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<br /> <br /> <br />1 <br />CNAPTERFOUR Enuironmenta~ consequences <br />This chapter of the Environmental Impact Statement (EIS) provides an analysis of the effects <br />(environmental consequences) that would result from implementation of the proposed Yankee <br />Gulch Project or its alternatives. Certain measures that would avoid or reduce impacts have been <br />included in the action alternatives, as discussed in Chapter 2. The environmental impact analysis <br />documented in this chapter took these measures into consideration. <br />' An environmental effect (also termed impact) or consequence is defined as a modification or <br />change in the existing environment brought about by the action taken. Impacts can be direct or <br />indirect in nature and can be temporary (short term) or permanent (long term). Impacts can vary <br />in degree from only a slight discernible change to a drastic change in the environment. For the <br />purpose of this EIS, temporary effects aze defined as those that would occur during the <br />construction phase. Short term effects would occur during the mining of a mine panel. Long <br />term impacts aze impacts caused by construction and operations that would remain for the life of <br />the project or longer. <br />The impact analysis evaluated the effects that would occur in the Project Area, regardless of land <br />ownership. However, the Bureau of Land Management's (BEM's) decisions on this project <br />would only apply to federal lands. Mitigation on state or private lands cannot be required by the <br />' BEM. The landowner/manager of other lands would specify such measures. The impacts <br />reported for non-federal lands may occur regardless of the BEM's decision. Impacts on non- <br />federal lands aze included to provide a full disclosure of effects for the complete project and to <br />support other environmental revisions and permitting associated with the project. <br />4.1 GEOLOGY <br />' 4,1.1 Introduction <br />' The Mine Plan calls for initial recovery of 1.4 million tons per yeaz of nahcolite from a series of <br />solution mining wells. Mining is anticipated to be conducted for at least 30 years. The recovery <br />of nahcolite is considered a significant consequence to geologic resources since the nahcolite <br />' would no longer be available for future use. <br />Additional impacts to geologic resources for this study focus on: (1) potential effects on future <br />mineability of oil shale and (2) surface topographic changes, both of which are related to the <br />process known as subsidence. Subsidence, as defined for this analysis, involves sinking or <br />collapse of subsurface geologic formations caused by the removal of material by subsurface <br />activities, in this case removal of nahcolite by the proposed in situ leaching process. Incases <br />where sinking or collapse of subsurface materials occurs to such an extent that effects are evident <br />at the ground surface, this is called surface subsidence. Surface subsidence has the potential to <br />' cause structural as well as environmental damage. <br />Agapito Associates, Inc. (Agapito) evaluated potential effects on subsurface formations and <br />' surface topographic changes related to the proposed mining project (Agapito 1998a and 1998b). <br />Agapito conducted thermomechanical numerical analyses to evaluate the stability of both single- <br />cavity (axisymmetrical) and multiple-cavity (plane-section) scenarios. Rock mechanics <br />information for these evaluations came from test cavity production well 20-3 and from <br />subsurface rock cores from boreholes 20-9, 29-3, and ] 9-2. <br />' Geology 4-1 <br />