Geologic Analogs of Engineered Barriers: Natural Examples of Very Long- Term Performance of Layered Geologic Materials

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1997

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Materials in the vadose zone are modified through time by a number of processes. These processes have the potential to adversely affect the performance of engineered barriers capping waste disposal facilities. In this study, the alteration of layered natural materials from 10 sites was described. In order for the materials to serve as partial analogs for typical arid-region engineered barriers, sites were selected to (1) have a difference between mean annual evaporation and rainfall greater than 40 inches rainfall and less than 50 days continuously below freezing; (2) be multilayered with maximum grain size contrasts (fine-grained materials overlying gravel); (3) be well-dated with ages less than 500,000 years. Site access and quality of exposure were also critical variables.

The selected sites are composed of fluvial and lacustrine deposits and range in age from 500 to 22,000 years. Fine-grained materials include clay, silty clay, diatomite, and calcareous lake deposits. Coarse-grained materials include gravel, gravel with sand matrix, and gravel with mud matrix.

Modification of sediments includes penetration by roots, cracking of fine-grained materials in response to shrink/swell, infiltration of fines into gravel, precipitation of carbonate, limonite, manganese oxides and hydroxides, gypsum and halite, and oxidation and reduction of iron associated with fines. The two main variables that can be related to intensity of alteration are (1) geomorphic setting and (2) composition of fine materials. The most alteration was observed in deposits now in topographic lows; deposits on hillsides underwent the least alteration. Clayey deposits showed more evidence of shrink-swell and cracking than diatomites. Topographic effects and mineralogy have more influence on the amount of alteration than the age of the deposits. Infiltration of fine materials into gravels is common but little can be deduced about the rates and processes driving this type of alteration. Relationships to constrain the timing of infiltration could not be developed. It is probable that much or all of the infiltration of fine material into gravel occurred in the depositional environment.

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