GROUNDWATER MOVEMENT IN A SAND AND PEAT AQUIFER AND ITS EFFECT ON ROAD SALT MIGRATION

We studied the stratigraphy and hydrogeology of an aquifer composed of sand and peat to predict the movement of road salt from an adjacent road during the winter and spring of 2009. The sand was deposited in the form of parabolic dunes during the late Pleistocene. Sphagnum peat moss bogs formed in depressions between the dunes, with peat thicknesses ranging from 0 – 8 m. The peat overlies and is adjacent to dune sand. Where the peat overlies the sand, downward-moving organic acids from the peat have formed a gray leached zone in the sand below the peat and a lower semi-cemented zone. The cement is derived from precipitated Fe-oxides that originally coated the sand grains. A combination of the low hydraulic conductivity of the peat and possibly the presence of the semi-cemented zone allows the bogs to have hydraulic heads higher than surrounding areas. Shallow groundwater flow directions appear to be radial away from the bog. Within the bog, head gradients are downward.

We installed a series of 12 piezometers into the sand aquifer between the road and the bog. The piezometers were installed to a depth of 3 m and spaced at 0.3 m intervals. We measured heads and sampled groundwater periodically from November 2008 to April 2009, with more frequent sampling in the spring when the seasonal snowpack melted. The samples were analyzed for major anions using ion chromatography. Our results showed that there is a regional gradient away from the bog during most of the study term. However, periodic reversals of the head gradient caused chloride from road salt to move away from the road in an up-gradient direction. The short-term reversals of the gradient appear to correlate with melting and precipitation events in which large amounts of water flowing from the road surface enter the aquifer and cause a temporary mounding of groundwater near the road. The mound dissipates with time, but it allows chloride from road salt to move in unexpected directions and at higher rates.