alt concentra- tions will kill the plants. Neither concern has proven to be of much consequence. The crust appears to be no serious barrier to the upward movement of the sewage, although some homeowners feel that periodic breaking up of the crust is a necessary precaution. The second concern, that of increased salt concentrations in the sewage killing plants, may or may not represent a real problem. If the local water supply has a high salt concentration to begin with, and if a considerable amount of salt is added from household activities (such as recharging water softeners), nonsalt-tolerant plants will probably die after several years. A solution is to replant with a salt-tolerant species of plant. During periods when evapotranspiration is insufficient to dispose of the effluent, sewage will accumulate in the bed and the level of saturation will move closer to the surface. Excessive rainfall during this period will increase the probability of sewage reaching the bed surface. It is necessary to provide a reserve volume in the bed to provide storage for such reasonably foreseeable circumstances. The LOW-IMPACT SYSTEMS depth of the bed, and the amount of storage obtained by increasing the depth of the bed, is limited by the height to which the capillary action of the sand can draw the effluent to the bed surface. Design There are two basic questions you must ask before you begin to design your ET system. The first is how much money do you have, and the second is how often are you willing to put up with exposed sewage in your yard. The extra capacity you add will depend on the importance which you or the health department attach to these two items. Estimates on the ability of ET systems to dispose of sewage vary from 0.3 gal/ft2/day to 0.03 gal/ft2/day. Variables affecting this ability include climate, quality of sand used, vegetation used, bed siting, and quality of construction. What all this means is that (1) there is no single and simple way to design an ET system that will assure satisfactory performance wher