Subsoil Guidelines for Green Zone Sites, Proposed Exclusion Depths for the Ecological Direct Contact Exposure Pathway at Remote Alberta Green Zone Sites

The Alberta Tier 1 guidelines document allows the exclusion of the ecological direct contact exposure pathway from the Tier 1 guideline value for petroleum hydrocarbon fractions F1 to F4 at depths greater than 3 m. Following discussions with key regulatory personnel in Alberta Environment and Sustainable Resource Development (ESRD), it was determined that there may be scope for this exclusion depth to reflect more closely the maximum rooting depth for trees and shrubs at forested sites that are remote from human habitation. This project was initiated to develop defensible maximum rooting depths for typical Alberta coniferous and deciduous tree species and associated shrub understory. The findings of this document are applicable to sites in the Green Zone of Alberta.

The approach taken to determining maximum rooting depths was firstly to compile and summarize all relevant rooting depth data from the literature, and then to conduct an extensive field verification program across the Green Zone of Alberta. This document reports the results of both these tasks.

Literature Review

Relevant species were identified by considering characteristic tree and shrub species native to forested natural regions of Alberta. Available information on rooting depth was compiled for relevant species. Available information on the distribution with depth of soil invertebrates and soil microbes was also compiled. The data were used to develop conservative values for the maximum rooting depth for coarse and fine soils, for all species in forested areas of the Green Zone.

Field Verification Study

A total of 16 sites with fine grained soils across the Green Zone of Alberta were identified for verification of the maximum rooting depth values developed in the literature review. Trenches were excavated at each of these sites adjacent to mature trees and the maximum rooting depth recorded for each.

Summary and Application

Maximum effective rooting depth values developed in this project for trees and other plant species in the Green Zone of Alberta, based on literature and new field data are:

  • 1.5 m for fine grained soils; and,
  • 3.0 m for coarse grained soils.

Available data presented for soil invertebrates and soil microbes indicate that the vast majority of these biota are present in soils at depths less than 1.5 m.

PTAC Proposed Eco Contact Exclusion Depths – Green Zone February 2013

The ecological direct contact exposure pathway in the Alberta Tier 1 guidelines can be excluded at 3.0 m for F1 F4 hydrocarbon fractions at any site. The data provided herein provide a scientific rationale for excluding the ecological direct contact pathway at 1.5 m at sites that meet all of the following conditions:

  1. The site is located in a forested area of the Green Zone of Alberta.
  2. The site is fine grained.
  3. The site is on public land administered by Alberta Environment and Strategic Resource Development (ESRD).
  4. The site is located remote from human habitation and future disturbance of the subsoil is not anticipated.
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