SBCT Final EIS - Govsupport.us

Chapter 3 ⎯ Affected EnvironmentUSACE 2004) supplemented NRCS soil survey information. According to the BAX and CACTFFEIS, the Eddy Drop Zone study area has a low permafrost occurrence and less permafrost comparedto the Donnelly Drop Zone sub-area and the North Texas Range sub-area. The Donnelly Drop Zonestudy area has less permafrost than the North Texas Range study area. Although geotechnical surveyswere not conducted for the Donnelly Drop Zone or North Texas Range study areas, several other factorswere used to supplement the NRCS soil survey information to determine the probability of permafrost,including previous investigations in the area.Erosion ManagementAs with FRA, soil resources management on DTA is mandated by and detailed in the USAGAKINRMP and ITAM Five-Year Management Plan.Chemical Constituents in SoilThe USAGAK 2006 BAX and CACTF FEIS detailed existing hazardous materials and other contaminantsin soils on the DTA, which are summarized here. Sampling at DTA firing points has detectedlow levels of 2,4-DNT on the surface, but not at depth in the soils, and not in groundwater orsurface water. The low mobility of 2,4-DNT in the soil is likely due to its low solubility, low precipitationin the region, and frozen soil conditions most of the year. Various heavy metals (including lead,zinc, antimony, copper, manganese, and iron) are used in munitions components and found in primers,shell casings, and various projectile components. These metals are also naturally found in backgroundlevels on soils in DTA. Soil samples collected from various training areas of DTA detectedlow levels of zinc, copper, lead, and antimony within impact areas and target berms where munitionswere used. The concentrations were above natural background levels, but were well below levels ofconcern (Walsh 2004). Soils on DTA in the areas of firing points and target berms are primarilysandy and gravelly with neutral pH values, making them unconducive to dissolution and mobilizationof metals deposited from munitions components. While soils in permafrost areas covered by blackspruce and sphagnum moss are often acidic (pH 4.0 to 5.0), the shallow active layer and impermeableunderlying permafrost limit the mobility of dissolved metals in these areas.Geologic Hazards and SeismicityOn November 3, 2002, a magnitude 7.9 earthquake affected most of the State of Alaska, with thestrongest ground motion felt in the region north of the Alaska Range. The areas around DTA were allin the moderate zone (V–VII) on the Mercalli Scale of earthquake intensity (see http://www.aeic.alaska.edu/Seis/Denali_Fault_2002/, for more information). Minor to moderate damageoccurred to roads, runways, and some buildings. Support structures for the Trans-Alaskan pipelinewere also damaged. Portions of the Richardson Highway between DTA and Fort Wainwright (FWA)were closed or partially closed for 2 days. The epicenter of this earthquake was located along theDenali Fault, about 40 miles south of DTA. Movement was felt in a large area from north of Fairbanksto the Kenai Peninsula south of Anchorage. This was the largest earthquake on record to strikethe area.FWA and DTA are located on a geologic terrain bounded to the north and south by active faults.Unlike the subduction zone tectonics that cause earthquakes in southern portions of the state (seeFRA discussion), the activity along Denali Fault is strike-slip. The areas bounding this fault and therelated Tintina Fault are characterized by seismic zones that likely are a result of block rotation betweenthese two larger faults, ultimately driven by the collision of the North American and PacificPlates further south in the state (USARAK 1999a; Page et al. 1995).February 2008 3–99 2/25th SBCT Final EIS