side of the watershed where their valleys have cut down into the Upper Cambrian Eau Claire formation. Phosphatic, inarticulate brachiopod shells are abundant fossils in these rocks and are a potential source of phosphorus.

To test the hypothesis of bedrock phosphorus contribution, we compared the water chemistry results to land use and location. These results showed significant correlations between conductivity and latitude (r²=0.722, p<<0.001), TP and latitude (r²=0.337, p<<0.001), and for soluble reactive phosphorus (SRP) and latitude (r2=0.292, p<<0.001). The correlation between TP and percent cropland was statistically significant, but much smaller (r²=0.085, p=0.03).

The correlation with conductivity is likely due to the outcropping of more soluble Ordovician carbonate rocks along the western edge of the watershed, compared to the siliciclastic rocks along the east side. The relationship of TP and SRP to latitude may be a result of the southwest dipping beds; the phosphatic units are closer to the surface or exhumed along the eastern edge of the watershed, but deeper in the subsurface to the west.

We collected bedrock samples representing characteristic lithofacies for each geologic unit across the region and performed a sequential phosphorus extraction on 200 mg aliquots of pulverized bedrock. For all steps of the extraction (DI-Water, 1M NaOH, 0.5M HCl), the Eau Claire formation samples released the most phosphorus.

These results demonstrate the role of bedrock geology as a potential source of phosphorus and a “one size fits all” approach to phosphorus loading may lead to problems in regions with heterogeneous bedrock materials.