VERTICAL ADVECTIVE AND DIFFUSIVE SOLUTE TRANSPORT IN A BOREHOLE, ALTONA FLAT ROCK (CHAZY, NEW YORK)

Altona Flat Rock Field Site, north of Plattsburgh, NY, has over 25 wells drilled into Cambrian Potsdam and Altona Formations. The Potsdam Formation (Ausable Member) is a feldspathic sandstone/conglomerate. The Altona Formation consists of sandstones, shales and dolostone. Post glacial flooding from Lake Iroquois denuded the landscape, leaving exposed bedrock over much of the site. The primary focus of this study was on well 102 with a depth of 140 m, which fully penetrates the Potsdam and Ausable Formations and extends about 20 m into the underlying Precambrian basement rock (diabase and meta-anorthosite).

This well has been studied since 2012 when a transition front was identified in borehole water. This transition front is defined by low specific conductivity water above the front (≈100 μS/cm) and high specific conductivity water below the front (≈250 μS/cm). We hypothesize that movement of the front is driven by changes in groundwater flows in different fractures to and from the well. These changes may be affected by the seasonal hydrologic cycle. For much of the year the zone moves between two fractures intersecting the well at depths 24 and 34 m. However, in the summer the front moves much deeper in the well.

During fall 2014 we measured vertical flows in the well using a heat-pulse flow meter. Below the 34 m deep fracture, flow was upwards (≈0.5 L/m). Flow was below instrument detection limit between the two fractures. This suggests that the upward movement of the transition zone is diffusion driven. Downward movement must be advective, although we have not monitored flows as the transition front moved downwards. Simple advective/diffusion models support these interpretations.

Wells nearby, even those intersecting fractures common to well 102 do not exhibit the transition zone observed in well 102. This suggests that processes affecting the development and movement of the transition zone are occurring deeper in the well from deep fractures. Analysis of major cations from water samples below and above the transition zone show that water sources are different. Relative percent concentrations of major cations show that differences in specific conductivity above and below the front is not due to dilution or evaporation. Water below the transition front shows evidence of dolomite dissolution from the Altona Formation.