Date of Award

1994

Document Type

Thesis

Degree Name

Master of Science (MS)

Department

Geology

First Advisor

J.R. Reid

Abstract

Contamination of groundwater by agricultural chemicals is of concern to the population of the Northern Great Plains. Preferential flow (macropore, finger, and funnel) allows such chemicals to move relatively rapidly through materials considered to be aquitards. Preferential flow mechanisms at two sites in North Dakota, one in till, the other in glaciolacustrine sediments were determined by application of blue dye (FD&C No 1) solution to framed sites (9m2 & 4m2) with the topsoil removed. The sites were then cored and blocks hand-excavated to depths of 1.5 and 2.5-meters. The three types of preferential flow (macropore, finger, and funnel) were analyzed in the excavated blocks. The relative importance of the flow mechanisms varied with depth. At the till site fractures provided the primary pathway for dye solution to be moved to textural boundaries at depth where funnel flow occurred. Over 90 percent of preferential flow occurred along fractures with minor root channel and finger flow. Funnel flow observed in cores was important from 2.3 to 3.8m-depth. At the glaciolacustrine site about 80 percent of preferential flow occurred along fractures and as fingers to ~80cm. Beyond this depth, >90 percent of flow was along root channels, and funnel flow also was important to 3.8m. The extent of development of preferential flow mechanisms, and thus their ability to transport a solution, is dependent on factors such as lithology, stratigraphy, climate, and agricultural practice. These conditions are site-specific and determine which preferential flow mechanisms have major or minor effects on solute transport. The interaction of these preferential flow mechanisms allowed relatively rapid movement of dye solution through sediments typically thought to be poorly permeable. The type of preferential flow mechanisms, their development, relative importance, and how they interact, should be a key part of evaluating the potential for contaminant transport in all unlithified aquitards.

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