A dry well is a dry excavated pit, backfilled with aggregate, usually pea gravel or stone. It is used as an infiltration system for receiving and temporarily storing runoff from rooftops in a single lot. Discharge of stored runoff occurs through infiltration into surrounding soils, allowing for filtering and recharge of groundwater to occur. Dry wells reduce impervious runoff volume and rate thereby reducing flooding and erosion downstream.
Dry wells can be constructed in two different forms, either a structural chamber that is assembled or inserted into an excavated pit or an excavated pit filled with aggregate. It is important that the location of the dry well is adequately planned so that it does not cause basement seepage or flooding or ponding at the ground surface. Dry wells should drain runoff within 72 hours and therefore must be sized with consideration of both drainage area (1 acre maximum) and soil type (sandy soils will drain much more quickly than clay dominated soils). Additionally, the bottom of the well should be at least 2 feet above the seasonal high water table and be as level as possible in order to uniformly distribute infiltration to the surrounding soil. Dry wells can fairly easily be installed by homeowners.
An infiltration trench is an excavated trench that has been back filled with stone to form a subsurface water catchment area. Stormwater is diverted into a trench and stored until it can be infiltrated into the soil over several days. Infiltration trenches are used for collecting surface water runoff from a few lots, whereas dry wells are only used for single lots. To do this the trench surface is depressed or a berm is constructed on the down-gradient side of the trench. These structures are most effective when accompanied by some sort of pretreatment, such as swales or vegetated filter strips. As a result infiltration trenches reduce runoff volume and rate of flow, thereby reducing flooding problems, while simultaneously recharging groundwater. Since infiltration trenches help control stormwater runoff from several lots they are often used in large parking areas or along roadways.