SITE INFORMATION AND CORRESPONDENCE

Home Browse Search

Mr. Ben Hall • - 2 - • 1 July 2002 The SWPPP needs to support its assertion that storm water from Area 1 is typically uncontaminated. In addition, the SWPPP must specify how this determination will be maintained. At a minimum, water quality sampling must be conducted regularly to determine the quality of discharge from Area 1. The SWPPP states that it used the Rational Method to determine the amount of storm water discharged from each area. The Rational Method uses two basic assumptions: (1) The maximum runoff rate is a function of the average rainfall during the time of concentration; and (2) The maximum rate of rainfall occurs during the time of concentration. Thomas Debo and Andrew Reese state in Municipal Storm Water Management(1995) that for large drainage areas, the time of concentration can be so large that constant rainfall intensities for such long periods do not occur and shorter more intense rainfalls can produce larger peak flows. In addition, they state that the use of the Rational Method often limits the drainage area to 100 acres and some regulations prohibit its use for areas larger than 20 acres. Therefore, the SWPPP must justify the use of the Rational Method in calculating peak flows for Areas 1-3. In addition, the SWPPP should provide (1) detailed information on how the time of concentration for each area was obtained, and (2)justification for the runoff coefficients used. Area 2 consists of several land application areas. Therefore, peak flow and volume from each sub-region should be determined individually. Since these land application areas also drain areas from upstream, the SWPPP should clearly explain how each drainage area is defined. Table 1 should include this information and the time of concentration of each area and all its sub-regions. 2. Page 4, Stormwater Originating from the Facility The SWPPP states that contaminated storm water runoff from Area 2 would be eliminated by collecting it in a 350-acre foot storage pond. However, the SWPPP also states that when storm water quality is within acceptable limits, storm water can be diverted to a bypass channel and allowed to flow past the retention pond into the natural drainage. Runoff can be discharged only when it will not adversely affect the beneficial uses of the receiving water. Therefore, the SWPPP needs to specify the conditions or criteria when Musco would be able to discharge runoff from land application areas and how Musco will make the determination that discharge would be appropriate. In addition, the SWPPP should outline the control measures Musco would take if discharge of runoff is inappropriate. One way to ascertain that discharge of runoff is appropriate is by containing all runoff in the retention pond and discharging runoff only after tests show that it is appropriate for discharge. 3. Page 10, Description of Potential Pollutant Sources, Petroleum Products The SWPPP states that potential storm water contamination may occur in three specific areas: two areas where runoff is contaminated and collected, and in one area where runoff may be discharged from the Musco property. The SWPPP should clarify the disposition of contaminated runoff from two areas. In addition, the SWPPP should explain how runoff from one area is appropriate for discharge. If the petroleum products are stored in aboveground storage tanks (AGTs) with a storage capacity of