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A note regarding the use of buffered alum as a pond management option
Matthew Albright
INTRODUCTION
Recreational and aesthetic uses of ponds are often impaired by excessive algal
growth. This condition is generally manifested by low transparencies and scum
conditions leading to odor problems and, upon decomposition, anoxic conditions in deep
waters. Historically, nuisance algae has been controlled with algicides, most commonly
copper-based chemicals (i.e. copper sulfate). The mechanism of action involves
disruption of photosynthesis. While the effect of this treatment may be immediate, it is
also temporary and may, in fact, compound the problem. Algal densities are usually
limited by either nutrient (phosphorus or nitrogen) concentrations or by grazing by
zooplankton. Upon their death, algal cells release nutrients, allowing for subsequent
blooms. Since copper compounds also kill zooplankton, which rebound more slowly than
algae, decreased grazing may benefit the rebounding algae. A sudden, widespread algal
die off often results in massive reductions in dissolved oxygen, leading to fish mortality.
Many species of blue-green algae develop a tolerance to algicides, leading to a situation
where these undesirable organisms are favored unless treatment rates and frequencies are
increased. Also, copper compounds are non-selective, killing vascular plants (through
interruption of photosynthesis) and certain animals (through direct toxicity) (Anon.,
1990).
An alternative method for managing algal populations (as well as non-algal
controlled water clarity) involves the use of buffered alum (aluminum sulfate buffered
with sodium bicaronate). Buffering is necessary to maintain pH between 6-8; aluminum
may become toxic when pH drops below 4.5-5.5. When dissolved, aluminum sulfate
dissociates, releasing aluminum ions. In the appropriate pH range, these ions immediately
undergo a series of hydrolysis reactions resulting in the formation of aluminum hydroxide
(AI(OH))). This compound forms a visible floc which, over several hours, settles through
the water column. As it falls, inorganic phosphorus adsorbs to the floc; also, the water is
clarified as particulate matter (algae cells, particulate organic and inorganic material)
becomes entrapped in the coagulated aluminum hydroxide. The potential for long-term
control exists because after phosphorus is removed from the water column it remains
adsorbed to the aluminum hydroxide in the sediments, preventing intemalloading which
is often a significant phosphorus source in shallow waterbodies (Cooke et at., 1993).
To evaluate the effectiveness of alum treatment it was applied on a small (~0.3
acre surface area, 2.3 m maximum depth, estimated volume 2 acre feet), privately owned
pond in August 99. Artificial mixing, using a submerged sump pump, began concurrent
with the treatment to oxygenate bottom waters. Baseline data were collected in August
98; post treatment work occurred in August and September 99.
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METHODS
The pond was visited, upon the owner's request, on 20,25 and 31 August 98.
Temperature, oxygen and conductivity were collected in profile at one meter intervals
and samples were collected at the surface and at two meters for total phosphorus,
nitrite+nitrate and alkalinity analysis. Transparency was measured with a standard Secchi
disk.
The pond was treated with alum on 5 August 99. Two "T" shaped manifolds were
built according to McMomas and Price (1993) using one inch (2.64 cm) PVC (Figure 1).
The arm of one was 120 cm, the other 180 cm. The crosspieces were 190 cm. The ends of
the cross pieces were capped and 0.4 cm holes were drilled at 10 cm intervals along the
plane of the arm. The arm of each manifold was attached with duct tape to the hose of a
hand operated bilge pump.
On site, aluminum sulfate and sodium bicarbonate (acquired from United
Horticultural Supply® ) were separately added to 20-gallon garbage cans. Pond water was
added and each was mixed into a slurry. From a small john boat the slurries were pumped
into the pond, the longer manifold used to distribute the sodium bicarbonate slightly
deeper than the alum (the rationale being that the alum slurry would sink into the
buffered water). Sixty pounds (27.2 kg) of each compound were used.
The pond was revisited on 10 August and 17 September 99. Temperature, pH,
conductivity and oxygen profiles were made. Mid water samples were collected for total
phosphorus determination. Secchi transparencies were collected. On 17 September a
qualitative zooplankton sample was collected by pulling a 23 cm plankton net (63 um
mesh) behind the john boat for several minutes.
Figure 1. Manifold and pump uses to apply alum and sodium bicarbonate.
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RESULTS AND DISCUSSION
Data collected on three dates prior to alum treatment were quite consistent.
Temperature ranged from 17.58° C near the bottom on 31 August to 20.88° C at the
surface on 25 August. pH was 7.07 to 7.78 through the water column and 6.61 to 6.82 at
the bottom. From 2 m to the surface waters were well oxygenated; near the bottom
conditions were anoxic. Conductivity ranged from 174 to 215 umho/cm and alkalinity
from 82 to 87 mg/l (as CaC03). Phosphorus was between 50 and 55 ug/l. Nitrite+nitrate
was below detection « 0.05 mg/l). Secchi transparency ranged from 0.6 to 0.7 m.
Following alum treatment (10 August and 17 September 99) dissolved oxygen
was near saturation throughout (likely due to artificial circulation). pH was between 7.22
and 7.65, indicating that the buffering provided by the sodium bicarbonate was adequate.
Mid water total phosphorus concentrations were 27 and 32 ug/l on the two dates. A
Secchi disk was readily visible when on the pond bottom (2.3 m). The pond, which had
appeared murky prior to treatment, had become aqua blue. A plankton haul revealed high
densities of large bodied copepod and cladoceran crustacean zooplankton.
Further observations are needed to evaluate the long-term effectiveness of this
treatment. However, this waterbody seems a good candidate for success because: 1)
alkalinity was relatively high, allowing for natural buffering if the sodium bicarbonate
dose was inadequate, 2) external phosphorus loading is minimal, and 3) protection from
wind by surrounding forests reduces sediment suspension and nutrient release.
REFERENCES
Anonymous. 1990. Diet for a small lake- a New Yorker's guide to lake management.
New York State Department of Environmental Conservation and Federation of
Lake Associations. Rochester, NY.
Cooke, G.D., E.B. Welch, S.A. Peterson and P.R. Newroth. 1993. Restoration and
management of lakes and reservoirs, 2 nd ed. Lewis Publishers, Ann Arbor, MI.
McComas, S. 1993. Lake smarts- the first lake maintenance book. Terrene Institue,
Washington, D. C.