The Bronx and Manhattan

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The Bronx has been the subject of limited geologic explorations and, as a result, few geologic reports (Bulletin GW-32, 1953, Geology in the Bronx and Richmond Counties ... by N.M. Perlmutter and T. Arnow) exist that cover the geology of that borough. Similarly with Manhattan, the existence of a "city" in these boroughs has limited the amount of classic geologic studies that can be made. Charles Baskerville has produced a set of engineering geology maps that cover both the Bronx and Manhattan. Mr. Baskerville has used engineering data including data developed from the numerous tunneling projects, such as water tunnels, subway tunnels and power lines. Data from foundation borings for numerous projects such as building and bridge construction were also used. The mapping is intended to assist engineers in foundation design and is not intended to be a definitive geologic investigation. Mr. Baskerville does not devote much space to the identification and in depth description of geologic units (lithology). Instead, he does provide generalized descriptors for the various rock types that exist in the Bronx and Manhattan. More importantly, however, Mr. Baskerville has mapped the existence of bedrock features that bear more relevance to geothermal system. The locations of bedrock faults, bedding contacts and strata folds have been mapped. These features can improve the chances of developing higher than average yielding wells, which will, in turn, reduce the amount of drilling necessary to develop the necessary thermal exchange for the project. Please note that for "closed loop" systems the existence of highly productive fracture zones is irrelevant.

EXPLANATION OF MAP SYMBOLS SUBSURFACE ENGINEERING STRUCTURES

---Old PitMl fi:-il Aquectucïi— rorniur li shown incomfllace núnh û(

Mflíor Dtcgí n fcypresswoy because It .runenlly Is not In ust

SuWuv ' (or) rtihmui tunnel CtinsylitNiH'íl I i'iui Ciiin£hin> '..A- 1 unnel -----Sewer tunnel

FORMER DRAINAGE AND SHORELINE

Form« drainage and shoreline—In bkie. Shown only where different from present drainage and shoreline. Areas formerly under water are shown by dot pattern. Where extremely smalt, former ponds Are lahelSod "p " Straight line segments probably are furrows and ditch« buift to tower ihe water labfo

Former awarnp or marsh —Ln blue. Da?.hed line deflins where jmajjup or ma»h adjoins higher ground, Extremely small occurrence are labelled

GEOLOGY-SURFACE AND SUBSURFACE

_ Contact between geologic unit*—Dashed where approximately located;

dotted where under water; queried where uncertain Where shown sofid under water, was located by test borings and tunnel data

- v r ■ FiMih-Psiwd arrows show relative movement; U, upthrown side; D, 0 down thrown side- Dashed where approximately located; doited where underwater. riufiriod u/hftre unc#ruin Wh<tro iuwn in tunnal, arrow *howt Inclined dip, and short line normal lo fault trace shows vertical dip

—*-A— Thrust fault—Sawteeth on upper plate Djwhed whoro approximately located; dotted where under water When.1 seen fn tunnel b shown solid Alternating soUH and open saw&mth indicate thru« inults coincident in map view fnear Roosevelt (Welfare] Island: see sheel I) ~~1*—Ovenumed thrust fauh —Sawteeih are on lower plate, but point In direction trf movement of overturned tipper plate, bars are on upper plate <see sheet

1, cross section A-A') Crush or shear rout encountered in underground workings— Shows dip where known

Contours on the bedrock surf»«—Based on seme datum os topographic contours. Closed, hachured areas indicate depressions. Contour interval

10 A

f Single outcrop or area of closely spaced outcrops

The above map is a section of the Baskerville Bronx map centered on the Mosholu Park in north central Bronx. The map symbols are explained to the right of the map. As can be seen, this map contains a wealth of information usable to the designer of geothermal systems. The blue letters (A,B and C) indicate hypothetical locations considered for a geothermal system. Location "A" is in an area that indicates significant fracturing, shown by the black line that runs from the upper left corner of the map to the lower center of the map. The line is labeled with U/D at several locations. The U infers that that side of the fault is displaced upwards with respect to the other side of the fault, labeled D. In addition to the main line discussed above is a shorter east west fracture that is partially obscured by the letter A. Wells at the intersection of two, or more, fracture systems typically produce more ground water than wells that are located in areas free of fractures or adjacent to single fractures.

Site A is located over the mapped path of one of the City's water tunnels. Before attempting to drill a well in any location in New York, it is imperative that the possible existence of any underground utilities be discovered. Utilities can include currently active water tunnels, sewers, telephone and power lines, subway lines and steam pipes. In addition to active utilities, "old "unused or abandoned utilities may be found that may include any of the above listed. Therefore, before the drilling starts, the One Call Center for New York and Long Island should be called. Their number is 1-800-272-4480. For additional information and to setup safety seminars call 1-718-631-6700. They also have a WEB page with the address: WWW.OCUC.net. The One Call Center will mark the project site with the location of "member" utilities. A list of their members is provided on the WEB page. The New York City agencies, such as sewer and water, are not members and need to be contacted separately. Site B on the above map is not directly on any mapped faults or fracture systems. The results of drilling at this location should produce a well with an average yield of 20 gallons per minute, or less. However, the key term above is "mapped" fracture system. Just because a fracture system is not mapped it does not mean that a fracture system does not exist at that location. Generally a fracture is mapped when there is evidence of its existence, such as a linear valley, bedrock outcrops, or data developed from subsurface work. When such evidence is not available, the geologist may not know of the existence of a fracture system and, therefore, cannot map it. Therefore, the preliminary design should be for a standing column well, assuming that a low yield well will be drilled, but sufficient flexibility should be built into the design to allow for an alternate design if a high yield well system is drilled. Similarly, in location A, the existence of mapped fractures does not guarantee the production of high yield wells and the system design should be sufficiently flexible to allow for alternate configurations.

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