Delineation of salt contamination in patterned ground

In this study we examined the distribution of salt contamination in hummocky and polygonal permafrost terrain of the Mackenzie Delta region. We hypothesize that contaminant movement in patterned ground is preferentially along trough features. Analysis of soil chloride distribution adjacent to salt-contaminated sumps showed that in polygonal terrain, water-soluble contaminants were more abundant in ice-wedge troughs that were likely enlarged by site disturbance. Sites in hummocky terrain exhibited more uniform chloride concentrations between hummocks and troughs that may be linked to reduced microtopographic differences due to terrain disturbance. In patterned ground, a sampling plan that accounts for surface microtopography will likely better delineate salt contaminants. RÉSUMÉ Dans cette étude nous avons examiné la distribution de contamination de terrain charactérisé par une abondance de hummocks et polygons de pergélisol de la région du delta de Mackenzie. Nous présumons que le mouvement de contaminant en terre modelée est préférentiellement le long des dépressions. L'analyse de la distribution de chlorure de sol à côté des bassins à boues de forage a prouvé que dans le terrain polygonal, les contaminants hydrosolubles étaient plus abondants dans les dépressions glace-coincent qui ont probablement été agrandis par perturbation d'emplacement. Les emplacements dans le terrain charactérisé par une abondance de hummocks ont montrés des concentrations plus uniformes en chlorure entre les hummocks et les dépressions qui peuvent être liés aux différences microtopographic que réduites dues à la perturbation du terrain. En terre modelée, un plan de prélèvement explique la surface microtopographic extérieur tracera probablement mieux des contaminants. 1 BACKGROUND AND OBJECTIVES Patterned ground is ubiquitous in the Canadian arctic and refers to the systematic arrangement of microrelief that is usually the result of freezing and thawing soil (Williams and Smith 1989). Patterned ground includes plot-scale landforms that are formed by cryoturbation, such as earth hummocks and high center ice-wedge polygons (Williams and Smith 1989, van Everdingen 2005). Both landforms result in site microtopography characterized by elevated land surfaces (Eg. hummock or polygon center) surrounded by a network of interconnected troughs that concentrate drainage and influence water movement (Lawson 1986, Quinton and Marsh 1998). Patterned ground is also associated with small-scale variations in soil texture between trough and inter-trough features that affects their permeability and influences subsurface drainage (Quinton and Marsh 1998, Barnes and Filler 2003). In the thaw season, water movement occurs preferentially through the slightly depressed, highly permeable, saturated layer of the interconnected trough features (Lawson 1986, Quinton and Marsh 1988). For instance, in hummocky terrain, relatively impermeable, clay-rich hummocks deflect water flow into interconnected, organic-rich troughs and in some cases water may flow perpendicular to the topographic gradient. Water-soluble contaminants associated with spills are often highly soluble and easily transported by water through the soil, so that mechanisms controlling water movement also control contaminant movement (Alberta Environment 2001). Soil sampling strategies to delineate contaminants are often based on coarse-scale topography (Lund and Young 2005); but, this strategy may be less effective in patterned ground because it does not account for preferential water flow associated with surface microtopography and variation in soil characteristics between trough and inter-trough features. We hypothesize that in patterned ground, specifically high-center polygons and earth hummocks, watersoluble contaminants move preferentially through troughs. In northern Canada, salt contamination has been associated with seepage from drilling-mud sumps (Dyke 2001). Briny circulating fluids used in exploratory hydrocarbon drilling are often encapsulated in sumps that are designed to remain frozen, but several studies have noted unfrozen conditions and the movement of drilling-muds away from the sump through the active layer (Dyke 2001, Jenkins et al. 2008, Kokelj et al. 2010). We used saline seepage from drilling-mud sumps as a tracer to study contaminant movement in patterned ground. The objectives of this paper are to 1) evaluate the vertical and lateral distribution of salts in patterned permafrost terrain, 2) compare and explain differences in the distribution of salts in hummocky and polygonal landforms. The goal of this research is to contribute to