539 14 Event-Driven Wetlands The treatment of stormwaters of various origins is of grow- ing concern as attempts to rectify point-source pollution reach maturity. Runoff from urban, agricultural, and indus- trial sources comprises a sizeable fraction of the total pollu- tion load to receiving waters in many locations. Often, these sources are relatively dilute compared to primary or second- ary domestic sewage but, nevertheless, they may negatively impact receiving waters. Urban stormwater wetlands were rst surveyed by Strecker et al. (1992), who documented the performance of 25 natural and constructed wetlands treating runoff. The implementation of the technology and the knowledge base continued to build, resulting in a compilation of data from 76 wetlands worldwide by Wong et al. (1999), and from 49 wetlands in the United States by Carleton et al. (2001). In North America, these are all FWS systems, and that is the predominant wetland type elsewhere as well. A typical con- guration consists of a sedimentation basin as a forebay, fol- lowed by some combination of marshes and deeper pools. Design guidelines have now been promulgated by a number of sources (for example: Schueler, 1992; Breen and Lawrence, 1998; Wong et al., 1999; LEC, 2000; Center for Watershed Protection, 2001). Agricultural stormwater occurs as runoff from crops and pastures. Early work on constructed wetlands for row crop runoff control was centered in the state of Maine (Higgins et al., 1993) (Wengrzynek and Terrell, 1990; U.S. Department of Agriculture, 1991). Runoff from sugarcane is being treated in the huge FWS wetlands (16,000 ha) called stormwater treat- ment areas (STAs) of South Florida (Goforth, 2001). However, there has also been signicant application of constructed wet- lands at a more modest scale for vegetable farming (Rushton and Bahk, 2001). Nonetheless, effective control of runoff may require consideration of the entire watershed (Crumpton, 2000). More recently, attention has been on controlling pas- ture runoff (Tanner et al., 2003). Industrial stormwaters have received less attention than urban and agricultural sources. These typically will con- tain contaminants specic to the industry in question. For instance, rain runoff from a fertilizer plant site will poten- tially contain high levels of nutrients, whereas rain run- off from a petroleum facility will carry hydrocarbons. For instance, in South Africa, a number of reed bed wetlands were established to treat waters generated from truck wash- ing operations at oil industry depots (Wood, 1993). An HSSF wetland treated runoff from a 0.8-ha vehicle yard in Surprise, Arizona, with 54–92% removal of oil and grease (Wass and Fox, 1993). Although rainfall runoff is the source of water for many event-driven wetlands, there are other situations in which a treatment wetland is subjected to episodic ows. These include wetland systems that treat stormwater, urban or agricultural runoff, combined sewer overows, and treat- ment systems that are operated in batch mode. Whatever the source, wetlands that receive pulsed ows utilize the suite of wetland removal mechanisms in different ways than their continuous-ow counterparts. 14.1 SOURCE CHARACTERIZATION I NCOMING FLOWS The amount of water to be expected from a given water- shed is variable and is keyed to rainfall in the contribut- ing basin. The land uses and soils in the contributing basin are an important modier of the runoff and inltration. The antecedent dryness in the basin is also a contributing factor. Detailed methods of estimating runoff are avail- able, for instance, the SCS (U.S. Soil Conservation Service) method (McCuen, 1982), which has been summarized by Novotny (1995). There are also numerous computer models that account for very detailed features of the contributing watershed and produce both the quality and quantity of the runoff to be treated. The focus here is the treatment wetland, and it will be presumed that the incoming ows will be char- acterized to the appropriate level of detail. For purposes of rough estimation, the rational formula may be used to estimate peak ow, Q P (ASCE, 2006): QCIA PR b (14.1) where A C b R watershed area, m runoff coefficient, 2 dimensionless peak runoff flow, m /h a P 3 Q I vvera g e rainfall intensit y , m/h The values of C R are a function of land use and storm inten- sity, as well as the climatological region. An example of the range of C R for an arid region is shown in Table14.1. To illustrate the regional effect, consider that agricultural land in Arizona is rated at C R 0.1–0.2 for high return-frequency events, whereas the value for agricultural land in South Florida is C R y 0.5. Water is used consumptively in arid regions, whereas the high water table and rainfall in a wet region leads to more runoff. Modications to this simplest formula have been presented on a site-specic basis (see, for example, Brezonik and Stadelmann, 2002; ASCE, 2006). © 2009 by Taylor & Francis Group, LLC 540 Treatment Wetlands For modeling purposes, the time series of water ows entering the treatment wetland inlet hydrograph is sometimes needed. This time series is driven by the pattern of rainfall associated with a particular event (represented by a hyeto- graph), together with the collection and transfer character- istics of the basin. In some instances, such transfer is solely by gravity, but in some cases pumps may be used. Usually, the inlet hydrograph contains both a rising and falling limb, although the rising limb is normally quite steep (for an exam- ple, see Figure 14.1). In some instances, the events are separated by interevent periods of no inow to the treatment wetland. These periods are important because the wetland will act as a batch reactor during much of these no-inow durations. A typical sequence is (1) wetland lling with no outow, (2) ow through with both inow and outow, (3) draining with no inow, and (4) nally, a batch-holding mode with neither inow nor outow. The rates and duration of these inows and outows are in part controlled by structures. The volume of water in the wet- land is also in part controlled by structures, but evapotrans- piration may be an important component during interevent periods. The durations of no-ow periods are very much a function of the climatological region in which the system is located. For instance, in Florida, during the rainy season, the most frequent periods are measured in hours (4–20 hours, Wanielista and Yousef, 1991). In contrast, the dry season of central north island New Zealand has periods of months with no rain, and interevent periods can be several months (Tanner et al., 2005b). Many stormwater wetlands are fed by pumps. These may range in size from small sump pumps for small local- ized urban systems to the huge pumps that send water to the Florida stormwater treatment areas (a.k.a. STAs, or treat- ment wetlands). Those Florida inow and outow pumps are among the largest in the world, up to three stories tall, with capacities up to 10,000,000 m 3 /d (Figure 14.2). A key feature of pumped systems is intermittent feed to the treatment wet- land, usually at xed but incremental rates, corresponding to the number of pumps that are operated, for periods of time dictated by conditions in the contributing watershed. There is typically a hydraulic limitation to the event size that may be treated in a given wetland. It is generally not feasible to size a wetland to treat the 100-year return fre- quency storm because of cost and footprint considerations and because of the need for maintenance water for the (large) wetland under normal conditions. Consequently, part of the design decision process is the determination of the maximum design storm to be treated. Even if the system is sized to treat TABLE 14.1 Runoff Coefficient (C) for Use in the Rational Method in Maricopa County, Arizona Return Period Land Use 2–10-year 25-year 50-year 100-year Streets and Roads Paved roads 0.75–0.85 0.83–0.94 0.9–0.95 0.94–0.95 Gravel roadways and shoulders 0.6–0.7 0.66–0.77 0.72–0.84 0.75–0.88 I ndustrial Areas Heavy 0.7–0.8 0.77–0.88 0.84–0.95 0.88–0.95 Light 0.6–0.7 0.66–0.77 0.72–0.84 0.75–0.88 B usiness Areas Downtown 0.75–0.85 0.83–0.94 0.9–0.95 0.94–0.95 Neighborhood 0.55–0.65 0.61–0.72 0.66–0.78 0.69–0.81 R esidential Areas Lawns—at 0.1–0.25 0.11–0.28 0.1–0.3 0.13–0.31 Lawns—steep 0.25–0.4 0.28–0.44 0.3–0.48 0.31–0.50 Suburban 0.3–0.4 0.33–0.44 0.36–0.48 0.38–0.5 Single family 0.45–0.55 0.5–0.61 0.54–0.66 0.56–0.69 Multi-unit 0.5–0.6 0.55–0.66 0.6–0.72 0.63–0.75 Apartments 0.6–0.7 0.66–0.77 0.72–0.84 0.75–0.88 Parks/cemeteries 0.1–0.25 0.11–0.28 0.12–0.3 0.13–0.31 Playgrounds 0.4–0.5 0.44–0.55 0.48–0.6 0.5–0.63 Agricultural areas 0.1–0.2 0.11–0.22 0.12–0.24 0.13–0.25 Bare ground 0.2–0.3 0.22–0.33 0.24–0.36 0.25–0.38 Undeveloped desert 0.3–0.4 0.33–0.44 0.36–0.48 0.38–0.5 Mountain terrain (slopes 10%) 0.6–0.8 0.66–0.88 0.72–0.95 0.75–0.95 Source: Adapted from the Drainage Design Manual for Maricopa County, Volume 1. © 2009 by Taylor & Francis Group, LLC Event-Driven Wetlands 541 a modest-sized storm ow, there remains the possibility that dry conditions might persist in some years to the point that the integrity of the wetland ecosystem is jeopardized. If such detrimental dryout conditions are expected, then a source of irrigation water for ecosystem maintenance should be identied. INCOMING CONCENTRATIONS AND LOADS Concentrations of most parameters in stormwater are time dependent, as are the ows. Stormwater concentrations and loads are episodic due to periods of dryfall and deposition, followed by the rst ush of runoff after rain, followed by exponential decreases in runoff constituent concentrations as storages rinse from the landscape, and nally, dry conditions and deposition until the next storm event. The time series of concentrations in the inow to the wetland is called the chemograph. An example chemograph for an agricultural runoff wetland, targeting nitrogen reduction, is shown in Figure 14.3. In some watersheds, the chemograph is not synchronized with the hydrograph, but instead provides higher concentra- tions early in the inow event. This phenomenon is termed rst-ush behavior, referring to the surge of pollutants con- tained in the rst water to leave the contributing basin. For instance, Wanielista and Yousef (1991) report that, in Flor- ida, the rst 25 mm of runoff from urban systems typically carries 90% of the pollution. However, for the large agricultural 0.0 0.5 1.0 1.5 2.0 2.5 3.0 0 50 100 150 200 Days from May 1, 2002 Flow (L/s) or Rain (cm) Inflow Rainfall FIGURE 14.1 Time series of ows entering a treatment wetland from an improved pasture in Toenepi, New Zealand. Several rain events occurred during this winter wet season, as indicated by the repeated spikes in inow. The runoff coefcients were 0.23–0.30 and were the result of tile drains. (Adapted from Tanner et al. (2005b) Agriculture, Ecosystems and Environment 105(1–2): 145–162.) FIGURE 14.2 The outow pump station from STA1E (Stormwater Treatment Area 1 East) of the Everglades Protection Project. The capac- ity of this pump station is 9,740,000 m 3 /d, and it serves to drain a 2,700-ha FWS wetland. © 2009 by Taylor & Francis Group, LLC 542 Treatment Wetlands watershed of South Florida, there is no such rst-ush effect. Despite the site-specic nature of the chemograph, there is no reliable database documenting dynamic time series of concentrations in any given watershed. Of necessity, average concentrations of some sort must be used, of which the ow- weighted concentration is most useful. This may be evalu- ated on a long-term basis or as an event mean concentration for the inlet water: C QCdt Qdt e ¯ ¯ (14.2) where C C instantaneous concentration, mg/L event e mean concentration, mg/L instantaneous fQ llow, m /h time, h 3 t and where integration is over the period of one event. The numerator is the mass of pollutant entering or leaving during the event. As described in Chapter 6, the performance of the wetland may be described in terms of the loads applied to and emanating from the system. Tables 14.2 and 14.3 provide long-term mean concen- trations for constituents in urban stormwater. The averages are ow-weighted to provide realistic estimates of the total constituent load that escapes during multiple storm events. Instantaneous concentrations may rise considerably higher than these averages. Pollutant concentrations and loads gen- erally range from low levels from undeveloped and park lands to low-density residential and commercial, to agri- cultural, to higher-density residential and commercial, and nally to high-density commercial, industrial, and agricul- tural land uses. Mean concentrations per event for BOD 5 vary from below detection for undeveloped lands to 20 mg/L for high-density urban areas. Total suspended solids concen- trations vary from about 10 mg/L for undeveloped areas up to 150 mg/L for high-density urban areas. Typical concentra- tions for other stormwater pollutants are also summarized in Tables 14.2 and 14.3. The mass loading rates represent normalized pollut- ant loads that are somewhat independent of local rainfall amounts. Because pollutant loads per area per time are rela- tively constant between similar land use areas, variable local rainfall washes these loads off the land in a few large events or over many smaller events. Urban pollutant loads increase with the imperviousness of the watershed. Although 20 to 40% of the material on street surfaces is organic, it does not biodegrade easily because it comes from leaf and wood litter, rubber, and road-surface material (Novotny, 1992). The high metal content of highway solids comes from vehicle emis- sion. Novotny (1992) reported that the average total nitro- gen load from urban lands is 5 kg/ha·yr (1 to 38.5 kg/ha·yr), and the total phosphorus load averages 1 kg/ha·yr (0.5 to 6.25 kg/ha·yr). Constructed wetlands are being increasingly used to treat runoff from intensive animal operations (CH2M Hill and Payne Engineering, 1997; Tanner et al., 2003) and from crops (U.S. Department of Agriculture, 1991; Crumpton, 2000). Concentrations and loads from agricultural land uses vary considerably. Flows and loads are typically highest from areas with high animal densities or high fertilization rates. Runoff pollutant concentrations from animal feedlots can be extremely high unless runoff is collected and treated. Pollutants from feedlot runoff typically include high levels of organic and inorganic solids and associated nutrients. Nutrient concentrations and loads from row crops and pas- tures depend on fertilization practices and type of soil. There exist many computer models for the amounts of contaminants that may be expected in runoff from different 0 5 10 15 20 25 30 35 40 45 –5–4–3–2–10123456789 Time (days) NO x -N (mg/L) FIGURE 14.3 The time series of nitrate nitrogen arriving at an agricultural runoff treatment wetland in McDowell County, North Carolina. The rain event started at time zero and drove runoff that entered the wetland by stream ow. (Adapted from Kao and Wu (2001) Water Sci- ence and Technology 45(3): 169–174.) © 2009 by Taylor & Francis Group, LLC Event-Driven Wetlands 543 landscapes. Novotny (1995) summarized the characteristics of six models for urban watersheds, and seven for agricul- tural watersheds, and the number has grown since that time. A large part of the design of event-driven treatment wetlands is the determination of the ows and loads to be treated. TABLE 14.2 Pollutant Concentrations for Source Areas for Stormwaters Constituent TSS a (mg/L) TP b (mg/L) TN c (mg/L) E coli a (1,000 #/mL) Cu a (Kg/L) Pb a (Kg/L) Zn a (Kg/L) Residential roof 19 0.11 1.5 0.26 20 21 312 Commercial roof 9 0.14 2.1 1.1 7 17 256 Industrial roof 17 — — 5.8 62 43 1390 Comm./res. parking 27 0.15 1.9 1.8 51 28 139 Industrial parking 228 — — 2.7 34 85 224 Residential street 172 0.55 1.4 37 25 51 173 Commercial street 468 — — 12 73 170 450 Rural highway 51 — 22 — 22 80 80 Urban highway 142 0.32 3 — 54 400 329 Lawns 602 2.1 9.1 24 17 17 50 Landscaping 37 — — 94 94 29 263 Driveway 173 0.56 2.1 17 17 — 107 Gas station 31 — — — 88 80 290 Auto recycler 335 — — — 103 182 520 Heavy industrial 124 — — — 148 290 1,600 Source: Data from Center for Watershed Protection (2001) New York State Stormwater Management Design Manual. Report by the Center for Watershed Protection (CWP) for the New York State Department of Environmental Conservation, Albany, New York. a Data from Claytor and Schueler (1996) Design of Stormwater Filtering Systems. Center for Watershed Protection: Ellicott City, Maryland. b Average of data from Steuer et al. (1997) Sources of contamination in an urban basin in Marquette, Michigan, and an analysis of concentration, loads, and data quality. Water Resources Investigation Report 97–4242, U.S. Geological Survey; Bannerman et al. (1993) Water Science and Technology 28(35): 241–259; Waschbusch (2000) Sources of phosphorus in stormwater and street dirt from two urban residential basins in Madison, Wisconsin, 1994–1995. Proceedings of the National Conference on Tools for Urban Water Resource Management and Protection; U.S. Environmental Protection Agency: Washington, D.C, pp. 15–55. c Data from Steuer et al. (1997) Sources of contamination in an urban basin in Marquette, Michigan, and an analysis of concentration, loads, and data quality. Water Resources Investigation Report 97–4242, U.S. Geological Survey. TABLE 14.3 Typical Concentration Data for Pollutants in Urban Stormwater Constituent Units Urban Runoff Source TSS mg/L 54.5 Smullen and Cave (1998) TP mg/L 0.26 Smullen and Cave (1998) TN mg/L 2.00 Smullen and Cave (1998) Cu Mg/L 11.1 Smullen and Cave (1998) Pb Mg/L 50.7 Smullen and Cave (1998) Zn Mg/L 129 Smullen and Cave (1998) Fecal coliforms 1,000 CFU/mL 1.5 Schueler (1999) Source: Data from Center for Watershed Protection (2001) New York state stormwater management design manual. Report by the Center for Watershed Protection (CWP) for the New York State Department of Environmental Conservation, Albany, New York; pooled NURP/USGS Smullen and Cave (1998) Updating the U.S. nationwide urban runoff quality database. 3rd Interna- tional Conference on Diffuse Pollution, 31 August–4 September 1998; and Schueler (1999) Watershed Protection Techniques 3(1): 551–596. HYDROLOGY OF PULSED AND SEASONAL SYSTEMS Event-driven wetlands are dynamic in all respects, and the principal underlying hydraulics exhibit variable water depths and ows. The behavior is strongly conditioned by the nature © 2009 by Taylor & Francis Group, LLC 544 Treatment Wetlands of inow and outow structures that may be designed to improve detention and treatment (Somes and Wong, 1997). The most general situation may have several complicating factors, but here a simple and common case is explored for purposes of illustration. It will be presumed that the wetland is relatively small and consequently behaves as a level-pool system, with no gradients in stage. Stormwater wetlands experience event ows and concentrations, followed by peri- ods of batch operation. It is then necessary to account for the dynamics of water storage within the wetland. The dynamic, level-pool water mass balance for the wet- land for unsteady state inows and meteorology is dAh dt QQ APETI () () io (14.3) where A Q Q wetland wetted area, m inflow, m /d 2 i 3 o ooutflow, m /d rainfall, m/d evapotransp 3 P ET iiration, m/d water depth, m infiltration h I ,, m/d time, dt The stage is often controlled by an outlet structure, here sup- posed to be a rectangular weir. Thus the outow-stage rela- tion would be given by the Francis formula (French, 1985): Qh oow B A () (14.4) where hhHH H ow ow w height over the weir, m ( ) wet lland stage, m elevation of weir, m out w o H Q fflow, m /d calibration exponent, dimensio 3 A nnless calibration constant, (m /d)/m 3 B A It is noted that the wetted area is not constant but changes with stage according to the bathymetry of the wetland, represented by the stage–area–volume relationship (see Chapter 2). Fur- ther, the wetland catches rainfall over an area typically greater than the wetted area but loses water to ET from just the wetted area. Inltration further contributes to water losses and may be a signicant fraction of the output from the wetland, as in the case of the Hidden River wetland in Tampa, Florida (Carr and Rushton, 1995). An example of this analysis is illustrated in Figure 14.4. For illustration, a wetland of 2,500 m 2 is subjected to a 5-cm rain event and the associated runoff from the contrib- uting basin. The presumed conditions and wetland design are shown in Table 14.4. The wetland does not have a level bottom; rather, there is a presumed quadratic stage–vol- ume relation, which implies that the wetted area increases linearly with stage. The hypothetical sequence of events is as follows: The wetland starting condition is partially full, with the water level 10 cm below the outlet weir. A steady rain begins at time zero and persists for one day. Runoff from the watershed begins after t 0.5 days and persists for 1.5 days. The wetland loses some water to ET (1.0 cm/d), and some to inltration (1.0 cm/d). For the rst half day (t 0–0.5 days), the wetland lls due to direct collection of rain. At t 0.65 days, the wetland has lled to the top of the outow weir, and outow commences. For the next half day (t 0.5–1.0 day), the wetland lls because of direct rain and incoming runoff. For the next day (t 1.0–2.0 days), the wetland lls because of incoming runoff. During t 2.0–3.1 days, there is no inow, and the wetland is draining over the outow weir. At t 3.1 days, the level has decreased to the top of the weir, and outow stops. After time t 3.1 days, the wetland loses water to ET and inltration, and the level continues a slow drop. • • • • • • • • • • TABLE 14.4 Parameters for a Hypothetical Stormwater Wetland Event Scenario Basin Area 120,000 m 2 Runoff coefcient 0.5 Wetland WWAR 2 % Nominal maximum depth 1.00 m Wetland area full 2,500 m 2 Wetland volume full 1,250 m 3 Stage area: A aH, a 2,500 m 2 /m Weir height 0.90 m Weir coefcient 31,623 (m 3 /d)/(m 1.5 ) ET rate 1.00 cm/d Inltration rate 1.00 cm/d Rain catchment 3,000 m 2 Rainfall Period 1.00 d Rain rate 5.00 cm/d Basin catch 6,000 m 3 Inow start 0.50 d Inow end 2.00 d Inow rate 2,000 m 3 /d Runoff 3,000 m 3 Storm/wetland volume ratio 2.40 © 2009 by Taylor & Francis Group, LLC Event-Driven Wetlands 545 Because of the assumed bathymetry, the wetted area rst grows and then shrinks through this course of events. This simple model has been found to t event wetland data quite well (Kadlec, 1994; Hey et al., 1994; Kadlec, 2001a). However, there are several different inlet and outlet structures that may be used, and the level-pool model must be modied accordingly for other situations. FLOW AND CAPTURE Event-driven wetlands operate with periods of ow through and periods of water-holding or batch processing. Any water that does not escape the wetland during a particular event will be held until the next event, and possibly longer. Therefore, it is subject to the water quality improvement functions of the wetland for not only the event duration but also the interevent period. Conversely, water that enters and leaves the wetland during the event is subject to treatment only dur- ing the (possibly brief) period of detention during the event. Therefore, at the rst level of consideration, it would seem desirable to contain the entire volume of water associated with an event within the wetland. This requirement is often referred back to the contributing watershed in terms of the rain amount and runoff coefcient that generated the storm volume. The wetland may be sized to nominally contain the runoff volume associated with a rain event of specied amount or return frequency. One limit on performance is the expulsion of water that resides in the wetland at the time of event initiation. This water is at a background concentration if the events are widely spaced. In this example, the time for background to be reached was on the order of two or three days. The length of time for the wetland water surcharge to dissipate, and thus for stage to reach the weir elevation, was also of the order of two or three days. If the events are more closely spaced, then the resident antecedent water will not be at background but rather will exhibit a concentration representing only partial treatment of the previous event. A second limit on performance is the 0 500 1,000 1,500 2,000 2,500 3,000 0.0 1.0 2.0 3.0 4.0 5.0 Time (days) Flow (m 3 /d) or Wetted Area (m 2 ) Inflow Outflow Area (a) FIGURE 14.4 Response of a hypothetical stormwater wetland to a one-day steady rain. Runoff into the wetland begins after half a day. The wetland lls, and outow persists for just over three day. Inltration and ET deplete the water after the event. See Table 14.4 for parameters. 0.7 0.8 0.9 1.0 1.1 0.0 1.0 2.0 3.0 4.0 5.0 Time (days) Wetland Water Stage (m) Rain Inflow Outflow (b) © 2009 by Taylor & Francis Group, LLC 546 Treatment Wetlands ultimate removal that would be associated with a continuous water input at the event average ow rate. This is likely to be a relatively low percentage reduction because of the high ow in the event. A complicating factor in analysis is that wetlands are not hydraulically simple and do not operate on a basis of plug- ow displacement. Therefore, the fraction of incoming water that remains in the system for a particular event is not deter- mined merely from displacement. Depending on the congu- ration of the wetland, some of the very rst water that enters the wetland may nd its way to the exit far in advance of nominal retention time during the event. This is partly due to preferential ow paths, and partly due to the positioning of water-level control structures. If it is assumed that the wetland behaves hydraulically as several mixed tanks in series, it is reasonable to use the known detention time distribution to compute how much of the incoming water is still in the wetland after a period of ow of known magnitude. Figure 14.5 shows the results as a function of the nominal number of wetland volumes of water that have passed through the system. If the ow were plug ow, then the retention is 100% until the nominal detention time is reached, after which the new event water begins to exit. For less ideal ow conditions, the amount of the new event water that is held is less. The presumption of Figure 14.5 is that all of the wetland water is involved in ow. That can be far from true because of wetland design factors (see Appendix B). For instance, if there is a point inlet to the system, and a point outlet aligned directly opposite, then the incoming pulse of water can travel directly to the outlet, never reaching zones to the side of the most direct ow path. That renders the corner areas essentially out of the ow path and reduces the volumetric efciency quite mark- edly (Agunwamba, 2006). Walker (1998) investigated this sit- uation using computational uid mechanics (simulations) of unvegetated FWS basins. He determined that aspect ratio (L:W) was the major determinant of such shortcircuiting and calculated a retention-displacement chart (Figure 14.6). For individual, separated inow events, the concept of retained volume provides a strong correlating factor for per- formance. If the event is small, it is wholly contained and held until the next displacing ow. Treatment proceeds in the batch mode for the interevent duration. If the event is large, then it is processed by ow through at the detention time of the event ow. Because such ows are typically large, the wetland will have a short detention time, which results in decreased treatment performance. Therefore, mass reduc- tions decrease (exponentially) as the number of nominal dis- placements caused by the event increases. The storage and treatment potential for individual events may ultimately be linked to the sequence of events that are likely to occur over a long period of record. The three required probability distributions are for the event duration, the event intensity, and the interevent duration (Wong and Somes, 1995). To further complicate matters, when the wet- land basin is “full,” water will be diverted away (bypassed). Thus, a wetland that drains quickly, i.e., one with short deten- tion time, will be in a position to detain more water upon relling than a wetland that drains slowly. Wong and Geiger (1997) examined the runoff patterns for Melbourne, Austra- lia, and concluded that the hydrologic effectiveness (the per- centage of runoff that is exposed to treatment) was inversely proportional to the wetland detention time. These factors all point to the existence of a set of perfor- mance determinants that go beyond those for continuous-ow treatment wetland systems. Batch interevent rate coefcients and background concentrations are important. The inter- nal ow patterns are also important as determinants of the concentration and timing of the ushed antecedent water. The dynamics of water ow and storage of the wetland are important because they determine the volumetric constraints on performance. 0.0 0.2 0.4 0.6 0.8 1.0 0123456 Size of Event (Wetland Volumes) Inflow Fraction Retained Plug Flow N = 10 N = 3 N = 1 FIGURE 14.5 Fraction of the inow retained for a range of numbers of tanks in series for event volumes of different nominal displacements. © 2009 by Taylor & Francis Group, LLC Event-Driven Wetlands 547 Ultimately, the computed description of performance will either be expressed as long-term mass removal or by dynamic modeling that produces time series of outow and efuent concentrations in response to the inlet time series. Most of the stormwater wetland literature reports mass removals over some period of record. However, there are now rst-genera- tion dynamic models for some applications. For instance, there is a ow and phosphorus model for application in south Florida called the Dynamic Model for Stormwater Treatment Areas (DMSTA) (Walker and Kadlec, 2005). 14.2 TECHNOLOGY STATUS Data reporting falls into two general categories: event time- series analyses, and summaries of removal efciencies. Two concepts are needed to organize these bodies of information: event mean concentrations and mass balancing. Because the concentration pulses and ow pulses are often out of sync, the event mean concentration (EMC) is used: EMC VC V 3 3 () (14.5) where C concentration in a water parcel, g/m = m 3 gg/L water volume in the parcel, m 3 V In effect, the EMC is the mass average concentration over the course of an event and may be calculated for both inlet and outlet ows of various types for the wetland. If only the directed inow and outow are considered, the EMC reduc- tion is dened as Concentration Reduction io i ()EMC EMC EMC (14.6) The outow from the wetland corresponding to a given inow event may total less than the inow due to inltration and evapotranspiration. The mass load reduction for a given pollutant is Load Reduction iioo ii ()V EMC V EMC VEMC (14.7) URBAN STORMWATER Runoff from roofs, lawns, parking lots, and other urban land- scape features often contains several classes of pollutants (see Table 14.2). There will be small amounts of nitrogen and phosphorus and biochemical oxygen demand (BOD), but possibly relatively greater amounts of total suspended solids (TSS), metals, and perhaps pesticides. An important subset of urban runoff control is focused on highway runoff (Shutes et al., 2001; Bulc and Sajn Slak, 2003; Pontier et al., 2003). However, the greater number of systems are used to treat rainwater runoff from more generalized urban areas. Here the focus is upon gravity-driven constructed systems, as opposed to pumped or natural systems. Such systems are often quite small, serving small catchments (Figure 14.7). In this section, attention is on gravity-fed, constructed systems treating urban stormwater. Pumped systems have been included in other summaries, such as Carleton et al. (2001) and Wong et al. (1999), but many of these do not reect the random and episodic nature of wetlands receiv- ing rain-driven events and are omitted here, as are natural wetland systems. A sampling of TSS performance is given in Table 14.5, in which it is clear that there is a wide spectrum of performances for all common constituents, with median removals far from 100%. Removal percentages are insufcient for design pur- poses, and hence it is necessary to nd relationships between system characteristics and performance measures. Carleton et al. (2001) concluded that long-term pollutant removals could be described in terms of the same kinds of rst-order, steady-ow design equations currently employed for waste- water treatment wetlands. They present rate coefcients that 0.0 0.2 0.4 0.6 0.8 1.0 1.2 0.0 1.0 2.0 3.0 4.0 Size of Event (Wetland Volumes) Fraction of Inflow Retained Plug Flow L:W = 8 L:W = 4 L:W = 2 L:W = 1 L:W = 0.5 FIGURE 14.6 Fraction of an inow event contained in a wetland as a function of the event volume and the aspect ratio (L:W). (From Walker (1998) Ecological Engineering 10(3): 247–262. Reprinted with permission.) © 2009 by Taylor & Francis Group, LLC 548 Treatment Wetlands are intended to be used in a procedure such as that described by Wong and Geiger (1997) and Wong et al. (2006). As dis- cussed in Chapter 6, that method has inherent drawbacks, and calibrations for event-driven systems provide only cen- tral tendencies. Additionally, rate coefcients derived from single events may not be adequate for a time series of meteo- rologically driven ows. Data sets analyzed by Strecker et al. (1992) and extended by Carleton et al. (2001) include about half natural wetland systems that receive stormwater. Strecker et al. (1992) in fact found that natural wetlands performed somewhat better than constructed wetlands, but the use of natural wetlands is not encouraged because of real and perceived negative impacts (U.S. EPA, 1993e). The median areal rate coefcients reported FIGURE 14.7 (A color version of this gure follows page 550) An urban stormwater wetland in the city of Blue Mountains, Australia. TABLE 14.5 Suspended Solids Reduction in Constructed Urban Runoff Treatment Wetlands Name Location Reference WWAR Area Ratio (%) HLR (cm/d) Reduction (%) Crookes Australia Raisin et al. (1997) 0.1 21.83 12 Mays Chapel Maryland Carleton et al. (2001) 0.6 5.55 11 Shop Creek Colorado Carleton et al. (2001) 0.6 — 25 Franklin Farms Virginia Carleton et al. (2001) 0.8 17.16 62 Lake Munson Florida Maristany and Bartel (1989) 1.1 5.19 93 Slovenia Highway Slovenia Bulc and Sajn Slak (2003) 1.1 — 74 DUST Marsh California Meiorin (1989) 1.8 — 64 Crestwood Virginia Carleton et al. (2000) 2.4 3.69 58 Greenwood Florida McCann and Olson (1994) 2.5 2.57 68 Queen Anne Maryland Carleton et al. (2001) 3.8 — 65 Clear Lake Minnesota Carleton et al. (2001) 4.9 1.71 76 Tampa Ofce Pond Florida Carleton et al. (2001) 5.1 8.16 55 West Lafayette Indiana Harbor et al. (2000) 6.3 — 75 Lake McCarrons Minnesota Carleton et al. (2001) 6.6 7.38 83 Hidden River Florida Carr and Rushton (1995) 19.5 1.04 86 Elbow Valley Calgary Amell (2004) — — 72 Hallam Valley Low Australia Wong et al. (2006) — 400 94 Hallam Valley High Australia Wong et al. (2006) — 220 94 Kaohsiung China Kao et al. (2001a) — 7.10 37 Villanova Pennsylvania Rea (2004) — 8.16 70 Median 7.10 68 Note: All are FWS except for Slovenia, which is HSSF. © 2009 by Taylor & Francis Group, LLC [...]... Fulda, Germany, over a 23-event, two-year period was more than 90% removal of chemical oxygen demand (COD), BOD, TSS, and ammonia, and 70–90% of phosphorus 14. 3 TSS IN EVENT-DRIVEN WETLANDS Event-driven treatment wetlands utilize the same suite of TSS processes as continuous-flow systems, but there are important differences in the mode of operation In contrast to continuous-flow wetlands, there is a greater...Event-Driven Wetlands 549 by Carleton et al (2001) for FWS constructed gravityfed systems (N 9) are • Total Phosphorus: 8.3 m/yr • Ammonia: 5.0 m/yr • Nitrate: 6.7 m/yr These are very low compared to the k-values reported for continuous-flow systems in the preceding chapters Individual events are afforded much better treatment Carleton et al (2001) also suggest that wetland-to-watershed area... 6.27 8.90 2.85 — 8.61 5.70 3.60 21.60 3 4 6 14 15 −11 48 19 53 12 26 28 36 −6 8 — 67 33 54 7 13 1.70 1.16 23 8.40 5.70 26 Note: All are FWS except for Conestoga, which is SSF; Rem = Removal Treatment Wetlands 6/19/08 11:57:45 AM © 2009 by Taylor & Francis Group, LLC Event-Driven Wetlands 567 TABLE 14. 11 Metals Reduction in Constructed Urban Runoff Treatment Wetlands Percent Reduction Name Shop Creek... characteristics that may be expected There is also the question of the trans- © 2009 by Taylor & Francis Group, LLC Treatment Wetlands ferability of rate coefficients from continuous-flow systems to event-driven systems It is also possible to use dynamic first-order modeling to predict phosphorus concentrations in stormwater wetlands (Walker and Kadlec, 2005) This approach is difficult because it is... preceded by an oxidation ditch pretreatment plant that conducted full nitrification The mean monthly influent ammonia-nitrogen to the treatment wetland averaged 0.42 mg/L, and the mean monthly effluent ammonia-nitrogen from the treatment wetland averaged 0.35 mg/L over a five-year period However, these averages were influenced by two WWTP upsets that sent high ammonia to the treatment wetland over 68 days... are excluded, the mean monthly influent ammonia-nitrogen to the treatment wetland averaged 0.085 mg/L, and the mean monthly effluent ammonia-nitrogen from the treatment wetland averaged 0.086 mg/L The average k-value during the period 1999–2003 was 12.0 m/yr, which is close to the median expected value based on other wetlands The year 2000 event had an event-mean wetland inlet ammonia concentration of... Days June 2002 15 20 FIGURE 14. 23 Response of the Commerce Township, Michigan, treatment wetland to an ammonia pulse from the wastewater treatment plant in June 2002 (From Commerce Township unpublished daily monitoring data.) 564 Treatment Wetlands 100 NO3-N Concentration (mg/L) Inlet Outlet Flow, L/s 10 1 0.1 0.01 0 100 200 300 400 500 Days from March 31, 2001 600 700 FIGURE 14. 24 Nitrate nitrogen entering... point-source-driven marshes, it is reasonable to expect that stormwater wetlands would perform somewhere near the average for the continuous-flow FWS database set For instance, the median reduction for nitrate in 72 continuousflow FWS wetlands was 53% at an average HLR 7.1 cm/d; the median reduction for constructed stormwater marshes, with an average HLR 5.4 cm/d, was 45% (Table 14. 9) The median plug-flow... displacements becomes large, the system approaches continuous-flow performance at the event flow rate EVENT SEQUENCES In gravity-fed systems and some pumped stormwater facilities, events are not necessarily isolated but occur in a time series with event-to-event overlap at times The time series of event treatment may be viewed on a storm-by-storm basis, in which the mass reduction for each event is... lowest-effect level at two stormwater wetlands of age four and nine years Thus, although there are buildups to be expected, the amounts of trace metals appear to generally be below levels that would cause concern 14. 7 PESTICIDES IN EVENT-DRIVEN WETLANDS A wide variety of pesticides are often found in agricultural runoff For some, the removal rates in wetlands have been determined, as discussed in Chapter . the k-values reported for continuous-ow systems in the preceding chapters. Individ- ual events are afforded much better treatment. Carleton et al. (2001) also suggest that wetland-to-water- shed. over a 23-event, two-year period was more than 90% removal of chemical oxygen demand (COD), BOD, TSS, and ammonia, and 70–90% of phosphorus. 14. 3 TSS IN EVENT-DRIVEN WETLANDS Event-driven treatment. 539 14 Event-Driven Wetlands The treatment of stormwaters of various origins is of grow- ing concern as attempts to rectify point-source pollution reach maturity.