Executive Summary
The management goal for Waughop Lake is to reduce the
intensity and duration of Cyanobacteria blooms resulting in harmful algal
blooms (HAB) events, while improving the overall water quality and aquatic
habitat that supports a recreational fishery. To attain this goal there are 6
potential alternatives that could be implemented; 4 include dredging
alternatives to remove phosphorus-rich sediment and 2 are alum treatment
alternatives that are directly targeting phosphorus to prevent HABs through
phosphorus inactivation (making phosphorus unavailable to aquatic plants). Due
to the uncertainty of the sediment depth needed to be removed to meet the
management goals, there is a shallow dredging and deep dredging alternative,
each with and without direct phosphorus inactivation via alum treatments.
The dredging alternatives 50-year life-cycle cost range from
$7,900,000 to $34,500,000 with a probability of success (increasing water
quality while reduced HABs) ranging from 20% to 90% over the 50-year period.
The 50-year life-cycle cost for the two alum treatment alternatives range from
$2,500,000 to $3,300,000 with a probability of success ranging from 75% to 85%
over the 50-year period.
The dredging alternatives would result in no direct use of the
lake for almost 1 year during dredging activities and no use of 60 to 100 acres
of park land for 1 year, plus limited lake access for 1 week per year from year
2 through 50. The alum treatment alternatives would result in limited lake
access for 1 week each year for the 50-year period.
General Discussion
Based upon the available information presented in the Waughop
Lake Management Plan by Brown and Caldwell the following is a brief outline of
a dredging approach for Waughop Lake. Due to data gaps and necessary
assumptions, this discussion includes alternative dredging actions with
additional alternative management actions relative to effectiveness in
controlling HAB events and water quality improvements. The assumed management
goal is to reduce the intensity and duration of HAB events, while improving the
overall water quality and aquatic habitat within Waughop Lake. Previous studies
and reporting have assumed that dredging will give the lake a 50-year period of
good water quality without additional significant efforts needed. This
assumption does not truly account for the role of groundwater phosphorus inputs
and internal sediment phosphorus loading from sediment not removed in the
proposed dredging process. It also does not consider the necessity to
inactivate sediment phosphorus (remove the potential
for phosphorus to become biologically available) from the newly exposed lake
water/sediment interface, (Gibbons, et al 1983).
The purpose of lake sediment
removal (i.e. dredging) is to remove the reservoir of phosphorus that is
assumed to be currently contributing to excessive production of cyanobacteria.
The effectiveness of dredging in Waughop Lake is dependent upon two undefined phosphorus
loading potentials; 1) depth of sediment phosphorus contributing to the
cyanobacteria production and 2) the impact of groundwater relative to its
direct contribution of phosphorus to the lake and groundwater inflow through
the lake sediments contributing to sediment phosphorus availability to the
water column. This translates into two dredging alternatives depending upon
depth of sediment to be removed. It also requires the inactivation of remaining
sediment phosphorus that will be exposed after dredging and the inactivation of
phosphorus brought into the lake via groundwater.
A viable dredging approach is
to employ a hydraulic dredge to minimize water contamination and to remove
phosphorus most efficiently. This approach would assume that sediment would be
dredged and removed from the lake at a 5% solids content. This dredgate would
be pumped from the hydraulic dredge to a treatment pond near the lake.
Assuming a pond depth of 9.8 ft
(3 m), the area of the pond would be 18 to 20 acres (7.3 to 8.1 ha) (Figure 1).
The treatment pond would need to be constructed above grade and lined to
prevent interaction with groundwater. As the dredgate enters the pond, a
flocculate (either a polymer or alum) would need to be added to aid in
dewatering the dredgate and retaining phosphorus in the dredge spoils. The pond
would be designed to have four cells. Three cells would store one day’s worth
of the treated dredgate (approximately 26,150 yd3 (20,000 m3) at 1,307 yd3 (1000
m3) of lake sediment) to allow the dewatering to occur over 24 to 72 hours.
Clarified water would then be
transferred via inverted syphon to the remaining pond cell for additional
clarification. Water would then be removed from the fourth pond cell and
returned to the lake after passing through a sediment curtain. This would
translate to approximately 20,915 yd3 (16,000 m3) per day of water returning to
the lake.
Dewatered dredgate would be
transferred to a composting disposal site assuming that metal and other
contaminate concentrations in the sediment do not require hazard waste
disposal. Note that sediment metal concentrations reported would not allow
disposal of dredgate to crop lands. Therefore, disposal would be to a landfill,
or non-crop landscape, i.e. park and golf course sites. Landfill disposal cost
would be very high due to limited land fill capacity and this could drive
dredging cost higher than potentially stated in this memo.
For cost savings, it was
assumed that the park would be the site for the dewatering pond and dredgate
disposal. Hence, the dredgate would be transferred to a soil spreader to place
6” of dredgate over 66 acres or 12” over 33 acres as shown in Figure 1. This
would be done at a rate of 1” per 7 seven days.
Dredging will result in a
direct impact to park use in the areas of the treatment pond and disposal site
for the period of operation. Park use and access would also be severely limited
post dredging to allow the plant community to recover. See Figure 1 for
proposed dredgate disposal areas and treatment pond location. It would be at
least a year before park use within these areas could be allowed. The estimated
total area of park to be impacted, excluding the lake, is approximately 60 to
100 acres. If off site dredgate disposal is utilized the impacted park area
would be approximately 30 to 40 acres.
Small Dredge Volume Alternative
Due to the lack of current
depth profile sediment data, it was assumed that approximately 3.3 ft (1 m) of
sediment will need to be removed over 30 acres (12.1 ha). It was also assumed
that groundwater inflow will replace any water removed from the lake via the
hydraulic dredging process. Specifically, at 1,307 yd3 (1,000 m3) of sediment
removed per day, with a total dredgate of 26,150 yd3 (20,000 m3), 24,850 yd3 (19,000
m3) of lake water would be removed per day. Total volume of sediment removed
would be 158,700 yd3 (121,400 m3). Dredging would take approximately 30 days
for mobilization including temporary treatment pond construction and dredge
pipeline installation, 120 days for dredging, and another 30 days for final
dredgate disposal and pond deconstruction with both sites replanted.
Following the dredging
operation within the lake, a phosphorus sediment inactivation, combined with a
water stripping treatment using alum, would be necessary to bind newly exposed
sediment phosphorus, with the added benefit of clearing the water column.
Again, due to the lack of sediment profile of phosphorus fractions the dose of
that treatment was assumed to be 4 mg Al/L for water column phosphorus removal
and 40 mg Al/L for sediment inactivation, for a one-time total dose of 44 mg
Al/L. This sediment inactivation treatment would take place within 5 days of
the sediment removal completion within the lake.
To address on-going loading of
phosphorus to the lake via groundwater, an annual spring alum treatment at a
dose of 4 mg Al/L would be needed to remove and inactive phosphorus for
external sources (mainly groundwater) to prevent extreme HAB events. This
annual dose would be further refined with lake and groundwater monitoring data.
Small Dredge Volume Alternative
without Phosphorus Inactivation or Annual Control
This alternative is the same as
the dredging option described above to remove 158,700 yd3 (121,400 m3) of
sediment, but without the alum addition to inactivate newly exposed sediment
phosphorus and annual phosphorus loading to the lake via, i.e. groundwater.
Large Dredge Volume Alternative
Due to the lack of current
depth profile sediment data, it was assumed that approximately 6.6 ft (2 m) of
sediment will need to be removed over 30 acres (12.1 ha). It was also assumed
that groundwater inflow will replace any water removed from the lake via the
hydraulic dredging process. Specifically, at 1,307 yd3 (1,000 m3) of sediment
removed per day, with a total dredgate of 26,150 yd3 (20,000 m3), 24,850 yd3 (19,000
m3) of lake water would be removed per day. Total volume of sediment removed
would be 317,400 yd3 (242,800 m3). Dredging would take approximately 30 days
for mobilization including temporary treatment pond construction and dredge
pipeline installation, 240 days for dredging, and another 30 days for final
dredgate disposal and pond deconstruction with both sites replanted.
Following the dredging
operation, a phosphorus sediment inactivation treatment would be necessary to
bind newly exposed sediment phosphorus. As stated above, the dose of that
treatment is assumed to be 4 mg Al/L for the water column phosphorus removal
and 40 mg Al/L for the sediment inactivation for a total dose of 44 mg Al/L.
This sediment inactivation treatment would take place within 5 days of the
sediment removal completion within the lake.
To address the on-going loading
of phosphorus to the lake via groundwater an annual spring alum treatment at a
dose of 4 mg Al/L would be needed to remove and inactive phosphorus from external
sources to prevent extreme HAB events. This annual dose would be further
refined with lake and groundwater monitoring data.
Large Dredge Volume Alternative
without Phosphorus Inactivation or Annual Control
This alternative is the same as
the large dredging option described above to remove 315,400 yd3 (242,800 m3) of
sediment, but without the alum addition to inactivate newly exposed sediment
phosphorus and annual phosphorus loading to the lake via groundwater.
Phosphorus Inactivation and/or
Annual Phosphorus Control without Dredging
The dose for an alum treatment
without dredging is assumed to be 4 mg Al/L for water column phosphorus removal
and 80 mg Al/L for sediment inactivation, based on the limited sediment
phosphorus data available. If sediment cores were collected and analyzed for
detailed phosphorus fractions and these data showed less phosphorus potential,
the estimated dose of 80 mg Al/L could be reduced. Based on the currently
available data, the total dose would be 84 mg Al/L. Due to cost and a
relatively high dose, the alum treatment could be conducted over a 2 to 4-year
period with an annual maintenance alum treatment to address on-going loading of
phosphorus to the lake, mainly via groundwater inputs. An annual spring alum
treatment at a dose of 4 mg Al/L would be needed to remove and inactive
phosphorus from external sources on an annual basis following the sediment
phosphorus inactivation dosing.
See Table 1 for relative costs
and effectiveness assessment of the alternatives. Costs are based upon recent
lake dredging operations and alum treatments conducted within the state.
Effectiveness assessments are based upon both literature and direct experience
with lake dredging projects since 1979 and alum lake treatments since 1974
throughout the US to improve water quality and control phosphorus
Insights and Recommendations
It is highly recommended, to both save costs and ensure the
potential of achieving the lake management goal of reducing the number and
intensity of HAB events in Waughop Lake, that three sediment cores of at least
6.6 ft (2m) depth be collected and analyzed every 5 cm for phosphorus
fractions. This would cost about $15,000 but could result in 25 to 50% alum
treatment costs savings.
To assess the effectiveness and plan for dredging and/or alum
treatment, the sediment composition of Waughop Lake must be characterized.
Specifically, the amount of phosphorus that is available to be released from
the lake sediments (and must be removed by dredging and/or inactivated by alum)
must be quantified. Therefore, all forms of phosphorus in every 5-cm sediment
layer for at least 2-meter depth of sediment must be characterized including,
total phosphorus (TP), mobile phosphorus (Mobile- P), organic phosphorus,
biogenic phosphorus, Aluminum bound phosphorus (Al-P), and calcium bound P
(Ca-P). Total phosphorus is a sum of all the components together. Mobile-P is
the phosphorus that is susceptible to changes in the oxidative conditions and
is the sum of iron-bound phosphorus (Fe-P) and loosely-bound phosphorus.
Mobile-P can also consist of a portion of organic and biogenic phosphorus that
is released through mineralization.
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