STABLE ISOTOPE APPLICATIONS TO UNDERSTAND SOIL RECLAMATION COVER HYDROLOGY

Precipitation is partitioned into vertical and lateral flowpaths within hillslopes. In engineered hillslopes, such as reclamation covers overlying mine waste or overburden material, quantifying flow partitioning is key in assessing the cover performance. Approaches to characterizing cover performance include lab testing of materials, hydrological modeling and monitoring of test covers. However, flow partitioning of flow in reclamation covers may undergo significant change over time as a result of weathering (e.g. freeze-thaw and wet-dry cycles) and vegetation growth (e.g. increasing rooting depth and density).

We present the distribution of the stable isotopes of water within three prototype soil covers on a saline-sodic shale overburden dump at the Syncrude Canada Ltd. Mildred Lake mine, north of Fort McMurray, Canada. A main concern in cover performance at this site is the continued supply of salts from the overburden material into the overlying cover, which is greatly affected by the hydrological processes in the covers.

Since their construction in 1999, soil properties, hydrological response to atmospheric and vegetative demands, and vegetation properties have been extensively monitored. The three covers have undergone substantial evolution due to freeze-thaw processes and aggrading vegetation. In field campaigns from 2003 to present, the deuterium and 18-oxygen isotope composition of the soil water, precipitation and collected interflow has been measured to complement hydrometric data. Isotopic signatures have e.g. been used to estimate the fractions of snowmelt water and soil water in interflow (Kelln et al., 2007) and shown how mixing between these two streams increases during the interflow season (Baer, 2014). In our most recent field campaign (June 2014) we found that the soil water in the three test covers showed various degrees of enrichment, with the thinnest cover being most and the thickest cover being least depleted.

In this work, we re-visit the extended dataset to further quantify hydrological processes in the reclamation covers, focusing on inter- and intra-annual patterns. We also explore the extent to which isotope composition can be used to determine the water pools available for vegetation growth at these reclamation sites.