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Appendix
Geologic Setting
The geologic setting for The Orchards proposed development property (the "Site ") and
surrounding area consist of a bedrock layer that is overlain by a series of unconsolidated deposits
of glacially- derived sediment.
Based on information obtained from the " Hydrogeologic Framework of the North Fork and
Surrounding Areas, Long Island, New York" (2004) prepared by the US Geologic Survey, the
bedrock in the Site vicinity consists of metamorphic rock of Paleozoic and Precambrian age. In
the area beneath the Site, the bedrock occurs at a depth of approximately 550 feet below grade
and slopes gently to the south.
Overlying the bedrock are Upper Cretaceous and Pleistocene -aged sediment deposits consisting
primarily of sand, silt, and clay. The soils beneath the Site are part of outwash plain deposits.
Based on the field survey by John C. Ehlers, L.S. and elevation data shown on the proposed
subdivision map dated June 18, 2019 (see Appendix G of the DEIS), the elevation of the Site
above mean sea level ranges from approximately 19.4 feet (which corresponds to NAVD 88
elevation of 18 feet) at the southwest corner of the site to approximately 11.4 feet (which
corresponds to NAVD 88 elevation of 10 feet) at the southeast corner of the site.
Hydrogeologic Setting
All municipal and private potable water supplies on Long Island are supplied by Long Island's
sole source aquifer. Groundwater in the Site area consists of multiple aquifers. The deepest
aquifer is the Lloyd Aquifer, which overlies the bedrock. The Lloyd Aquifer is overlain by the
Raritan confining clay unit. The Magothy Formation and corresponding Magothy Aquifer
overlie the Raritan unit. The Upper Glacial Formation and corresponding Upper Glacial Aquifer
overlie the Magothy Formation and represent the upper hydrogeologic unit in the Site area. Some
clay layers of limited areal extent may exist within the Upper Glacial formation in the Orient
area.
The Upper Glacial Aquifer is an unconfined aquifer. The lower confining unit of the Upper
Glacial Aquifer is the Glacial Lake Clay layer. The upper surface of the Glacial Lake Clay layer
is estimated to be at a NAVD 88 elevation of -71 feet. The Upper Glacial Aquifer is expected to
have a hydraulic conductivity rate of approximately 200 feet per day and, therefore, the Upper
Glacial aquifer beneath the Site is expected to be highly productive.
Based on test wells installed on site, the water table elevation at the Site area is at a NAVD 88
elevation of approximately +2 feet.
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Groundwater at the Site and throughout Long Island exists as a freshwater lens that floats above
the zone of saltwater that exists beneath the freshwater lens. According to the U.S.G.S.,
freshwater in the Orient flow system lies above the lower confining unit, i.e. the Glacial Lake
Clay layer. The vertical hydraulic conductivity through the Glacial Lake Clay layer is very low
(estimated at 0.1 feet per day). The clay layer acts as a physical barrier between the freshwater
lens above the layer, and the saltwater below. In addition, even if the freshwater and saltwater
layers were not physically separated by the clay layer, due to the density differential the two
layers would not mix appreciably. Instead of a clear line of separation between the freshwater
and saltwater, there is a zone of diffusion that exists between the freshwater and saltwater zones.
Therefore, in the lower portion of the freshwater lens, the chloride concentrations will slowly
increase with depth as the saltwater zone is approached.
The depth to the saltwater interface at the Site can be estimated using the Ghyben- Herzberg
Ratio which states that for every foot of groundwater (freshwater) that exists above mean sea
level, there is approximately 40 feet of fresh water below mean sea level. At the Site, the
groundwater water table is presently at an elevation that is approximately 3.4 feet above mean
sea level (which corresponds to a NAVD 88 elevation of +2 feet). Therefore, the depth to the
saltwater interface is estimated to be at a depth of 136 feet below mean sea level (which
corresponds to NAVD 88 elevation -134 feet). Results from on -site test wells indicate the
groundwater underlying the Site at a NAVD 88 elevation of approximately -53 feet has an
average chloride concentration of 33 mg/l1.
1 Appendix N of DEIS