Nutrient removal from eutrophic lake water by wetland filtration

Lake Apopka is a large (125 km(2)). shallow (mean depth 1.6 m) lake in Florida. USA. The lake was made hypereutrophic by phosphorus loading from floodplain farms and has high levels of nutrients, phytoplankton (Ch1 a 80 mug l(-1)), and suspended matter. The restoration plan developed by the St. Johns River Water Management District encompasses the biomanipulation concept in which the critical step for large shallow lakes is increasing the transparency of the water to allow the re-establishment of submerged macrophytes. Restoration includes operation of a treatment wetland, reduction in external P loading, harvest of fish, fluctuation of lake levels. and littoral planting. The District constructed a 2-km(2) pilot-scale treatment wetland to test nutrient-removal and hydraulic performance, Lake water was recirculated for 29 months. and the removal of suspended solids and particle-bound nutrients was assessed. Hydraulic loading rate varied from 6.5 to 65 m year(-1) with a mean hydraulic residence time of about 7 days. The inflow contained 40-180 mg l(-1) TSS, 80-380 mug l(-1) TP (mostly particulate organic). and 3-9 mg l(-1) TN (mostly dissolved and particulate organic). Overall, particulate matter was removed (> 90% by the wetland, and soluble organic compounds were unaffected. Soluble inorganic compounds such as nitrate, ammonia, and soluble reactive phosphate (SRP) were low in the lake water but increased during passage through the wetland. Particulate matter at the outlet was enriched in both N (2-fold) and P (5-fold) compared to particles in the inflow. Mass removal efficiencies were 89-99 (TSS), 30-67 (TP), and 30-52% (TN), but efficiency fell when hydraulic short-circuiting occurred. First-order removal coefficients were 107 (TSS), 63 m year(-1) (TP) and 98 m year(-1) (particulate N). Areal particulate removal rates were 5.4 g dry matter m(-2) day(-1), 0.18 g PON m(-2) day(-1), and 0.006 g POP m(-2) day(-1). The ratio of N:P removal was 28:1. Total sedimentation rate was 0.4 mm day(-1) of very light matter 4.4 g dw l(-1)). About 40% of the dry matter and nitrogen removed and about 80% of the phosphorus was found in the new sediments. Relative to the inflow of lake water, evapotranspiration (4.3%), seepage (2.6%). and rainfall (2.8%) were low. Major problems were initial leaching of SRP. but not ammonia, from native organic soils and vegetation when this former farmland was flooded hydraulic short-circuiting via former drainage ditches: and low inflows under drought conditions. After 6 months SRP release declined, and initial SRP leaching could be prevented with soil treatment. Hydraulic short-circuiting occurred only after modifications were made. Low gravity flows were augmented with pumped inflows. With these improvements P-removal should increase from the measured 0.48 to at least 3 g P m(-2) year(-1). Based on the pilot project results, the first phase of an improved 14-km(2) wetland filter has been constructed. This project should accelerate improvements in the water quality of Lake Apopka Mid. ultimately. create a new, large wildlife-rich marsh. (C) 2002 Elsevier Science B.V. All rights reserved.