Maryland State Highway Administration – Environmental Programs Division
- Post-Construction Monitoring
Contract PB-C involved the construction of bioswales, which are stormwater best management practices (BMPs). The sites were designed and constructed as part of the ICC Environmental Stewardship (ES) efforts within the Upper Paint Branch Special Protection Area (SPA). PB-C consists of approximately 70 individual bioswales at 21 sites. Representative bioswales were selected for monitoring.
The PB-C bioswales were constructed as retrofit installations in existing housing developments constructed prior to current stormwater regulation. Design treatment areas for the bioswales were based on site availability and were not necessarily sized to meet current Maryland Department of Environment (MDE) water quality volume standards. Instead PB-C bioswales were designed to provide as much treatment as possible given site constraints in location that lack sufficient stormwater infrastructure.
The overall project goals were to:
- Provide infiltration of stormwater to slow runoff and lessen its affect on receiving streams,
- Increase water quality treatment for adjacent lands,
- Recharge groundwater using increased infiltration,
- Decrease thermal impacts from runoff and increase cooling by groundwater, and
- Show the feasibility of further watershed restoration beyond traditional efforts by implementing smaller projects at a micro-scale level throughout the watershed.
To evaluate the effectiveness of the bioswales in meeting the project goals, temperature and water level data were collected at each of the representative bioswales or bioswale series. A pressure transducer was deployed to measure water elevations within the observation wells at each of the bioswales. Temperature data loggers were deployed to measure inflow and outflow temperatures through the bioswale or bioswale series. The inflow temperature loggers were placed in the driveway culverts at the upstream end of the bioswale or bioswale series. The outflow temperature loggers were placed in the under drain pipe near the outfall of the bioswale or bioswale series. Local precipitation data was collected monthly when data loggers were downloaded to use in conjunction with water level and temperature data.
Recharge rates were determined by measuring the falling head water level elevations in the observation wells as measured by the pressure transducers. Inflow and outflow temperatures were analyzed when the storage volume of the facility was exceeded to determine impacts of the bioswales on runoff temperature. Analysis of this data was then used to develop:
- Infiltration rates that represent periods of recharge within each bioswale for selected storms representative of typical runoff conditions.
- A calculation of the percentage of storms and runoff events, where the facility detained 100% of the runoff it received.
- Representative graphs showing overlays of inflow and outflow temperatures when the storage volume of the facility was exceeded.
- A volume of water treated by the facility for representative storm events.
A Summary Monitoring Report was prepared. The results indicated that during the monitoring period, the bioswales provided significant treatment of runoff. Although it was not possible to provide complete treatment for contributing impervious areas according to current MDE regulations at all sites, the project goals were achieved based on data from the representative sites. Several of the bioswales were sized to current MDE standards and provided water quality volume and water quality treatment for of the contributing imperious areas and exceeded the MDE water quality volume by 200-732%. By meeting and exceeding water quality treatment and water quality volume design goals, these bioswales met all of the design goals for the monitoring period. In most cases, there was no significant difference between inflow and outflow temperatures from the bioswales. However one large storm event showed runoff inflow temperatures as high as 94 degrees Fahrenheit and outflow temperatures were reduced to less than 78 degrees. Overall, the data suggests that the project was successful and it should be considered a model for implementing small scale watershed restoration projects in existing developments.