GROUNDWATER TABLE CHARACTERIZATION USING ELECTRICAL RESISTIVITY IN THE EASTERN LAKE ONTERIO DUNES DURING THE 2016-2017 RECORD HIGH LAKE LEVEL PERIOD, OSWEGO COUNTY, NEW YORK

The high-water level on Lake Ontario over the past year has impacted the lake margin habitats for many species (positive and negative), in addition to substantial damage to land and property for people living on the lake shore. Another concern is the impact on the unique sensitive dune environment at the eastern margin of the lake, located on a series of spits between Sandy Pond and the main lake. It is proposed here that the artificially higher lake level has contributed to an increase in the height of the water table within the dunes, thereby establishing newly observed pools and flowing springs, that have resulted in increased dune erosion. These surface effects were observed for the first time on several dunes in the Sandy Island Beach area of Oswego County, NY. In the summer of 2017, the distribution of groundwater within select dunes was examined using non-invasive electrical resistivity methods, to establish baseline information associated with the high lake level. A series of parallel ERT surveys were run over two dunes at Sand Island Beach State Park using an ER meter with automated switching system. Each survey included a Wenner array with 24 nodes, spaced 3 meters apart resulting in 7 data levels and about 10.5 meters of penetration. The depth of penetration exceeded the thickness of the dune deposits and underlying till, and most likely reached the bedrock. The survey locations were chosen to cover the highs and lows of the dunes, with a grid pattern that was used to ultimately produce a map of the groundwater table. The ends of ERT survey lines were georeferenced with GPS coordinates and tied to available air photographs and high resolution DEM’s based on recent LIDAR. The near-surface dune sand produced very high ER values (>1500 ohm-m), that contrast well with much lower values of the saturated sane (100-300 ohm-m). This sharp contrast in ER clearly delineated the groundwater table, shows where it approaches the surface, and correlated with regions of saturated sand and pools. Although further documentation will be needed to reach definitive conclusions, it appears that the high lake level has prohibited the dune groundwater discharge to slow, thereby causing the overall groundwater table within the dunes to rise, reach the surface forming pools, spring and flows that are currently eroding the dunes at a higher than typical rate.