s those described in the chapter on water conservation, your ET bed may never overflow. Now let's assume that you have a tight soil with a percolation rate of 120 min per in. according to carefully performed percolation tests in the disposal field area. Let's further assume that you can't dig deep enough to get below it because bedrock is just 3 ft below the soil surface. The Manual of Septic Tank Practice says that soil which percolates water any slower than 60 min per in. is unsuitable for any type of soil absorption system, and 120 min per in. is on the very edge of acceptability by the EPA Manual. Finally, let us assume that 186 ONSITE WASTEWATER DISPOSAL Table 7.2 ETA System Loading Capacities Percolation Rate Loading Rate (min/in.) (gpd/ft2) 60-120 121-200 201-300 301-600 601-1000 your site is perfect for an ET system. You are a good candidate for an ETA system. The first item you must address is if you will be violating any regulations by putting in an ETA system whose bottom is only 1 ft over bedrock. Let's suppose that you can convince the local officials that 1 ft of tight clay will filter sewage as well as several feet of sand, and you get permission to install the ETA system. In deciding how big to make the bed, you must realize that even though the water runs out of the percolation test holes very slowly, the soil will accept a certain proportion of your sewage output. The question is how much and over what period of time. Table 7.2 shows loading rates that can be assumed for the absorption portion of the sewage disposal process. It is very important to realize that these absorption capacities are too conservative under some conditions and too liberal under other conditions. System failures in very tight soils appear to relate less to loading rate than to construction practices, rainfall, and surface drainage capacity. Because many of the variables affecting ETA sys- tem performance are very difficult to control, it is wise to have a backup plan in case the ETA system doesn't work out. To complete th