 per day divided by 61 gal). This is a reasonable frequency for sandy soils. To decrease the dosing frequency for natural soils with slow per- colation rates, simply increase the dosing volume. Ten times the distribution lateral volume per dose is a minimum. You can increase that volume up to the daily discharge volume (the system design flow), although this is not recommended. Such a large dose deliv- ered over a short period of time could overload the absorption sys- tem within the mound or cause leakage at the base of the mound. See Chapter 4 for a discussion of dosing frequencies. Example As a final example, let's design a complete system for a difficult site (refer to Figure 6.9). Assume: slope = 12% design flow = 300 gpd fill depth below excavation = 2 ft fill infiltration rate = 0.77 gpd/ft2 natural soil percolation rate = 42 min per in. This is your dream vacation cottage in the mountains. You had to scrape away all of the sandy loam top soil to make a level site for the house, and you had to blast a hole in the bedrock to set your septic tank and dosing chamber. The area where the disposal field has to go has only 2 ft of loam above bedrock. The state regulations call for a minimum 4-ft clearance. The mound fill will be the sandy loam scraped from the house site. ONSITE WASTEWATER DISPOSAL Figure 6.9 Example mound design. If the natural soil percolation rate was measured at 42 min per in., then the natural soil infiltration rate and the fill infiltration rate are 5 V42 = 0.77 gpd/ft2 Table 6.1 shows us that we should use trenches inside the mound because the slope is close to the maximum permissible. The trench bottom area required is 300 gpd 0.77 gpd/ft2 = 390 ft2 Let's add another 10 ft2 to round off the bottom area to 400 ft2. With this slope, the trenches should be long and narrow. We can select two parallel trenches, each 100 ft long and 2 ft wide. The distance between trenches is 2 ft X 0.77 gpd/ft2 - 2 ft = 0 ft 0.77 gpd/ft2 CONVENTIONAL SYSTEM MODIFICATIONS In other words, we can use a single 100