Studies of solute transport through fractured till in Iowa.

摘要

Fractures may provide pathways for agricultural chemicals to reach aquifers through till units that have traditionally been considered effective barriers to contaminant transport. Till units were studied at three sites in Iowa that represented three landform regions, till ages from 12.5 to >730 ka, and depths from 1 to 27.5 m. Fractures were present at all study locations and at all depths, including one site where fractures intersected an aquifer at 30 m depth. Laboratory experiments using eight large (0.40 to 0.45 m in length and 0.43 m in diameter), undisturbed columns of till showed bulk hydraulic conductivity (K_b) ranging from 7.7 × 10−10 to 3.8 × 10−5 m/s, which is generally greater than the matrix hydraulic conductivity reported in the literature. Laboratory experiments with KBr, PFBA, PIPES, KNO ₃, and atrazine as tracers were used to produce breakthrough curves (BTCs). First arrival velocities of Br ranged from 0.004 to 64.8 m/d—10 to 100 times faster than predicted using the equivalent porous medium (EPM) assumption. Similar velocities of NO₃ and atrazine were observed for columns collected from depths of less than 3 m. In deeper columns, sorption (atrazine) and degradation (NO₃ and atrazine) retarded transport. Tracers were not affected significantly by fracture origin or orientation. Separation of conservative tracers with different aqueous diffusion coefficients was observed during the rising and tailing limbs of BTCs, which indicates that matrix diffusion was a controlling process. Laboratory BTCs were compared against model-simulated BTCs using three approaches: the Mobile-Immobile Model (MIM), the Parallel-plate Discrete Fracture Model (PDFM), and a 3-Dimensional Discrete Fracture Model (3-D DFM). All three model approaches were reasonable predictors of the BTCs (goodness-of-fit statistic d₁ ranged from 0.751 to 0.959).; The results of this study demonstrate that fractures may have a controlling influence on solute transport through till units in Iowa. Contaminants may be transported rapidly through thin aquitards of fractured till, but non-conservative compounds are likely to be retarded or degraded in thicker till units. Fractures should be considered in groundwater studies in glaciated regions and in assessments of aquifer vulnerability to non-point source pollution.