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

Abstract

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.

Impact of artificial subsurface drainage on groundwater travel times and baseflow discharge in an agricultural watershed, Iowa (USA)

“…Drain conductance was selected as 8.7 × 10 −4 m 2 /s, close to typical literature values (Goswami and Kalita, ). Effective porosity used for particle tracking was 0.3 (Helmke et al ., ).…”

Evaluation of analytical and numerical approaches for the estimation of groundwater travel time distribution

“…In our model, we assigned aquifer porosity as 0.3, based on data reported for pre-Illinoian till in eastern Iowa (Helmke et al, 2005). The watershed-wide average saturated thickness was estimated to be 7 m. Annual recharge to the aquifer was estimated to be 129.5 mm based on the long-term average baseflow reported for the Walnut Creek watershed .…”

“…We recognize that this might not necessarily be true; however since the purpose of the paper is to compare alternate methods of TTD estimation, the assumption was considered to be adequate. Effective porosity used for particle tracking was estimated to be 0.3 (Helmke et al, 2005).…”

Preferential Flow in a Pedological Perspective

“…In this respect, the modified scheme implicitly assumes that the effects of finger flow can be equated with weak macropore flow (see Weiler and Flühler, 2004) in terms of accelerating flow and transport velocities. Jørgensen et al, 1998;Mayes et al, 2003;Helmke et al, 2005) are sometimes FIGURE 7 Predicted macropore flow class compared with the degree of preferential flow measured in short column breakthrough curve experiments; as indicated by p 0.05 , the pore volumes drained when 5% of the applied solute amount has leached. However, as a first approximation for sandy soils, the answer to the question as to whether the horizon is water repellent could be made dependent on land-use and management practices (e.g.…”