Statewide Assessment of New York’s Karst Aquifers With an Inventory of Closed-Depression and Focused-Recharge Features

The New York State Departments of Environmental Conservation (NYS DEC) and Health (NYS DOH) are concerned about groundwater contamination in the carbonate-bedrock aquifers in New York, especially relating to the unintended introduction of volatile organic compounds (VOCs) and liquid manure to these aquifers. These carbonate rocks form extensive aquifers that transmit, and can yield, water from solution-enlarged fractures, bedding planes, and other openings (Olcott, 1995). Groundwater contamination from liquid manure application (fecal coliform bacteria [E. coli] and nitrate) has been on the increase over the past decade as these incidents have been reported to the NYS DEC. Nitrate is highly water-soluble and thus can move readily through the soil and enter the groundwater system. E. coli bacteria are not water soluble, but because of their small size, can move through larger soil pores and in bedrock fractures, especially solution-widened fractures in the carbonate bedrock. The high water content and organic nature of liquid manure may enhance movement of some contaminants under certain conditions. Carbonate-bedrock aquifers have also been contaminated by improper chemical disposal or spills (i.e., TCE – trichloroethylene).

 

Contamination issues continue to be reported along and within carbonate-rock units throughout New York. The bedrock units include: Limestone – Onondaga, Helderberg Group, Trenton Black River Group, and Beekmantown Group; and Dolostones –Bertie Formation, and Lockport Group (fig. 1). While not a carbonate unit, the Camillus Shale, north of the Onondaga Limestone, from Erie to Madison Counties in western and central New York, has an evaporite (gypsum) unit where gypsum is present as beds or as scattered deposits throughout the upper 100 feet of the unit and has the potential for groundwater contamination issues similar to the carbonate-rock units in New York. The Camillus Shale, can develop karst-like subsidence features due to dissolution of gypsum deposits and can result in localized groundwater contamination via these karst-dissolution features.

 

In 2008, the US Geological Survey (USGS), in cooperation with the NYS DEC Division of Water, initiated a pilot study to characterize the recharge settings of the carbonate bedrock in Genesee County after a number of groundwater contamination issues were reported over the previous decade (Reddy and Kappel, 2010). A series of 1:24,000 maps were constructed showing areas where focused recharge (surface water that can recharge the groundwater system through karst features or other closed-depression features in the land surface) can most readily infiltrate into the karst carbonate bedrock and potentially contaminate the groundwater-flow system (fig. 2). Similar to the Genesee County case, gently dipping carbonate-rock crops out along a 2-to-5 mile wide band that trends east-west for 250 miles across the State (fig.1). Karst features such as sinkholes, swallets, solution channels, and caverns can locally transmit large amounts of groundwater into these formations, which can move quickly and over large distances. Many homeowners, farms, municipalities, and businesses use these carbonate aquifers for potable water.

 

The carbonate-rock units that run from east-to-west in the western and central part of the state (Onondaga-Helderberg and their Dolostone counterparts), along with their associated aquifers are bounded on the south by unconsolidated deposits, bedrock or both, which may be in hydraulic contact with the underlying carbonate rock. This southern “buffer zone” is where karst-like development may occur in these overlying units when they are generally less than 50-75 feet thick. The Onondaga Limestone forms an escarpment at its northern outcrop in several locations across the western part of New York State and is typically near land surface along its escarpment due to glacial erosion. South of the escarpment, the carbonate bedrock becomes buried by progressively thicker glacial deposits (mostly till and glaciolacustrine fine sand, silt, and clay, but locally by sand and gravel). All carbonate units described above are at risk for rapid, focused recharge and possible aquifer contamination. The Trenton-Black River, and Beekmantown Group limestones throughout northern New York have variable regional bedrock dips, different than the units in western and central New York, so they may not have as wide of a buffer zone as the east-west trending carbonate-rock units.

 

Potential focused recharge to carbonate-bedrock aquifers is greatest where the rocks crop out at, or near to the land surface. Sources of recharge include (1) precipitation that directly falls over the aquifer and infiltrates into the fractures, (2) channelized runoff that seeps into the rocks through swallets, sinkholes and fractures at the land surface, and (3) runoff derived from adjacent uplands, generally to the south of the carbonate unit, in the western and central part of the state, which lose water where it flows northward onto the carbonate-bedrock aquifer units. Any contaminant associated with the recharge water would also readily infiltrate into these aquifers.