Lake Okeechobee Technical Advisory Committee

Water Quality Data Analysis

Long-Term Trends

Spatial Variations

Bloom Frequency vs. Total P

P Mass Balance Modeling

Mass-Balance Data Analysis

Model Formulation

Calibration

TMDL Applications

Presentation to

W. Walker

April 6, 2000

Long-Term Monitoring Stations

#S

#S#S

#S#S

#S

#S

#S

L001

L002

L003

L004L005

L006

L007

L008

Median Values by Station & Year Variable: TP Chl-a TurbidityTrend (%/yr): 2.1% 1.5% 2.3%

Long-Term Water-Quality Trends at Open-Lake Stations

0

20

40

60

80

100

120

140

73 75 77 79 81 83 85 87 89 91 93 95 97 99T

ota

l P (

pp

b)

0

5

10

15

20

25

30

35

40

73 75 77 79 81 83 85 87 89 91 93 95 97 99C

hlo

rop

hyl

l-a

(p

pb

)

0

5

10

15

20

25

30

35

73 75 77 79 81 83 85 87 89 91 93 95 97 99T

urb

idit

y (

ntu

)

North ---------------------------------------------------------->South Central

Trend (%/yr)Station 73-99 80-99 90-99

L001 1.8 * 0.5 1.8 L002 2.3 * 0.5 1.1 L003 2.7 * 0.8 1.4 L004 2.5 * 0.7 1.0 L005 3.6 * 1.6 5.6 *L006 3.3 * 1.7 * 4.0 *L007 2.8 * 1.7 * 4.3 *L008 2.9 * 1.7 * 4.9 *

* significant at p < .05, seasonal kendall test

Long-Term Phosphorus TrendsLake Okeechobee Open-Lake Stations

0

1

2

3

4

5

6

L001 L002 L003 L004 L005 L006 L007 L008

Station

To

tal P

Tre

nd

(

% /

yr )

73-99

80-99

90-99

North -------------------------------------> West --- South -- West Central

Median Concentrations vs. Station & Time Interval

Spatial & Temporal Water Quality Variations at Open Lake Stations

0

20

40

60

80

100

120

140

L001 L002 L003 L004 L005 L006 L007 L008 Lake

To

tal P

(p

pp

)

73-81

82-90

91-99

0

5

10

15

20

25

30

35

40

L001 L002 L003 L004 L005 L006 L007 L008 Lake

Ch

loro

ph

yll-

a (p

pb

)

73-81

82-90

91-99

0

5

10

15

20

25

30

35

40

L001 L002 L003 L004 L005 L006 L007 L008 Lake

Tu

rbid

ity

(ntu

)

73-81

82-90

91-99

Long-Term Trends in Algal Bloom Frequency & Total P

0%

5%

10%

15%

20%

25%

30%

35%

40%

73 75 77 79 81 83 85 87 89 91 93 95 97 99

Blo

om

Fre

qu

ency

(%

)

Chl-a >40 ppb

Chl-a > 60 ppb

0

20

40

60

80

100

120

140

73 75 77 79 81 83 85 87 89 91 93 95 97 99

To

tal P

(p

pb

)

No Data

Algal Bloom Frequencies vs. Total P in Open Lake

0%

1%

2%

3%

4%

5%

6%

7%

8%

9%

10%

0 20 40 60 80 100 120 140

Median Annual Total P (ppb)

An

nu

al B

loo

m F

req

uen

cy (

%) Chl-a > 60 ppb

0%

5%

10%

15%

20%

25%

30%

35%

40%

0 20 40 60 80 100 120 140

Median Annual Total P (ppb)

An

nu

al B

loo

m F

req

uen

cy (

%) Chl-a > 40 ppb

Rainfall35

Inflow Outflow460 Area = 1730 150

Conc = 86Knet = 2.3

Retention 345

Phosphorus Fluxes in metric tons / yearAreas in 1000 acresConcentrations in ppbSettling Rates in m/yr

Phosphorus Mass Balance1973-1998 Average

LAKE

2.0

2.2

2.4

2.6

2.8

3.0

3.2

1970 1975 1980 1985 1990 1995 2000

Year

Mea

n D

epth

(m

)

4-Yr Rolling

1 Year

0

20

40

60

80

100

120

140

1970 1975 1980 1985 1990 1995 2000

Year

Lak

e P

(p

pb

)

4-Yr Rolling

1 Year

0

100

200

300

400

500

600

700

800

900

1000

1970 1975 1980 1985 1990 1995 2000

Year

To

tal P

Lo

ad (

mt/

yr)

4-Yr Rolling

1 Year

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

1970 1975 1980 1985 1990 1995 2000

Year

P R

esid

ence

Tim

e (

yrs

)

4-Yr Rolling

1 Year

0.0

2.0

4.0

6.0

8.0

10.0

12.0

1970 1975 1980 1985 1990 1995 2000

Year

Wat

er R

esid

ence

Tim

e (

yrs

)

4-Yr Rolling

1 Year

Net Settling Rate 1 Year & 4-Year Rolling Averages

0.0

1.0

2.0

3.0

4.0

5.0

6.0

7.0

8.0

9.0

2.0 2.2 2.4 2.6 2.8 3.0 3.2

Mean Depth (m)

Kn

et (

m/y

r)

0.0

1.0

2.0

3.0

4.0

5.0

6.0

7.0

8.0

9.0

1970 1975 1980 1985 1990 1995 2000

Year

Kn

et (

m/y

r)

Latmos

Linflow Qnet Clake

Lremoved

Knet Area Clake

Water Balance:

Qnet = Qout + dV/dt = Qin + Qprec - Qevap

Mass Balance:

Linflow + Latmos - Lremoved = Qnet Clake + Knet Area Clake

TMDL Calculation:

TMDL = Qnet Ctarget + Knet Area Ctarget

= Linflow + Latmos - Lremoved

Linflow = TMDL - Latmos + Lremoved

Model Calibration to 4-Year Rolling Averages, 1973 - 1998:

Knet = 4.76 exp [ -.063 ( Year - 1975 ) ] R2 = 0.97

Clake = Total Load / ( Qnet + Knet Area ) R2 = 0.90

Hydrologic Conditions for TMDL Calculations (1973-1998 Means):

Qnet = 1751 hm3/yr

Area = 1730 km2

Clake

Lake Okeechobee Steady-State P Balance Model

Model: Knet = 4.76 exp [ -.063 ( Year - 1975 ) ] R2 = 0.97

Clake = Total Load / ( Qnet + Knet Area ) R2 = 0.90

Lake Okeechobee Phosphorus Balance ModelFour-year Rolling Averages

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

1975 1980 1985 1990 1995 2000

Year

Kn

et (

m/y

r)

0

20

40

60

80

100

120

1975 1980 1985 1990 1995 2000

Year

Lak

e P

Co

nc

(pp

b)

Lake Okeechobee Phosphorus Balance ModelOne-Year & Four-Year Rolling Averages

0

20

40

60

80

100

120

140

1970 1975 1980 1985 1990 1995 2000

Year

Lak

e P

Co

nc

(pp

b)

Obs - 1 Yr

Obs - 4 Yr

Pred - 4 yr

0

1

2

3

4

5

6

7

8

9

1970 1975 1980 1985 1990 1995 2000

Year

Kn

et (

m/y

r)

Obs - 1 Yr

Obs - 4 Yr

Pred - 4 yr

Residuals Plots Observed - Predicted Concs 4-Year Rolling Averages

-10

-8

-6

-4

-2

0

2

4

6

8

10

1970 1975 1980 1985 1990 1995 2000

Year

Co

nc

Res

idu

al (

pp

b)

-10

-8

-6

-4

-2

0

2

4

6

8

10

2.2 2.4 2.6 2.8 3.0

Mean Depth (m)

Co

nc

Res

idu

al (

pp

b)

-10

-8

-6

-4

-2

0

2

4

6

8

10

250 350 450 550 650

External Load (mt/yr)

Co

nc

Res

idu

al (

pp

b)

-10

-8

-6

-4

-2

0

2

4

6

8

10

1.5 2.0 2.5 3.0 3.5

Inflow (e9 m3/yr)

Co

nc

Res

idu

al (

pp

b)

-10

-8

-6

-4

-2

0

2

4

6

8

10

-20 -10 0 10 20 30 40

Load Increase (mt/yr over 4 years)

Co

nc

Res

idu

al (

pp

b)

-10

-8

-6

-4

-2

0

2

4

6

8

10

100 150 200 250 300

Iinflow Conc (ppb)

Co

nc

Res

idu

al (

pp

b)

Rainfall

3336

73-77 94 - 98

Inflow Outflow

441 Conc = 54 106 83

502 305

Knet = 4.3 1.2

Retention 391 82%

233 43%

Phosphorus Loads in metric tons / year

Concentrations in ppb

Settling Rates in m/yr

Phosphorus Mass BalancesHistorical Conditions

Net Settling Rate Decreasing @ ~ 6% / year

From 4.3 m/yr in 1973-1977 to 1.2 m/yr in 1994-1998

Model residuals independent of depth, flow, & load

Explains 90% of variance in 4-yr rolling average conc

Implied time scales of sediment response:

Half-life (50% Response) 11 years90% Response 36 years

Potential Causal Factors:

Gradual accumulation & recycling of P

Decrease in lake area caused by dike

Decrease in calcium levels (?)

Recycling aggravated by increased water depths

wave action - resuspension

intermittent stratification &O2 loss (?)

increased light-limitation of algal growth

loss of rooted vegetation

increased horizontal transport

Reversible?

Decrease external P load

Decrease water depth

Remove sediment

Re-establish vegetation

Conservative Assumption for TMDL Calculation:

Use 1994-1998 settling rate

Steady-State Settling Rate may be lower

Effect of changes in water level?

Lake Okeechobee P Balance Results

Variable Units 73-98 73-77 94-98 84-96

Knet m/yr 2.32 4.31 1.22 1.65

Design Conditions ( 1973 - 1999 ) Means

Qnet hm3/yr 1750.6 1750.6 1750.6 1750.6

Qin hm3/yr 2564.5 2564.5 2564.5 2564.5

Area km2 1729.8 1729.8 1729.8 1729.8

Volume e9m3 4.7 4.7 4.7 4.7

Zmean m 2.70 2.70 2.70 2.70

Qs m/yr 1.01 1.01 1.01 1.01

Atmos P Load mg/m2-yr 20 20 20 20

Atmos P Load mt/yr 34.6 34.6 34.6 34.6

Historical Linflow mt/yr 460.2 460.2 460.2 460.2

TMDL Calculations

Lake Target Conc ppb 40.0 40.0 40.0 40.0

Ltotal = TMDL mt/yr 230.9 368.3 154.1 184.2

Latmos mt/yr 34.60 34.6 34.6 34.6

Linflow mt/yr 196.3 333.7 119.5 149.6

Cinflow ppb 76.5 130.1 46.6 58.3

Loutflow mt/yr 70.0 70.0 70.0 70.0

Load Reduction mt/yr 263.9 126.5 340.7 310.6

Load Reduction % 57% 27% 74% 67%

P Retention mt/yr 161 298 84 114

WC P Res Time years 0.81 0.51 1.21 1.01

Calibration Period

Lake Okeechobee TMDL Calculation

0

50

100

150

200

250

300

350

400

73-98 73-77 94-98 84-96

Calibration Period

TM

DL

( m

ton

s / y

r )

Tributary Atmospheric

0

20

40

60

80

100

120

140

73-98 73-77 94-98 84-96

Calibration Period

Tri

b In

flo

w C

on

c (p

pb

)

0%

10%

20%

30%

40%

50%

60%

70%

80%

73-98 73-77 94-98 84-96

Calibration Period

Lo

ad R

edu

ctio

n

0.0

1.0

2.0

3.0

4.0

5.0

73-98 73-77 94-98 84-96

Calibration Period

Kn

et (

m/y

r)

Rainfall35

1973-1998Historical Inflow Outflow

460 120 Area = 427 70Conc = 40

Reduction Knet = 1.2341

Retention 84Load Reduction 74%TMDL 154 mt/yrTarget Inflow Load 120 mt/yrTarget Inflow Conc 47 ppb

Phosphorus Fluxes in metric tons / year, Areas in 1000 Acres, Concentrations in ppb

Settling Rate in m/yr, Calibrated to 1994-1998 Data

LAKE

Phosphorus Mass BalanceTMDL - E X A M P L E

Area = 0Cin = 47

Cout = 4Knet = 20

Rainfall0 0 0 35

Post-Historical BMP Inflow

460 120 120 0BMP's Area = 0

Area = 427 Cin = 0341 C = 40 Cout = 4

Outflow Knet = 1.2 Knet = 40BMP Load Reduction 74% 70 0TMDL 154 mt/yr

Target Inflow Load 120 mt/yr Retention 84 0Target Inflow Conc 47 ppb ?

Sediment Removal ?P Loads in metric tons / year, Areas in 1000 acres, Concentrations in ppb, Settling Rates in m/yr

STA's

LAKE STA

Phosphorus Mass BalanceTMDL with Controls - Example 1

Area = 20Cin = 69

Cout = 21Knet = 20

Rainfall84 26 59 35

Post-Historical BMP Inflow

460 178 120 0BMP's Area = 0

Area = 427 Cin = 0282 C = 40 Cout = 4

Outflow Knet = 1.2 Knet = 40BMP Load Reduction 61% 70 0TMDL 154 mt/yr

Target Inflow Load 120 mt/yr Retention 84 0Target Inflow Conc 47 ppb ?

Sediment Removal ?P Loads in metric tons / year, Areas in 1000 acres, Concentrations in ppb, Settling Rates in m/yr

STA's

LAKE STA

Phosphorus Mass BalanceTMDL with Controls - Example 2

Area = 20Cin = 114

Cout = 33Knet = 20

Rainfall138 40 98 35

Post-Historical BMP Inflow

460 292 194 101BMP's Area = 20

Area = 427 Cin = 50168 C = 40 Cout = 13

Outflow Knet = 1.2 Knet = 40BMP Load Reduction 37% 70 27TMDL 154 mt/yr

Target Inflow Load 194 mt/yr Retention 84 74Target Inflow Conc 76 ppb ?

Sediment Removal ?P Loads in metric tons / year, Areas in 1000 acres, Concentrations in ppb,

STA's

LAKE STA

Phosphorus Mass BalanceTMDL with Controls - Example 3

TMDL = External Load + Atmospheric Load - Load Removed by STA

TMDL Components:

External Load Requirements:

Load Allocations with Parallel STA

-200

-150

-100

-50

0

50

100

150

200

250

300

0 5 10 15 20 25 30 35 40

STA Area (acres)

P L

oad

(m

ton

s/yr

)

TMDL

External Inflow

Atmospheric

STA Removal

0

20

40

60

80

100

120

0 5 10 15 20 25 30 35 40

STA Area (acres)

Tar

get

In

flo

w C

on

c (p

pb

)

0%

10%

20%

30%

40%

50%

60%

70%

80%E

xter

nal

Lo

ad R

edu

ctio

n (

%)

Inflow Conc

Load Reduc

Sensitivity of TMDL Calculation to Atmospheric LoadsCalibration Period: 94-98

PreviousVariable Units Assumed StudiesAtmospheric Load mg/m2-yr 20.0 43.3Atmospheric Load mt/yr 34.6 75.0Average Rainfall m/yr 1.07 1.07Bulk Concentration ppb 18.7 40.5

Atm Load Knet Conc R2 TMDL L Reduc Linflow Cinflowmg/m2-yr m/yr -- mt/yr % mt/yr ppb

1.22 0.90 154 74% 120 4710 1.12 0.89 148 72% 130 5120 1.22 0.90 154 74% 120 4730 1.31 0.90 161 76% 109 4240 1.40 0.90 167 79% 98 3850 1.50 0.90 174 81% 87 3460 1.59 0.90 180 83% 76 3070 1.69 0.90 187 86% 66 26