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Report: Comparison of DRASTIC and DRASTIC-Fm methodologies for evaluation of intrinsic susceptibility of coastal bedrock aquifers and the adjustment of DRASTIC-Fm Fractured Media parameter, Southern Gulf Islands, British Columbia

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This paper assesses the differences in application of the DRASTIC method adopted by the Province of BC for assessing the intrinsic susceptibility of aquifers in BC by two studies conducted on an area of overlap on Gabriola Island. An additional Fractured Media parameter applied to one study carried out on the southern Gulf Islands was incorporated into original DRASTIC parameters such that the resulting overall susceptibility range would be consistent with the DRASTIC standard range.

Author:  Glenna Erlandson

Date Published:  Mar 2014

Report ID:  42387

Audience:  Government and Public

Regional mapping of the intrinsic susceptibility of aquifers (commonly referred to as aquifer vulnerability mapping) to surface contaminants provides a tool to supply decision makers and water resource managers with quantifiable and visual representations of risk to aquifers. With increased development pressure along the coast of Vancouver Island and of specific interest to this study, the southern Gulf Islands, it has become increasingly important to quantify, analyze and classify risk to aquifers as many residents use groundwater as a main source of drinking water (Denny et al., 2007). The DRASTIC methodology, originally developed by the US Environmental Protection Agency, utilizes seven parameters of groundwater physical characteristics that impact groundwater pollution potential. Making up the DRASTIC acronym, these parameters include: Depth to Water (D), Net Recharge (R), Aquifer Media (A), Soil Media (S), Topography (T), Impact of the Vadose Zone (I) and Hydraulic Conductivity (C) of the aquifer. The DRASTIC method has been applied to regional study areas in BC including areas of Vancouver Island and the southern Gulf Islands where sufficient well data are available to meet parameter inputs. A modification to the original DRASTIC methodology titled DRASTIC-Fm (Denny et al., 2007) incorporates the structural characteristics of fractured bedrock aquifers as an additional influencing parameter. This modified method was applied to a local study of the southern Gulf Islands where structural bedrock is an important element to the hydrogeological characteristics of the islands (Denny et al., 2007). This revision to the methodology produced an overall possible range of intrinsic susceptibility values inconsistent with other DRASTIC studies; DRASTIC employs a 23-230 range, and DRASTIC-Fm employs a 26-260 range. In 2010 a DRASTIC study carried out on Vancouver Island created an area of DRASTIC evaluation overlap with the results of the 2007 study on Gabriola Island. This project compares the results of the two DRASTIC methods used for the study area and utilizes sensitivity analysis techniques to identify the variation of input parameters between the two studies. Parameter evaluation results are then used guide the incorporation of Fm characteristics into the C parameter to re-generate a new intrinsic aquifer susceptibility map that has the same overall range as the rest of the province that was assessed with the original DRASTIC methodology. This document is formatted to represent the consecutive flow of the project phases. Part 1 of this report outlines the methodology and results of comparing the differences in parameter ratings between the two studies. Of particular interest, the study area for Part 1 of this project is the area of overlap between the two studies occurring on Gabriola Island. Part 2 of this report describes the techniques used to reweight the results of the DRASTIC-Fm study to incorporate the Fm rated values into the C parameter rated values and the resulting updated intrinsic susceptibility values for the southern Gulf Islands.

Report Type
  Water Information - Groundwater

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