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have failed. This rate is for long-term failures of the septic system <br /> absorption fields and does not include temporary problems such as sewage <br /> backup into a house. <br /> To estimate an average annual failure rate, the time series approach <br /> proposed by the 1986 EPA report "Forecasting Onsite Soil Adsorption System <br /> Failure Rates" was used. This approach considers an annual failure rate <br /> (percent per year of operation) , future population growth estimates, and <br /> system replacement rate to forecast future overall failure rates. <br /> An annual septic tank failure rate of between 2% and 3% Ter year was <br /> assumed for the Lake Michie and Little River Reservoir watersheds. This is <br /> somewhat conservative as literature values reported for areas across the <br /> U.S. range from about 1% to 2%. Population estimates assume an annual <br /> population growth rate of 5% per year. Average household size was assumed <br /> to be 2.6 people. Based upon these assumptions, an overall failure rate of <br /> about 50% is predicted for the period 1960-1987, which is consistent with <br /> the information provided by the Durham County Health Department. The <br /> predicted overall failure rates for the periods 1960-2005 and 1960-2050 <br /> (buildout) are 57% and 62%, respectively. For average annual conditions, <br /> it was assumed that septic tank systems failures would be unnoticed or <br /> ignored for five years before repair or replacement occurred. Therefore, <br /> during an average year, 10% to 15% of the septic tanks systems in the <br /> watershed were .assumed to be failing. <br /> The "impact zone" or -the "zone-of-influence"_for_-failing:septic_tanks--.was <br /> assumed to be limited to areas within 300 ft of a stream. This is based <br /> upon overland flow lengths characteristic of Piedmont watersheds. The area <br /> within the--"impact-zone" as-determined by-multiplying by the 300-ft width <br /> by the total length of perennial and intermittent streams in each <br /> watershed. The "impact zone" for Lake Michie is about 22,300 acres or 21% <br /> of the watershed. For Little River Reservoir, the impact zone is 14,300 <br /> acres or about 23% of the watershed. <br /> 3-25 <br /> • <br />