B-6195 08-08 Soil Storage and Infiltration System Justin Mechell, Extension Assistant, and Bruce Lesikar, Extension Program Leader for Biological and Agricultural Engineering, The Texas A&M University System soil storage and infiltration system collects rainfall runoff from the roofs of buildings and directs it underground Awhere it infiltrates into the soil. The system consists of gutters and downspouts to collect roof runoff, a catch basin to capture trash and fine particles, underground trenches that store the water while it soaks slowly into the soil, and an observation port to aid in maintenance. When the trench is filled with water during a storm, the excess water flows from the gutter and onto the ground surface. A soil storage and infiltration system decreases the volume of run- off, contains potential pollutants, and increases the amount of water entering the ground to re- Figure 1. Rainwater soil storage and infiltration system charge our groundwater systems. components. Conserving and Protecting Water As more land is developed for new houses and In its natural state, the land can absorb much of businesses, the problem becomes worse be- the rain that falls. Water absorbed into the soil cause development covers much of the land helps recharge groundwater supplies. Growing with impervious surfaces. Rainwater that falls urban populations are straining groundwater on parking lots, buildings, driveways and roads supplies by using water faster than it can be re- is not absorbed by the soil. Instead, it runs off plenished. into storm drains, streams and rivers. Stormwa- Feasibility of a ter runoff increases urban flooding and erodes Soil Infiltration System the banks of rivers and streams. Table 1 shows just how much runoff is generated from imper- It is feasible to install a storage and infiltration vious surfaces during common storms. system at many homes and businesses. Most buildings already have gutters and downspouts, and at many sites the storage and infiltration Table 1. Volume of rainwater captured in a 100-square-foot trench can be located relatively close to build- area (assuming 100 percent collection). ings. The location of the system and its proxim- ity to other structures is discussed in the next Rainfall amount Volume Volume (inches) (ft3) (gallons) sections. 0.5 4.2 31 There are sites where a soil storage and infil- tration system should not be installed. For ex- 1.0 8.3 62 ample, installing such a system in heavy clay 1.5 12.5 94 soil is not a good idea because clay swells when 2.0 16.7 125 it is wet and shrinks when it is dry. This can 2.5 20.8 156 put stress on foundations and cause major prob- lems. 3.0 25.0 187 A system should not be installed where the land slopes toward the building. If the land slopes An average-size house has a 1,500- to 4,000- away from the structure, pumps and additional square-foot impervious roof. Thus, you must piping are not necessary and water moving un- multiply the volumes in Table 1 by at least 15 to derground will not be not trapped by the foun- estimate the amount of water a typical roof will dation. shed. Areas with no slope can be used if the system is In addition to the flooding it can cause, excess designed properly. However, on land that slopes rainfall runoff has properties that can harm the steeply away from the building it is not feasible environment. Rainwater is heated by the pave- to install such a system because of the depth of ment and roofs on which it falls. When this excavation that would be necessary. heated water enters a stream or river it raises the temperature of the water in the stream and Installing a system in an existing landscape also speeds the growth of bacteria. This, in turn, might not be feasible because of the disruption depletes the amount of dissolved oxygen in the it would cause. It is best to install a system be- water. Oxygen depletion can kill some fish and fore any landscaping is done. other aquatic organisms. Selecting a Site Urban runoff also carries many pollutants into When choosing a location for a soil infiltration streams and rivers. Water flowing over roads system, consider geography, topography, vegeta- and parking lots picks up oils, heavy metals and tive cover, separation distances, water move- other chemicals that leak from vehicles. Water ment and soil characteristics. flowing over yards picks up excess fertilizers, chemicals, animal wastes and grass clippings. Geographic Location These pollutants also harm aquatic life. The geographic location of the site determines the climate and rainfall patterns. Almost any Water is a valuable resource and every effort location can be suitable for a rainwater soil stor- should be made to conserve and protect it. In- age and infiltration system if it is properly mon- stalling a soil storage and infiltration system is itored and maintained. In cold climates, there one way to do that. may need to be a cover over the catch basin and insulation around it to keep water from freez- 2 ing within the system. In the northern United Onsite wastewater treatment system separation States, systems must be installed below the frost distances can be observed as a conservative depth if they are to function during the winter. measure to ensure safe operation of the system. Limited guidance is currently available for sepa- Topography ration distances from soil storage and infiltra- Topography is important because rainwater in- tion systems to landscape features and drinking filtration systems are usually gravity fed. Thus, water system components. Therefore, separa- the surrounding land must slope away from the tion distances associated with onsite wastewater building and foundation. Otherwise, serious treatment systems are shown in Table 2. foundation and ponding problems can occur if the soil near the building is periodically satu- The proper separation distance between the rated. infiltration trenches and structures depends on the soil texture, the soil moisture, and the sub- Vegetative Cover surface hydrology of the site. In areas where There should be vegetative cover over the in- infiltration is rapid (sandy soils) and moisture filtration trenches at all times to help remove is transported away from the foundation, infil- moisture from the soil and to prevent erosion. tration trenches may be as close as 5 feet from However, it is not wise to have woody plants or a structure. In areas with clayey soils, greater trees growing directly over or near the infiltra- distances will be necessary, if an infiltration sys- tion trenches because their roots are attracted tem can be used at all. to water and can plug the piping in the system. A distance of at least 25 feet downslope from Separation Distances foundations is suggested by Applied Ecologi- The system must be at the correct separa- cal Services, Inc. in their Fulton Neighborhood tion distance from other objects to protect the Rainwater Management Fact Sheets (2002). structural integrity of nearby buildings and to prevent possible water contamination problems. Water Movement and Soil Characteristics Table 2 shows the separation distances required To accurately design an infiltration system you by the Texas Commission on Environmental must know the water movement (where the Quality. water is coming from and where it will go) and soil characteristics of the site. Table 2. Separation distances between rainwater soil storage and infiltration systems and other site features (adapted from Table X of Title 30 TAC Chapter 285, TCEQ 2005). to soil absorption Distance from systems (feet) Public water wells1 150 Public water supply lines1 10 Wells and underground cisterns 100 Private water lines 10 Wells (pressure cemented or grouted to 100 ft. or pressure cemented or grouted to water table if water table is less than 100 ft. deep) 50 Streams, ponds, lakes, rivers and creeks (measured from normal pool elevation and water level); salt water bodies (high tide only) 75 Foundations, buildings, surface improvements, property lines, easements, swimming pools and other structures 5 Slopes where seeps may occur 25 1 For additional information or revisions to these separation distances, see Chapter 290 of this title (relating to Public Drinking Water). 3 Soil texture (which affects permeability) and There must be at least 2 feet between the bot- soil profile influence water movement. A soil’s tom of the infiltration trench and an imperme- textural class is determined by the sizes of the able layer or groundwater. To make sure a site inorganic particles (sand, silt and clay) it con- is suitable, dig an observation hole 2 feet deeper tains and the percentage of each size particle. than the bottom of the proposed infiltration trenches. Then look at the different horizons of Figure 2 shows the soil texture triangle with the the soil profile and note any impermeable layers different soil classifications. Sands have very and any gray soil colors that may indicate the coarse particles, while silty or clayey soils have presence of groundwater. very fine particles. Sandy soils are permeable, which means that water moves through them You can look up the soil texture and soil profile rapidly. Soils with lots of clay or silt restrict the in your area in the Natural Resource Conserva- movement of water. tion Service (NRCS) Soil Survey Reports (avail- able at your county Extension office). Or, you Acceptable soils for a soil storage and infiltra- might seek help in analyzing your site from tion system include sand, loamy sand, sandy someone with advanced knowledge of soils, loam and loam; all others are considered imper- such as an NRCS employee. meable and should be avoided (EPA, 1999).