Assessment of Water Quality Responses to Sediment Removal in Lake Hancock

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Assessment of Water Quality Responses to Sediment
Removal in Lake Hancock

• David Tomasko, Ph.D.
• Emily Hyfield, M.S.
• Doug Robison, M.S., PWS

2
Background information related to influence of
sediments on water quality

• Nutrient concentrations in the lake dont balance
  with calculated loads for TN (ERD 1999 FDEP
  2005)
• Water quality in lake so poor it doesnt lend
  itself trophic state modeling (Brenner et al.
  2002)
• Significant N-fixation occurs in lake
• Are sediments a sink or a source for nutrients?

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Why a mesocosm study to predict effects of
sediment removal on water quality?

• Would water quality be expected to benefit with
  sediment removal?
• Would benefits be expected downstream from lake?
• Recommendation of B-MAP working group for a
  manipulative sediment removal study
• Combined with manipulative N-fixation study
• How does sediment removal fit within the context
  of TMDLs and potential water quality restoration
  activities?

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Manipulative Nitrogen Fixation Study
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Rapid response of N-fixation and C-fixation to
phosphorus addition, but not with pre-incubated
samples. Saturation at 5 uM PO4.
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N-fixation results

• Measured rates can account for excess TN within
  timeframe of residence time of Lake Hancock
• Phytoplankton adapted to low light conditions
• Rapid response of N-fixation to TP enrichment
• Potential saturation of N-fixation at 0.16 mg TP
  / liter
• TP levels presently ca. 0.52 mg / liter (1992
  2004 mean)
• Substantial reductions in TP needed to reduce
  presence of N-fixing blue-greens

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Manipulative Sediment Removal Study

• 6 Customized Aluminum cylinders were constructed
  for Lake Hancock
• Paired cylinders installed at three locations in
  the Lake (LH1-North LH2-Middle LH3-South)

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Cylinder Dredging

• One cylinder at each location was randomly
  selected for dredging
• A MudHog pump was used to dredge the three
  cylinders

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Sampling Methodology

• Samples were collected in Winter 2006 and Summer
  2007
• December 6th, 2006 -May 7th, 2007
• December 8th, 2006 -May 9th, 2007
• December 11th, 2006 -May 14th, 2007
• December 13th, 2006 -May 16th, 2007
• The water column within the cylinder was mixed
  on the 2nd and 4th sampling date to simulate
  windy day conditions.

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TSSMean Std. ErrorMay 2007
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Total PhosphorusMean St. ErrorMay 2007
12
Total NitrogenMean St. ErrorMay 2007
13
Chlorophyll a (corrected) Mean St. ErrorMay
2007
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TNTPMay 2007
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Percent Change of Dredged vs. Undredged
Increase in NP ratio may decrease probability
of blue-green algal dominance
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Proposed Lake Hancock paradigm

• Elevated external TP loads accumulate (29 ERD
  1999) in lake
• Lake is shallow (getting shallower) and large
• Windy conditions suspend TP (more so than TN)
  into lake
• This is in addition to apparent TP
  re-mineralization
• Elevated TP drives down the TNTP ratio
• Decreased TNTP ratio favors dominance of
  blue-greens
• Blue-greens manufacture their own TN
• Result is more TN in lake (and exported out of
  lake) than what is loaded into it
• Role of sediments on water quality is indirect,
  and temporally disjunct, but still there

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Dissolved Oxygen
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Biological Oxygen DemandMean Std. ErrorMay
2007
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Context Upper Peace River
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P-11 Discharge vs. Bartow DO
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Drawdown of DO associated with high BOD is not
eliminated by agitation
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Empirical model of TP vs. potential forcing
functions
TP 5.93133 (0.633035rainfall) (0.062019
windspeed) (0.0581623 stage)
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Scenario examined for TP and water depth

• Present configuration
• District lake level modification factored
• 98.7 feet elevation raised to 100.0 feet
• Increase in water depth of 1.3 feet
• Three feet of muck removal
• Total of 4.3 feet greater effective depth

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How would TP be expected to change with changes
in effective water depth?

• Present day TP of 0.54 mg / liter
• Based on empirical equation of TP vs. effective
  water depth
• Potential future TP of 0.30 mg / liter
• 44 reduction
• Average TP of dredged cylinders (n 4) was 0.31
  mg TP / liter
• Similar expectations

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Changes in TP would result in a more favorable
TNTP ratio
3 feet of muck removal plus 1.3 feet increase in
effective water depth predicted to give TNTP
ratio of 13
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Predictions based on Bachman et al. (2000)
Dynamic Ratio concept

• Not physical oceanographers
• However, Dynamic Ratio has been shown to be
  able to predict water quality in Florida lakes
• Dynamic Ratio Square Root of 18.4 km2 / 1.5
  meters 2.86
• Present condition expect whole lake bottom
  resuspension ca. 1 / 6.7 days
• With increase effective water depth of 4.3 feet,
  whole lake bottom resuspension ca. 1 / 13.5 day
• Whole lake bottom resuspension half as frequent
• Less frequent resuspension less frequent pulses
  of TP

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Potential lake responses to sediment removal if
results can be extrapolated to whole lake

• TSS down 35 to 78
• Chl-a down 5 to 31
• TN down 13 to 53
• TP down 21 to 77
• TNTP up 11 to 111
• But TSI down by 1 to 12

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Can we reach critical TP values?

• Saturation of N-fixation at 0.16 mg / liter
• TNTP gt 291 at 0.08 mg / liter
• TNTP gt 161 at 0.21 mg / liter
• Empirical model of TP vs. effective water depth
  (with 4.3 feet increase) gives 0.30 mg / liter TP
• TNTP of 131
• Average (n 4) TP of dredged cylinders 0.31 mg
  / liter TP
• TNTP of 131
• TP levels presently ca. 0.52 mg / liter (1992
  2004 mean)
• TNTP of 91
• Similar results of both approaches
• Potential to reduce dominance of blue-greens with
  sediment removal
• But eliminate???

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Contact Information

• David Tomasko
• DATomasko_at_pbsj.com 813-281-8346
• Emily Keenan
• ECGHKeenan_at_pbsj.com 813-281-8378
• Doug Robison
• DERobison_at_pbsj.com 813-281-8379