349 10 Phosphorus A great deal has been learned about wetland phosphorus pro- cessing over the last 15 years (Kadlec, 1999c, 2005c; Toet, 2003; Reddy et al., 2005). Treatment wetlands are capable of phosphorus (P) removal from wastewaters, on both short- term and long-term bases. Phosphorus is a nutrient required for plant growth, and is frequently a limiting factor for vege- tative productivity. A measure of relative ecosystem require- ments is the proportion among the nutrient elements in the biomass, which is often represented as a molar proportion of C:N:P 106:16:1, or 41:7:1 on a mass basis (the Redeld ratio). Wastewaters do not have this ratio except by rare chance, and most often, there is excess phosphorus in municipal or domestic wastewater. The introduction of trace amounts of this element into receiving waters can have profound effects on the structure of the aquatic ecosystem. When a wetland, either natural or constructed, is given a new supply of water and phosphorus, it responds by read- justing storages, pathways, and structure. If those new sup- plies are variable and within the stochastic band of historic inputs, a mature ecosystem will not change in character or function. But, treatment of water for phosphorus removal in the wetland implies that additions will signicantly exceed the historic stochastic band of the natural wetland, and a newly constructed wetland will require a successional period to adapt to the intended inputs. In both cases, a period of adaptation and change is to be expected. Thereafter, the wet- land functions in a long-term sustained mode, which is tuned to the inputs, but displays probabilistic variation as well. In general, treatment wetlands built specically for phosphorus removal are area-intensive compared to conventional waste- water treatment technologies. For the purpose of understanding phosphorus cycling, wetlands may be visualized as consisting of several compart- m e nts: water, plants, microbiota, litter, and soil (Figure 10.1). Naturally occurring inputs of phosphorus are from surface inows, and atmospheric deposition that consists of both wet deposition and dryfall. Outputs may be in the form of surface outows or inltration to groundwater. Inputs from groundwater and gaseous release to the atmosphere are less common or probable. Animal migration, ranging from insect movement to sh and bird travel, has been identied as a potential contribution to the phosphorus budget; but to date no quantication of this process exists. A large number of transfer and alteration processes occur, as indicated in Figure 10.1, but only soil-building pro- vides a net long-term storage of phosphorus. Sediment and soil accretion provides phosphorus storage that can alternate between deposition and erosion on a short-term basis. The wetland environment provides appropriate conditions for net long-term buildups, because inundation slows oxidative pro- cesses. Historical accumulations in natural wetlands are the genesis of peatlands. Natural accretions are on the order of a few millimeters per year (Mitsch and Gosselink, 1993). 10.1 PHOSPHORUS FORMS IN WETLAND WATERS Wetland science has evolved to focus on categories of phos- phorus compounds that are dened by methods of analy- sis (Table 10.1). In every case, the analytical procedure is reported as the elemental phosphorus content of the category, which is in contrast to the agricultural practice of reporting as P 2 O 5 . The most reactive forms are the dissolved phos- phates, which change hydration in response to pH. The most common species are mono- and dibasic phosphates, which dominate at all typical wetland pH values (4 pH 9) (Morel and Hering, 1993): HPO HPO H pK 7.2 24 4 2+ W (10.1) The generic term used for these inorganic phosphate ions is orthophosphate (PO 4 –P). The molybdate analytical test nom- inally nds this form of phosphorus, but has been shown to also detect exchangeable phosphorus and colloidally bound phosphorus in eutrophic wetlands (Baldwin, 1988; Hens and Merckx, 2002). However, phosphorus readily combines with, and may be part of, dissolved organic materials, and in that form has the designation of dissolved organic phosphorus (DOP). In fact, DOP has been characterized in great detail for treat- ment wetland situations, and found to consist of several kinds of organics (Turner and Newman, 2005). Some of them are readily hydrolyzed by soil enzymes, and together with PO 4 –P are called soluble reactive phosphorus (SRP). The organic components of SRP can move readily in soils and sediments (Anderson and Magdoff, 2005). Phosphorus also may be associated with suspended particles, and is called PP. Wetlands provide an environment for the interconversion of all these forms of phosphorus, with the eventual sink being one or more of the wetland solid compartments. A variety of cations can precipitate phosphate under certain conditions. The important potential mineral precipitates in the wetland environment include apatite (Ca 5 (Cl,F)(PO 4 ) 3 ) and hydroxyl- apatite (Ca 5 (OH)(PO 4 ) 3 ) (Reddy and D’Angelo, 1994). In addition to direct chemical reaction, phosphorus can co-pre- cipitate with other minerals, such as ferric oxyhydroxide and © 2009 by Taylor & Francis Group, LLC 350 Treatment Wetlands TABLE 10.1 Forms of Phosphorus in the Wetland Environment Dissolved forms (ltered (0.45 Mm) samples): • Orthophosphate (PO 4 –P). • Condensed phosphates. These consist primarily of pyro-phosphate, meta-phosphate, and poly-phosphates. • Soluble reactive phosphorus (SRP). PO 4 –P, together with some condensed phosphates. • Total dissolved phosphorus (TDP). Phosphorus that is convertible to PO 4 –P upon oxidative digestion. • Dissolved organic phosphorus (DOP). Phosphorus, in forms other than SRP, that is convertible to PO 4 –P upon oxidative digestion ( TDP–SRP). Dissolved plus associated with suspended solids. The procedures above performed on unltered samples yield, by analogy: • T otal reactive phosphorus (TRP) • Total acid hyrolyzable phosphorus (TAHP) • Total phosphorus (TP) • Total organic phosphorus (TOP) ( TP–TAHP) •P articulate phosphorus (PP) (TP–TDP) Sorbed to the surface of soil particles: • Sorbed phosphorus is remo ved using extractants such as water, or solutions of KCl or bicarbonate. Contained in the structure of biomass: • T otal phosphorus may be found by analyzing for PO 4 –P in digests of biomass samples. Digestion may involve dry or wet ashing, followed by re-dissolution. Contained in the structure of soil particles: • Structural, internal forms of phosphorus in the solid are removed (solubilized) using harsh extracts of wet soil samples. Typical extractants include: • Sodium hydroxide (0.1 M). The SRP in the extract is representative of iron and aluminum bound phosphorus. The balance of the TP in the extract (TP–SRP) is representative of organic phosphorus associated with humic and fulvic acids. • Hydrochloric acid (0.5 M). The SRP in the extract is representative of calcium bound phosphorus. • Total soil phosphorus may be found by analyzing for PO 4 –P in digests of soil samples. Digestion may involve dry or wet ashing, followed by re-dissolution. Chemically bound P Structurally bound P Chemical precipitation Sedimentation Mass transfer Litterfall Litterfall Volatilization Microbiota Porewater DP Sorbed P Solubilization Diffusion Uptake Sorption Uptake Infiltration Decomposition Decomposition Root zone Subsoil Litter Water Outflow CombustionMacrophytes Rainfall and dryfall Inflow PO 4 -P PO 4 -P PP PO 4 -P PH 3 PP DOP PO 4 -P FIGURE 10.1 Phosphorus storages and transfers in the wetland environment. PO 4 –P = orthophosphate; PP = particulate phosphorus; DP = dissolved phosphorus; PH 3 = phosphine. PP may consist of all the forms shown in the root zone. (From Kadlec and Knight (1996) Treatment Wetlands. First Edition, CRC Press, Boca Raton, Florida.) © 2009 by Taylor & Francis Group, LLC Phosphorus 351 the carbonate minerals, such as calcite (calcium carbonate), CaCO 3 (Reddy and D’Angelo, 1994). The overall P-mineral chemistry is very complex; consequently, quantitative calcu- lations of solubilities are generally not possible and computer models are used as estimation tools. Trends for phospho- rus movement in wetland soils are as follows (Reddy and D’Angelo, 1994): In acid soils, phosphorus may be xed by alumi- num and iron, if available. In alkaline soils, phosphorus may be xed by cal- cium and magnesium, if available. Reducing conditions lead to solubilization of iron minerals and release of phosphorus co-precipitates. If free sulde is present due to sulfate-reducing conditions, iron sulde can form and preclude iron mineralization of phosphorus. 10.2 WETLAND PHOSPHORUS STORAGES Phosphorus compounds are a signicant fraction of the dry weight of wetland plants, detritus, microbes, wildlife, and soils, although they are about ten times less than nitrogen compounds. The mass of these phosphorus storages varies in different wetland types, and with the season of the year. A general idea of the relative size of these various storage com- partments is necessary to understand the phosphorus uxes discussed herein (Figure 10.2). The dominant fraction of phosphorus is contained in the wetland soils and sediments. Plants and litter comprise most of the remainder, with very little mass contained in microbes, algae, and water. • • • 20 cm 25 cm Deep Soil Floc 50 g/m 2 at 0.5% P 0.25 g/m 2 Mineral suspended matter 2 g/m 2 at 0.5% P 0.01 g/m 2 Water 250 L/m 2 at 5 mg P/L 1.25 g/m 2 Soil (root zone) 40,000 g/m 2 at 0.15% P 60.00 g/m 2 Roots 1,000 g/m 2 at 0.25% P 2.50 g/m 2 Plankton and organic suspended matter 2 g/m 2 at 0.5% P 0.01 g/m 2 Periphyton 30 g/m 2 at 0.5% P 0.15 g/m 2 Live plants 2,000 g/m 2 at 0.25% P 5.00 g/m 2 Structural and mineral 45.00 g/m 2 Decomposable 15.00 g/m 2 Sorbed and porewater 0.25 g/m 2 Standing dead 500 g/m 2 at 0.25% P 1.25 g/m 2 Litter 500 g/m 2 at 0.25% P 1.25 g/m 2 Note: Dry mass is in italics and standing stock is in bold. FIGURE 10.2 Example of phosphorus storages in a treatment wetland. SRP is taken up by plants and converted to tissue phos- phorus or may become sorbed to wetland soils and sediments. Organic structural phosphorus may be released as soluble phosphorus if the organic matrix is oxidized. Insoluble pre- cipitates form under some circumstances, but may redissolve under altered conditions. A large amount of the phosphorus in the soil and sedi- ments is present in the organic fraction. Organic phosphorus forms can be generally grouped into: 1. Easily decomposable organic phosphorus (nucleic acids, phospholipids, and sugar phosphates) 2. Slowly decomposable organic phosphorus (inosi- tol phosphates or phytin) Organic phosphorus can be classed in decreasing order of bioavailability: microbial biomass phosphorus, labile organic phosphorus, fulvic acid-bound phosphorus, humic acid-bound phosphorus, and residual organic phosphorus. In general, large quantities of organic phosphorus can be immo- bilized in wetland soils, and only a small portion of the total organic phosphorus content is bioavailable. A major portion of organic phosphorus is stabilized in relatively recalcitrant organic phosphorus compounds (Dunne and Reddy, 2005). PLANT BIOMASS The phosphorus content of living biomass in marsh wetlands varies among species, among plant parts, and among wet- land sites. There is little variation from location to location within a homogeneous stand (Boyd, 1978). Example ranges of dry weight phosphorus percentages in natural wetlands are: 0.14–0.30% for emergent plants; 0.14–0.40% for oating © 2009 by Taylor & Francis Group, LLC 352 Treatment Wetlands leaved plants; and 0.12–0.27% for submersed plants (Boyd, 1978). Bedford et al. (1999) report a range of 0.1–0.64% for live plant tissue in 41 marshes. The biomass of either live or dead microbiota is virtually impossible to measure, but it is small compared to the biomass of live or dead macrophytes, or macrodetritus (litter). Treatment wetlands are often nutrient-enriched, and dis- play higher values of tissue nutrient concentrations than natu- ral wetlands. For instance, live cattail leaves in the discharge area of the Houghton Lake wetland averaged 0.18% phos- TABLE 10.2 Examples of Phosphorus Content of Various Wetland Plants (mg/kg Dry Weight) Wetland Name Concentration Plant Community Above Live Above Dead Below Reference Low P Wetlands Boney Marsh, Florida 25–50 Mg/L Mixed emergents 741 327 — Unpublished data Houghton Lake, Michigan 40 Mg/L Typha latifolia 895 — — Unpublished data ENRP, Florida 50–150 Mg/L Typha spp. 1,343 709 1,945 Unpublished data Aiken, South Carolina “Infertile” Typha latifolia 1,460 — — Boyd (1971) Lauwersoog, The Netherlands 1 mg/L Phragmites australis 1,870 520 — Mueleman et al. (2002) Alderfen, United Kingdom ~150 Mg/L Typha angustifolia 2,000 — — Mason and Bryant (1975) Alderfen, United Kingdom ~150 Mg/L Phragmites australis 2,800 500 1,000 Mason and Bryant (1975) Median 1,460 510 1,473 Tr eatment Wetlands Houghton Lake, Michigan Lagoon Typha angustifolia 1,842 — — Unpublished data Brisbane, Australia Secondary Baumea articulata 1,900 — 2,300 Browning and Greenway (2002) Sacramento, California Secondary Schoenoplectus acutus 2,000 2,000 4,000 Nolte and Associates (1998b) TVA Mussel Shoals, Alabama 15 mg/L P Schoenoplectus acutus 2,000 1,000 2,000 Behrends et al. (1996a) TVA Mussel Shoals, Alabama 15 mg/L P Schoenoplectus atovirens 2,000 1,000 3,000 Behrends et al. (1996a) TVA Mussel Shoals, Alabama 15 mg/L P Schoenoplectus cyperinus 2,000 1,000 2,000 Behrends et al. (1996a) TVA Mussel Shoals, Alabama 15 mg/L P Phalaris arundinacea 2,000 1,000 2,000 Behrends et al. (1996a) TVA Mussel Shoals, Alabama 15 mg/L P Phragmites australis 2,000 1,000 2,000 Behrends et al. (1996a) TVA Mussel Shoals, Alabama 15 mg/L P Typha spp. 2,000 1,000 3,000 Behrends et al. (1996a) Malham, United Kingdom Secondary Phalaris arundinacea 2,000 1,000 6,000 Hurry and Bellinger (1990) Brisbane, Australia Secondary Phragmites australis 2,400 — 3,300 Greenway (2002) Brisbane, Australia Secondary Philydrum lanuginosum 2,400 — 3,100 Browning and Greenway (2002) Listowel, Ontario Lagoon Typha latifolia 2,440 — — Herskowitz (1986) Sacramento, California Secondary Typha latifolia 2,500 2,500 3,800 Nolte and Associates (1998b) Brisbane, Australia Secondary Carex fascicularis 2,700 — 1,900 Browning and Greenway (2002) Sacramento, California Secondary Schoenoplectus acutus 2,800 2,800 4,000 Nolte and Associates (1998b) Houghton Lake, Michigan Lagoon Typha latifolia 3,259 2,570 — Unpublished data Kirinya Jinja, Uganda Primary Cyperus papyrus 3,540 — 5,890 Okurut (2001) Brisbane, Australia Secondary Schoenoplectus mucronatus 3,900 — 3,500 Browning and Greenway (2002) Kirinya Jinja, Uganda Primary Phragmites mauritianus 3,910 — 2,600 Okurut (2001) Sacramento, California Secondary Typha latifolia 4,200 4,200 5,300 Nolte and Associates (1998b) Median 2,400 1,000 3,050 phorus; those in nutrient-poor control areas averaged 0.09% pho sphorus (Table 10.2). In general, the median aboveg- round tissue phosphorus concentration of example wetlands increases from 0.15–0.24% as the water phosphorus increases from 0.1–0.5 mg/L to 3–15 mg/L. If the nutrient status of a wetland is increased from low (oligotrophic) to high (eutro- phic), there is a pronounced increase in tissue phosphorus concentration. The standing dead leaves have lesser phos- phorus concentrations than their live counterparts. Litter may have slightly greater or slightly lesser concentrations. © 2009 by Taylor & Francis Group, LLC Phosphorus 353 The phosphorus content of periphyton and plankton is higher than that for macrophytes, especially in phosphorus rich waters. Vymazal (1995) reports values typically rang- ing from 0.2–2.0% dry weight (2,000–20,000 mg P/kg) for a dozen different periphyton and plankton species. The nutrient content of many plant species, includ- ing Typha spp., display increasing tissue phosphorus with increasing phosphorus availability ranging from 0.05–0.5% dry weight. There is also an increase in standing crop with an increase in nutrient (phosphorus) availability, ranging from about 1,000 g/m 2 of biomass at low nutrient to 6,000 g/m 2 of biomass at high nutrient conditions. Aboveground biomass collected at the end of the grow- ing season displays much lower phosphorus content than in spring (Figure 10.3). Klopatek (1978) has shown trends of the same magnitude for cattail shoots, growing under lower nutrient availability. It is apparent that the timing of veg- etation sampling can greatly affect subsequent calculations of phosphorus storage and biomass translocation to below- ground rhizomes. Different plant parts may show large differences in phos- phorus content, and the seasonal variability may be very large. The extent of this variability is shown in Table 10.3 for Phragmites australis, for a reed stand near Grifth, New South Wales, Australia, with a warm dry continental climate and a water phosphorus concentration was 12 mg/L. There is about a factor of two difference among samples due to plant part and location within the plant part. Plant growth changes the proportions of stored phospho- rus in various plant parts as the seasons progress. The growth patterns vary with climate, as discussed in Chapter 3 (see Figures 3.10 and 3.11). The time-varying phosphorus concen- trations may be combined with the time-varying biomass for each compartment, which in total represent the phosphorus storage on a seasonal basis. Typically that storage follows the pattern of a growing-season increase to a maximum, followed by a senescence-season decrease to a minimum, with the cycle repeated each year (Figure 10.4). After the plants are fully mature, there is, on average, no net year-to-year increase in plant storage. However, on a seasonal basis, plant uptake can be a very large part of phos- phorus removal or release. For instance, the Lauwersoog, Netherlands, system receives an annual phosphorus loading of 30–40 g P/m 2 , or an instantaneous loading of 0.08–0.11 g P/m 2 ·d. In Figure 10.4, it is seen that the plants use about 10 g P/m 2 over a growing period of 100 days, or an instan- taneous removal of 0.10 g P/ m 2 ·d. Thus it seems that 100% 0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45 0.50 0 30 60 90 120 150 180 210 240 270 Days Growing Season Phosphorus Content (% dry weight) Typha glauca Phragmites australis (Australia) Phragmites australis (Netherlands) Schoenoplectus spp. L1 (New Zealand) Schoenoplectus spp. L2 (New Zealand) FIGURE 10.3 The decline of phosphorus content in aboveground tissues of wetland plants. The Typha marsh was unenriched, but the Phragmites stand was nutrient-enriched. The Schoenoplectus (Scirpus) wetlands were loaded at 0.2 g P/m 2 ·d and 0.9 g P/m 2 ·d for L1 and L2. (Typha data from Bayly and O’Neill (1972) Ecology 53(4): 714–719. Phragmites Australia data from Hocking (1989b) Australian Journal of Marine and Freshwater Research 40: 445–464. Phragmites Netherlands data from Mueleman et al. (2002) Wetlands 22(4): 712–721. Schoenoplectus data from Tanner (2001a) Wetlands Ecology and Management 9: 49–73.) TABLE 10.3 The Variation of Phosphorus Content with Plant Part for Phragmites australis at the Peak of the Standing Crop Stem Leaves Whole Shoot Top 0.135 0.173 0.163 Middle 0.095 0.148 0.125 Bottom 0.090 0.106 0.092 Source: Data from Hocking (1989b) Australian Journal of Marine and Freshwater Research, 40:445–464. © 2009 by Taylor & Francis Group, LLC 354 Treatment Wetlands 0 5 10 15 20 25 0 30 60 90 120 150 180 210 240 270 300 330 360 Yearday Phosphorus Content (g/m 2 ) Above Below Total FIGURE 10.4 Variation of phosphorus storage in the Lauwersoog, Netherlands, VF treatment wetland over the growing season. The amount in belowground organs is relatively constant, but storage in aboveground parts increases, and then decreases. This wetland received water at 10.6 mg/L. (Data from Mueleman et al. (2000) Wetlands, 22(4): 712–721.) of the applied phosphorus is utilized by the plants during the growing season. This phosphorus is returned during autumn senescence, less some fraction retained as permanent accre- tion of nondegradable organic and mineral matter. SOILS AND SEDIMENTS Most of the phosphorus in the soil column is structural phosphorus, both organic and inorganic. Very small frac- tions are found in pore water or as sorbed phosphorus. Most wetland treatment systems will build an organic sediment and soil layer with time. The rst step in soil-building is often the formation of an unconsolidated material of low density, called oc. This material partially decomposes, and consolidates with time, forming new soil layers in a FWS wetland. The initial soils in a constructed treatment wetland are the choice of the wetland designer. One choice is to use on-site mineral soils; another is to use imported organic materials, such as peats or compost. Typically, about 30 cm of soil is placed, and becomes involved as the root zone of the wetland plants. Selected soils may be de- cient in phosphorus if infertile mineral soils are used, or may contain a surplus of phosphorus if fertile, agricultural soils are employed. Here, we examine the phosphorus con- tent of soils that have adapted to treatment conditions over a period of several years. Floc It has only been in recent years that the presence of a loose and movable layer of material in FWS treatment wetlands has been recognized in the literature. Terminology varies: Kadlec and Bevis (1990) referred to it as “a grey gelatinous material (ooze)”; Nolte and Associates (1998b) as “Layer A”; DeBusk et al. (2001) as “muck or unconsolidated peat”; and SFWMD (2006) “oc.” Floc is easily disturbed, and ma y then move with water until it resettles (see Figure 7.9). Floc is harvested from tube corers by pouring or vacuum- ing it out. The phosphorus content is typically 0.1–0.4% dr y weight (Table 10.4), but the bulk density is low, about 0.02–0.04 g dry weight/cm 3 . Nevertheless, thicknesses up to 20–30 cm can contain 4–40 g P/m 2 . Movement rates have not been measured, but are speculatively about 1,000 m/yr in slow-moving wetland waters. Because most of the phospho- rus is structural rather than sorbed in the oc, these solids provide for the slow transport of phosphorus even in waters of extremely low dissolved phosphorus (Gaiser et al., 2005). However, decomposition can later release the phosphorus transported with oc. Base Soils The phosphorus content of organic soils that have experi- enced only low water-phase phosphorus (about 50 Mg/L) are generally in the range 300–500 mg/kg. However, if the ecosystem has long been exposed to higher phosphorus con- centrations (e.g., greater than 3 mg/L), then organic soil TP is 1,000–2,000 mg/kg (Table 10.4). These values pertain to the upper-most layer, which may still be experiencing decomposition. Because of the competition among plant uptake, detrital decomposition, and the transpiration ux in the root zone, there may exist a vertically decreasing prole of soil TP and pore water phosphorus in the soil column. An example is given in Figure 10.5, for the lightly fertilized zone of WCA2A in south Florida. The top soil layer (0–10 cm) typi- cally contains the most roots (see Chapter 3), which remove phosphorus from this zone of high concentration. Those roots also undergo a cycle of growth, death, and decom- position, and hence the return ux of available phosphorus is greatest in this top zone. Lower layers (10–20 cm and 20–30 cm) become progressively depleted by plant uptake, © 2009 by Taylor & Francis Group, LLC Phosphorus 355 TABLE 10.4 Example Phosphorus Contents of Wetland Flocs and Sediments System Water Vegetation Horizon Bulk Density (g/cm 3 ) TP (mg/kg) Storage (gP/m 2 ) Reference Low P Wetlands WCA2A, Florida 10 Mg/L Cladium jamaicense 0–5 cm 0.05 666 1.8 Reddy et al. (1991) WCA2A, Florida 10 Mg/L Cladium jamaicense 5–10 cm 0.10 380 1.9 Reddy et al. (1991) Boney Marsh, Florida 25–50 Mg/L Mixed Emergents 0–5 cm 0.18 652 5.9 Unpublished data Boney Marsh, Florida 25–50 Mg/L Mixed Emergents 5–10 cm 0.39 487 9.5 Unpublished data STA6, Florida 19 Mg/L Mixed Emergents 0–10 cm 0.60 492 29.7 Unpublished data STA5 C1A, Florida 80 Mg/L Mixed Emergents 0–10 cm 0.65 446 28.8 Unpublished data STA2 C3, Florida 26 Mg/L Mixed Emergents 0–10 cm 0.24 526 12.7 Unpublished data M oder ate P Wetlands Sacramento, California Secondary Schoenoplectus acutus 0–5 cm 0.6 147 4.4 Nolte and Associates (1998b) Houghton Lake, Michigan 3 mg/L Typha latifolia 0–5 cm 0.2 1,268 12.7 Unpublished data Houghton Lake, Michigan 3 mg/L Typha latifolia 5–10 cm 0.2 1,180 11.8 Unpublished data Tres Rios H1, Arizona 3 mg/L Scirpus spp. 0–5 cm 0.6 1,242 37.3 NADB database (1998) Tres Rios H2, Arizona 3 mg/L Scirpus spp. 0–5 cm 0.6 1,365 41.0 NADB database (1998) Tres Rios C1, Arizona 3 mg/L Scirpus spp. 0–5 cm 0.6 958 28.7 NADB database (1998) Tres Rios C2, Arizona 3 mg/L Scirpus spp. 0–5 cm 0.6 1,294 38.8 NADB database (1998) Ar eal Density (g/m 2 ) TP (mg/kg) Storage (gP/m 2 ) Floc Houghton Lake, Michigan 3 mg/L Typha latifolia — 634 4,088 2.6 Unpublished data Houghton Lake, Michigan 0.1 mg/L Typha latifolia Carex spp. — 266 1,767 0.5 Unpublished data Sacramento, California Secondary Schoenoplectus acutus — 11,630 4,000 46.5 Nolte and Associates (1998b) Sacramento, California Secondary Schoenoplectus acutus — 6,320 4,000 25.3 Nolte and Associates (1998b) STA6, Florida 19 Mg/L Mixed Emergents — 4,398 991 4.4 Unpublished data STA5 C1A, Florida 80 Mg/L Mixed Emergents — 3,509 1,075 3.8 Unpublished data STA2 C3, Florida 26 Mg/L Mixed Emergents — 5,700 699 4.0 Unpublished data Note: Bulk densities in italics are estimated. and because of lesser root biomass. The top layer is also the site of newest depositions of detrital and particulate phos- phorus (PP). It is not common for the new sediments and soils in a treatment wetland to be inorganic in character. However, systems treating runoff may receive considerable quantities of inorganic solids from soil erosion in the watershed, which then combine with organic materials generated within the wetland. An example is Chiricahueto marsh in Mexico (Soto-Jiménez et al., 2003). Agricultural runoff brought water at about 10.2 mg/L of total phosphorus (TP) to the marsh for over 50 years. The soil column is now mostly inorganic, with less than 5% carbon. Both carbon and phos- phorus decreased together as depth increased, indicating © 2009 by Taylor & Francis Group, LLC 356 Treatment Wetlands that most of the soil phosphorus was associated with the organic content. The phosphorus content of the upper 10 cm at Chiricahueto was 1,200 Mg P/g, decreasing to 300 Mg P/g at 60–70-cm depth. Soil Phosphorus Speciation Some of the accreted phosphorus occurs as minerals that are not particularly susceptible to remobilization, but new organic substances are subject to degradation if conditions change within the wetland. For instance, dry-out can cause oxidation of some of the stored organics, which may be com- pletely stable under submerged conditions. The associated organic-bound phosphorus may then be mineralized, and redissolve on rewetting (Olila et al., 1997; Pant and Reddy, 2001a). Phosphorus that is bound to ferric iron compounds under oxic conditions may be released if reducing conditions sub- sequently occur. Under reducing conditions, ferric iron can be converted to soluble, ferrous forms. Soto–Jiménez (2003) speculate that under reduced conditions, decomposition pro- cesses of organic matter use manganese oxides and iron oxy- hydroxides as electron acceptors, releasing PO 4 −3 ions to pore water solution. These PO 4 −3 ions are then transported from the 0–8-cm sediment layer to the water column, where they are transformed into suspended particulate oxides and trapped in the surface layer. This upward migration and reincorporation into the sediment favored the enrichment of phosphorus in the upper sediments. Sorbed orthophosphate may partially desorb if the water concentrations become lower. However, under continuous inundation and stable water chemistry and redox conditions, stored phosphorus is not likely to be released. The forms of soil phosphorus vary depending upon the chemistry of the wetland that formed them. Figure 10.6 –40 –30 –20 –10 0 10 20 30 40 0.0 0.2 0.4 0.6 0.8 1.0 1.2 Total Phosphorus (TP) in Water (mg/L) and Soil (mg/g) Depth (cm) Surface Water Porewater Soil TP FIGURE 10.5 Vertical proles of phosphorus concentrations in water and soil at a station in Water Conservation Area 2A of the Everg- lades, Florida. The overlying water was at 100–200 µg/L, without much vertical variation. Pore water under the surface was at about four times higher concentration, but rapidly became less concentrated with increasing depth. (Data from Reddy et al. (1991) Physico-chemical properties of soils in the Water Conservation Area 2 of the Everglades. Report to the South Florida Water Management District, West Palm Beach, Florida.) Ruck’s Stream Labile Iron + Aluminum Calcium + Magnesium Organic Residual (a) FIGURE 10.6 Speciation of phosphorus in soils. The upper panel shows a stream bed that is dominated by iron- and aluminum-bound phosphorus. The lower panel shows a cattail wetland dominated by calcium- and magnesium-bound phosphorus, plus a large fraction of refractory organic phosphorus. ENRP Test Cell Labile Iron + Aluminum Calcium + Magnesium Organic Residual (b) shows two distinct speciations, one is dominated by iron and aluminum compounds, and the other is dominated by cal- cium and magnesium compounds. The ferruginous system is likely to respond to lowering redox conditions by releasing sorbed phosphorus, but the calcitic system will not. © 2009 by Taylor & Francis Group, LLC Phosphorus 357 10.3 PHOSPHORUS PROCESSING IN FWS WETLANDS There are three principal categories of phosphorus removal processes in wetlands: sorption, utilization to build a bigger biomass compartment, and storage as newly created, refrac- tory residuals (burial). The rst two mechanisms (sorption and biomass storage) have nite phosphorus retention capacities, whereas the third, accretion, is a sustainable process. Sorp- tion and biomass-building are sometimes termed saturable mechanisms, because they reach that limited capacity, and then can remove no more phosphorus. However, sustainable mechanisms continue, with no capacity limit. In addition to these, secondary processes such as particulate settling and movement among storage compartments also exist. Particulate settling can rapidly remove large amounts of phosphorus from runoff waters that carry high amounts of suspended sediment. There may also be rearrangements of the phosphorus storages within the wetland that affect its availability and mobility. SORPTION There is a misconception that wetlands provide phosphorus removal only through sorption processes on existing soils. It is true that most soils do have sorptive capacity for phospho- rus, but this storage is soon saturated under any long-term increase in phosphorus loading. A number of different sorp- tion isotherms have been proposed, with the Langmuir for- mulation among them: S S C C k max eq eq 1 (10.2) where S sorbed phosphorus concentration, mg P/kg ssoil maximum sorbed phosphorus concent max S rration, mg P/kg soil equilibrium phosph eq C oorus concentration in water, mg/L 1/ halfk ssaturation concentration, mg/L It is sometimes the case that a soil sample contains sorbed phosphorus when it is retrieved from the eld. The amount of pre-sorbed initial phosphorus (S i ) corresponds to a water concentration termed the initial equilibrium phosphorus concentration (at S i , C eq EPC o ), and these are related by Equation 10.2. If water at concentration EPC o is added to the soil, there is neither adsorption nor desorption. Water at lower concentrations causes desorption, while water at higher concentration causes adsorption. A hypothetical example of sorption behavior is shown in Figure 10.7. Typical values of the Langmuir sorption parameters are shown in Table 10.5. The maximum sorbed concentration (S max ) of soils for binding phosphorus have been found to depend strongly upon the amount of iron and aluminum in the soil (Lijklema, 1977; Reddy et al., 1991, 1998): S max a (C Fe C Al )(10.3) where proportionality constant, mg P/mmola C FFe oxalate-extractable iron concentration iin soil, mmol/kg oxalate-extractable al Al C uuminum concentration in soil, mmol/kg The constant a was reported as 0.24 r 31 7.44 (Reddy et al., 1991), and for a different set of soils 0.17 × 31 5.27 (Reddy et al., 1998). An alternative sorption formulation is the Freundlich isotherm: SKC n eq (10.4) Sorption is generally described as a two-step process (Kadlec and Rathbun, 1984): 1. Phosphorus rapidly exchanges between the soil pore water and soil particles or mineral surfaces (adsorption). 2. Phosphorus slowly penetrates into solid phases (absorption). –200 0 200 400 600 800 1,000 0 5 10 15 20 25 Water Phase Phosphorus (mg/L) Sorbed Phosphorus (mg/kg) EPC o FIGURE 10.7 Hypothetical example of Langmuir adsorption of phosphorus on a wetland sediment. The initial amount sorbed is S o = 200 mg/kg, and EPC o = 1.23 mg/L (arrow). The Langmuir parameters are S max = 1,500 mg/kg and 1/k = 8 mg/L. © 2009 by Taylor & Francis Group, LLC 358 Treatment Wetlands TABLE 10.5 Summary of Langmuir Isotherm Parameters for Natural and FWS Wetlands System Type Material Type Material Location Half- Saturation Concentration, 1/k (mg P/L) S max (mg P/kg) Sorbed P @1mg/L (mg P/kg) Estimated Bed Life a (years) Reference FWS Soil Dunhill Ireland 0.3 1,463 1,125 166 Dunne et al. (2005b) FWS Soil Johnstown Castle Ireland 0.8 618 343 51 Dunne et al. (2005b) FWS Soil Byron Bay Wetland (original) NSW, Australia 3.1 4,484 1,094 161 Sakadevan and Bavor (1998) FWS Soil Byron Bay Wetland 2 NSW, Australia 5.6 4,237 642 95 Sakadevan and Bavor (1998) FWS Soil Byron Bay Wetland 3 NSW, Australia 4.6 5,208 930 137 Sakadevan and Bavor (1998) FWS Soil Carcoar soil NSW, Australia 5.3 934 148 22 Sakadevan and Bavor (1998) FWS Soil Richmond subsoil NSW, Australia 15.2 1,727 107 15.9 Sakadevan and Bavor (1998) FWS Soil Richmond topsoil NSW, Australia 42.5 1,153 27 4.1 Sakadevan and Bavor (1998) FWS Soil BEEC wetland Nova Scotia, Canada — 1,270 — — Stratton et al. (2005) FWS Soil Eureka wetland Nova Scotia, Canada — 1,862 — — Stratton et al. (2005) FWS Soil Banweol high marsh South Korea 1.3 227 99 15 Yoo et al. (2006) FWS Soil Banweol low marsh South Korea 0.7 260 153 23 Yoo et al. (2006) FWS Soil Donghwa high marsh South Korea 1 227 114 17 Yoo et al. (2006) FWS Soil Donghwa low marsh South Korea 2.8 165 43 6.6 Yoo et al. (2006) FWS Soil Samhwa marsh South Korea 2 155 52 7.8 Yoo et al. (2006) Natural Soil Biscayne wetland soil Florida 74.6 3,156 42 6.3 Zhou and Li (2001) Natural Soil Fisheating Creek Basin Florida 4.3 98 18 1.8 Reddy et al. (1995) Natural Soil Immokalee soil Florida 4.7 355 62 9.4 Graetz and Nair (1995) Natural Soil Indian River (aerobic) Florida 0.7 133 78 4.0 Pant and Reddy (2001b) Natural Soil Indian River (anaerobic) Florida 2.3 32 10 0.7 Pant and Reddy (2001b) Natural Soil Istokpoga Basin Florida 2.1 28 9 0.4 Reddy et al. (1995) Natural Soil Lower Kissimmee River S-154 Basin Florida 7 297 37 1.9 Reddy et al. (1995) Natural Soil Lower Kissimmee River S-65D Basin Florida 9 418 42 2.3 Reddy et al. (1995) Natural Soil Myakka soil Florida 1.7 505 187 28 Graetz and Nair (1995) Natural Soil Pomello soil Florida 6.8 168 22 3.4 Graetz and Nair (1995) Natural Soil Chao Lake (mesotrophic) Jiangsu, China 1.8 211 75 11.3 Wang et al. (2006b) Natural Soil East Taihu Lake (mesotrophic) Jiangsu, China 0.7 208 122 18.2 Jin et al. (2005) Natural Soil Hongze Lake (mesotrophic) Jiangsu, China 3.2 714 170 25.2 Wang et al. (2006b) Natural Soil Poyang Lake (mesotrophic) Jiangsu, China 1.7 432 160 23.7 Wang et al. (2006b) © 2009 by Taylor & Francis Group, LLC [...]... phosphorus doses in mesocosms A total dose of 1 ,100 g/L was reduced to 50 g/L 360 Treatment Wetlands TABLE 10. 6 Summary of Freundlich Isotherm Parameters for Various Substrates in Treatment Wetlands Material Sand Peat Shellrock Quartz gravel Peat Peat Mineral soil 0 10 cm unimpacted peat 10 20 cm unimpacted peat 0 10 cm impacted peat 10 20 cm impacted peat Half-burned dolomite sand Kd (mg/kg)*(mg/L)n 4.79... in-stream wetland in North Carolina, and found both uptake and release at different times and places, which they attributed to hydroperiod and time-varying input phosphorus Dry-Out and Rewetting Continuous-flow FWS treatment wetlands maintain standing water over the entire year, which allows the maximal rates of accretion (see Table 10. 7) This distinguishes them from natural wetlands and stormwater wetlands, ... after dry-out in the performance records of large treatment wetlands Figure 10. 12 shows the fifth year of operation for STA6, one of the phosphorus-removal treatment wetlands in south Florida The very first waters to enter the system after the dry season produce a surge of phosphorus in the water, which declines as the wetland refills, prior to any outflow This batch-filling mode produces treatment, ... and degree of hydraulic detail Twenty-eight percent of these (79/282) have been tracer tested to determine hydraulic efficiency (number of tanks-in-series, NTIS) Full-scale FWS treatment wetlands with phosphorus removal data span five orders of magnitude in size, from about 0.01 to 2,000 ha (see Table 10. 10 for examples) They are loaded from less than 1 to over 10 cm/d Inlet phosphorus concentrations... associated with each inlet concentration (Figure 10. 17) For any specific inlet concentration, or a narrow inlet concentration range, the slope of the data cloud is about 0.33 (Figure 10. 17), but the resultant outlet concentration range moves upward to higher values The right-hand asymptote of 376 Treatment Wetlands TABLE 10. 10 Examples of Phosphorus Treatment Wetlands Site Warangal Listowel 4 Linköping... 2.39 6.2 10. 06 48.6 73 88.7 95.5 263 498 1,545 1,800 4.28 1.95 4.00 6.32 8.88 9.31 0.89 7.00 4.52 1.39 2.01 10. 96 0.75 5.61 3.14 0.52 2.61 3.02 7.375 3.165 0.281 2.438 0.870 3.090 3. 210 0 .103 0.525 0.667 0.052 0.175 0. 110 0.280 6.080 8.365 0 .104 0.499 1.742 0.624 0.078 2.287 0.576 3.350 0.550 0.035 0.265 0.364 0.021 0.090 0.070 0 .106 2.320 4.181 0.021 0.232 115.21 22.57 4.11 56.20 28.20 105 .00 10. 43 2.59... 200 100 0 0 365 730 1,095 Time (days) FIGURE 10. 13 Floodwater phosphorus during the start-up of STA-1W Cell 5 of the Everglades Protect project Antecedent land use was agriculture (Unpublished data from South Florida Water Management District.) © 2009 by Taylor & Francis Group, LLC 372 Treatment Wetlands Total Phosphorus Removal 100 % 80% 60% 40% 20% 0% 0 365 730 1,095 1,460 Time (days) FIGURE 10. 14... cold-climate wetlands, nonwoody aboveground growth occurs from a zero starting point in early spring Most aboveground leaves and stems persist through the growing season, and are measurable as the end-of-season standing crop Under these circumstances, the turnover of plant material is the ratio of the end-of-season standing crop to the average growing-season GPP rate Macrophyte turnover in cold-climate... dimensionless The Jackson Bottoms data fit Equation 10. 10, for areal rate constant (k) with R2 0.94 Listowel, Ontario, Canada, systems 4 and 5 received the same water over a four-year period, and fit Equation 10. 10 with R2 0.90 (Figure 10. 22) Total Phosphorus kV (d–1) 0.8 System 4 System 5 Regression 0.6 0.4 0.2 0.0 0 5 10 15 20 25 30 Depth (cm) FIGURE 10. 22 Dependence of the volumetric phosphorus rate... explorations Examples are discussed later in this chapter Annual Rate Constants Results across systems are given here, for measured P-values, or for the mean of those (P 3.4) when not measured The value C* 0.002 mg/L is used, and the remaining model parameter is the k-value, selected to fit Equation 10. 9 The median annual rate constant was k 10. 0 m/yr (Table 10. 11) The 10th–90th percentile range is 1.4–60 m/yr . Figure 10. 4, it is seen that the plants use about 10 g P/m 2 over a growing period of 100 days, or an instan- taneous removal of 0 .10 g P/ m 2 ·d. Thus it seems that 100 % 0.00 0.05 0 .10 0.15 0.20 0.25 0.30 0.35 0.40 0.45 0.50 0. macrophytes, or macrodetritus (litter). Treatment wetlands are often nutrient-enriched, and dis- play higher values of tissue nutrient concentrations than natu- ral wetlands. For instance, live cattail. measurable as the end-of-season stand- ing crop. Under these circumstances, the turnover of plant material is the ratio of the end-of-season standing crop to the average growing-season GPP rate.