Phosphorus Retention Capacity of a Pilot Stormwater Treatment Area in the Lake Okeechobee Basin Y. Wang 1, K.R. Reddy 1, V. Nair 1, O. Villapando 2, and A.L. Wright 1 1 Soil and Water Science Dept., University of Florida, Gainesville, FL 2 South Florida Water Management District, West Palm Beach, FL INTRODUCTION Stormwater Treatment Areas (STAs) located strategically in the Lake Okeechobee Basin (LOB) are used to reduce phosphorus (P) loads into the lake. It is important to document baseline soil conditions of STAs to allow evaluation of changes in physical, chemical and biological functions across time. The objectives of the study were to: 1) determine the maximum P retention capacity of soils under aerobic and anaerobic conditions, and 2) establish the relationship between P retention and soil physico-chemical properties. MATERIALS AND METHODS Single-point isotherms (SPI) (100 mg P L -1 ) were measured for 140 samples to assess the P retention capacity of soils collected from Taylor Creek STA (Fig. 1a). Batch multi-point P sorption isotherms were conducted under aerobic and anaerobic (flooded) conditions along an inflow-outflow transect of the STA (Fig. 1b). Phosphorus sorption was measured using 1 g of air-dried soil equilibrated with 10 mL of 0.01 M KCl containing various levels of P (0, 0.1, 1, 5, 10, 25, 50, and 100 mg P L -1 ) in a mechanical shaker for 24 hours. The solutions were filtered through 0.45 µm membrane filters and analyzed for P by a Technicon Autoanalyzer 3 segmented-flow system (EPA 365.1). Anaerobic isotherm soil samples were pre- incubated for four weeks and purged with N 2 weekly, followed by extraction and P analysis as described above. 1 M HCl-extractable Ca, Mg, Fe, and Al were measured by ICAP (EPA 200.7) and related to soil properties (Table 1). Phosphorus sorption parameters were determined by the Langmuir equation: C/S=1/(k*S max )+C/S max S’ where S = the total amount of P sorbed (mg kg -1 ) (S’+So), C = P concentration after 24 hr equilibration (mg L -1 ), S max = P sorption maximum (mg kg -1 ), and k = a constant related to the bonding energy (L mg -1 ). At low equilibrium concentrations, the relationship between S’ and C are typically linear. The P sorption parameters were estimated by a least square fit of data using the following equation: S’ = Kd*C – So, where S’ = P sorbed by the solid phase (mg kg -1 ), So = P originally sorbed on the solid phase (mg kg -1 ), and Kd = a linear adsorption coefficient (L kg -1 ). EPC = So/Kd where EPC=equilibrium P concentration (mg L -1 ) when S’=0. The maximum P retention capacity of soils was estimated using single-point isotherms, S max -aerobic, and S max -anaerobic (Fig. 6). These estimates suggest that Taylor Creek STA soils are capable of adsorbing an average of 60 g P m -2. Assuming the STA is loaded at 2- 5 g P m -2 yr -1, these soils represent a P retention expectancy of 12 to 30 years. Long-term P retention in Taylor Creek STA depends not only on soil P sorption capacity, but also on the physico-chemical characteristics of newly accreted organic and mineral matter. CONCLUSIONS The physico-chemical properties of Taylor Creek STA soils exhibited significant P retention capacity within the top 30 cm. Low EPC values suggest that these soils potentially function as sinks for P at relatively low surface water P concentrations. The P-retention capacity was not adversely impacted by flooding as indicated by minimal differences in S max between aerobic and anaerobic conditions. The SPI was a suitable method to estimate S max. The relationships of Langmuir parameters and soil P forms show that Fe/Al-P and Ca/Mg- P are excellent indicators of P retention capacity. The soil P sorption capacity was significantly correlated with HCl- extractable Fe+Al and Ca+Mg, indicating that these cations were responsible for P sorption. The long-term effectiveness of the STA soils depends on P loading and contact of water column P with underlying soils. In addition, vegetation and the extent of particulate matter accretion, would also affect the overall capacity of the Taylor Creek STA to retain P. RESULTS Fig. 6. Phosphorus retention maxima estimated from the Langmuir model and the single point isotherm (SPI). There was no significant difference in P sorption capacity between soils incubated under aerobic and anaerobic conditions (Fig. 3a). The EPC, k, and initial P present in the soil adsorbed phase (So) were not affected by flooding (Fig. 3b and 3c). Phosphorus retention capacity measured using the SPI was shown to be good predictor of S max (Fig. 4). Wetland Biogeochemistry Laboratory Fig. 1. Soil sampling locations in the Lake Okeechobee watershed. The red color (b) indicates the sample locations from inflow to outflow. Cell 1 Cell 2 The significant relationship between EPC and 0.01 M KCl-extractable Pi is presented in Fig. 5. The soil extractable Pi may serve as a simple way to estimate EPC, but more research on different soils is needed. The SPI was significantly related to HCl-extractable Ca, Mg, Fe, and Al under aerobic conditions, and gave the following equation (R 2 =0.82, n=138): SPI (mmol/kg) = 0.01 HCl (Ca+Mg) (mmol/kg) HCl (Fe+Al) (mmol/kg) – 2.9 The P sorption capacity soils was mainly attributed to soil Fe and Al concentrations (P<0.0001), but also to extractable Ca and Mg (P<0.0004). The SPI can be estimated as 10% of the HCl-extractable (Fe+Al) (mmol/kg) + 1% of the HCl-extractable (Ca+Mg) (mmol/kg). Cell 2 Cell 1 a b Fig. 2. Soil P forms in 0-10 cm and cm soil depth intervals. Langmuir parameters k, S max, and EPC were significantly correlated with soil P forms (Table 2 and Fig. 2). The controlling factors for EPC included both inorganic and organic P forms, but these concentrations of these forms differed between aerobic and anaerobic conditions. Negative correlation of k with Fe/Al bound P was observed. Ca/Mg bound P and organic P were the main contributors to soil P retention maximum under aerobic conditions, but Fe/Al bound P was the most important factor under anaerobic soil conditions. Fig. 3. Comparison of the P sorption curve (a), the Langmuir P sorption parameter S 0 (b), and k (c) under aerobic and anaerobic conditions. a b y = 2.25 x R 2 = 0.75** n= S 0 _Aerobic (mg kg -1 ) S 0 _Anaerobic (mg kg -1 ) k_Aerobic (L mg -1 ) k_Anaerobic (L mg -1 ) c 1:1 a b Fig. 4. Relationship between S max and P sorbed at 100 mg P L -1 (SPI) under aerobic and anaerobic conditions. Fig. 5. Relationship between EPC and 0.01 M KCl- extractable Pi (at P=0) under aerobic conditions. y = 0.24 x R 2 = 0.96, n= M KCl extractable Pi - aerobic (mg kg -1 ) EPC-aerobic (mg L -1 ) Table 2. Multivariable linear correlations for Langmuir parameters, EPC, k, S max, and various P forms in the soil. ParameterConditionRegressionnR2R2 EPC Aerobic EPC = (Exch P) (Ca/Mg P) (Residue P)260.87** Anaerobic EPC = (Fe/Al P) (Organic P)230.64** k Aerobic k = (Fe/Al P)260.32* Anaerobic k = (Fe/Al P)230.32* S max Aerobic S max = (Exch P) (Ca/Mg P) (Organic P)260.81** Anaerobic S max = (Exch P) (Fe/Al P) + 22 (Ca/Mg P)- 2.8 (Organic P)230.74** Cell Sites from Inflow to Outflow P Retention Maxima at 0-30 cm (g P m -2 ) SPISmax_AerobicSmax_Anaerobic Table 1. Selected characteristics of 0-10 and cm soil (n=140). BD=bulk density, LOI=loss on ignition, TN=total nitrogen, TC=total carbon, TP=total phosphorus, TPi=total inorganic phosphorus, WEP=water extractable Pi, M1=Mehlich 1-P, SPI=single point isotherm DepthBDpHLOITNTCTPTPiWEPM1-PSPI cm g cm -3 -% g kg mg kg Cell 2 Cell 1