Studies of solute transport through fractured till in Iowa

Helmke, Martin F.

doi:10.31274/rtd-180813-14331

Cited by 7 publications

(21 citation statements)

References 28 publications

(49 reference statements)

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“…Column temperature was maintained at a constant 12°C to simulate in situ conditions. Flow rates were monitored and K b was calculated using the Darcy equation (Helmke, 2003).…”

Section: Methodsmentioning

confidence: 99%

“…Fractures or zones of preferential flow in till have been reported in Iowa (Kemmis et al, 1992) and elsewhere (Connell, 1984; Simpkins and Bradbury, 1992; Brockman and Szabo, 2000; Helmke, 2003), Canada (Keller et al, 1988; McKay et al, 1993a), and Denmark (Klint and Gravensen, 1999). Fractures promote greater velocities than would otherwise be expected under porous media assumptions (Freeze and Cherry, 1979; Grisak and Pickens, 1980; J⊘rgensen and Spliid, 1992; McKay et al, 1993a, 1993b).…”

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confidence: 92%

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

2005

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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.

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“…Column temperature was maintained at a constant 12°C to simulate in situ conditions. Flow rates were monitored and K b was calculated using the Darcy equation (Helmke, 2003).…”

Section: Methodsmentioning

confidence: 99%

mentioning

confidence: 92%

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

2005

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“…Hydrogeologists have long considered fractures in till to act as preferential flowpaths and facilitate rapid transport of contaminants. The bulk hydraulic conductivity ( K b ) of fractured till is commonly 1 to 3 orders of magnitude greater than the hydraulic conductivity of the till matrix ( K m ) (Freeze and Cherry 1979; Helmke 2003). Fracture porosity ( n f ) is frequently 1 to 4 orders of magnitude less than the total porosity ( n T ) of the till (Jørgensen and Spliid 1992; McKay et al 1993a).…”

Section: Introductionmentioning

confidence: 99%

“…Fractures are also well documented in till throughout the world and constitute a ubiquitous feature of these deposits. In the United States, till fractures have been documented in Iowa (Kemmis et al 1992; Helmke 2003), Wisconsin (Connell 1984; Simpkins and Bradbury 1992), and Ohio (Brockman and Szabo 2000). Fractures in till have also been reported throughout Canada (Keller et al 1988; McKay et al 1993a) and Denmark (Klint and Gravensen 1999).…”

Section: Introductionmentioning

confidence: 99%

Simulating Conservative Tracers in Fractured Till under Realistic Timescales

Helmke¹,

Simpkins

Horton

2005

Groundwater

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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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“…We describe a series of experiments on eight samples of unfractured late Wisconsinan and Pre‐Illinoian till from Iowa where the radial diffusion cell method was applied to estimate diffusion properties. Diffusion parameters were used to simulate solute transport through fractured till as part of a larger study (Helmke, 2003; Helmke et al, 2004). The diffusion experiments were conducted under saturated conditions because most of the till sequence (1 to >30 m depth) remains saturated throughout the year in Iowa.…”

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confidence: 99%

Experimental Determination of Effective Diffusion Parameters in the Matrix of Fractured Till

Helmke

Simpkins

Horton

2004

Vadose Zone Journal

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Diffusion is often the dominant mode of solute transport in soils when advection is minimal. This paper describes the application of a radial diffusion cell method to estimate the effective diffusion coefficient (De) and effective diffusive porosity (θDe) for use in solute transport models for fractured‐porous media. Twenty‐four experiments were conducted for 28 d using three conservative solutes (Br, PFBA, and PIPES) on eight late Wisconsinan and Pre‐Illinoian till samples from Iowa. The mean value of the total porosity (θT) of the till samples was 30.0%. Concentrations of the three tracers in the reservoir decreased with time and eventually approached equilibrium concentrations. A model simulated the observed concentration data and the modified goodness‐of‐fit (d1) values ranged from 0.878 to 0.950. Mean values of θDe from the model were 28.3 (Br−), 26.5 (PFBA), and 21.6% (PIPES) and there were significant differences in θDe among the three tracers (p = 0.05). Mean values of De were 5.6 × 10−10 m2 s−1 (Br−), 2.9 × 10−10 m2 s−1 (PFBA), and 1.3 × 10−10 m2 s−1 (PIPES). Values of De differed significantly by compound and were significantly different (p = 0.05) from the aqueous diffusion coefficient (D0). Calculated mean values of the first‐order mass exchange coefficient (α) were 8.4 × 10−7 (Br−), 4.1 × 10−7 (PFBA), and 1.6 × 10−7 s−1 (PIPES); they differed by compound (p = 0.05) and generally decreased with increasing molecular weight of the tracer. This study confirmed that the radial diffusion cell method is an efficient method to estimate effective diffusion parameters necessary to accurately model solute transport in fractured till and soil.

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Studies of solute transport through fractured till in Iowa

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Cited by 7 publications

References 28 publications

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

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

Simulating Conservative Tracers in Fractured Till under Realistic Timescales

Experimental Determination of Effective Diffusion Parameters in the Matrix of Fractured Till

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