Saint Mary's College of California
OP-27: Rainwater Management

Saint Mary’s has incorporated control measures similar to the “Minimum Control Measures” identified by the U.S. EPA for Phase II MS4 NPDES permitees. Though Saint Mary’s is not subject to the MS4 Phase II requirements, U.S. EPA’s Minimum Control Measures have provided guidance to Saint Mary’s as it developed its own “Control Measures,” listed below: • College Community Education and Outreach on Stormwater Impacts • College Community Involvement / Participation • Non-Stormwater Discharge Detection and Elimination • Pollution Prevention / Good Housekeeping for Facilities Operation and Maintenance • Construction and Post-construction Site Stormwater Runoff Control Saint Mary’s goal in developing these control measures is to reduce the discharge of pollutants to the extent feasible, considering technological availability and economic practicability and achievability, and to identify activities or structural improvements that help reduce the quantity and improve the quality of the stormwater runoff. The best management practices (BMPs) that have been included were selected by Saint Mary’s to achieve this reduction in pollutant discharge to the storm drain system. BMPs include treatment controls, operating procedures, and practices to control site runoff, spills and leaks, sludge or waste disposal, or drainage from raw material storage. BMPs will be updated as appropriate to conform to emerging best practices and actual experience

Saint Mary’s College has developed a Stormwater Pollution Prevention Plan (“SWPPP”) in order to implement its commitment to reduce the chances of pollutants from entering into the storm drain system from its campus to the extent feasible, considering technological availability and economic practicability and achievability, and consistent with measures being implemented by other similarly-situated academic institutions. Though Saint Mary’s does not fall under Non-traditional MS4 regulations, it has drawn from those regulations and related guidelines and best practices in the developed of this SWPPP. This SWPPP identifies the potential on-campus sources of pollutants that might affect the quality of stormwater discharges and provides best practices in order to reduce non-stormwater discharges in the storm drain system.

No rainwater harvesting is currently being employed at this institution.

Saint Mary’s College has installed several rainwater filtering systems; each consists of the following: • a storm drain system which collects runoff from impervious areas and conveys it to • a vegetated bioretention filter; and • an outlet structure which connects the bioretention filter to the outlet pipe leading to the storm drain system. Each bioretention filter provides flow control and infiltration. Two also provide detention to accommodate the increase in peak runoff in the 10-year storm. Please see below for a brief description of stormwater retention and/or detention ponds employed by Saint Mary’s College. Each bioretention filter functions as a soil and plant-based filtration device that removes pollutants through a variety of physical, biological, and chemical treatment processes. The bioretention filter consists of a vegetated surface on which water may pond to the depth of a few inches (as determined by the elevation of the overflow pipe or grate), an organic layer or mulch layer, a special “sandy loam” soil mix, an underground gravel storage layer (not always used, depending on the design) and a perforated underdrain system. The runoff’s velocity is reduced by being distributed across a wide ponding area and by interacting with the soil medium, vegetation, and soil microbes, as it passes through to the underdrain. In this same process, contaminants are extracted from the runoff. Depending on whether a storage layer is used, exfiltration of the water from the bioretention filter into the underdrain system can occur over a period of days (after significant storm events). In light rains, the water soaks down through the bioretention filter into the perforated underdrain and thence to the overflow structure. In heavier rains, or if the soil is already saturated, water will also pond up on the surface of the bioretention filter and go out through an overflow grate into the overflow structure. In both cases, the water is conveyed through the overflow structure into the outfall storm drain pipe.

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Saint Mary’s College has installed several rainwater retention ponds, referred to as bioretention filters. Each bioretention filter functions as a soil and plant-based filtration device that removes pollutants through a variety of physical, biological, and chemical treatment processes. The bioretention filter consists of a vegetated surface on which water may pond to the depth of a few inches (as determined by the elevation of the overflow pipe or grate), an organic layer or mulch layer, a special “sandy loam” soil mix, an underground gravel storage layer (not always used, depending on the design) and a perforated underdrain system. The runoff’s velocity is reduced by being distributed across a wide ponding area and by interacting with the soil medium, vegetation, and soil microbes, as it passes through to the underdrain. In this same process, contaminants are extracted from the runoff. Depending on whether a storage layer is used, exfiltration of the water from the bioretention filter into the underdrain system can occur over a period of days (after significant storm events). In light rains, the water soaks down through the bioretention filter into the perforated underdrain and thence to the overflow structure. In heavier rains, or if the soil is already saturated, water will also pond up on the surface of the bioretention filter and go out through an overflow grate into the overflow structure. In both cases, the water is conveyed through the overflow structure into the outfall storm drain pipe.

The North Parking Lot structure (total impervious area = 19,097 sf, including concrete curbs, flatwork, etc.) slopes towards a low point. The impervious areas are disconnected from this parking lot by a large swale (property of Central contra Costa Sanitary District), which directs flow into Las Trampas Creek.

Saint Mary’s College has incorporated the following rainwater management technologies and strategies as Control Measure: • College Community Education and Outreach on Stormwater Impacts • College Community Involvement / Participation • Non-Stormwater Discharge Detection and Elimination • Pollution Prevention / Good Housekeeping for Facilities Operation and Maintenance • Construction and Post-construction Site Stormwater Runoff Control Saint Mary’s goal in developing these Control Measures is to reduce the discharge of pollutants to the extent feasible, considering technological availability and economic practicability and achievability, and to identify activities or structural improvements that help reduce the quantity and improve the quality of the stormwater runoff. The best management practices (BMPs) that have been included were selected by Saint Mary’s to achieve this reduction in pollutant discharge to the storm drain system. BMPs include treatment controls, operating procedures, and practices to control site runoff, spills and leaks, sludge or waste disposal, or drainage from raw material storage. BMPs will be updated as appropriate to conform to emerging best practices and actual experience.