PROBLEM
The valley‐fill aquifer in the lower Fall Creek valley (designated as aquifer 4, fig. 1), within the Towns of Dryden and Groton, was mapped by Miller (2000) and identified as one of 17 unconsolidated aquifers in Tompkins County that need to be studied in more detail. The east end of the valley (near the Tompkins and Cortland County border) is on the backside of a large morainal plug, which is part of the Valley Heads Moraine. A large system of springs discharge from the backside of the moraine and forms part of the headwaters to Fall Creek. The valley‐fill aquifer thins and pinches out to the west (figs. 1 and 2)— where the valley is floored by bedrock and becomes a “hanging valley” to Cayuga Lake trough.
The moraine in the east part of the valley is comprised of a thick sequence of heterogeneous sediments (typically sand and gravel interbedded with layers or lenses of fine‐grained material) more than 200 ft thick. In the west part of the valley, coarse‐grained ice‐contact deposits become overlain by a thick sequence of fine‐grained lacustrine deposits (as much as 200 ft thick) that confines a sand and gravel aquifer in the central part of the valley.
There has been moderate development during the last several decades over the aquifer in the lower Fall Creek valley. The residents in the Village of Freeville and Hamlet of Etna, many businesses, farms, and rural homeowners obtain their water supply from wells that tap sand and gravel aquifers in the valley. The Town of Dryden, Tompkins County, and Cornell University need hydrogeologic information to help planners develop a more comprehensive approach to water‐resources management in Tompkins County. Recent (2016) drought conditions along Fall Creek (the potable water source for Cornell university) further heightened awareness of a need for a better understanding of the groundwater‐surface water connection of this resource.
APPROACH
Hydrogeologic data will be collected and maps will be compiled that depict the aquifer boundaries, generalized geologic framework, recharge areas, potentiometric surface(s), and direction of groundwater flow in the aquifers. A report will describe general aquifer characteristics, groundwater and surface‐water interactions, and groundwater quality.
Individual work elements
Hydrogeologic framework
1. Construct a base map using ArcGIS (or a geographic information system (GIS)) with topography derived from lidar, surficial geology, bedrock geology, and well information.
2. Inventory wells in the valley and enter data into the USGS national well database-Ground-Water Site Inventory (GWSI). Groundwater use from the aquifer will be estimated from inventorying metered pumping data from large pumping wells and by estimating the groundwater use by homeowners that tap the aquifer.
3. Conduct passive seismic surveys to complement well log data and determine the thickness of the valley-fill deposits and the configuration of the bedrock floor that underlies the valley.
4. Install at least two pairs of nested piezometers (two wells in the upper unconfined system and two wells in the lower confined system) to (a) gather subsurface data where there is little or no data, (b) serve as control points (ground-truthing) for interpreting seismic-refraction data, and (c) collect water-level and water-quality data that will determine variations between the confined and unconfined systems. Drilling costs will be paid separately by Tompkins County Planning Department and/or the Towns of Dryden and Cornell University. USGS will help Tompkins County and/or the Towns of Dryden and Cornell University in writing the drilling contract, drilling oversight, and sample collection.
5. Construct geohydrologic sections showing the stratigraphy of the glacial deposits and aquifer boundaries.
6. Compile a surficial geology map using existing maps, well logs, surficial survey data and field visits.
Potentiometric surface and direction of groundwater flow
7. Conduct synoptic groundwater-level measurements in wells in the valley-fill aquifer. The data will be used to construct generalized potentiometric-surface maps of geohydrologic units that compose the aquifer system. The generalized direction of groundwater flow will also be indicated on the maps.
8. Install at least two water-level recorders. Seasonal water-level fluctuations will be represented by hydrographs.
Groundwater/surface‐water interaction
9. Conduct streamflow measurements in several major tributaries and in Fall Creek to determine whether groundwater discharges into the stream (gaining reaches) or the streams recharge the aquifer (losing reaches).
Hydraulic properties
10. Collect and analyze available aquifer-test data (if any) from drillers’ records to determine specific capacity of wells and estimate aquifer transmissivity and hydraulic conductivity (permeability of aquifer materials).
Water quality
11. Collect and compile available chemical data from community and non-municipal community wells from water suppliers, the County Health Department, and the USGS NWIS database.
12. Collect four or more groundwater samples to determine the water quality in the aquifers. Water samples will be analyzed for pH, specific conductance, alkalinity, major ions, and nutrients.
13. Collect five surface-water samples—four from tributary streams and one from the main stream (Fall Creek)—and analyze for the same water quality parameters. These samples will be collected during baseflow conditions and will represent the general chemical characteristics of the groundwater contribution of streamflow.
REFERENCES
Miller, T.S., 2000, Unconsolidated aquifers in Tompkins County, New York: U.S. Geological Survey Water-Resources Investigations Report 00-4211, 1 sheet
