Influence of Thermokarst Failures on Hillslope and Stream Water Quality

The occurrence of thermokarst terrain has increased with the warming of climate and degradation of permafrost. In 2007 we observed numerous hillslope thermokarst failures in the Noatak Preserve (western Brooks Range), Alaska. The observed failures were shallow translational landslides that had displaced massive amounts of active layer soils (often 10's m wide, 100's m long and a few m deep). Hillslopes had been structurally reconfigured and surface layers of permafrost were thawing. Therefore, we expected shallow subsurface water to be altered with respect to its biogeochemistry, becoming in general, richer in dissolved solute concentrations compared with soil water from outside the thermokarst failure. We also expected that headwater streams at the base of these hillslopes would reflect the changed chemical loading. We sampled hillslope water upslope and within 5 thermokarst failure features. We also sampled 3 receiving streams up and downstream of where we expected hillslope water affected by thermokarst to enter the stream. Water samples were analyzed for total suspended sediments (TSS), major ions (Cl, SO4, Na, K, Ca, Si), as well as TDN, TDP, DOC, and specific conductivity. TSS increased by more than 3 orders of magnitude within thermokarst failures, and more than 2 orders of magnitude in receiving waters. Water collected within thermokarst failures was consistently higher in concentrations of Cl, Na, TDN, and DOC (on average, 341%, 263%, 88%, and 47%, respectively), consistently lower in specific conductivity (-18%). Concentrations of SO4, K, and Si in thermokarst waters were, on average greater than above thermokarst waters, whereas those for Ca and TDP were on average, lower. In receiving streams concentrations of Na, TDN, and DOC were all consistently greater downstream (on average 30%, 43%, and 13%, respectively) of thermokarst-affected inflows. These results suggest that the greatest changes to aquatic ecosystems resulting from thermokarst failure features are likely to be associated with increased suspended sediment load, rather than changes to dissolved chemistry.