e bed sizing process, simply calculate the loading rate permitted if you were to use only evapotranspiration, then add the appropriate loading rate from Table 7.2. If the soil has a percolation rate of 120 min per in., then the absorption capacity of the soil would be used by loading it at a rate of 0.2 gpd/ft2. Since 120 min per in. is at the extreme limit of the percolation rate range which accepts 0.2 gpd/ft2, split the difference to the next lower loading rate (0.1 gpd/ft2 and use a loading rate of 0.15 gpd/ft2. Then if we calculate a loading rate of 0.04 gpd/ft2 depending on evapotranspiration alone, we simply add the two rates to get the final rate to be used in sizing the ETA bed. LOW-IMPACT SYSTEMS 0.15 gpd/ft2 + 0.04 gpd/ft2 = 0.19 gpd/ft2 If we expect a flow of 300 gal per day, the area required for the ETA bed is 300 gpd 0.19 per ft2 = 1579 ft2 This is only 21% of the surface area needed for an ET system with no absorption. Again, a caution is in order. Remember that, in the example, bedrock was just 3 ft below the surface? That meant we would not use deep trenches. Wherever possible, deep trenches should be used in tight soils in preference to ETA systems. They are more reliable and cheaper. On a more positive note, ETA systems can be used over a broader geographic area than ET systems and have been made to work from Canada to Mexico. One more variation of the ET system deserves to be mentioned. Under some circumstances, the volume of sewage to be discharged onto a site is prohibitively large compared to the site's ability to accept sewage. In this situation, an ET/overflow system might be a solution. The idea is that an undersized ET bed is designed to get rid of most of the discharge, but small volumes are allowed to overflow into an absorption trench during certain periods when evapotran- spiration is low. The disposal trench acts as a safety valve to prevent sewage from ever coming to the surface. Check with the health department. Materials 1. ET liners of 10- to 12-mil polyethylene sufficient to 