Sustainable Stormwater Management: Validating Water Quality and Quantity

Transcription

1 Sustainable Stormwater Management: Validating Water Quality and Quantity Progress Report from University of Virginia November 12, 2008 UVA Undergraduate Student Participants Andrew T. Smith Civil & Environmental Engineering Kate E. Abshire Michael J. Downey Nathaniel C. Farrar Environmental Sciences UVA Faculty Participants Joanna C. Curran Teresa B. Culver Civil & Environmental Engineering Janet S. Herman Environmental Sciences UVA Office of Environmental Health and Safety Staff Participants Suzanne M. Perala Jeffrey A. Sitler Jessica S. Wenger Judith Nitsch Engineering Participants Judith Nitsch Nicole Holmes

2 Executive Summary Before the construction of the Dell Pond in 2004, stormwater from the Meadow Creek watershed was diverted into an underground pipe constructed during the 1950s, often leading to flooding along Emmet Street. Today, 1100 feet of Meadow Creek have been daylighted, following an engineered channel and floodplain designed to reduce flow rates and improve water quality. In addition to the riparian reconstruction, a large detention basin in the form of the Dell Pond was constructed. and a wetland area was created downstream of a large sports arena. Construction of the new stomwater management system was completed 4 years ago. Preliminary observations indicate that during high flow events, the pond serves to remove sediment, hold water and then slowly release it over a period of time, effectively reducing the peak flow rate and allowing water to infiltrate to recharge groundwater. Both stream and pond are designed to improve water quality by reducing nutrient levels and sediment load, yet quantitative evaluation of the system s success has not yet been completed. The objective of our current project is to evaluate the effectiveness of the stormwater management system. By implementing a regional stormwater plan, UVA is working to ensure both the protection and remediation of rivers that are currently stressed systems. A useful outcome of working with the designers of this system (Nitsch Engineering) is the opportunity to verify the success of the design for future installations. Additionally, students are being trained in monitoring, data analysis, chemical analysis, and assessment of best management practices for a design feature right on their own campus. Interaction across academic departments and between faculty and staff support a long-term commitment to UVA s sustainable stormwater management.

3 Introduction This project involves the detailed monitoring of Meadow Creek, its reconstructed riparian areas, and the associated newly constructed stormwater controls to verify the efforts undertaken by the University of Virginia (UVA) to control stormwater thereby reducing downstream flooding while improving surface-water quality. With funding from the BSA, a team of 4 students, 3 faculty, and 2 staff of UVA seeks to monitor, understand, and evaluate the actual performance of the installed stormwater management system for both water quantity mitigation and water quality treatment capacity. By collecting stream flow, precipitation, and water quality information, the team is measuring the attenuation of peak discharge rates and the removal of problematic water quality constituents. Whereas this project is not complete and was delayed by an extended drought this past summer, significant progress has been achieved by installation of monitoring sites, deployment of equipment, training of students, and the initial collection of stream discharge and chemical information to support load determinations. The long-term goal is to evaluate the stormwater management practices at UVA with respect to regional BMPs such that validation of sustainable stormwater management practices might be achieved at UVA. Research Methods Monitoring of this surface-water system is being accomplished through the use of both automated samplers and water-level indicators coupled with hands-on verification and calibration. To measure water levels, five stilling wells (Fig. 1) were installed to allow for the continuous monitoring of the water levels in Meadow Creek, the Dell Pond, and the Dell Pond outflow culvert (Photo 1). A Solinst Levelogger was installed in each stilling well (Fig. 1). The Levelogger is a single unit that combines a pressure transducer and a data logger. Each unit records the water level in the stilling well every five minutes (Fig. 2) and the data is downloaded

4 approximately once a month. To covert the water levels or stage heights to flow, stream cross section is measured with meter stick and tape at the same time that water velocity is measured with a Marsh-McBirney Flow Meter (Photos 2 and 3). Such measurements are made over a range of flow conditions and used to create a stage-discharge rating curve (Fig. 3). The rating curve is applied to the Levelogger stage data to determine actual stream discharge at any given time. To evaluate stream response rainfall, local rainfall data (Fig. 2) was obtained from the UVA weather station located in the headwaters of Meadow Creek as short distance from the site. To assess water quality conditions during storm events, water samples were collected and analyzed from two stations, one upstream of the site and one at the outfall where Meadow Creek leaves the site (Photo 4). Sampling upstream of any of the stormwater management feature (Site 1) and at the Dell Pond outfall (Site 5) was accomplished using Hach Sigma 900 automatic samplers that collect 24 samples at programmed time intervals over the course of a single storm event (Tables 1 and 2). A subset of the collected samples was analyzed by UVA staff and students for turbidity and for nitrate, nitrite, and phosphate concentrations within 24 hours of collection (Fig. 4A&B). Preliminary Findings Preliminary evaluation shows that Meadow Creek responds very quickly to modest rain events (Fig. 2). In addition, for a small event in September 2008, all water-quality parameters were dramatically reduced from their upstream concentration (Fig. 4A) to that at the outfall from the Pond (Fig. 4B). Turbidity and phosphate were most dramatically reduced in concentration, but some improvements in nitrate were also noted. Preliminary assessment indicates that the stormwater management features are ameliorating water-quality factors.

5 Ongoing Work and Future Plans Data collection and analysis will continue. As more data from the chemical analysis is correlated with individual storm events, average rates of pollutant removal by the stormwater management area can be determined with greater accuracy. The level of flood attenuation through the system will be better determined as more storms occur and are monitored. With conversion of stream stage data to stream discharge, the chemical concentrations can be used with discharge to calculate load delivered downstream. Load information is what typically constrains management practice for stormwater discharge, so progress toward this type of data analysis is central to our ongoing research. In addition to the current monitoring around the Dell, discharge monitoring will be initiated in the area around the JPJ arena where parking lots are a larger portion of the landscape than in the Dell. Results of the water-quality and quantity analyses between the two individual sites will be compared in order to assess the effectiveness of the individual BMPs. More importantly, the site as a whole will be evaluated for overall performance and for actual versus predicted mitigation levels. Communication of Findings In addition to learning the methods of monitoring and data analysis appropriate to stormwater management systems, the students are engaged in communicating their findings to the UVA community. A poster presentation was made at the Annual Undergraduate Research Forum on November 11, 2008, and another presentation will be made at the Annual UVA Recycles Community Awareness event later in November. All four students were co-authors of the posters.

6 Photograph aerial photograph of the Dell with surveyed monitoring locations. Photograph 2. Faculty and students measure stream profile and water velocity at Site 3.

7 Photograph 3. Faculty and students measure stream profile at upstream Site 1. Deep water conditions necessitate the use of remote measurements to assure student safety. Photograph 4. Staff and student retrieving water samples from the Simga automatic sampler located at the Dell Pond outfall (Site 5).

8 Stilling Well Design Vented locking cap Stream bank Water level Water Levelogger Figure 1. Schematic of stilling well design showing suspension of pressure transducer (Levelogger) below water level in well.

9 Figure 2. Rainfall (in mm, right axis) and relative stream stage (in ft, left axis) for a small rainfall event on September 6, Stage data were collected by a pressure transducer placed in a stilling well at Site 3. Rainfall data were obtained from the Virginia State Climatology meteorological monitoring station on Observatory Hill, University of Virginia. Figure 3. A rating curve provides a relationship between stream stage which is measured continuously using pressure transducers with a correction for barometric pressure and stream discharge which is measured rarely (see Photographs 2 and 3).

10 Figure 4A. Water quality data collected at the upstream Site 1 before Meadow Creek enters the new stormwater management features. Figure 4B. Water quality data collected at the outfall from the Pond in the Dell. Significant improvements in water quality have been derived from the stormwater management features.

11 Table 1. Dates and completion times for sampled rain events at the upstream Site 1 and at the Pond outfall Site 5. Automatic samplers triggered by rainfall (at Pond outfall) and by rising stream stage (at upstream site) each collected 24 bottles of water at programmed time intervals. Dell Pond Upstream Date Time Date Time 06/04 05:02 No samples collected No evidence of trigger 06/05 07:45 No evidence of trigger 06/15 06:38 06/23 04:00 06/23 04:55 08/28 08:51 No evidence of trigger 08/28 23:44 08/29 00:47 09/06 11:33 09/06 12:24

12 Table 2. The water-quality parameters analyzed and the number of samples analyzed (always a subset of the 24 bottles collected) for the rain events that were collected (Table 1). Date Data Obtained Number of Samples Run Dell Pond: nitrate, nitrite, phosphate Dell: 2 (1, 24) Dell Pond: nitrate, nitrite, phosphate Upstream: nitrate, nitrite, phosphate Dell Pond: nitrate, nitrite, partial phosphate, turbidity Upstream: nitrate, nitrite, partial phosphate, turbidity Dell Pond: partial nitrite, phosphate, turbidity Upstream: partial nitrite, phosphate, turbidity Dell Pond: nitrate, nitrite, phosphate, turbidity Upstream: nitrate, nitrite, phosphate, turbidity Dell Pond: nitrate, nitrite, phosphate, turbidity Upstream: nitrate, nitrite, phosphate, turbidity Dell Pond: nitrate, nitrite, phosphate, turbidity Upstream: nitrate, nitrite, phosphate, turbidity Dell: 2 (1, 24) Up: 2 (2, 4, 6) Dell: 12 (1-12) Up: 12 (2-24 even) Dell: 12 (1-23 odd) Up: 12 (2-24 even) Dell: 14 (1, 2, 3, 4, 5, 6, 7-11 odd, odd) Up: 14 (2, 3, 4, 5, 6-24 even) Dell: 6 (1, 8, 12, 15, 20, 22) Up: 6 (1, 8, 12, 16, 20, 24) Dell: 12 (1-23 odd) Up: 12 (2-24 even)