Salt-Affected Wellsite Closure Project, Scientific Rationale

Millennium EMS Solutions Ltd.

July 2016

Executive Summary

Introduction

There are a large number of oil & gas sites with salts in soil that exceed one or more generic guideline values. The nature of generic guidelines is that, by definition, they do not uniquely consider site specific conditions and therefore may indicate the potential for an adverse effect when in actuality there is no such potential. Undertaking remedial activities where no potential for adverse effect exists will result in a negative net environmental benefit. Given these concerns, an alternative method of managing such sites is needed. Petroleum Technology Alliance Canada (PTAC) has initiated a project to look for solutions to these issues for all relevant exposure pathways and land uses. The current document is focussed specifically on issues relating to the ecological direct contact exposure pathway (growth and reproduction of plants and soil invertebrates) in native grasslands areas of Alberta, and develops an Alternate Closure Protocol to identify sites which have no current, and no likely future adverse effects on native grassland ecosystems.

Relevant background data on native grassland ecosystems and on the factors controlling the movement of moisture and salts in prairie soils was compiled and summarized. A regional moisture flux modelling study was completed for south eastern Alberta to identify the predicted variation of water table indicator depth across the region. The term water table indicator depth is used in this document to reflect a water table depth below which salt accumulation at surface is considered unlikely.

Alternative Closure Protocol

An alternative closure protocol was developed in this document for salt-affected wellsites in native grassland habitats in south eastern Alberta.

The Alternate Closure Protocol has three main steps:

Demonstrate that the site conditions are having no current adverse effect on the grassland plant community.
Demonstrate that the site conditions are not likely to have an adverse effect on the grassland plant community in the future.
Demonstrate that the site conditions are not likely to have an adverse effect on any other receptors via relevant exposure pathways.

Successful completion of Step #2 requires a favourable outcome from all three of the following tasks:

use a modelling approach to demonstrate that long-term net upward migration of moisture in the vadose zone is unlikely;
demonstrate that there is no long term tendency for natural salts to accumulate at the soil surface by using the vertical profile of sulphate concentrations in soil; and
demonstrate that there is no long-term tendency for salts to accumulate within the root zone even in cases where there is no salt accumulation at surface.

If all the above steps are successfully completed then the site can proceed to regulatory closure via the Alternate Closure Protocol. Further details on each of the three steps are provided in the document.