An evaluation of the use of natural stable isotopes of water to track water movement through oil sands mine closure landforms

Abstract

Surface mining of oil sands results in extensive land disturbance, earth movement and water usage. After mining, the disturbed landscapes must be reconstructed and reclaimed as natural landforms. There are numerous challenges associated with understanding the responses of these landforms over time, including a need to track and characterize water movement through closure landforms to understand the hydrological responses of these landforms over time. This study attempted to use natural stable isotopes of water (δD and δ18O) to identify and characterize source waters from various closure landforms at an oil sands mine site.

The study area is Syncrude‟s Mildred Lake mine, an open pit oil sands mine located in northern Alberta. A variety of groundwater, surface water and soil samples from a variety of landforms (overburden dumps, composite and mature fine tailings areas, tailings sand structures and freshwater reservoirs) were collected in an attempt to fully represent the isotopic distribution of waters across the mine site. Laboratory analysis of δD and δ18O was done on all samples.

The local meteoric water line first established by Hilderman (2011) was redeveloped with additional precipitation data and calculated to be δD=7.0(δ18O) -18.6‰. A natural evaporation line having a slope of 5.3 was calculated for the mine site with samples collected from three surface water ponds on the mine site.

Five primary source waters were identified on the mine site: process affected water/tailings, rainfall, snow, interstitial shale water and Mildred Lake water. It was found that these sources of water generally have unique natural stable water isotope signatures. Process affected water at the site generally had an enriched signature compared to other mine waters. The enrichment was attributed to fractionation from the recycle water circuit and natural evaporation.

The characterizations of these source waters were then used in several hydrogeological examples to demonstrate that natural stable water isotopes can be applied to water balance estimates and to identify water movement processes related to closure landforms.