2002-07-08_GENERAL DOCUMENTS - M2002004 (4)

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INTRODUCTION Commercial mining of lignite in North Dakota began to increase in the early 1960s in response to the increased demand for energy and the declining supply and increased costs of other sources of fossil fuels. The Fort Union formation, located in Montana, North Dakota, South Dakota and Wyoming, has been estimated to contain more than 40 percent of the total United States coal reserves. Much of western North Dakota, more than 28,000 square miles, is underlain by lignite. Strippable lignite reserve estimates refer to coal seams 5 feet or thicker overlain by 100 feet or less of overburden. Strippable reserves in western North Dakota are estimated to underlie as much as 2 million acres. A general discussion of North Dakota lignite reserves and development potential has been given by Dalsted and Leistritz (1974). Public concem about the degradation of land by strip- mining increased concurrently with increases in production. Beginning in 1970, North Dakota pass- ed legislation requiring reclamation of strip -mined lands. The state reclamation laws were strengthened by each succeeding legislative assembly until the current 1979 North Dakota law was passed to bring the state into compliance with the 1977 Federal Stripmining Act. The 1979 law requires that strip - mined lands be restored to productive levels which are equal or better than before mining. Initial attempts to revegetate leveled spoils were generally successful only on nonsodic spoils (Power et al., 1974). This led to experiments on highly sodic spoils in which various chemical treatments, mulches and topsoil replacement were compared. It soon became apparent that even 2 inches of topsoil markedly increased vegetative establishment and dry matter production. These experiments showed that reclamation success would be directly related to chemical and physical characteristics of the underlying spoil materials and the amount and quali- ty of soil materials available for replacement. Further studies were initiated to 1) characterize overburden materials in the potential stripmining areas of North Dakota, 2) evaluate soil materials available for use in reclamation, and 3) develop reclamation technology to assist in meeting legal requirements for land restoration. The purpose of this review is to evaluate published and unpublished North Dakota research relating to the depth and quality of topsoil and sub- soil (first and second lift) needed for successful reclamation of North Dakota strip -mined lands and to identify areas where further research may be needed. OVERBURDEN CHARACTERISTICS Most of the lignite reserves in North Dakota are located in the Bullion Creek and Sentinel Butte for- mations of the Fort Union Group and consist of alter- 4 nating layers of lignite, soft shales, silts and some sands (Sandoval et al., 1973). Soils in the area are derived from these Fort Union geologic materials, from overlying glacial till or from alluvial or aeolian deposits. Studies of the characteristics of the lignite strip - mine spoils in North Dakota as related to reclama- tion potential have been reported by Gee et al. (1978); Sandoval et al. (1973); Schroer (1976 and 1978) and in ARS -NDSU Progress Report 20 (1975) and the suc- ceeding Update (1977) and Supplement to the 1977 Update (1979). No attempt will be made in this discussion to summarize all the results of these studies; instead, the variability in the properties of these spoil materials which relate to soil replace- ment depths will be emphasized. Schroer (1976 and 1978) analyzed several hundred overburden samples for various physical and chemical properties and noted that these properties varied widely with depth both within and between mine areas. Because of this wide variation, he stated that site sampling will be needed at each mine or permit location to assess pertinent properties so that the appropriate reclamation procedure can be developed. He also sampled and analyzed reshaped spoils at four mine sites and noted that the resulting properties of the reshaped spoils could have been predicted from pre -mine overburden sampling. Sandoval et al. (1973) reported that minespoils derived from the Fort Union geologic materials are frequently fine - textured, with clay contents as high as 40 percent. Smectite is the predominate clay mineral, with appreciable quantities of vermiculite and illite, with traces or low amounts of chlorite and kaolinite (Bauer et al., 1976; Klages and Hopper, 1982). Sandoval et al. (1973) and Schroer (1976 and 1978) reported that these overburden materials tend to be nonsaline to moderately saline (EC below 8) but are often sodic (SAR above 20). However, the sodium content is variable with SAR values varying from 2 to 70. In some areas, the Fort Union stratified materials are overlain by glacial drift and /or aeolian materials. These materials are generally low in soluble salts and sodium but are occasionally coarse - textured with a low water - holding capacity. When the sodium content increases in a clay high in expanding - lattice clay minerals (such as smectite), the particles disperse, the surface becomes sealed, and water intake may be reduced to nearly zero. Gee et al. (1976) reported that when top- soil was spread over sodic spoil and excess water was applied, the moisture content of the topsoil was still above field capacity after 30 days. The results cited above and those to be discussed later show the importance of spoil properties In determining the optimum depth of topsoil and sub- soil to be replaced. The most important spoil proper- ties are sodium content, salinity, clay content and