A The Hydrogeology of New York City

The geology of the New York City counties is complex and varied. Geologic formations range from Precambrian bedrock the age of which can be measured in billions of years to glacial deposits that are less than twelve thousand years old to land filled areas that were created in the recent past. The stratigraphy of geologic units tends to be ordered with the younger formations resting on top of older formations. The oldest formations, the crystalline bedrock, such as granites and gneisses, form the "basement" rocks, with younger, softer rocks and unconsolidated deposits, such as sand, gravel and clay, resting on top of the basement. In New York City, the basement-rocks are generally inclined so that they are close to the land surface, or are the land surface, in the Bronx and Manhattan and parts of Queens and Richmond and slope downward towards the southeast. Southeastern portions of Queens and eastern portions of Kings counties are covered with sedimentary deposits with the bedrock as much as 1000 feet below the land surface in south Brooklyn and Queens.

If the "project site" is located in Manhattan or the Bronx, it is expected that the geo-exchange wells will be drilled into the bedrock since the likelihood of encountering usable quantities of granular aquifer materials in these counties is unlikely. Conversely, if the project site is located in southern Brooklyn or Queens, several productive unconsolidated aquifers exist in these locations. Therefore, either the project designer has the option of using extraction/diffusion wells or standing column wells.

Site C as shown on the map below is located adjacent to a mapped thrust fault, a fault in which one block (referring to the section of rock on one side of a fault and not to city blocks) is pushed over the top of another block. The symbol for a thrust fault used in the map above is a heavy line with a saw tooth pattern. The points of the saw tooth refer to the downward movement of the fault plane. Since these fault types generally produce shallow fault angles, a well drilled on the downward side of the fault line will encounter the fault plane further down as the location of the well is moved further away (in the direction of the saw tooth pattern, from the fault line. If the well is located on the upward side of the fault, the main (mapped) fault plane will be missed. However, since thrust fault systems tend to have multiple planes (like a stack of papers pushed from the side), it may be possible to develop a productive well on the "wrong" side of the fracture line. Again, just because the fault is mapped does not guarantee a productive well.

The section of the Baskerville Manhattan map, "Bedrock and Engineering Geology Maps of New York County and Parts of Kings and Queens Counties, New York, and Parts of Bergen and Hudson Counties, New Jersey", shown below is for an area north of Central Park. The box on the map indicates an area that is currently being investigated for geothermal potential. An 8 story apartment building with 130 dwelling units is planned for the site. The map indicates that the project site lies over a former swamp or marsh associated with a drainage channel or stream. During drilling of test wells, a productive gravel zone was discovered at the western end of the project site. The gravel was found to be absent less than 75 feet to the east. The presence and absence of the unconsolidated aquifer corresponds very well with the mapping. The mapping also shows a fault zone that crosses Manhattan starting at the Hudson River at 125th street and trends towards the southeast. The mapping indicates that the fault zone does not cross the project site, but turns south, away from the mapped drainage area, avoiding the project site. Several deep wells were drilled as test wells, for this project. The wells ranged from 600 feet to 900 feet deep and had yields ranging from 150 to 30 gallons per minute. It is clear from these results that these wells are tapping significant fractures, even though none are mapped in the immediate area of the project site. This illustrates the need for a thorough and careful analysis of any given site.

Working in bedrock areas, the Bronx and Manhattan and portions of Staten Island, requires design flexibility since the actual outcome of the drilling program cannot be predicted in advance. The system designer should assume that the wells to be drilled will not produce sufficient water for the project requirements and, therefore, the wells should be designed as standing column wells with minimal extraction. If, as described above, high yielding wells are developed, the depth of the wells can be limited in proportion to the yield of the wells.

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