Stormwater September 2012 : Page 31
the temperate climate regions were on the order of 50 to 100 mg/L. Perhaps the performance goal should be 90% for cold climate regions. As indicated in Figure 8, a better effl uent quality can be reached with a larger basin, relative to that needed in temperate regions. It has been found that the TSS in sediment and chloride laden melts settles at about half the rate experi-enced during less-contaminated sum-mer melts. This was observed in small vaults. Differential settling rates may not be a factor in large basins. It is commonly believed that lower performance occurs during melts, but the fi eld data are inconsistent. While this is a legitimate concern, fi eld studies suggest that lower performance may be due more to bottom scour at the en-trance than degraded removal process-es. However, the studies of scour are inconsistent, likely due to differences in the geometry of the entrance. A deep entrance or a hardened entrance can prevent scour. The strength of stratifi cation and its effect on processes within the wet pool are affected by whether the ice remains throughout the winter. Intermittent melting of the ice may result in the re-oxygenation of the basin, particularly with winds, and the re-precipitation or sorption of metals previously dissolved. Thus, the impact of ice cover depends upon the latitude and nearness to large bodies of water that tend to moderate atmospheric temperatures. Biological activity is slower in the winter. However, with little fl ow, the is-sue is not particularly relevant except during the spring melt. There are two biological activities of importance. One is the nitrogen processes. Nitrifi cation (conversion of ammonia to nitrate) and denitrifi cation (nitrate to nitrogen gas) slow signifi cantly below 15 ° C (59 ° F) and essentially cease below 5 ° C (41 ° F). However, ammonia concentrations in stormwater are not at concentrations suffi cient to be toxic to receiving wa-ter biota. Ammonia is relevant only if the discharge is to nitrogen-limited streams. The other biological activity is the formation of sulfi des, which, as with nitrogen processes, are performed by specialized bacteria. Sulfi de is impor-tant for the sequestering of dissolved metals, drawing the metals into the soil. While plant activity reduces signifi -cantly at low temperatures, plants were found to be benefi cial with respect to performance at water temperatures as low as 4 ° C (39.2 ° F). These tests were conducted with small pilot units. A reasonable question is whether poorer performance during winter or spring melts is of concern. At very low water temperature, plants as well as fi sh and other aquatic organisms are dor-mant or absent. The metabolism of fi sh in cold receiving waters is low and there-fore less affected by toxic pollutants. Swales and Strips The issue with fl ow-through swales is dormancy of the grass during the spring melt. Several spring storms may occur before the grass is fully func-tional. As a consequence, grass swales may not be an appropriate as the main Improving Water Quality from Above and Below... at Stormcon 10 See Us Booth Otterbine AERATING FOUNTAINS OFFERING BOTH FUNCTION AND BEAUTY these surface spray aerators are engineered to provide results, and are supported by a 5-year warranty and Pattern Guarantee . Widely used throughout the world, Otterbine’s Aerating Fountains improve and maintain water quality in ponds and lakes by controlling algae and eliminating odors while providing attractive fountain-like displays. To learn more call us at 1-800-237-8837 or visit us online at www.otterbine.com/aerating_fountains Aerating Fountains by Otterbine® September 2012 www.stormh2o.com 31