Intra- Inter-Storm Variability in Effluent Turbidity from Passive Treatment Systems (PTS) from Construction Development Projects

摘要

The promulgation of the new Effluent Limit Guidelines (ELG) for the Construction and Development (C & D) industry issued by the U.S. Environmental Protection Agency is a wake-up call to the construction industry. The ELG requires that outfalls from C & D sites be monitored for a numeric turbidity standard for precipitation events that are equal to or less than the 2-year, 24-hour storm. The EPA guideline specifies that the maximum daily average from each outfall should not exceed 280 Nephelometric Turbidity Units (NTU). Enforcing a performance threshold for construction runoff forces the industry to re-evaluate its current arsenal of treatment methodologies, or Best Management Practices (BMPs). Many of these BMPs are also categorized as Passive Treatment Systems (PTS). Passive Treatment Systems (PTS) include conventional BMPs that rely on settling and filtration to reduce turbidity. In order to meet the goals under the ELG, understanding the performance efficacy of non-proprietary and proprietary erosion and sediment controls is imperative. Turbidity readings from construction-site PTS can widely vary, both during a single storm event, as well as from event to event. Can engineers design treatment trains that can dampen the vast majority of runoff turbidity from a given storm, in order to consistently maintain the overall daily average below the 280 NTU limit? This work introduces data from two separate field test sites in Middle Tennessee, initiated in 2009, in cooperation with Williamson County Government, the Tennessee Department of Transportation, and a large geotextiles manufacturer. Comparative studies at both field sites, utilizing automatic samplers and flow meters, evaluating turbidity readings from multiple storm events discharging from various types of PTS, are currently under evaluation. The PTS units chosen for evaluation for these field tests include straw wattles, rock filters, and a proprietary geocell material with shredded wood mulch. Data from both intra-storm variations in turbidity readings from each PTS as well as turbidity variations between storm events from these systems are presented.