nsported over great distances and, consequently, are of variable geochemistry (Dollhopf, 1983, p. 593). In the region's semiarid climate, annual precipitation can range down to as low as 20 centimeters. With infrequent hydrologic infiltration or infiltration of low intensity dissolved salts and exchangeable sodium from mine spoil below the topsoil can potentially move upward into the root zone. Such upward movement may occur along temporary hydraulic, thermal, chemical, or osmotic gradients (NRC, 1981, p. 146). This chemical pollution can disrupt soil microbial activity and also prevent revegetation. In particular, high quantities of sodium may be particularly harmful. Sodium can cause swelling of any clay minerals which are present. Such swelling may impede the movement of air and water into the root zone, and thus may have various negative effects on agriculture. Furthermore, although this problem often takes years to develop, it tend
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