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dissolved nutrient load. In addition, the delivery ratio contours were <br /> used to identify critical management zones in each watershed. As shown in <br /> Figure 3-3, the area exhibiting 100% delivery of suspended pollutant loads <br /> (less than a 12-hour travel time to the reservoir) was designated as the <br /> "most critical zone," and the area upstream of the 50% delivery ratio <br /> contour (travel times of about 30 hours or greater) was designated as the <br /> "least critical." The most critical zone extends about 10 miles upstream <br /> from the water supply reservoirs, while the least critical zone begins <br /> about 17 to 18 miles upstream from the impoundments. The most stringent <br /> nonpoint pollution controls (e.g. , land use controls) are recommended for <br /> the most critical zone, while the most intensive land uses and structural <br /> controls are most suitable for the least critical zone. <br /> 3.4.2 FAILING SEPTIC TANK IMPACTS <br /> Most of the residential developments within the Lake Michie and Little <br /> River watersheds rely on household septic tanks and soil absorption fields <br /> for wastewater treatment and disposal. The nonpoint pollution loading <br /> factors for low density residential areas (Table 3-2) , which are typically <br /> served by septic tank systems are based on test watershed conditions where- <br /> the septic systems were in good working order and the septic tanks systems <br /> made no significant contribution to the monitored nonpoint pollution loads. <br /> In fact, septic tank systems typically have a limited useful life <br /> expectancy and failures are known to occur which cause localized water <br /> quality impacts. This section presents a methodology for estimating <br /> long-term and average annual septic tank failure rates, the areas most <br /> likely to be impacted by septic tank failures, and the additional nonpoint <br /> pollution loadings from -failing septic systems located close to waterways <br /> in the Lake Michie and Little River watersheds. -- <br /> A limited number of case histories for-septic-tank failures-were -obtained <br /> from the Durham County Health Department. According to Health Department - <br /> staff, most septic systems that are presently failing in Durham County were <br /> installed in the late 1960's and the 1970's. Typically these systems have <br /> either conventional- or sand filter fields. Local staff estimate that about - _ <br /> 50% of the systems installed in Durham County during these time periods • <br /> 3-23 <br />