North-Central Section - 39th Annual Meeting (May 19–20, 2005)

Paper No. 17
Presentation Time: 1:20 PM-5:20 PM


CARTER, Jonathon T.1, SIMPKINS, William W.1, THOMPSON, Michael L.2 and PARKIN, Timothy B.3, (1)Dept. of Geological and Atmospheric Sciences, Iowa State Univ, Ames, IA 50011, (2)Agronomy Department, Iowa State Univ, Ames, IA 50011, (3)USDA-ARS National Soil Tilth Lab, 2150 Pammel Drive, Ames, IA 50011,

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.