An examination of Mactaquac's artesian aquifer and pressure relief system
University of New Brunswick
Of primary consideration in the construction of the Mactaquac Hydro-electric dam was the response of the artesian aquifer. The presence of this artesian aquifer, containing artesian head to about 11 meters above river level, required some method of depressurizing to prevent heave during construction. Concern was that foundation heave would occur upon dewatering of the dam site. To ensure against this, a permanent system of six pressure relief wells was designed for the aquifer. These wells have provided sufficient drawdown during construction as well as long term artesian pressure relief. Over the last 25 years the flow, or yield, from the wells has decreased by 50% (Hanscom, 2000). Little has been substantiated as to why this has occurred. Video inspections indicate clean well screens which are free of obstructions; flushing of the wells has not significantly increased well yield. The Dumas-Neyrpic current meter used to measure vertical flow in the wells has not been calibrated since its purchase in the late 1960's. NB Power wants to know i f equipment error is influencing the downward trend of the flow data, as they want to ensure that what they have been measuring is the accurate yield from the aquifer. The objectives of this report were to: examine the hydraulic properties of the Mactaquac artesian aquifer and its relief system, construct a vertical flow model which simulates the well flow under artesian influence and calibrate the Dumas-Neyrpic borehole current meter. In order to effectively calibrate the vertically mounted current meter, a vertical flow system was simulated. The constructed model has a 1:2 scale and produces an maximum upward flow of 945 m3/day, which is approximately 50% of the yield from the relief wells at Mactaquac. For comparison, the current meter was also tested in the University of New Brunswick's hydraulic flume. The flume produces a continuous horizontal flow in a rectangular trough. The results of the calibration confirmed that vertical flow testing in the constructed model yielded better results than horizontal testing. Calibration in the vertical model found that the current meter is over-registering velocity by 10%. This is twice the value stated by the manufacturer, but the deviation is still within reason. That being said, the velocity and therefore the yield for each well is actually 10% lower than what is measured using the Dumas-Neyrpic current meter. The 50% decrease in yield that has been noted over the last 25 years is in actuality a 60% decrease. It has been concluded that the Dumas-Neyrpic borehole current meter is not responsible for this downward trend in data.