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TECHNICAL REPORTS

Ground Water Quality

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

^* Corresponding author (helmkem{at}dickinson.edu)

Received for publication June 12, 2003. 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 () 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.

Abbreviations: , first-order exchange coefficient • _e, effective porosity • _f, fracture porosity • BTC, breakthrough curve • C₀, influent tracer concentration • D₀, aqueous diffusion coefficient • d₁, modified index of agreement • DML, Des Moines Lobe • IES, Iowan Erosion Surface • KBr, potassium bromide • K_b, bulk hydraulic conductivity • K_d, equilibrium sorption coefficient • MIM, mobile–immobile model • PFBA, pentafluorobenzoic acid • PIPES, 1,4-piperazinediethanesulfonic acid disodium salt • PV, pore volume • SIDP, Southern Iowa Drift Plain

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JEQ 2005 34: 1-6. [Full Text]