APPLYING THE DISSOLVED OXYGEN ALTERATION METHOD WITH HYDRAULIC ISOLATION TO MEASURE WATER QUALITY IN INDIVIDUAL FRACTURES IN OPEN WELLBORES

To accurately assess contaminant migration in fractured bedrock, it is necessary to obtain water quality samples from individual transmissive fractures intersecting a wellbore. Spatially averaged samples (such as with low flow or tap sampling) do not account for water quality changes with depth and consequently fail to produce a three-dimensional evaluation of contaminant distribution. This can result in an inaccurate water quality assessment and potentially ineffective remediation.

This study demonstrates a method for assessing water quality in individual fractures in open crystalline bedrock wellbores utilizing naturally-occurring flow conditions in the well. The technique uses the dissolved oxygen alteration method (a borehole dilution technique), hydraulic isolation using a single control pump, and a simple sampling device. The method was demonstrated in two wells containing one and two transmissive inflowing fractures, and has been described conceptually for nine bedrock wells with varying flow conditions containing one, two, or three transmissive inflowing fractures. The method can provide preliminary information for long-term monitoring, and offers a low cost means of sampling discrete zones in wells where permanent or long-term multi-level sampler installation may not be feasible. Furthermore, the technique can identify fractures that may be an ideal candidate for sealing or long-term monitoring if contaminant concentrations are particularly high.