A DYNAMIC SPECIFIC CONDUCTIVITY FRONT CAUSED BY CHANGES IN RELATIVE GROUNDWATER INPUTS FROM FRACTURES

Altona Flat Rock Field Site, north of Plattsburgh, NY, has over 25 wells drilled into denuded Cambrian Potsdam Sandstone. One well (102) is 140 m deep, fully penetrating the Potsdam and extending into the Precambrian basement rock.

As part of an ongoing investigation, we monitored specific conductivity profiles in well 102. We observed a transition front characterized typically by low specific conductivity above the front with higher conductivity below the front. This front appeared to move between two fractures at depths of 24 and 34 m during the late summer and early fall. During spring, 2013, the front moved to a depth of 70 m.

To better document temporal changes in the front, we installed a pair of specific conductivity loggers between the two fractures at 26 and 30 meters depth in the well. Specific conductivity profiles measure during the fall (2012 and 2013) showed similar patterns in the movement of the front. In late summer, the front was anchored to the fracture at 34 m. The front decreased rapidly in depth to the same depth as the shallow fracture (24 m). However, the specific conductivity loggers showed that the movement of the front between the two fractures involved reversals of the front with higher specific conductivity above the front.

Water samples collected above and below the transition front indicate there is mixing of three end member waters occurring at the wells. In early fall, cation composition showed distinct waters above and below the front. Late in the fall, chemistry suggested greater mixing, with less concentration above the front. We have been able to identify end members for the water above the front; we have not identified one end member for water mixing below the front.

The depth of the transition front appears to be determined by changes in groundwater fluxes at different fractures intersecting the well. Nearby wells, that intersect the same fractures, have a uniform specific conductivity throughout the depth of the well. These wells have no transition front. The mechanics affecting the formation and movement of the transition front are not completely understood. The broader implication of these observations is that in some circumstances ground water quality in wells may change rapidly. We have observed changes in specific conductivity in a 10 m length of this well occurring as rapidly as a few days.