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Thaw Settlement in Soils of the Arctic Coastal Plain, Alaska

ABSTRACT On the Beaufort Coastal Plain of northern Alaska, thaw settlement in permafrost soils occurs whenever natural or human disturbances result in an increase in the depth of seasonally thawed soil (the active layer). Knowledge of the potential magnitude of thaw settlement is important for assessing the long-term recovery of disturbed land and for developing rehabilitation plans and performance standards. To address this need, we analyzed thaw strain and thaw depth data from soil cores distributed across the central Beaufort Coastal Plain to evaluate potential thaw settlement at landscape (e.g. terrain units) and regional (e.g. ecodistricts) scales in connection with various oilfield studies during the period 1998–2003. Mean thaw strain values ranged from 1% for meander active channel deposits to 55% for delta inactive overbank deposits, and tended to be highest at 1–2 m below the ground surface. The potential thaw settlement of specific terrain units was evaluated based on thaw strain of soils and the range of changes in active layer depths typically found after disturbance. Mean estimates for potential thaw settlement for an active layer adjustment to 110 cm after disturbance varied from ∼0 cm in sandy soils with thick active layers associated with meander active channel and overbank deposits to 86 cm in very ice-rich silty soils with thin active layers associated with delta inactive overbank deposits. Potential thaw settlement also varied by region, with values for terrain units tending to be higher in the central Beaufort Coastal Plain (Prudhoe Bay and Kuparuk Oilfield areas) than in the western Beaufort Coastal Plain. The highest estimate for potential thaw settlement was 103 cm for delta inactive overbank deposits on the Colville Delta. The thawing of ice wedges will further contribute to thaw settlement and effect local hydrology and topography following thermokarst.