SAMPLING CVOC IN FRACTURED BEDROCK

Chlorinated volatile organic compounds (CVOC) that contaminate fractured bedrock exist as aqueous phase, sorbed phase, and pure phase. When wells are pumped in fractured bedrock, the water sample collected usually contains only aqueous phase CVOC that flows within from the most conductive fractures which intercept the well. Aqueous phase CVOC within primary porosity, micro-fractures, and low conductive fractures generally is not sampled. In addition, sorbed and pure phase CVOCs are usually not sampled while pumping.

Pumping stresses the aquifer to intercept contaminated water from many surrounding fractures. However, when pumping is replaced by passive diffusion samplers, then the aquifer around the well is not stressed, and the water sample collected comes from the higher conductive fractures near the sampling point within the well.

When rock cores are crushed, immersed in methanol, and analyzed for CVOCs, the extract may contain aqueous phase CVOCs from the primary porosity, micro-fractures and adsorbed phase CVOC from organic compounds (bitumens) and inorganic compounds (clays, zeolites and other minerals).

When CVOCs are detected as pure phase using dye tracers, the CVOCs in many cases represents a newly intersected low conductive fracture in a freshly drilled well.

What are the best methods/practice for displaying CVOC concentrations in a fractured bedrock aquifer? Maps, sections, and graphs visually display the data to show to research team, stakeholders, and regulators the breadth and extent of the contamination. Remediation plans may then follows a logical sequence from initial phase to final phase site treatment.

At the former Naval Air Warfare Center (USGS National Research Site to investigate CVOCs in fractured bedrock) water samples and flux meter samples have been used for aqueous phase sampling in the high conductive fractures. Core samples, packers sampling, and recently flux samplers have been used to detect CVOC as sorbed phase, and as aqueous phase within micro-fractures and primary porosity. Dye tracers and activated carbon felt strips have been used for pure phase detections in macro- and micro- fractures.