Agenda - 11-27-1995 - C-1-b - Laserfiche WebLink

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034 In the zone of saturation, the size and type of openings or interstices depends in part on the type of rock, and the quantity of water that a rock can hold depends on its porosity and permeability. In sedimentary rocks, porosity is governed by the size, shape, and arrangement of individual grains. In crystalline rocks, porosity is governed by the size and numbers of fractures. On the other hand, the permeability of a rock type is governed by the size and shape of the spaces between grains or fractures, and the degree to which they are interconnected. Topography also has an important bearing on the quantity of ground water that a well will yield. Lower yields can be expected on hills because they are underlain by massive, poorly jointed rock that has few pore spaces for water storage. Lower yields can also be expected because the depth to the water table is greater beneath hills than surrounding valleys and slopes. Finally, a greater percentage of precipitation becomes runoff on hills, contributing more to the recharge of valleys and flats below. Ground water yields are important because 29 percent of the county's housing units rely on this source for home and farm needs. As shown on the accompanying table, most of these units are located in rural areas outside of municipalities. WATER SOURCE All Public System or Township Housing Private ompany Individual Well Units Number Percent Number Percent Bingham 2,310 164 7.1% 2,146 92.9% Cedar Grove 1,485 13 0.90/0 1,472 99.1% Chapel Hill 25,708 22,083 85.9% 3,645 14.1% • Chapel Hill 14,271 14,071 98.6% 200 1.4% • Carrboro 6.108 6,084 99.6% 24 0.5% • Remainder 5,329 1,928 36.2% 3,401 63.8% Cheeks 2,154 1,045 48.5% 1,109 51.5% • Mebane 216 216 100.0% - - • Remainder 1,938 829 42.7% 1,109 57.3% Eno 2,164 493 22.8% 1,671 77.2% Hillsborough 4,069 3,516 86.4% 553 13.6% • Hillsborough 1,783 1,756 98.5% 27 1.5% • Remainder 2,286 1,760 77.0% 526 23.0% Little River 793 20 2.5% 773 97.5% Total 38,683 27,334 1 70.7% 11,349 29.3% Source: U.S. Census of Population&Housing, U.S. Census Bureau, 1990. Ground water in the county is obtained from openings such as fractures or from the overlying porous mantle of soil and weathered rock. Dug wells generally are not deep enough to intersect the water table during long periods of low rainfall. Consequently, drilled wells constructed far below the water table are a more dependable sources of ground water and less susceptible to surface water contamination.