Fractures in till may provide pathways for agricultural chemicals to contaminate aquifers and surface waters. This study was conducted to quantify the influence of fractures on solute fate and transport using three conservative and two nonconservative tracers. The conservative tracers were potassium bromide (KBr), pentafluorobenzoic acid (PFBA), and 1,4-piperazinediethanesulfonic acid disodium salt (PIPES); the nonconservative tracers were nitrate and atrazine [6-chloro-N-ethyl-N'-(1-methylethyl)-1,3,5-triazine-2,4-diamine]. Three sites in Iowa were investigated, including four late Wisconsinan and Pre-Illinoian tills. Laboratory tracer experiments were conducted using eight large (0.4-0.45 m long by 0.43 m in diameter), undisturbed columns of till collected from depths of 1 to 28 m. The tills were densely fractured, with fracture spacing ranging from 3.8 to 10.4 cm. First arrival velocities of Br- ranged from 0.004 to 64.8 m d(-1), 10 to 100 times faster than predicted for unfractured media. Nitrate behaved as a conservative tracer in weathered till columns, but degraded during experiments using deeper tills. Sorption caused retardation of atrazine in the shallowest four columns. Atrazine degradation occurred in deeper columns as demonstrated by deviations between atrazine and the conservative tracers. Mobile-immobile model (MIM) simulations estimated first-order exchange coefficients (alpha) ranging from 1 x 10(-8) to 1.7 x 10(-2) s(-1), sorption coefficients (K(d)) for atrazine ranging from 2.6 x 10(-5) to 1 x 10(-3) m3 kg(-1), and degradation half-lives ranging from 0.24 to 67 d (nitrate) and 1.6 to 277 d (atrazine). This study suggests that aquifers and surface waters associated with thin, fractured till units may be vulnerable to contamination, yet deeper aquifers may be protected by these materials due to increased residence times provided by matrix diffusion.

Discrete-fracture and dual-porosity models are infrequently used to simulate solute transport through fractured unconsolidated deposits, despite their more common application in fractured rock where distinct flow regimes are hypothesized. In this study, we apply four fracture transport models--the mobile-immobile model (MIM), parallel-plate discrete-fracture model (PDFM), and stochastic and deterministic discrete-fracture models (DFMs)--to demonstrate their utility for simulating solute transport through fractured till. Model results were compared to breakthrough curves (BTCs) for the conservative tracers potassium bromide (KBr), pentafluorobenzoic acid (PFBA), and 1,4-piperazinediethanesulfonic acid (PIPES) in a large-diameter column of fractured till. Input parameters were determined from independent field and laboratory methods. Predictions of Br BTCs were not significantly different among models; however, the stochastic and deterministic DFMs were more accurate than the MIM or PDFM when predicting PFBA and PIPES BTCs. DFMs may be more applicable than the MIM for tracers with small effective diffusion coefficients (De) or for short timescales due to differences in how these models simulate diffusion or incorporate heterogeneities by their fracture networks. At large scales of investigation, the more computationally efficient MIM and PDFM may be more practical to implement than the three-dimensional DFMs, or a combination of model approaches could be employed. Regardless of the modeling approach used, fractures should be incorporated routinely into solute transport models in glaciated terrain.

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a solute diffuses may cause the observed, or effective diffusion coefficient (D e ) to be lower than the diffusion Diffusion is often the dominant mode of solute transport in soils coefficient of a compound in water (D 0 ) (Rao et al., when advection is minimal. This paper describes the application of a radial diffusion cell method to estimate the effective diffusion coeffi-1980). Moreover, exclusion of a solute from some pore cient (D e ) and effective diffusive porosity ( De ) for use in solute trans-classes may reduce the effective diffusive porosity ( De ), port models for fractured-porous media. Twenty-four experiments or that portion of a porous medium that is accessible were conducted for 28 d using three conservative solutes (Br, PFBA, to diffusing molecules (van der Kamp et al., 1996). These and PIPES) on eight late Wisconsinan and Pre-Illinoian till samples effective parameters are likely to be specific to each from Iowa. The mean value of the total porosity ( T ) of the till samples soil-solute combination, and therefore require direct was 30.0%. Concentrations of the three tracers in the reservoir demeasurement for the solute and material of interest.creased with time and eventually approached equilibrium concentra-Mathematical solutions to the diffusion problem are tions. A model simulated the observed concentration data and the well established (Crank, 1975). In addition, models are modified goodness-of-fit (d 1 ) values ranged from 0.878 to 0.950. Mean now capable of simulating solute diffusion coupled with values of De from the model were 28.3 (Br Ϫ ), 26.5 (PFBA), and 21.6% (PIPES) and there were significant differences in De among advection through macroporous soil and fractured till the three tracers (p ϭ 0.05). Mean values of D e were 5.6 ϫ 10 Ϫ10 m 2 (Therrien and Sudicky, 1996; Toride et al., 1999). These s Ϫ1 (Br Ϫ ), 2.9 ϫ 10 Ϫ10 m 2 s Ϫ1 (PFBA), and 1.3 ϫ 10 Ϫ10 m 2 s Ϫ1 (PIPES). models require that effective diffusion parameters be Values of D e differed significantly by compound and were significantly provided as input. Laboratory methods, including the different (p ϭ 0.05) from the aqueous diffusion coefficient (D 0 ). Calcuhalf-cell, the reservoir-cell, and radial diffusion cell, lated mean values of the first-order mass exchange coefficient (␣) have been proposed for determination of D e . The most were 8.4 ϫ 10 Ϫ7 (Br Ϫ ), 4.1 ϫ 10 Ϫ7 (PFBA), and 1.6 ϫ 10 Ϫ7 s Ϫ1 (PIPES); widely used is the half-cell method, where two cells, they differed by compound (p ϭ 0.05) and generally decreased with one spiked with a solute, are pressed together, allowing increasing molecular weight of the tracer. This study confirmed that diffusion to take place (Li and Gregory, 1974; Robin et the radial diffusion cell method is an efficient method to estimate al., 1987). Work by van Rees et al. (1991), however, effective diffusion parameters necessary to accurately model solute transport in fractured till and soil.

Earthen waste storage structures (EWSS) associated with large confined (concentrated) animal feeding operations (CAFOs) were evaluated for their potential to impact water resources in Iowa. A representative sample of 34 EWSS from a digital database of 439 lagoons and basins permitted between 1987 and 1994 was analyzed. Eighteen percent (6 of 34) directly overlie alluvial aquifers that are used widely for potable water supply. Ninety‐four percent (29 of 31) were constructed below the water table based on EWSS depth data. At 65 percent of EWSS (22 of 34), 50 percent or more of the manure‐spreading area (MSA) has a water‐table depth less than 1.6 m. At 74 percent of EWSS (25 of 34), 90 percent or more of the MSA contains soil with vertical K exceeding 25.4 mm/hr. Seventy‐one percent (24 of 34) occur where 10 percent or less of the MSA is frequently flooded. No significant differences were found among leakage rates due to aquifer vulnerability class or surficial material. However, at least 50 percent of EWSS (14 of 28) leaked at rates significantly greater than 1.6 mm/d under the new construction standard. The estimated 5,000 unregulated CAFOs may have a greater potential to impact water resources in Iowa.