Using Construction Storm Water as a Resource – What We Learned From the 2019-2020 Rainy Season
Years of challenging weather patterns including droughts and floods have forced the construction industry to shift our management of storm water runoff at construction sites away from being thought of as a waste, to being reused as a resource. What once was a priority to design a site in a way to very efficiently route storm water off-site via a storm drain system, we now find ourselves designing creative approaches to retaining storm water runoff on-site, even during construction. This discussion will evaluate several benefits of storing construction storm water on-site for reuse, to include incorporating it into the erosion control sprays, dust control practices, irrigation use, infiltration, and other design elements.
One practice commonly used in the past was to dewater ponded storm water by pumping into the storm drain system. This activity would entail installing a sump pump and pumping water out of a containment area either directly into a storm drain cleanout or off-site. In doing so, the City of San Diego’s Storm Water Standards Manual, dated October 2018, provides strict criteria, to include sampling the effluent to ensure it does not exceed 20 nephelometric turbidity units (NTUs) as well as advance notice to the City. The 20 NTU criteria effectively prohibits the ability to discharge ponded storm water from a construction site as the threshold is incredibly low as compared to the 250 NTU criteria of the California Construction General Permit.
Another option that was explored was to dewater into the sanitary sewer. This method promotes greater philosophical questions, such as is construction storm water to be treated as a waste, that will be conveyed through a labyrinth of treatment systems to include treatment at the Point Loma Wastewater Treatment Plant or to the City of San Diego’s PURE Effect clean drinking water program. The City of San Diego’s regulations allow for either a single use batch discharge permit, or an ongoing discharge permit that requires the use of pretreatment systems. These systems, with mobilization and maintenance costs in the tens of thousands of dollars, typically include the use of frac tanks and various sand or bag filters.
The 2019-2020, with its back-to-back storms, motivated the construction industry to develop management practices to treat storm water as a resource instead of a waste that would require construction storm water to be stored on-site for a period of time until it will be reused. These practices took careful consideration into the potential for vector breeding habitat by incorporating ‘mosquito donuts’ and inspection criteria to monitor ponded water and promote the circulation of water. Vector Control Plans were incorporated into the SWPPP that would discuss these management practices.
Some of the reuse options explored was to retain water on-site and use it to mix with erosion control spray (in lieu of potable water). With the CGP requirements of incorporating erosion control for any inactive areas (ie undisturbed for more than 14 days) and for appropriate active areas such as prior to rain events (for Risk Level 2 and 3 projects), construction sites can use gallons upon gallons of potable water to mix with soil binders, hydraulic mulches or hydroseed. This discussion will go into a case study that details approximate potable water use (in quantity and price) that is commonly associated with erosion control sprays. Other successful reuse practices included substituting ponded storm water for potable water for dust control and compaction practices, as well as promoting ground water recharge through infiltration. As we find ourselves currently challenged to evolve our industry quickly during changing times, we also found ourselves challenged during the rainy season of 2019-2020 to evolve our management of storm water away from a waste and more into a resource.