ANALYSIS OF PHOSPHATE IN RIPARIAN SEDIMENT PORE-WATERS: DOES PHOSPHATE SEQUESTERED IN THE STREAM BED CONTRIBUTE TO THE PHOSPHATE LOAD OF THE KALAMAZOO RIVER?

The upper Kalamazoo River in south-central Michigan is a low-profile, primarily gaining, meandering stream with an adjacent wetland corridor. An item of regulatory concern is the dissolved phosphate levels in the river. We investigated the flux of phosphate from organic sediments to the river as a possible contributor to the dissolved phosphate load. Sediment total phosphate ranges from .05 to 1% P₂0₅. Phosphate has strong negative correlation with SiO₂ in sediment, and positive correlations with Fe₂O₃ and organic material. This suggests that a significant fraction of the phosphate is adsorbed on ferric hydroxide in organic sediments. This fraction is susceptible to desorption as ferric hydroxides dissolve under reducing conditions. Mapping of 18 km of river bed allows us to estimate that it contains 134,000 m³ of methane-producing sediment. A conservative estimate is that this reservoir could supply the annual dissolved phosphate load of the river for 35 years.

Profiles of pore-water show highest concentrations of dissolved phosphate between 10-20 cm into the sediment. At this depth, the average concentration of orthophosphate is 128 ppb (n=11), significantly higher than the average concentrations at all depths (73ppb, n=37), and river water (20ppb, n=10).

Pore- waters entering the stream averaged 143 ppb orthophosphate (n=8), 7 times that of the stream concentration. Calculated phosphate fluxes range from 3 x 10^-10 to 5 x10^-12 g PO₄^3-/cm²/sec. The higher fluxes appear to occur in locations with high methane effervescence, as inferred from pock-marked sediment surfaces, which result from repeated methane eruptions. This release of orthophosphate from sediment provides the river with a steady supply of phosphate. Calculations show that the highest flux would provide 200% of the observed phosphate load, the low flux would provide about 5% of the load. Without more data, the precise flux is not known, but these estimates show that the release of phosphate likely contributes significantly to the river’s dissolved phosphate load.

These data have implications for remediation efforts in the watershed. Because the sediment has accumulated such a high concentration of phosphate, any reduction of current sources of contamination may not have an immediate effect on the river’s phosphate levels.