TEMPORAL VARIATIONS IN A FRACTURE CONTROLLED SPECIFIC CONDUCTIVITY FRONT IN A BEDROCK WELL, ALTONA FLAT ROCK FIELD SITE (CHAZY, NY)

Altona Flat Rock Field Site near Plattsburgh, New York (northeastern New York State) has over 25 bedrock wells in the Potsdam Sandstone. Post glacial flooding denuded the landscape, leaving exposed bedrock over much of the field site. The exposed bedrock offers a great opportunity to study groundwater flow through fractures.

As part of a continuing study at the field site, we observed changes in depth profiles of well-water specific conductivity in one well (Well 102) over several months. The change in the conductivity profiles was characterized by a rapid increase with depth in the specific conductivity from 75 to 260 µS/cm. This transition front moves between two fractures at depths of 24 and 34 meters. We have not observed this occurrence in neighboring wells. Even though some of these wells have fractures common with Well 102.

During fall 2012, we studied the movement of this transition front. Approximately every week we logged fluid specific conductivity and temperature using a fluid resistivity and temperature probe with a geophysical well logger. Each time we identified the depth of the transition front and collected water samples above and below the front. Water samples were analyzed for major cations (Ca, Mg, Na, and K) using an ICP-EMS. We also measured hydraulic head, specific conductivity (30 m depth) and water temperature every hour with a data logger.

The depth of the transition front appears to be related to the hydraulic head of the well. As the hydraulic head of the well decreases, the transition front moves upwards. As the hydraulic head increases, the transition front decreases in elevation. We hypothesize that this transition front is an effect of changes in the relative flow of water to and from the well through the two fractures that define the depths between which the front migrates. Cation water chemistry shows that the water below the transition front has elevated percent concentrations (Meq/L) of Mg compared to the water above the transition front. This is consistent with local bedrock geology. The deeper fracture is contributing more water to the well from the basal member of the Potsdam Sandstone. As hydraulic head increase, the contribution of flow from the more shallow fracture increases, resulting in more water from the Ausable member of the Potsdam Sandstone flowing into the well.