Degradability of Sediments from the St. Lucie Estuary Florida
A Pilot Study
Mark Thompson, Thomas White and Greg Graves
Florida Department of Environmental Protection
Water Quality Program
Port St. Lucie
February 2001
Introduction
The St. Lucie Estuary (SLE) is an important ecological feature of east central Florida. The Estuary also
serves as a recreational and commercial resource for the area. Sea level rise since the end of the Wisconsin Glaciation 15,000 years ago flooded a portion of the St. Lucie River Valley forming the estuary (Graves
and Strom, 1992). The average tidal range is 3 feet near the ocean and 2 feet in the inner estuary. Tidal
currents are nominal throughout and the SLE may be classified as a low energy, microtidal estuarine
system (Haunert, 1988). The chemistry and relative quiescence of the estuary create a depositional
environment for sediments transported from upstream. Under normal conditions, the St. Lucie Estuary is
moderately stratified with a layer of fresh water flowing over a wedge of tidally pumped saline water. The
layers may flow at different velocities and directions resulting in a circulation pattern which enhances
deposition of heavier than water particles (Olson and Burgess, 1967). It is a normal condition for estuaries
to be turbid and act as sediment traps accumulating deposits at rates which estuarine species have become
adapted, but anthropogenic activities may adversely increase sedimentation rates and change sediment
characteristics.
The modern day St. Lucie Estuary has been impacted by major anthropogenic changes within its watershed. Natural streams and wetlands have been systematically altered and drained with canals and
channelization projects. Sediment-laden runoff from rainfall events is rapidly transported downstream into
the estuary. The C-23 and C-24 Canals discharge into the North Fork of the estuary and drain coastal
watersheds which are being converted to predominantly urban and intense agricultural land uses (Figure
1).
Figure 1. The St. Lucie Estuary with Significant Features.
Sample Site
1
The Okeechobee Waterway (C-44 Canal) empties into the South Fork of the estuary and serves as a
source for large influxes of freshwater and sediments from Lake Okeechobee. These activities have
resulted in a rapid increase in sedimentation rates. Fine particle sediments are accumulating at a rate of 1
to 2 cm/yr. (Davis and Schrader, 1984; Schrader, 1984). Turbidity and sediment smothering may adversely
affect benthic and plankton communities, which are the base of the estuarine ecosystem. Historical
accounts of SLE indicate aquatic species diversity was high, water clarity good and submerged aquatic
vegetation (SAV) and oysters were an important feature of the estuary (Chamberlain and Hayward,
1996). Currently the SLE is devoid of significant populations of oysters or SAV. Additionally, pesticides
and heavy metals associated with agriculture and urban activities adsorb onto sediment particles and
accumulate in low energy areas. These toxic substances accrue in benthic organisms and bioaccumulate
at higher trophic levels. Monitoring studies have found concentrations of some heavy metals and pesticides in SLE sediments to be above threshold effects levels (TELs) which are used as guidelines for
evaluating sediment quality (Table 1) (MacDonald et al. 1996; Haunert, 1988; Florida Department of
Environmental Protection, 2001).
Table 1. SLE Sediment monitoring data from Florida DEP Southeast District. Values exceeding
threshold effects level shown in Italics. Values exceeding probable effects level shown in bold.
Site Storet
TEL
PEL
NEST 28010037
SEST 28010229
NEST 28010037
SEST 28010229
NEST 28010037
SEST 28010229
NEST 28010037
SEST 28010229
NEST 28010037
SEST 28010229
NEST 28010037
SEST 28010229
NEST 28010037
SEST 28010229
Mean
Stdev
Min
Max
Date
9/14/100
9/14/100
7/25/100
7/25/100
4/20/100
4/20/100
11/23/99
11/23/99
8/25/99
8/25/99
6/28/99
6/28/99
9/30/98
9/30/98
Hg
As Cd
Cr
Cu
Pb
Mn
Ni
Zn
DDE
0.13
7.24 0.68 52.30 18.70 30.20
15.90 124.00 2.07
0.70 41.60 4.21 160.00 108.00 112.00
42.80 271.00 374
0.15 5.90 2.10 32.10 71.60 26.60 244.00 7.30 110.00
4.7
0.20 5.40 2.20 25.60 46.40 25.60 231.00 7.70 85.00
9.5
0.19 5.30 0.78 38.30 70.40 28.00 238.00 8.60 110.00
7.4
0.24 4.80 0.83 36.40 41.10 25.00 222.00 9.70 82.00
12
0.16 6.80 0.60 32.80 93.10 27.00 264.00 9.60 110.00
0.20 5.10 0.49 27.10 44.10 26.00 249.00 9.80 85.00
8.6
0.24 9.40 0.48 40.00 68.30 83.10 197.00 12.50 94.00
6.4
0.17 13.00 0.33 28.00 125.00 103.00 246.00 9.40 130.00
11
0.18 3.71 0.43 27.50 38.40 30.50 223.00 7.12 106.00
7.6
0.26 2.65 0.44 23.30 20.70 26.70 196.00 6.01 76.10
10
3.00 0.35 39.30 23.60 28.50 234.00 9.08 63.00
0.25 2.10 0.45 37.40 15.20 28.20 195.00 7.43 35.80
7.4
5.00
87.80
4.8
4.00
34.20
4.9
0.20 5.44
0.04 2.85
0.15 2.10
0.26 13.00
0.79
0.65
0.33
2.20
32.32 55.71 38.18 228.25 8.69 90.57 7.86
5.89 31.56 26.02 22.56 1.72 25.30 2.44
23.30 15.20 25.00 195.00 6.01 35.80 4.70
40.00 125.00 103.00 264.00 12.50 130.00 12.00
2
Sediment oxygen demand has been shown to be a significant component of the total oxygen demand on a
water column (Davis and Davis, 1958) and may produce anoxic conditions near the bottom during times of
high respiration rates and high water temperatures and salinity. Monitoring of water quality in the SLE has
shown anoxia to be a major concern (Chamberlain and Hayward, 1996; Florida Department of Environmental Protection, 2001). Pursuant to the directive of the Florida Surface Water Improvement and
Management Plan, goals have been established to maintain a salinity and flow regime within the SLE
which is conducive to restoring vital ecosystem components, and to maintain water quality to ensure
bottom dissolved oxygen (DO) concentrations of at least 4 mg/l (state standards). Turbidity and accumulation of sediments and pollutants within the modern day SLE will necessitate significant action to achieve
these goals.
The impacts of increased sedimentation rates, surges of freshwater inputs and the pollutants large pulses
of water carry have become a major concern to all St. Lucie Estuary stakeholders. Federal, state and
local governments are currently evaluating alternatives, under the auspices of the Comprehensive Everglades Restoration Plan (CERP), to improve water quality conditions in the estuary. These improvements
aim to help reestablish a viable estuarine ecosystem, including successful populations of oysters and expansion of SAV communities into areas where they are currently absent. However, deposits of
unconsolidated sediment may restrict, and in some areas prevent oyster and SAV recovery.
This analysis seeks to answer questions regarding the rate at which widespread deposits of unconsolidated
sediment may be expected to degrade. If the current rate of sediment input is reduced via CERP or Best
Management Practices (BMPs) implementation, how long will it take for the existing deposits to be transformed through in-situ biological processes into a substrate suitable for establishment of a viable ecosystem?
Will natural processes be capable of transforming these sediments into a usable substrate or will more
invasive management options such as dredging be required? This study will attempt to give some insight
into the degradability of SLE bottom sediments and the feasibility of allowing natural processes to convert
these to viable substrates. Samples of SLE sediments were subjected to extended Biochemical Oxygen
Demand (BOD) analyses at differing sample concentrations and larger samples of sediment submerged in
ambient SLE water were subjected to extended aeration within open containers (a bucket and beaker).
The BOD and Chemical Oxygen Demand (COD) results were analyzed and compared to estimate
degradability and toxicity. COD results for sediment samples taken before the extended aeration trials
were compared to results for samples taken after the trials to further establish the extent of sediment
degradability. Percent volatiles for samples before and after extended aeration were established and
related to the degradability study.
Alternate hypotheses for these experiments include: 1. The sediment BOD after an extended test (83
days) will approach the sediment COD value; 2. The COD before the extended aeration experiment will
be significantly greater than the COD after treatment; 3. The sediment volatile solids component before
the extended aeration treatment will be significantly greater than the sediment volatile solids after treatment; 4. There will be a significant correlation of sediment sample size with BOD value.
The results of these limited studies will be disseminated to become a component of a weight-of-evidence
case for establishing a proper sediment management alternative.
3
Methods
In August 2000, sediment samples were collected in a water depth of about 2.5 meters, using an Eckman
sediment sampler. The sample site was approximately 100 meters offshore in the South Fork of the SLE
between the Roosevelt Bridge and Palm City Bridge in Stuart Florida (Figure 1). Surface water samples
were collected from the same site for use as dilution water in the BOD analyses and as a medium for the
extended aeration experiment.
For the extended aeration experiment, two separate units were assembled to assure a sufficient volume of
sediment would be recoverable after the aeration process. One unit consisted of a 4000 ml beaker filled
with 100 grams of wet sediment and about 3500 ml of site water and fitted with a 2000 cc/min aquarium
aerator. The second unit consisted of a 5-gallon plastic bucket filled with about 1 kg of sediment and 4
gallons of site water and fitted with a 9000 cc/min aerator. The mixtures were then aerated continuously
for 82 days and samples were collected after the treatment by allowing the sediments to settle and decanting supernatant. COD analyses were performed in replicate on sediment subsamples before treatment
and subsamples taken after treatment using Standard Methods 5220B – Open Reflux Method (Standard
Methods, 1992). A sample size of 0.5 gram was found to give acceptable results when diluted to 50 ml
with deionized water. Before and after treatment sediment subsamples were also analyzed in replicate for
total solids and volatile solids per Standard Methods 2540G (Standard Methods, 1992).
BOD analyses followed a modified version of Standard Methods 5210. Water from the sample site was
filtered and used as the dilution water for initial runs. It was found that the oxygen demand of this water
was too great to be used as dilution water. Seawater was then collected immediately north of the St. Lucie
Estuary confluence with the Atlantic Ocean then filtered and diluted with demand-free water to obtain a
salinity concentration similar to that of the sample ambient conditions. The solution was aerated while
covered with a dark trash bag (to discourage algal growth) for a week to further lower its oxygen demand
and revitalize microorganism vitality. This mixture exhibited an acceptable level of oxygen demand and
was used as dilution water.
A random subsample of sediment was stirred to a uniform consistency and re-stirred prior to weighing and
introduction into the BOD bottles. Three sample aliquots of the same wet weight (weight set = 3 samples)
were transferred to individual 300 ml BOD bottles with a dilution water rinse of the weigh boat. The
weight sets were 0.1 – 0.5g in 0.1g increments. Bottles were diluted to volume, stoppered and capped.
Blanks accompanied the setup for subtraction of dilution water demand from the oxygen depletion of the
sample bottles.
Oxygen concentrations of sample solutions and blanks were read by probe about every four days and
recorded. As the DO concentration approached 1 mg/l, dilution water was decanted from each sample’s
flock, re-aerated and returned. Sample aliquots larger than 0.3g per bottle depleted oxygen at a rate too
great for the bottle size. These sample dilutions were omitted from the study to keep re-aeration demand
uniform. Samples were re-aerated two times during the study. Total oxygen consumed over the test
period was recorded and summed at the test’s end. The BOD test was performed over an 83-day period.
Plots of BOD vs. time showed the curves approaching an asymptotic value. The pooled BOD83 (mean
values) data was used as an estimate of the ultimate BOD (asymptotic value).
Mean sediment COD before aeration treatment was compared to mean BOD83 for significant difference
using a t-test on Box-Box transformed data (α= 0.05). The difference in sediment COD before and after
4
aeration was tested for statistically significant difference using a t-test (n= 4, α= 0.05). The difference in
volatile solids before and after the aeration treatment was tested for significant difference using a t-test
(n=6, α= 0.05). BOD verses sample concentration (weight) was analyzed for significant correlation using
Pearson’s test on Box-Cox transformed data. All data was tested for normality using the Ryan-Joiner test.
BOD data (individual and pooled) was plotted vs. time to estimate ultimate BOD graphically.
Results
Plots of BOD vs. time show BOD values approaching an asymptotic value (Figure 2). The mean BOD83
value for the pooled data was 67879 mg/kg (Table 2). The mean COD/mean BOD5 ratio was 26.6/1 while
the COD/BOD83 ratio was 4.95/1. Mean sediment COD before the extended aeration treatment (335908
mg/kg) was significantly greater than the mean BOD83 (67870mg/kg) (t-test, t = 25.59, p = 0.0000). The
average sediment COD before the extended aeration treatment was not significantly different than COD
after treatment (t-test, t = 2.99, p = 0.0058). Sediment volatile solids composition before aeration was not
significantly different than sediment volatile solids after treatment (t-test, t = 1.00, p = 0.362). The average
change in sediment COD during the 82 day aeration experiment was significantly less than the average
BOD83 (t-test, t = -4.89, p = 0.009). A significant correlation was found between increasing sample size
and decreasing BOD5, BOD26, and BOD45. No significant correlation was found between increasing
sample size and decreasing BODs after day 45 (Pearson’s, p > 0.1).
Table 2. COD and Volatile Solids Analyses Results for SLE Sediment Samples (Before and
After Extended Aeration Experiment).
Initial COD (mg/kg)
321554
323181
349365
349531
Mean
335908
St. Dev.
15649
Final COD (mg/kg)
317042
309657
316431
319336
315617
4165
Mean
St Dev
Initial % Volatiles
36.9
38.4
23.7
35.3
25.31
23.47
30.51
7.06
5
Delta COD (mg/kg)
4512
13524
32934
30195
20291
13573
After % Volatiles
27.04
26.34
24.66
26.41
30.8
28.37
27.27
2.10
Bod vs. Time for SLE Sediment
140000
Series1
2
120000
3
4
100000
BOD (mg/kg)
5
80000
6
7
60000
8
9
40000
10
11
20000
12
Mean
0
0
5
10
14
19
26
29
33
36
39
45
48
52
55
62
66
69
76
83
Days
Discussion
General findings of these experiments suggest that the SLE sediments are composed of a high proportion
of organic matter which is very resistant to degradation. BOD results show that the sediments exert a
BOD which approaches an asymptotic value. After 83 days of testing, the BOD values were on the
asymptotic segment of the curve, yet these values were significantly lower than the sediment COD. COD
values are often used as an estimate of the ultimate BOD (the asymptotic value). COD/BOD5 ratios
normally range from 2 to 5 in domestic wastewater with increasing ratios corresponding to less degradable
influent (industrial components) (WPCF, 1975). The COD/BOD5 ratio for the SLE sediments was 26.6/
1 suggesting the sediments are very resistant to degradation.
The significant correlation between increasing sample size and decreasing BOD for the first 45 days also
suggests the sediments were exerting a toxic affect on the decomposer community. After 45 days, any
toxic affect on the decomposers seemed to have dissipated or it was consistent with regards to sample
concentration. The change in sediment COD over the duration of the 82 day extended aeration experiment would be expected to approximate the BOD exerted over a similar period. The fact that mean
BOD83 was significantly greater than the change in COD over the 82 day duration of the extended aeration
experiment may be explained by the difference in proportion of sample size to volume of dilution water.
The range in the proportion of sample size to dilution volume in the BOD test was 0.36 to 1.07 g/l. The
range of proportions in the extended aeration experiment was 29 to 66 g/l. Because of the other confounding factors (aeration variability, dilution water and seed differences, etc.), toxicity can only be recognized
as one of the possibilities for the great difference between expected change in COD and actual change in
COD over this period.
6
Sediment monitoring within the SLE has revealed threshold effects-level (exceed TEL’s) concentrations
of metals and pesticides exist in these sediments (Table 1) (MacDonald et al., 1996). Sampling has also
revealed that probable effects level (PEL’s) concentrations of Copper may exist in SLE sediments. At
least six different parameters exceeded PEL’s during seven sampling events at two DEP sites within the
SLE since 1998. The additive effects of several pollutants at concentrations above effects-levels may
contribute to overall sediment toxicity.
Combined with the negative correlation of sample volume and BOD, sediment monitoring results suggest
SLE sediments exert toxic effects on the SLE benthic microbial environment. However, toxicity may not
be exclusively related to contaminants associated with the sediments. Under anaerobic conditions, ammonia produced by microbial activity can accumulate to toxic levels in the benthic region (Beaton, 2001). In
our study, all samples were well aerated and toxicity affects were probably not related to ammonia buildup.
Volatile solids analyses offer a rough estimate of the amount of organic matter present in the solid fraction
of sediments (Standard Methods, 1999). Though the availability of volatile solids data for estuarine sediments in the southeastern United States is limited, the results of analyses on SLE sediments at our sample
site is high (Table 2) compared to the existing data (McCabe and Hinson, 1996). Previous studies have
found the volatile solids component of St. Lucie Estuary sediments to vary spatially according to hydrographic characteristics (Schrader, 1984). Volatile solids ranged from less than 10% in higher energy areas
to near 30% in low energy regions with smaller sediment sizes. Our sample site fell within a low energy
area of the SLE.
The high volatile solids component of the sediments suggests there is a large amount of potentially degradable material available in the sediments. Degradation of this material would change the physical and
chemical characteristics of the sediments and perhaps produce a substrate more amenable to ecosystem
development. However, degradation of so much material would cause sediment oxygen demands detrimental to normal ecosystem function. Our study suggests that even though there is a large proportion of
organics in the sediments much of it is not readily available to degradation. Organic material originating
from plant cell walls has been shown to be highly resistant to degradation (Richard, 1996). Lignin, a major
component of cell walls has been shown to be particularly resistant with typically only 20% available to
aerobic decomposition and even less under the anaerobic conditions possible in the SLE benthos (Richard,
1996).
Releases of sediment-laden fresh water from Lake Okeechobee may be a major source of the nondegradable organic material found in the SLE sediments. The organic component of sediments released
from the lake bottom may have already been oxidized as much as possible in a benthic environment. The
released material may be primarily lignin and cellulose based materials which degrade little with continued
biological activity. The low energy St. Lucie Estuary provides an ideal settling area for these already
degraded organics. Land clearing and burning of large tracts of woodlands associated with the conversion
of natural landscapes within the SLE watershed, may contribute additional masses of non-degradable
organics to the SLE sediment load. Anecdotal material supports these ideas. Letters to the editors of local
newspapers and recollections by long time area residents, describe the SLE as very productive with good
water clarity and a sandy bottom throughout much of its extent in the 1930’s and 1940’s. Some suggest
catastrophic releases of sediments from Lake Okeechobee into the estuary during hurricanes of that era
coated the estuary with thick deposits and changed its productivity rapidly (Stuart News, 1992).
7
Conclusions
Sediments from one sample site in the SLE show they contain a high proportion of non-biodegradable
organics. The origin of the organic portion is unknown, but may be associated with plant cell wall material
found in bottom sediments from Lake Okeechobee and from the effects of natural landscape conversion
to agricultural and urban land uses within the watershed. Even though a large portion of the sediment is
non-degradable, the BOD exerted by microorganisms in the sediments may be significant enough to promote anoxic conditions in the benthic environment. Additionally a significant toxic effect on decomposer
microorganisms was found at early stages of decomposition. These findings suggest SLE sediment characteristics will not change significantly due to biodegradation if allowed to remain without remediation. An
active management scheme may be required if we wish to improve the present quality of this once viable
ecosystem.
Our study was performed using only two samples taken from one sampling site in the SLE. Statistically
valid conclusions relating to sediment characteristics for the entire SLE cannot be drawn from our results.
These experiments were intended only to provide a basis for further study and a bit of additional insight into
selection of the most viable management options.
8
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9