Large-scale permafrost degradation as a primary factor in Larix sibirica forest dieback in the Khentii massif,northern Mongolia

The objective of this study is to investigate the potential causes of widespread Larix sibirica Ledeb.mortality observed in the Khentii massif of northern Mongolia.The ratio of deadwood to living trees in affected stands in the Goricho region,the southernmost study site situated close to the Gobi Desert,was as high as 3.6:1.Moisture fluctuations monitored over 2 years using electrical impedance spectrometry revealed that the Goricho study site had higher soil moisture levels than the two less affected sites Barun Bayan and Dzun Bayan.High soil moisture was recorded in an area characterized by highly skeletal soils,ones with more than 35%by volume of rock fragments,and comparatively shallow soil horizons,from valley to mountains.The layer of permafrost influencing hydrogeological processes is much deeper in the Goricho region compared to the undisturbed study sites.Redundancy analysis confirmed a significant number of dead L.sibirica on sites with developed soils.Live forest stands,however damaged,grow in this region on well-drained scree slopes or on rocky bastions.The mass mortality observed for L.sibirica may be directly linked to accelerated permafrost thaw in the area bordered by the Tuul and the Terelj Rivers.Our assumption is that L.sibirica root system necrosis occurred as a result of long-term waterlogging of developed soils with high spatial heterogeneity,normally able to absorb high quantities of groundwater.The areas unaffected were scree fields and rocky bastions characterized by adequate drainage.All of our findings support the primary stages of large-scale permafrost thaw,i.e.,correlating increases in soil moisture with increasing permafrost active layer thickness.     

Comparison of Two Methods for Assessment of In Situ Jet-fuel Remediation Efficiency

In 1997, total petroleum hydrocarbon (TPH) remediation started at a former Air Force Base, which operated from 1940 to 1991. TPH had been released to soil and groundwater at the site by military activities. The TPH was 70% jet fuel and the affected area covered 28 ha. Remediation involved a combination of technologies, including removal of volatile organic compounds using soil vapor extraction and air sparging, free product vacuum recovery and aerobic biodegradation of organics with oxygen supplied by the air sparging system, along with nutrient addition. The primary remedial method was found to be biodegradation, which has removed 93% of the contaminants from the site to date. A significant aspect of the remedial action was performance monitoring, including documentation of remediation efficiency. The goal of the research was to assess the relative accuracy of methods commonly used for monitoring in situ TPH remediation. Two such methods were selected for the research: monitoring change in soil TPH concentration (specified as non-polar extractable substances) and monitoring respiration activity in soil with a subsequent stoichiometric mass balance to estimate the mass of TPH destroyed. The study demonstrated that both of the methods provided comparable results regarding the effectiveness of in situ TPH remediation, despite the fact that their methodologies are very different.