The permafrost headwater catchments in the Tibetan Plateau (TP) have experienced extensive permafrost degradation, which may cause major changes in riverine solutes. However, surface water hydrochemistry and its influencing factors in such catchments are poorly understood. Hydrochemistry data for different surface waters were obtained for the Yakou catchment in the Northeastern TP. The results indicate that the ionic and organic concentrations of frozen soil seeps (FSS) were higher on the north-facing slope compared to the south-facing slope, and that FSS may be involved in streamflow generation processes and in determining the spatial pattern of riverine solutes. The north-facing slope of the catchment has a thin active layer and wet moisture conditions compared to the south-facing slope; hence supra-permafrost water, with high ionic concentrations, can drain to the ground surface as FSS in the riparian zone and then recharge the surface ponding water and the main stream water. The high ionic concentrations of the supra-permafrost water and FSS can be attributed to intense rock weathering and evaporative effect, together with the high mobility of elements and the transport of organic matter. The tributaries, with low ionic concentrations, comprise a mixture of infiltrating precipitation and diluted supra-permafrost water. Carbonate weathering is the dominant weathering type within the catchment, but the weathering of evaporite and silicate is more important on the north-facing and south-facing slopes, respectively, and chemical weathering on the north-facing slope may be enhanced by strong physical erosion during repeated freeze-thaw cycles due to the wet conditions. The results indicate that the surface water hydrochemistry is heterogeneous on the different hillslope units, and that a thicker active layer under climate change may lead to a shift of hydrological and hydrochemical pathways, and thus a decrease in water and solute flux from the hillslopes, with underlying permafrost, to the river channel.

The presence of permafrost has a strong influence on arctic hydrology, ecology, and engineering. Therefore, understanding the response of permafrost to arctic warming is critical to predicting the regional effects of global climate change. Recent research suggests that thaw depth may be increasing in response to warming, but physical thaw depth surveys in the Alaskan arctic are often not sensitive enough to detect incremental increases and cannot measure increases in the permafrost thaw bulbs beneath lakes and streams. Here we assess the use of geochemical tracers in stream water to identify changes in thaw depth in an arctic watershed. Based on marked differences in geochemistry with depth in soils and permafrost on the Alaskan North Slope, we used Sr-87/Sr-86 and elemental ratios in an arctic stream as tracers of increases in the maximum depth of soil water flow and therefore the integrated thaw depth in the watershed. From 1994 to 2004, stream water Sr-87/Sr-86, Ca/Na, and Ca/Ba at base flow showed significant trends with time, consistent with increasing depth of soil water flowpaths. Although long time series will be necessary to identify long-term trends, stream geochemistry may be useful as a qualitative indicator of changes in thaw depth in other areas where permafrost and active layer soil geochemistry differs. (C) 2010 Elsevier BM. All rights reserved.