WATER RESOURCE PROTECTION YOU CAN COUNT ON
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Client: TD Bank

Challenge
The Hockessin Fire Company (HFD) purchased an adjacent abandoned gas station as part of a plan to redevelop their four acre campus. The site redevelopment included reconfiguration and expansion of HFD's parking facilities, as well as development of a portion of the site for lease to TD Bank as a branch office. As the project's Water Resources consultant, Duffield Associates was challenged to include green technology in an environmentally sensitive WRPA and subsidence area which had unique site constraints.

Solution

The site was located in an area susceptible to sinkhole formation, within a regulated Water Resource Protection Area (WRPA), and near key public drinking water supply wells. County code required a WRPA Evaluation with Environmental Impact Assessment, as well as a Subsidence Evaluation. Duffield Associates' team performed these evaluations. The WRPA Evaluation was required to demonstrate that the proposed development would not impact recharge to critical area groundwater supplies. However, traditional stormwater management facilities were required to have an impermeable bottom to prevent rapid infiltration which could promote sinkhole formation. Furthermore, water quality standards required treatment of the stormwater runoff for water quality prior to any discharge from the site.

As the site stormwater consultant, Duffield Associates' team worked with TD Bank's site designer to develop a stormwater management program that utilizes diffuse recharge stormwater management practices to mitigate the potential for the development of sinkholes at the site while still protecting groundwater recharge volume and quality. Faced with this challenge, Duffield's designers designed and implemented on-site stormwater management practices that included a large rain garden and rain harvesting of rooftop runoff which was stored in underground tanks for use as irrigation water.

The rain garden accepts and treats run-off from the adjacent parking area for water quality. Stormwater that doesn't recharge exits the rain garden into rain tanks underneath the parking lot. These tanks manage peak rate discharges from the site and are underlain by an impermeable liner to avoid unmanaged high volume recharge. "Rain harvesting" of rooftop runoff utilizes a separate series of tanks to capture rainfall runoff from the TD Bank branch. The captured rainfall is then used to irrigate the rain garden and landscaped areas to promote diffuse infiltration. Infiltration testing was performed by Duffield Associates prior to site disturbance and after site grading was complete to insure natural recharge was maintained.

This project is a Storm Water Solutions Magazine Top Project for 2008 and an Honors Award Winner from the American Council of Engineering Companies Delaware.