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08/22/95 TUE 09:43 FAX WOODWARD CLYDE 0 <br /> to upstream as well as downstream areas of a watershed. The advantage of on -site <br /> infiltration over on -site detention is that it reduces total runoff volumes in addition to <br /> reducing runoff peak flow rates, and it permits more efficient utilization of the <br /> downstream conveyance system. One of the disadvantages to infiltration is that <br /> depending on the soil types, soil infiltration rates, and the frequency of maintenance <br /> conducted (i.e., removal and replacement of the filtration media or the top layer of soil), <br /> infiltration facilities may contribute to the contamination of groundwater. In other words, <br /> the problem may only be transferred from surface waters to groundwaters. <br /> Regional Facilities: Regional detention basins are generally considered to be more <br /> effective for flood control because they are designed using a watershed perspective. The <br /> problem of accumulated combined flows from numerous on -site detention facilities is <br /> therefore eliminated. A disadvantage of regional detention is that if located in <br /> downstream areas, it fails to control flooding in upstream headwater areas and, like on- <br /> site detention, regional detention reduces peak flow rates but fails to reduce total runoff <br /> volumes. <br /> Infiltration facilities, as opposed to detention basins, may be constructed on a regional <br /> basis, however they are considered less effective than their on -site counterparts because <br /> soils are usually less well drained in the downstream areas of a watershed, and runoff <br /> volumes are larger and hence more unmanageable for infiltration. As described above for <br /> on -site infiltration facilities, groundwater contamination issues should also be considered. <br /> 33 Effectiveness of Facilities for Water Quality <br /> On -site Facilities: One of the main advantages to on -site facilities for water quality is that <br /> they provide treatment of stormwater close to its source (i.e., in the upstream areas of <br /> watersheds). For example, if infiltration facilities are used (as described above for flood <br /> control) these facilities can filter pollutants and improve the quality of runoff before it <br /> ever reaches the streams. Several on -site best management practices (BMPs) strategically <br /> targeted to high pollutant source areas will likely be more effective (with respect to water <br /> quality treatment) than a large regional facility treating stormwater from a mix of low and <br /> high pollutant source areas. <br /> One of the organizational difficulties associated with requiring on -site controls for water <br /> quality is that the effectiveness of various controls will depend upon site - specific <br /> conditions (e.g., soil types, groundwater table, slopes). Therefore, a jurisdiction will need <br /> to certain amount of planning time to determine what types of contrg will be <br /> most appropriate in what areas. For example, dry wells may be appropriate for providing <br /> infiltration in some areas, whereas in other areas they may be discouraged because the <br /> soils are relatively impermeable, or the site is located over a drinking water source, or the <br /> site has potential slope stability problems. <br /> Regional Facilities: Since regional facilities are usually publicly owned and maintained, <br /> a more focused effort can be directed towards monitoring and adjusting the facility to <br /> 6 <br />