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Fracture-Controlled Nitrate and Atrazine Transport in Four Iowa Till Units

^a Department of Geology, Dickinson College, Carlisle, PA 17013
^b Department of Geological and Atmospheric Sciences, Iowa State University, Ames, IA 50011
^c Agronomy Department, Iowa State University, Ames, IA 50011

View larger version (42K): [in a new window]   Fig. 1. Map showing the locations of the three study sites on the Des Moines Lobe (DML), Iowan Erosion Surface (IES), and Southern Iowa Drift Plain (SIDP) landform regions. Other mapped landform regions are given in Prior (1991).

View larger version (33K): [in a new window]   Fig. 2. Plan view maps of fracture patterns at the (a) Des Moines Lobe (DML) site, (b) Iowan Erosion Surface (IES) site, and (c) Southern Iowa Drift Plain (SIDP) site.

View larger version (39K): [in a new window]   Fig. 3. Observed and modeled breakthrough curves generated from the DML-1, IES-1, IES-2, and DML-2 columns, where DML is the Des Moines Lobe and IES is the Iowan Erosion Surface. The dashed vertical line indicates the time for 1 pore volume (PV) to pass through each column. MIM, mobile–immobile model; PFBA, pentafluorobenzoic acid; PIPES, 1,4-piperazinediethanesulfonic acid disodium salt.

View larger version (39K): [in a new window]   Fig. 4. Observed and modeled breakthrough curves generated from the DML-3, SIDP-1, SIDP-2, and SIDP-3 columns, where DML is the Des Moines Lobe and SIDP is the Southern Iowa Drift Plain. The dashed vertical line for the DML-3 column indicates the time for one pore volume (PV) to pass through the column. The pore volumes were off scale for the SIDP-1, SIDP-2, and SIDP-3 columns.

View larger version (21K): [in a new window]   Fig. 5. Plot of velocity (log scale) versus depth for the eight till columns. Velocities are shown for time of first arrival of the bromide tracer at relative concentration (C/C0) = 0.02 (VBr, filled arrows) and for plug flow through the entire porosity of the sample calculated by Eq. [7] (VPM, open circles).

View larger version (40K): [in a new window]   Fig. 6. Plan view of a slice taken from the center of the DML-3 column (from the Des Moines Lobe) showing fractures (dark lines) and extent of FDC Brilliant Blue Dye no. 1 (gray zones) at a depth of 3.65 m.