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Memorandum 2 <br /> Impvervious Limits <br /> February 9, 1990 <br /> use scenarios. Except for Scenario (7) , which is the County' s <br /> new proposal, all were calculated with the lot size/impervious <br /> surface associations used previously (4% for 5-acres, 6% for 2- <br /> acres, etc. ) . As noted in the _ Table, total imperviousness for <br /> Scenario (7) was computed with the corresponding averages <br /> reported by the County. Chlorophyll concentrations for that <br /> scenario were not computed directly, but were interpolated from <br /> the graph in Figure 2. <br /> Figure 1 is a plot of these same data points, except for Scenario <br /> (7) , which was omitted for the reasons cited above. All were <br /> calculated directly with the CDM model. The figure is a good <br /> illustration of the general relationship between imperviousness <br /> and water quality: degradation (chlorophyll ) increases with <br /> imperviousness. ` <br /> Figure 2 is an expanded view of a portion of Figure 1. It shows <br /> the regressive effects on water quality resulting from <br /> incremental changes to CDM' s original 5-acre recommendation. It <br /> also provides an estimate of the water quality effects of the <br /> County' s proposed higher impervious limits. The 26+ ug/L- <br /> chlorophyll interpolation is based on two assumptions: <br /> 1. A direct relationship between total imperviousness 'and <br /> - chlorophyll in the range displayed by Figure 2; and, <br /> 2. Linearity between the two data points bracketing the <br /> "FEB PLAN. " <br /> Given the consistent pattern of Figures 1 and 2, that shows <br /> chlorophyll increasing with imperviousness, the estimate for <br /> Scenario (7) appears to be reasonable. <br /> What Does It All Mean? <br /> As noted in the CDM report, all future land use scenarios will <br /> degrade University Lake. Figure 3 shows deterioration in terms <br /> of the lake' s present condition, as represented by chlorophyll. <br /> University Lake is substantially worse than the Cane Creek <br /> Reservoir. Even the original 5-acre CDM recommendation (Scenario <br /> 3) represents a 17 percent deterioration in water quality - a <br /> level that is supposed to represent the "minimal degradation" <br /> goal. <br /> After adjusting the original 5-acre recommendation to reflect <br /> Chatham County' s 2-acre zoning, grandfathering all existing lots <br /> of record, allowing property owners to create up to 5 two-acre <br /> lots, and finally mitigating the administrative and political <br /> burden of stringent impervious limits, the "February Plan" <br /> (Scenario 7) represents a future water quality that is 36 percent <br />