Phosphorus cycling in a sedimentation pond of a constructed wetland. Santiago Clerici Michael Krom, Robert Mortimer and Sally MacKenzie

Transcription

1 Phosphorus cycling in a sedimentation pond of a constructed wetland Santiago Clerici Michael Krom, Robert Mortimer and Sally MacKenzie

2 WWT Slimbridge Effluents high in N, P, Organic Matter and suspended solids River Severn River Severn estuary downstream

3 Ditch The treatment wetland Sedimentation pond to remove suspended matter Inflow Reserve 100 mts Cascade PO 4 and excess suspended matter taken up by reed beds

4 Failing of the treatment wetland Excess P is being exported from the system Palmer Felgate et.al 2011: Sedimentation pond becomes source of P in summer Collapse of algal blooms in early Summer: P released by redox reactions when oxygen levels go down P released by decomposition of algal matter

5 Objectives To find the dominant processes controlling P cycling in the sedimentation pond in Spring and Summer To know whether the sediment is a source or a sink of Phosphorus? To find the major processes controlling P cycling in the sediment and whether they change seasonally?

6 Model of P cycling in the water column INLET Part P Inflow PO 4 Inflow Part P Outflow POND Part P Algal uptake Respiration POND PO 4 PO 4 Outflow Resuspension Settling Sediment fluxes Moles P / day

7 Chl.a (μg/l) DO (% saturation) DO (% saturation) Chlorophyll and Oxygen levels DO - March Bloom :00 18:00 00:00 06:00 12:00 18:00 time Chl.a 450 Bloom DO - June Crash 50 Crash 25 0 Mar-11 Apr-11 May-11 Jun-11 date 0 12:00 18:00 00:00 06:00 12:00 18:00 time Night hours

8 Sampling P at inlet and pond INLET Part P Inflow PO 4 Inflow Part P Outflow POND Part P Algal uptake Respiration POND PO 4 PO 4 Outflow Settling Resuspension Sediment fluxes

9 PO4 (μm P) PO4 (μm P) Calculating water column fluxes INLET Obscured incubation Part P Inflow PO 4 Inflow 25 Only respiration Part P Outflow POND Part P Algal uptake Respiration POND PO 4 PO 4 Outflow Settling Resuspension Sediment fluxes 20 12:00 18:00 00:00 06:00 12:00 18:00 time Open incubation 25 Photosynthesis + respiration 20 12:00 18:00 00:00 06:00 12:00 18:00 time

10 PO4 (μm P) PO4 (μm P) Calculating sediment fluxes Part P Inflow INLET PO 4 Inflow PO 4 in Benthic Chamber #1 March 1.5 Part P Outflow POND Part P Algal uptake Respiration POND PO 4 PO 4 Outflow Settling Resuspension Sediment fluxes 0 12:00 18:00 00:00 06:00 12:00 time PO 4 in Benthic Chamber #1 June :00 18:00 00:00 06:00 12:00 18:00 time

11 P fluxes March INLET Part P Inflow 14 PO 4 Inflow 2 Part P Outflow 12 Pond Part P Algal uptake 1 Pond PO 4 PO 4 Outflow 1 14 moles 2 moles Settling 4 Sediment fluxes 0.1/0.2 Fluxes in moles P / day

12 P fluxes June INLET Part P Inflow 16 PO 4 Inflow 47 Part P Outflow 13 Pond Part P 9 moles Settling 6 Algal uptake 2 Respiration 1 Pond PO 4 35 moles Sediment fluxes 10 PO 4 Outflow 55 Fluxes in moles P / day

13 Sediment phosphorus cycle (after Slomp et al 1996) PO 4 To outlets Water Sediment Organic P Pore water PO4 Fe~P Ca~P Apatite P Water: it transports PO 4 Solid: it retains P but it can be released it back into water Solid: it retains P indefinitely

14 depth (cm) depth (cm) Description of the solid phase sediment content 20x increase % O 2 saturation -15 porosity

15 depth (cm) depth (cm) Organic P Pore water PO 4 Increased respiration in June PO 4 um NH4 5 Organic P Pore water PO4 200 moles P released between March and June Organic P (μ mole P / litre) March June -16 june march

16 depth (cm) depth (cm) Iron~P Pore water PO 4 Increased dissolved iron in June PO 4 5 um Fe Pore water PO4 Fe~P moles P released between March and June Iron~P (μ mole P / litre) March June -16 june march

17 depth (cm) depth (cm) Apatite P Pore water PO 4 PO 4 Consumption of calcium in June Ca 2+ (μ mole Ca / litre) Pore water PO4 Apatite P 100 moles P consumed between march and June -6 0 Apatite P (μ mole P / litre) june march

18 depth (cm) depth (cm) Pore water PO 4 Water column PO 4 PO 4 (μm) - March 5 PO 4 (μm) - June Calculated: 0.3 mole P / day -15 Calculated: 7 moles P / day

19 Summary: control of P in the water column Pond retains 15% of Particulate P, both in March and June Algal bloom in March Oxygen saturation Algae and sediments take up SRP in March Collapse of algal bloom Oxygen levels very low in June Large plume of SRP flows in, in June Important release of P from sediments in June

20 Summary: release of P from sediments Release of PO 4 mainly by consumption of accumulated organic matter Redox related dissolution of Fe also contributes to the release of PO 4 Precipitation of apatite counteracts, in part, the release of PO 4