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

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9 A correlation between recharge rates and the underlying hydrogeology is not immediately apparent,except in the Triassic basin in southeastern Orange. Among the reasons for this is the fact that all of the basins contain multiple hydrogeologic units. Infiltration capacity of the soil, topographic relief, and depth of channel incision also play an important role. For example,the streams in the Morgan Creek basin are deeply incised, and significant topographic relief occurs in the basin. The mean recharge rate generally increases as one moves downstream in basins because of these factors. Use of Recharge/Storage Data for Plannino Information on the amount of ground water recharge and the amount of ground water in storage can be used to help in long-term ground water management planning. The ultimate limit to ground water extraction is the rate of recharge to the system. Ground water storage can sustain wells during dry periods, but long-term pumping at rates in excess of recharge will eventually cause well yields to decline.To ensure that this"carryover"between recharge and storage works, the location of wells becomes very important.Locating wells in areas with thick saturated regolith is likely to produce the best results. Examples are provided in the report that illustrate how recharge rate data can be used with projected demand to estimate needed recharge areas.Both use the data generated in the recharge rate analysis of the 12 basins in Orange County. However,the report notes that using the basin- wide average recharge rates may not work in every case,especially for small tracts. The first example assumes that no site-specific data are available for the creation of single- family lots on wells and septic tanks.This example uses the calculated mean recharge rate in combination with assumptions about the anticipated number of household occupants,per capita water demand,amount of impervious cover and consumptive loss of wastewater on site to generate an estimate of the area needed for recharge in the Cane Creek basin. It should be noted that some of the assumptions in this example related to typical household occupancy,per capita water demand,change in infiltration capacity and consumptive loss of wastewater in on-site treatment systems are conservative and thus may be subject to challenge and re-interpretation.Others factors may call for an additional adjustment that could increase the recharge area.These issues are summarized as follows: • 1990 Census data indicates that the typical rural Orange County household contains approximately three persons per family,which represents a potential twenty-five percent reduction in water demand from the example. • Water consumption data can be produced to show that typical per capital domestic water use may be nearer to seventy gallons per day,which represents a potential thirty per cent reduction in water demand. • It can also be shown that when on-site waste treatment systems-particularly ground absorption systems-are utilized,the waste water itself is a source of ground water recharge. That is,total water use does not represent a complete or consumptive loss of water from the ground water system. Some unknown percentage of water used,perhaps as much as fifty to eighty percent, is returned to the ground water system through the nitrification field.At the 6