BEsT MANAGEMENT PRACTICEs HANDbooK - Tahoe BMP

EFFECTIVENESS CONSIDERATIONSDry basins primarily rely on particle settling as a pollutant removal mechanismto improve the quality of stormwater that passes through the dry basin.Consequently, their effectiveness for removal of fine sediment particles andother pollutants of concern is questionable. In terms of treated effluentquality, dry basins are considered less effective relative to other stormwatertreatment BMPs that employ filtration as a pollutant removal mechanism, suchas wet ponds or media filters. However, dry basins also provide stormwatervolume reductions from evapotranspiration and infiltration, which can providesignificant reductions in pollutant loading. Furthermore, dry basins arerelatively easy to construct when adequate land is available, and are typicallyless maintenance intensive than BMPs relying on filtration of stormwater forpollutant load reductions.The Lake Tahoe BMP Monitoring Evaluation Process 13 analyzed results from fiveperformance monitoring studies conducted on Tahoe Basin dry basins andnoted the following:• TSS concentrations were consistently reduced, with at least a 68% eventmean concentration (EMC) reduction in TSS reported from influent toeffluent.• Total organic nitrogen (TKN) and particulate phosphorous (PP)concentrations were consistently reduced from influent to effluent.• Performance for reducing dissolved nutrient concentrations was highlyvariable. Based on review of the data, the author’s theorized that there maybe an achievable effluent quality limit, below which dry basins cannotprovide treatment for dissolved nutrients when influent concentrations areat or below: Nitrate-Nitrite (NOx) < 250 ug/L; Ammonia (NH4+) < 50ug/L, Soluble Reactive Phosphorus (SRP) < 50 ug/L; and DissolvedPhosphorus (DP) < 80 ug/L.• Fine sediment particle concentrations were not analyzed among the fivestudies reviewed.132NDNATURE, 2006, Tahoe Basin BMP Monitoring Evaluation Process: Synthesis of Existing Researchprepared for USFS Tahoe Basin Management Unit.Chapter 4: BMP ToolkitTRPA BMP Handbook4.4.1.3-Dry Basin 2012Page 6