AN IN-DEPTH ANALYSIS OF A WET DETENTION STORMWATER SYSTEM Betty T. Rushton, Ph.D. Craig W. Dye Southwest Florida Water Management District 2379 Broad Street Brooksville, Fl 34609 (904)796-7211 January 1993 The Southwest Florida Water Management District (District) does not discriminate upon the basis of any individual's disability status. This nondiscrimination policy involves every aspect of the District's functions, including one's access to, participation, employment, or treatment in its programs or activities. Anyone requiring reasonable accommodation as provided for in the Americans With Disabilities Act should contact Gwen Brown, Resource Projects Department, at (904)796-7211 or 1(800)423-1476, extension 4226; TDD ONLY 1(800)231-6103; FAX (904)754-6885/SUNCOM 663-6885. EXECUTIVE SUMMARY Urban development affects the quantity and quality of stormwater runoff by reducing soil infiltration and adding pollutants. Some of the changes include increased stormwater runoff volumes, larger peak flows, shorter times of concentration, accelerated channel erosion and greater pollutant loads. Stormwater management systems can be designed to reduce these impacts and also help meet other water resource objectives. The Southwest Florida Water Management District (SWFWMD, District) regulates stormwater systems under Chapters 4OD-4 and 4OD-40 FAC, Rules for Management and Storage of Surface Water (MSSW). In mid-1988 the District initiated a stormwater research program which by 1992 included six projects to evaluate the effectiveness of MSSW rules. This report presents the results from one of these studies. This study was designed to determine the efficiency of a wet detention pond in reducing pollutants found in stormwater runoff. Also investigated were hydrologic responses to rainfall, groundwater-pond interactions, constituent input from rainfall, relationships between constituents and the dynamics of constituent concentrations over the hydrograph. Discharge water quality was also compared to State of Florida Class M water quality standards (Ch 17-3 FAC). The study site is a small shallow wet detention pond built in 1986 at the SWFWMD service office in Tampa, Florida. It was instrumented to automatically collect hydrologic and water quality data at the inflow and outflow during rain events. The pond is 0.32 acres in size and receives runoff from a basin of 6.3 acres draining a light commercial development. The permanent pool holds 2796 cu. ft. and the detention storage or "treatment volume' is 10,945 cu. ft. (Definitions of terms on back-end cover) Hydrology Rain events for the summer of 1989 and all rainfall from May 1990 to May 1991 were recorded for volume, duration and intensity. The region received below normal rainfall for both data periods with the greatest disparity from normal in the summer of 1990. During this period the site received 38 percent less rain than the long-term normal for the area. Stormwater treatment for water quality is an evolving technology, and improved techniques are still being developed. When the Tampa Office pond was designed in 1986, the MSSW criteria required that wet detention ponds be sized for one-half inch of runoff from the drainage basin. Measurements showed that this design criteria provided storage volume for only 55 to 65 percent of the stormwater runoff entering the Tampa Office pond. A more recent rule, requiring ponds be sized for one inch of runoff would have detained more stormwater, but would not have increased the residence time. Instead of basing design on idealized storm events, some practitioners involved with stormwater treatment methods are now recommending a fourteen day
residence time in a permanent pool in addition to detention storage. As a comparison, the Tampa Office pond had a residence time in its permanent pool of 3.68 days when calculated on a yearly basis and 2.08 days when calculated for the four month summer rainy season. Water Quality One of the goals of the research was to determine if the Tampa Office wet detention pond met numerical state standards for Class III waters (Ch 17-3 FAC). Although water discharged from the pond was the major concern, for comparison purposes, all the water quality data were evaluated using state standards. Composite flow weighted samples showed that out of the eight metals measured (copper, iron, lead, zinc, cadmium, chromium, manganese, nickel) most were usually present in concentrations below the detection limit and thus met standards. However, two metals were of concern, zinc and cadmium. During one year of study (1990-91) zinc exceeded the state standard of 0.03 mg/i, 67 percent of the time in rainfall, 96 percent of the time at the detention pond inflow and 52 percent of the time at the outflow. The cadmium results were more difficult to interpret (cadmium has a standard below the laboratory detection limit), however, cadmium exceeded standards in at least 32 percent of the rainfall samples, 55 percent of the samples at the inflow and 45 percent of the samples at the outflow. Since rainfall seemed to be a significant input for both constituents, these pollutants may have to be cleaned up at the source to improve water quality. Field parameters of ph and conductivity always met state standards during the three months they were measured in the pond. Dissolved oxygen, however, was often below the state standard of 4.0 to 5.0 mg/l and this parameter should be studied in greater detail and an aeration device installed if warranted. Researchers have found that anaerobic bottom sediments promote more soluble (available) forms of phosphorus as well as some metals which could increase their release back into the water column. Although no numerical state standards exist for nitrogen and phosphorus, these constituents have an important bearing on algal productivity and accelerated eutrophication. When compared to 477 stations in Florida lakes, discharge water from the wet detention pond had median values for nitrogen that were slightly above the average for Florida lakes; and phosphorus concentrations that were higher than 80 to 90 percent of the lakes measured. ' Wet detention ponds have been shown to be effective as settling basins and this was also true of the Tampa Office pond where the concentration of suspended solids discharged were low (average 10 mg/1). Other investigators have found that this is generally the lowest level that can be achieved by settling alone. Rainfall Contribution Rainfall appeared to be a significant input of pollutants into the system. Nitrogen, especially ammonia and nitrate, were often found in greater concentration in rainfall than in stormwater entering the pond. Also, as noted above, much of the zinc and cadmium measured entered the system via rainfall. Other researchers have also documented high levels of nitrogen in rainfall. Efficiency A major objective of the study was to determine how well the wet detention pond reduced pollutants from the inflow to the outflow i.e. treatment efficiency. The efficiency of the system is relevant to the State Water Policy (Ch 17-40 FAC) which has a goal for new stormwater systems of 80 percent reduction in annual loads (95% if it discharges to Outstanding Florida Waters). When compared to other studies done in Florida, the Tampa Office pond was intermediate in its ability to reduce pollutants with removal rates of 30 to 60 percent for most constituents. More storage capacity in a permanent pool and a longer detention time would probably be necessary to meet the State goal of 80 percent reduction. In a comparison with other studies, their data suggested that a good
strategy for pollution removal was to retain as much stormwater as possible on site using a treatment train concept, which included swales and ditches, and to design for extended residence times in the pond. First Flush Effect An important concept of the MSSW Rule criteria, is the one that requires wet detention ponds treat the first inch of runoff from the contributing area. This is based on studies that have shown pollutant concentrations are greatest during the early part of the storm and that as rainfall continues, most of the surface pollutant accumulation has been flushed out of the system and diluted by the larger flow. In this study individual samples, taken at discreet intervals over the entire hydrograph, were collected for four rain events. Generally, concentrations were higher at the beginning of large storms (greater than one inch) for metals and phosphorus and gradually declined as the storm continued. Suspended solids followed the shape of the hydrograph with highest concentrations associated with the peak of the storm suggesting that velocities controlled suspended solids movement. No pattern was seen for nitrogen. Groundwater-Surface Water Interactions Wet detention ponds have the potential for recharging both the surficial and deeper aquifers by holding back storm flows and allowing infiltration. Data collected from a network of nine water table wells demonstrated the interaction of the wet detention pond with groundwater. After rain events, water was detained in the pond which caused a mounding effect whereby the water level in the pond was higher than the surrounding water table. The effect of the outlet structure was to detain flows, allowing time for water to percolate into the groundwater system. This was in contrast to the adjacent canals, typical of older stormwater systems, which drained water from the area much more rapidly and maintained a more constant level. Relationships Between variables Statistical analyses (regressions and correlations) were performed to determine processes which might contribute to constituent concentrations. Rainfall characteristics such as antecedent conditions, intensity, and duration were compared to constituent concentrations. Concentrations were also compared to each other to identify any associations. Relationships were not strong between most variables which may be the result of lower than average concentrations of metals and suspended solids entering the pond. However, regression analysis between total rainfall and ortho-phosphorus concentrations indicated that 48 percent of the phosphorus concentrations at the outflow could be attributed to the amount of rainfall. Also, concentrations of some nitrogen species and zinc increased at the outflow after extended dry periods. Iron, zinc and total suspended solids were often correlated with each other which can be explained by the fact that attachment sites on particles. Most heavy metals are adsorbed either or completed and transported with sediments. Consequently good removal efficiencies should be obtained in stormwater management systems which allow adequate detention time for sedimentation to occur. RECOMMENDATIONS An extended residence time should improve the efficiency of the pond and reduce pollutant loads in the effluent. A redesign of the pond is recommended to increase the residence time in accordance with current SWFWMD permit criteria, guidelines for conservation wet detention (TP/SWP-022). This design includes features such as a fourteen day residence time, maximum
mixing, and littoral zones for biological treatment. Specifically, in the wet detention pond studied, it would mean excavating a deeper permanent pool, providing a sediment sump, and installing a spreader weir to improve flow and increase mixing. Studies have shown that anaerobic conditions contribute to lower removal rates of metals and nutrients. A more detailed study of the effect of anaerobic conditions could determine if low dissolved oxygen is a problem in the Tampa Office pond. If anaerobic conditions in the pond do release pollutants from the sediments, it is suggested an aeration device be installed to see if conditions improve. Even if treatment efficiency is improved, the water discharged from the pond may not meet all state water quality standards. Additionally nutrient concentrations may still be above the levels considered necessary to prevent accelerated eutrophication of the receiving waters. Therefore, reuse of pond water for irrigation might be one method to reduce discharge volume and pollutant loading to receiving waters. Stormwater management systems may never be able to totally remove constituents to pristine background levels especially since many of the problem pollutants enter the surface waters through rainfall. Since atmospheric deposition was found to be a significant source of pollution for some nitrogen and metal species, a concerted effort should be made to improve discharges at the source for both water and air pollution. CONCLUSIONS The Tampa Office pond was about average in its ability to remove pollutants when compared to other stormwater systems with large grassed areas. The one exception was the removal of metals where it was less efficient. Lower concentrations entering the pond at the inflow for metals and suspended solids may account for the lower removal efficiency. The pond frequently became anaerobic which may reduce its ability to remove pollutants from the water column. Changes in the pond design should improve its ability to reduce pollutant loads. This theory is being tested with a redesign of the pond using SWFWMD technical procedure conservation wet detention (TP/SWP-022). ACKNOWLEDGEMENTS The amount of data collected for this research project would not have been possible without the cooperation of colleagues in the Environmental Section at SWFWMD. Special thanks go to Mark Kehoe, Lisa Henningsen, Ken Romie and Quincy Wylupek. Marcella Buickerood was helpful in giving advice and finding references. Also indispensable to the research efforts were the many laboratory personnel, especially Mark Rials, Lynn Olsson and Mike Carta, who worked tirelessly analyzing samples which could not be collected by any pre-determined schedule. Jeff Cunningham provided engineering assistance. Linda Eichhom demonstrated her skills at organizing tables and the final draft. Chip Neville designed the cover. Lois Bono of SWFWMD's Hydrologic Data Section provided area rainfall data. Clark Hull, Charlie Miller and Bruce Wirth of SWFWMD gave invaluable assistance in interpreting the rules and proof reading the final draft. Two people deserve special mention because of the long often tiresome hours spent working on this project. Steve Saxon corrected problems with weirs, built instrument shelters, installed and debugged equipment, installed wells, repeatedly rechecked surveyed levels, calibrated and deployed recording hydrolabs, made field measurements of flow data and well data, wrote and tracked purchase requisitions, cut weeds, cleaned shelters, conducted routine maintenance, changed tubing, charged batteries and collected samples at night, on weekends or whenever necessary for quality assurance. He also designed and built peak staff gauge recorders to use as backup systems. David Carr, as a part-time co-op student, downloaded and processed the tremendous amount of data generated by the automated recording devices. This was a confusing often frustrating job since data were collected at both the inflow and outflow
every fifteen minutes and had to be checked against field measurements and kept consistent with offsets and coefficients. That is, when the equipment worked. Otherwise it had to be calculated from strip charts or estimated from field measurements. This information was transferred to 120 floppy disks where calculation were made for the information reported in the hydrology tables as well as the graphs in Appendixes B and C. David also located and xeroxed necessary references from the University of South Florida Library.