HR: 1330h
AN: H13A-03    [Abstracts]
TI: Accumulation of Nitrogen in the Pore Water of Anoxic Lake Sediments
AU: * Iqbal, M Z
EM: m.iqbal@uni.edu
AF: University of Northern Iowa, Department of Earth Science, Cedar Falls, IA 50614 United States
AU: Fields, C L
EM: cfields@igsb.uiowa.edu
AF: Iowa Geological Survey, 109 Trowbridge Hall, Iowa City, IA 52242 United States
AB: The impact of soil runoff nitrogen on the Silver Lake of Iowa was assessed in this study. Currently, the lake cannot support its designated uses as a recreational water body. Extensive algal blooms characterize the lake in late summer, lowering the dissolved oxygen content in water (< 2.0 mg/L). The goal of this study was to map the buildup of nitrogen in the pore water of lake sediments and come up with recommendations for restoration strategies. Sediment cores were taken from 20 sites along 5 transects in the lake. In the top 5 cm of the sediments, the pore water nitrogen ranges between 1.8 and 733.1 micro-gm of nitrate per gm of sediments. The average concentration is 94 micro-g/gm. Vertically, nitrate concentrations were measured at 90 micro-g/gm at 0-10 cm, 95 micro-g/gm at 10-20 cm, and 19 micro-g/gm at 20-30 cm. The sharp decline in nitrate below the 20 cm depth in the sediment is attributed to biochemical reduction of nitrate through denitrification in relatively older, much anoxic sediments. The above results indicate that sediments in Silver Lake are heavily contaminated with N trapped in the pore water. The primary sources of N are the surrounding croplands and an active hog lot on the southeastern lakeshore. The average rate of sedimentation in the lake has been 1 cm/year in the last 32 years. Upon sedimentation, the pore water N is slowly released to the lake water, thereby dramatically limiting the lake's capability to process incoming nutrients. The mass distribution of N in the lake was estimated as 3.66 x 10³ kg (65%) in bottom sediments, 172 kg (3%) in suspended particulates, and 1.83 x 10³ kg (32%) in the dissolved phase. Some of the recommendations made through this study include dredging the top 25 cm of lake sediments, applying buffer strips along the lake's northern and eastern shorelines, and reducing the application of N and P-based fertilizers.
DE: 1806 Chemistry of fresh water
DE: 1815 Erosion and sedimentation
DE: 1831 Groundwater quality
DE: 1871 Surface water quality
DE: 1890 Wetlands
SC: Hydrology [H]
MN: 2005 Joint Assembly