2.1 Overview of the Proposed Project - Keystone XL pipeline - US ...

Draft Supplemental Environmental Impact StatementKeystone XL Projectenvironments are difficult to contain primarily because bentonite readily disperses in flowingwater and quickly settles in standing water. Further discussion of frac-outs is presented inSection 4.3, Water Resources; Section 4.7, Fisheries; and Section 4.8, Threatened andEndangered Species and Species of Conservation Concern.2.1.9.2 Wetland CrossingsConstruction across wetlands would be similar to typical conventional upland cross-countryconstruction, with modifications to reduce the potential for effects to wetland hydrology and soilstructure. The wetland crossing methods used would depend largely on the stability of the soils atthe crossing location at the time of construction. The 110-foot pipeline construction corridorwidth would be reduced to 85 feet for wetlands in Montana and Nebraska, and 75 feet forwetlands in South Dakota, unless conditions require a wider construction corridor. In instanceswhere the wetland is supersaturated or inundated, the corridor ROW may be increased to ensuresafe construction conditions. The operation ROW would be 30 feet in all three states, unlessconditions require a wider permanent ROW width.If wetland soils are not excessively saturated at the time of construction and can supportconstruction equipment without equipment mats, construction would occur in a manner similarto conventional upland cross-country construction techniques. Low ground pressure equipmentor conventional equipment supported by pre-fabricated matting would be used in all wetlands.Where wetlands are located at the base of slopes, permanent slope breakers would be constructedacross the ROW in upland areas adjacent to the wetland boundary. Temporary sediment barrierswould be installed where necessary until revegetation of adjacent upland areas is successful.Additional TWAs would be required on both sides of saturated or inundated wetlands to stageconstruction, fabricate the pipeline, and store materials. These additional TWAs would belocated in upland areas a minimum of 10 feet from the wetland edge. More information islocated in the Site-Specific Waterbody Crossing Plans located in the CMRP (Appendix G) withrecommended modifications to the CMRP provided in Section 4.4.4, Recommended AdditionalMitigation.Clearing of vegetation in wetlands within the construction corridor would be limited to trees andshrubs, which would be cut flush with the surface of the ground, leaving the root stock in place,and removed from the wetland. Scrub-shrub and forested communities would be allowed toregenerate outside of the maintained operations ROW, and in areas where HDD was used to spanopen water areas. To avoid excessive disruption of wetland soils and the native seed androotstock within the wetland soils, stump removal, grading, topsoil segregation, and excavationwould be limited to the area immediately over the trench line to the maximum extent practicable.Trench breakers would be installed where necessary to prevent the subsurface drainage of waterfrom wetlands. Trench width may vary in some wetlands to provide an even, safe work area,which depends upon topography, soil moisture content, and groundwater levels. Severetopography may require additional disturbance to create an even safe work area. More saturatedsoils usually require a wider trench in order to maintain a safe ditch and to avoid unstable trenchwalls. During clearing, sediment barriers, such as silt fence and staked straw bales, would beinstalled and maintained on down slopes adjacent to all wetlands and within additional TWAs, asnecessary, to minimize the potential for sediment runoff.Construction equipment working in all wetlands would be limited to the area essential forclearing the ROW, excavating the trench, fabricating and installing the pipeline, backfilling theProject Description 2.1-60 March 2013