Water Balance Modeling to Demonstrate On-Site Compliance Option Criteria
Increasingly stringent discharge limitations coupled with increased demand on water supply are motivating Dischargers to evaluate alternatives to minimize or eliminate storm water discharge as a compliance strategy. In addition to reducing pollutant loads in discharges, storm water capture and infiltration alternatives provide long-term regional benefits such as groundwater recharge, increased water supply, and reduced hydromodification effects.
The Industrial General Permit (IGP) was recently amended to incorporate total maximum daily load (TMDL)-based effluent standards and on-site and off-site compliance options which incentivize storm water capture, diversion, and infiltration to reduce pollutant loads to impaired waterbodies. In general, Dischargers which satisfy the compliance option criteria will be deemed in compliance with the IGP effluent limitations. As a result, Dischargers throughout California are evaluating alternatives to infiltrate, reuse, or divert storm water to the sanitary sewer as a storm water compliance strategy.
The storm water diversion system must be designed by a California-licensed Civil Engineer to eliminate discharge of the volume of runoff produced up to and during the 85th percentile, 24-hour storm event with a 24-hour drawdown time or with additional storage volume beyond the compliance storm standard to offset longer drawdown time.
In this presentation I will summarize how the volume-based standard achieves compliance with the mass-based TMDL Waste Load Allocations and how engineers can develop site-specific water balance models to demonstrate how storm water diversion systems satisfy the On-Site Compliance Option design criteria. The water balance modeling approach can be used for volume-based systems such as impoundment tanks, flow-based diversion systems such as sewer diversion systems, and a combination of volume and flow-based systems such as infiltration galleries.
This presentation will also review additional requirements to satisfy the On-Site Compliance Option including methods to adequately evaluate risks associated with infiltration and implementing pretreatment best management practices (BMPs) and monitoring methods to prevent adversely impacting soil and groundwater.
Mehran is a Senior Civil and Environmental Engineer with GSI Environmental Inc. He has over 10 years of environmental consulting experience focusing on developing storm water compliance strategies for a variety of industrial facilities regulated under California's Industrial General Permit for Storm Water Discharges. He has developed site-specific hydrologic simulation models and supported the design and optimization of advanced storm water treatment and infiltration systems. Mehran is a California-licensed Professional Civil Engineer and a Qualified Industrial Stormwater Practitioner Trainer of Record.