Agenda - 12-02-1996 - 15a - Laserfiche WebLink

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6 streamflow in the county. Recharge rates vary from basin to basin depending on precipitation, topography, soil, land use,etc. Ground water storage is a function of recharge, and of the characteristics of the aquifer. The majority of Orange County is underlain by extensively folded and fractured bedrock which is almost entirely covered by a layer of unconsolidated material called regolith.The characteristics of these geologic layers and their relationship determines the water-supply potential of the ground-water system in the county.The aquifer system in Orange County is composed of four tiers: an unsaturated zone in the regolith(organic soil layers),underlain by a saturated zone; followed by the lower regolith(which contains the transition zone),and finally the fractured crystalline bedrock.The regolith serves as a conduit that slowly channels water down into bedrock fractures. Recharge and Storage Ground water is a part of the hydrologic cycle,which is often quantified as a"water budget." A water budget can be expressed as the following equation: precipitation =evaporation+transpiration+streamflow±change in storage Streamflow and storage are the two areas from which ground water may be found.The two important components of the ground water system are ground water recharge and ground water storage. Ground water recharge occurs when water percolates down through the unsaturated regolith layer to either the saturated layer of the regolith,or further down to the fractures in the bedrock where the water begins to move laterally. The rate at which water does this is expressed as a volume per unit of area per period of time(for example,gallons/day/acre).Recharge rates vary from season to season and year to year,and are affected by changes in precipitation, evapotranspiration, land use and other factors.Most recharge to the ground water system occurs during late fall,winter and early spring. One important distinction about ground water in the Piedmont physiological province is that nearly all ground water storage is in the regolith zone-with very little storage in the bedrock.As previously noted,the regolith serves as a"reservoir"conducting water into the fractured bedrock.The level of storage is affected by the seasonal variations in recharge,and the water level can change by as much as 12 feet between seasons.There is very little annual variation, however,so ground water storage can be assumed to be relatively stable over time.The water table usually reaches its highest point in May or June. The amount of water in storage can be estimated by examining the saturated thickness of the regolith from wells.A previous report on wells in the Piedmont found that the average depth of well casing is 52 feet,although this can vary depending on topography.In general,depth to ground water is greatest beneath hills and ridges and least under valleys and draws. Work in the Piedmont areas of Georgia contains similarity of hydrogeology,such that equations to determine the amount of ground water in storage can be used with confidence. 3