Agenda 05-28-2002 - Laserfiche WebLink

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u Recharge for the Morgan Creek basin (upstream from White Cross and Chapel Hill) is higher than any other basin, explained in part by the soil properties in'this area that allow high infiltration rates (sometimes known as "Chapel Hill gravel'). The Haw River basin (Bingham Township and the northwestern edge of the County) and New Hope River sub-basin (northern and eastern Chapel Hill) has the lowest recharge, the latter largely due to the presence of Triassic sedimentary rocks and soils with low infiltration capacities. Finally, the report offers a method of using the recharge rate data for ground water management planning. The ultimate limit on ground water availability is the rate of recharge, i.e., the amount that can be sustainably withdrawn without overdrafting the supply. Two examples are provided that offer ways to translate the recharge rate data on a basin-by-basin approach to determine the recharge area needed and the sustainable lot size, given a known water demand. In summary, the report found that there are significant reserves of ground water in the County, but that the rate of recharge varies from one geographic area to another. Although ground water supplies are currently sufficient, the report notes the critical importance of proper management, conservation and planning for use of ground water supplies for the future. B. 2001 Ground Water Resource Investigation Where the 1996 Recharge Rate report looked at the County's ground water availability using streamflow records by basin, the 2001 Resource Investigation augments this report with actual well data. The new report provides a baseline on well yield in the County, from data gathered from 649 wells. These data include yield, depth, diameter, location, casing depth, and water level. Table 3 on page 11 of the USGS report offers a synopsis of the water availability findings. Well yield (in gallons per minute) ranged dramatically, as has been known for many years by those involved in well drilling and inspection. The lowest yield well in the 600-plus sample produced only 0.1 gallons per minute (gpm), while the high yield well produced 240 gpm. The average well in the sample was much lower, however, averaging 17.6 gpm. The depth for wells in the sample likewise varied significantly, from a minimum of 24 feet to a maximum of 805 feet. The average well depth was 208 feet. The well yield and well depth findings allowed USGS to perform sensitivity analysis to see what type of correlation well yield had to construction practices and siting of wells — and to well yield per foot of depth. 15