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Assessing Groundwater Behaviour in Fractured Rock

Terrascan was contracted by Geo-Logix to provide fracture mapping to assist with defining the vertical and lateral extents of trichloroethylene (TCE) in groundwater within fractured bedrock.

Terrascan's Optical and Acoustic Televiewers were crucial in demonstrating that groundwater flow is contained within low angle bedding partings within the Nowra Sandstone. Due to pinching out of bedding partings there is some lateral spreading of the TCE along strike direction. This has resulted in a plume that is wide and circular, as opposed to an elongate plume.

(Above: Extent of contamination plume)

The Televiewers have also identified the depths where bedding partings are more common and open to flow within the bedrock. The partings close and become less frequent with depth as rock becomes more competent. Importantly, they were able to identify steeply dipping joints which are considered vertical migration pathways connecting the low angle bedding partings.

“The interesting thing about the camera work is that it supports our conceptual model. We have noted that the groundwater plume flows north (uphill) and then takes a sharp left-hand turn and heads towards Bomaderry Creek. The northward migration of the plume is controlled by low angle bedding partings. This plume is very clearly truncated to the north and then turns west.” – David Gregory (Geo-Logix Director)

(Above: Terrascan conducting optical and acoustic imaging)

The plume direction change led Geo-Logix to investigate Bomaderry Creek to look for Geological Structures. They found that the creek cuts through the massive sandstone in a particular section that correlates well with the joints observed in the Televiewer work. In a particular location, the creek trends south along a joint plane, then does a small dog leg, then heads south again on the same joint plane. The joint has been offset by a dog leg in the Creek. Geo-Logix believes the dog leg is an east west trending fault. When the fault is transposed back to the site along the strike it lines up perfectly with the direction change of the groundwater plume. Therefore, they believe the plume heading north is truncated by the fault which then moves the groundwater west towards the creek. Upon inspection of the creek Geo-Logix found a spring halfway up the hill, approximately at the same R.L as we see the greatest TCE concentrations in groundwater. The spring was sampled and found to contain TCE from the site, 800m away.

“This is a major find and like the Televiewer work demonstrates the importance of structural analysis when assessing groundwater in fractured rock.” – David Gregory (Geo-Logix Director)

(Above: Groundwater flow along low angle bedding partings in sandstone.)

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