PHOSPHORUS GEOCHEMISTRY IN GROUNDWATER IN THE CLEAR LAKE WATERSHED, NORTH-CENTRAL IOWA

Groundwater flow and P geochemistry were characterized in the Clear Lake watershed in order to identify sources that could be contributing to the high P concentrations in the lake. Clear Lake is a 1468-ha glacial lake in north-central Iowa set in till and outwash of the Algona-Altamont moraine complex. Eutrophication in the lake has resulted in frequent algal blooms and loss of fish diversity . A study of the groundwater chemistry in 2000 observed a median total phosphorus (P) concentration of 173 μg/L. Because groundwater is a major component of inflow to and outflow from the lake its input may account for the high concentrations of P observed in the lake water. Hydraulic gradients observed from June 2004 to January 2005 showed Clear Lake as a flow-through lake with flow from west to east. Most groundwater sampled during the study period was under strongly reducing conditions. Concentrations of NO₃-N and dissolved O₂ were not detectable and the groundwater contained measurable concentrations of Mn, Fe, H₂S, and CH₄. Sulfide concentrations were generally < 0.1 mg/L, while CH₄ concentrations ranged from < 1 to 650 µmol/L. Calculated pE values ranged from -4.0 to -3.5. Total P and ortho-P concentrations were determined using the Murphy-Riley method and the difference between the concentrations was used to assess the dissolved inorganic, organic, and acid-hydrolyzable fractions. Data from a January 2005 sampling suggest that P is present in organic and inorganic forms and that they may emanate from different sources. Total P and ortho-P concentrations ranged from 30 to 930 μg/L and 20 to 710 μg/L, respectively, with median concentrations of 155 μg/L and 65 μg/L, respectively. Ortho-P concentrations were always less than total P. The USGS geochemical model PHREEQC was used to speciate the solution and calculate saturation indices with respect to P mineral phases. The model results suggest that groundwater at shallow depths is, on average, undersaturated with respect to P-bearing minerals such as vivianite and hydroxyapatite, but is close to equilibrium with these minerals at depth. Hence, more P may be available at greater depth in the groundwater system, although the source of this additional P is not known.