Phosphorus cycling in a

sedimentation pond

of a constructed wetland

Santiago Clerici

Michael Krom, Robert Mortimer and Sally MacKenzie

WWT Slimbridge

•Effluents high in N, P, Organic Matter and suspended solids

•River Severn estuary downstream

River Severn

The treatment wetland

•Sedimentation pond to remove suspended matter

•PO4 and excess suspended matter taken up by reed beds Reserve

Ditch

Inflow

Cascade

100 mts

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

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?

Model of P cycling in the water column

Moles P / day

INLET

POND Part P

POND PO4

Part P Inflow

PO4 Inflow

Algal uptake

Respiration

Part P Outflow

PO4 Outflow

Resuspension

Settling

Sediment fluxes

Chlorophyll and Oxygen levels

Chl.a

0

150

300

450

Mar-11 Apr-11 May-11 Jun-11

date

Ch

l.a (

μg

/l)

Bloom

Crash

DO - March 2011

0

25

50

75

100

125

12:00 18:00 00:00 06:00 12:00 18:00

time

DO

(%

sa

tura

tio

n)

Bloom

DO - June 2011

0

25

50

75

100

125

12:00 18:00 00:00 06:00 12:00 18:00

time

DO

(%

sa

tura

tio

n)

Crash

Night hours

INLET

POND Part P

POND PO4

Part P Inflow PO4 Inflow

Algal uptake

Respiration

Part P Outflow PO4 Outflow

Settling

Resuspension

Sediment fluxes

Sampling P at inlet and pond

INLET

POND Part P

POND PO4

Part P Inflow PO4 Inflow

Algal uptake

Respiration

Part P Outflow PO4 Outflow

Settling

Resuspension

Sediment fluxes

Calculating water column fluxes

Obscured incubation

20

25

12:00 18:00 00:00 06:00 12:00 18:00

time

PO

4 (

μM

P)

Open incubation

20

25

12:00 18:00 00:00 06:00 12:00 18:00

time

PO

4 (

μM

P)

Only respiration

Photosynthesis + respiration

INLET

POND Part P

POND PO4

Part P Inflow PO4 Inflow

Algal uptake

Respiration

Part P Outflow PO4 Outflow

Settling

Resuspension

Sediment fluxes

Calculating sediment fluxes

PO4 in Benthic Chamber #1

March

0

1.5

12:00 18:00 00:00 06:00 12:00

time

PO

4 (

μM

P)

PO4 in Benthic Chamber #1

June

15

25

35

12:00 18:00 00:00 06:00 12:00 18:00

time

PO

4 (

μM

P)

P fluxes March

INLET

Part P Inflow

14

PO4 Inflow

2

Algal uptake

1

Part P Outflow

12

PO4 Outflow

1

Settling

4

Sediment fluxes

0.1/0.2

Fluxes in moles P / day

Pond Part P

14 moles

Pond PO4

2 moles

P fluxes June

INLET

Part P Inflow

16

PO4 Inflow

47

Algal uptake

2

Respiration

1

Part P Outflow

13

PO4 Outflow

55

Settling

6

Sediment fluxes

Fluxes in moles P / day

Pond Part P

9 moles

Pond PO4

35 moles

10

Sediment phosphorus cycle (after Slomp et al 1996)

Fe~P

Water

Sediment

Ca~P

To outlets

Water: it transports PO4

Solid: it retains P but it can be released it back into water

Solid: it retains P indefinitely

PO4

Organic P

Apatite P

Pore water

PO4

Description of the solid phase

-15

-10

-5

0

0.75 1

porosity

de

pth

(c

m)

sediment content

20x increase

-5

0

5

0 50 100

% O2 saturation

de

pth

(c

m)

-15

-5

5

0 400

uM NH4

de

pth

(c

m)

March

June

Organic P ↔ Pore water PO4

Increased respiration in June

Organic P (μ mole P / litre)

-16

-8

0

0 16000

de

pth

(c

m)

june

march

200 moles P released between March and June

PO4

Organic PPore water

PO4

Iron~P (μ mole P / litre)

-16

-8

0

0 16000

de

pth

(c

m)

june

march-15

-5

5

0 300

uM Fe

de

pth

(c

m)

March

June

Iron~P ↔ Pore water PO4

Increased dissolved iron in June

25 moles P released between March and June

Fe~P

PO4

Pore water

PO4

Ca2+

(μ mole Ca / litre)

-12

-6

0

0 8000

de

pth

(c

m) Apatite P (μ mole P / litre)

-16

-8

0

0 16000

de

pth

(c

m)

june

march

Apatite P ↔ Pore water PO4

Consumption of calcium in June

100 moles P consumed between march and June

PO4

Apatite P

Pore water

PO4

-15

-5

5

0 80

de

pth

(c

m)

Pore water PO4 ↔ Water column PO4

-15

-5

5

0 600

de

pth

(c

m)

PO4 (μM) - March PO4 (μM) - June

Calculated: 0.3 mole P / day Calculated: 7 moles P / day

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

Summary: release of P from sediments

Release of PO4 mainly by consumption of accumulated organic matter

Redox related dissolution of Fe also contributes to the release of PO4

Precipitation of apatite counteracts, in part, the release of PO4