THE EFFECT OF TERRACE GEOLOGY ON GROUND WATER MOVEMENT AND ON THE INTERACTION OF GROUND WATER AND SURFACE WATER ON A MOUNTAINSIDE IN NEW HAMPSHIRE

The geologic composition of glacial terraces affects ground water movement and the interaction of ground water and surface water in hillslope settings. The west watershed of Mirror Lake in New Hampshire contains four terraces that are at different altitudes and have different geologic compositions. The lowest terrace has 5 m of sand overlying 8 m of till, the two next higher terraces, one of which is uphill of the other, consist entirely of sand, and the highest has 2 m of sand overlying 8 m of till. All terraces overlie fractured crystalline bedrock. Numerical models of hypothetical settings having terraces on a valley side indicated that the presence of terraces cause local flow cells to develop and that the flow differs based on the geologic composition of the terraces. Terraces consisting entirely of sand had more ground water flow from the bedrock to the glacial deposits than terraces that had till beneath the sand. The presence of these types of flow patterns were confirmed at Mirror Lake by head measurements from nested piezometers and packed-off fracture intervals in the bedrock, which were constructed on each of the terraces.

Stream W turns part way down the hillside and passes between the two sand terraces, essentially transecting the movement of ground water down the slope of the valley side. Head data from transects of wells in the stream’s floodplain above, at, and below the sand terraces indicated that the stream gains ground water on both sides above and below the sand terraces, but where it flows between the sand terraces it gains ground water on its uphill side and loses water on its downhill side. The loss of water from the stream affected the ground water chemistry beneath the lower sand terrace. Passive collector bags placed in the hyporheic zone on the gaining and losing sides of the stream during a time of baseflow conditions indicated that sulfate reduction is taking place on the uphill side of the stream and methanogenesis on the downhill side. These processes are related to the presence of organic-rich sediments in the hyporheic zone and has resulted in an anoxic zone down the ground-water gradient from the stream. Chemical analyses of water samples from wells in the lower sand terrace have confirmed the anoxic conditions in the ground water.