A Diffusive/Advective Model of a Pore Water Transition Front in a Borehole Connecting two Fractures

We report the findings of a diffusive/advective model of specific conductivity profiles in an uncased borehole.  The well in this study is 140 m deep, fully penetrating the Cambrian Potsdam Sandstone at our field site (Flat Rock, West Chazy, New York).  Over a period of several months we have documented the movement of a specific conductivity transition front.  This transition front is characterized by a change in specific conductivity from 75 to 260 µS·cm^-1.  The transition front moves between two fractures intersecting the well at depths of 24 and 34 m.  We have not observed this occurrence in neighboring wells. Even though some of these wells have fractures common with the well in this study.

We speculate that this transition front is driven by changes in the contribution of groundwater flow to and from the borehole between the two fractures.  Relative percent concentrations of major cations (Ca, Mg, Na, and K) of borehole-water samples collected above and below the transition front suggest different water sources for the fractures.   The water from the deeper fracture shows influence of increased Mg, consistent with dolomitic units in the Potsdam.  Preliminary analysis of the transition front suggest that average vertical flow between the fractures was 0.45 m·sec^-1 (6 mL·min^-1). 

The extended time over which this transition front has been monitored offers us the opportunity to model temporal changes in the flow to and from the borehole through these fractures.  We are developing a diffusive/advective model to characterize the movement of this transition front over time in response to changes in water flux through the fractures.  We will model several different configurations to test multiple hypotheses explaining the transition front.  The model is being developed using Stella ®.