How Watershed Management and Continuous Laminar Flow Inversion Reduce Phosphorus and Nitrogen in Lakes, Rivers and Reservoirs

Watershed management is an important tool in restoring polluted water bodies. The results, however, are often limited and further treatment is necessary to achieve the desired results. A major problem is that non-point source pollution may be difficult to control. This leads to the question, can in-situ treatment help attain the nutrient reductions needed for limitation of aquatic plant growth? If it can, it should become a welcome addition to any limnologist’s tool box.

Lake Weston in Orlando, Florida, The Helpe Minuere River in Fourmies, France and Sweeney Lake in Golden Valley, Minnesota are excellent examples of watershed management in combination with the CLEAN-FLO Continuous Laminar Flow Inversion and Oxygenation System for destratifying lakes. This combination in Sweeney Lake brought both orthophosphate and total phosphorus down to below the critical level of 30 micrograms per liter from surface to bottom. This post discusses sediment nutrient exchange as well as the combination of watershed management with much-needed sediment oxygenation.

Anaerobic Release of Nutrients from Sediments

While watershed management is often necessary, a major shortcoming of watershed management is that massive quantities of ammonia and phosphorus in the sediment are made soluble for both weed and algae use when the bottom becomes anaerobic. The amount of nutrients released usually dwarfs all incoming nutrients (Armstrong, 1979; Lazoff, 1983; Holdren, G.C., Jr., 1983; Meals, 1978; Scholz, et al, 1985; Miller, et al, 1985).

In testing hundreds of lakes over the past thirty-five years, we have found the pre treatment bottom water to be devoid of oxygen almost without exception. Dierberg et al (1985) found thirty out of thirty lakes tested in Florida to be low in bottom oxygen. These are all shallow lakes six to ten feet deep and the problem increases with deeper lakes. Contrary to popular belief, wind and wave action do not always keep shallow lakes oxygenated down to the bottom.

During the day, aquatic plants release oxygen into the water. Photosynthesis reverses at night, and aquatic plants absorb oxygen from the water. At night, especially after a few days of calm, cloudy weather, the bottom often becomes anaerobic. Since limnologists often do not measure bottom oxygen at 4:00 AM, they may not have measured the low oxygen that normally occurs at that time.

Current concerns about external loading may ignore the importance of internal loading that researchers have recognized since the early 1900’s. Reports of internal loading are numerous and date back to 1903 (Kofoid, 1903; Pond, 1905; Hutchinson, 1941; Lindeman, 1942; Mortimer, 1941, 1942).

Stefan and Hanson (1981) observed significant phosphorus transport from the anoxic bottom water to the surface waters associated with mixing in five shallow lakes in south central Minnesota.

Taylor (1978) found nutrients recycled from bottom sediment in a Connecticut lake during periods of anaerobic conditions to contribute 3.3 times more nitrogen and 3.6 times more phosphorus than all other influent sources.

Terry (1974) found 5l to l7l mg ammonia released per kg of sediment per day when anaerobic conditions existed.

Sonzogni, et al, (l977) measured a sediment release rate of 7 mg phosphorus per square meter per day for Lake Shagawa in Minnesota during two summer months, after nutrient diversion.

After a nutrient diversion project failed to restore Cedar Lake in Indiana, Jones (1985) found that 69 92% of the total phosphorus was entering the lake from the anoxic sediment.

Cooke et al (1977) calculated that 65 105% of the increase in phosphorus in Twin Lakes in Ohio came from the anoxic sediments during summer stratification. Ryding et al (1977) found phosphorus from the sediments to be as much as 400% of the external load per year in three Swedish lakes. Likewise, phosphorus from the sediment in Lake Wabumun in Alberta was over 4 times as high as incoming phosphorus (Mitchell, 1984).

Schneider, et al, (1969), estimated the top three feet of sediment in 31,000 acre Lake Apopka in Florida to contain approximately 500 million pounds of nitrogen (all forms) and 5 10 million pounds of phosphorus.

Many researchers have found large increases in ammonia and phosphorus during periods of anaerobic activity (e.g. Mortimer, l94l and 1942; Gunnison, et al, 1980;

Nurnberg, 1984; Summerfelt, et al, l967; McKee, et al, l963; Black, et al, 1954; Fillos, et al, l975; Robinette, l976; Fekete, 1973; and Pamatmat, et al, l973).

 

Natural Inversion of Lakes

In temperate climates, every spring and fall as the temperature of the surface water goes through 4o C, the surface water becomes heavier than the water under it. This causes a natural inversion of the lakes. Oxygenated surface water is carried down to the bottom. Anoxic, toxic gas-laden bottom water is brought to the surface where it is oxygenated and the gases neutralized by the atmosphere. Wetzel (1983) and Hutchinson (1957) both show seasonal declines in nutrients following natural destratification.

In tropic or sub-tropic zones, hurricanes, typhoons, and heavy rainfalls turn the lakes over or flush them out, producing aerobic lake bottoms. CLEAN-FLO duplicates nature by continuously duplicating spring and fall turnover without creating turbulence.

It has been shown that by maintaining oxygen over lake bottom sediments, the nutrient status of the lake can be improved (Fillos, et al, l976; Serruya, 1975; Kamp Nielson, l975; Viner, l975; Poon, et al l976; Fitzgerald, 1970, Mortimer, 1971; Nelson, et al, 1973; and Lynn, et al, 1972).

Latterell, et al, (1971) reported that under aerobic conditions, sediments in lakes adsorb orthophosphate from the lake water. Gaugush (1984) observed a decrease in nutrients and dissolved metal concentrations when oxygen was introduced into the bottom water of Eau Galle Lake in Wisconsin.

Hypolimnetic Aeration

Hypolimnetic aerators cannot move enough water to fully oxygenate the bottoms and rid them of toxic gases without a huge expenditure of energy and a large number of expensive aerators.

Hypolimnetic aerators are engineered to not oxygenate sediment surface. These aerators are usually suspended about six feet above the sediment to collect water above the bottom and release the oxygenated water at the same height. After CLEAN-FLO International restored Congress Lake in Canton, Ohio, a consultant advised the lake residents to move the CLEAN-FLO diffusers six feet above the bottom. The lake quickly reverted to its former condition, inundated with filamentous algae. With diffusers six feet above the bottom, bottom sediment cannot possibly be oxygenated.

A prime purpose of hypolimnetic aeration is to not bring warm surface water down to the bottom. The belief is that many fish cannot live in water that warm. This is a common misconception because no fish will feed or lay its eggs in the anoxic benthic region.

A further limitation of hypolimnetic aeration is that it cannot possibly reduce blue-green algae (Cyanophyceae) that only grows within a few inches of the surface. In contrast, complete inversion takes blue-green algae down to the bottom, where it cannot survive. So hypolimnetic aeration has very little purpose.

Continuous Laminar Flow Inversion

While most bottom diffusers often create turbulence and re-suspend phosphorus and nitrogen into the water column, the CLEAN-FLO Process was engineered to eliminate this problem. It is important to move as much water as possible without creating turbulence. The CLEAN-FLO Process does not try to put oxygen into the bottom of the lake by pumping air down to the bottom. Instead, it relies on tiny bubbles to drag water from the bottom to the surface without introducing turbulence that disperses sediment into the water column. The CLEAN-FLO Process causes oxygenated surface water to gently flow to the bottom of lakes without stirring up the sediment.

Keeping the interstitial water aerobic binds nutrients in the water column to the bottom sediment and prevents the release of nutrients from the sediments. Existing ammonia in the water can be converted to nitrogen gas through oxygenation and bacterial action. The nitrogen gas and carbon dioxide are then exhausted to the atmosphere through complete inversion, but are not efficiently reduced through hypolimnetic aeration which keeps a lake stratified.

Another problem is when limnologists find high nutrient levels when testing after heavy storm events and wrongly conclude that the increase in surface water nutrients was caused by bottom aeration diffusers when actually, it was caused by external loading.

Phosphorus and nitrogen reductions in lakes, ponds, canals reservoirs using the CLEAN-FLO Process were measured by Hestand and Carter, 1974; Laing, 1974; Albreckt, 1977; Padva, 1977; Environmental Protection Agency, 1977; Wenck and Albreckt, 1978; Kaleel and Gabor, 1978; Carr and Martin, 1978; Hickok, 1979; Laing, 1979; Cooley et al, 1980; Cowell, et al, 1987; Laing and Rausch, 1993; Ishizaki and Laing, 1995; Laing and Rausch, 1993; Langjahr, 1990; Morrison, 1984; Muller, 1990; and Sohoni, 2002.

The Combination of Watershed Management and CLEAN-FLO Non-Turbulent Inversion

1. Lake Weston

Lake Weston in Orlando, Florida had sewage from a sewage treatment plant diverted a year before the CLEAN-FLO system was installed. The lake remained highly eutrophic due to bottom oxygen depletion. The three-year water quality restoration goals set by the Orange County Pollution Control were met from three months to eighteen months after the CLEAN-FLO system was installed.

The Orange County Pollution Control Department in Orlando, Florida measured composite TKN, ammonia nitrogen, orthophosphate and total phosphorus. All parameters were reduced 97 percent or more (Figure 1) after the CLEAN-FLO inversion system was installed (Bateman & Laing, 1977).

2. Sweeney Lake

The Minnesota Department of Transportation (Heinz, 2004) monitored 76 – acre Sweeney Lake in Golden Valley, Minnesota from 1998 to 2004 after the city re-routed high-phosphorus storm water through an artificial wetland into the lake. The Highway Department then performed major construction of the highway near the entrance to the wetland. Construction was completed during 2003.

Sweeney Lake had a CLEAN-FLO inversion and oxygenation system installed in 1973. Before 1973, the lake experienced heavy growth of cyanophyceae (blue green algae). After the system was installed, blue green algae were never a problem.

Surface total phosphorus remained from 0.022 – 0.094 mg/l during the Highway Department construction period. Bottom total phosphorus ranged from 0.020 – 0.081 from 1998 – 2002, with a single high of 0.287 in July of 1998, probably due to a power failure. During 1998 – 2003, parts of the inversion and oxygenation system were down due to the equipment being 25 – 30 years old. Replacement of some worn out diffusers and compressors began in 2003. In 2004, some additional inversion equipment was added.

Influent and effluent total phosphorus levels were measured by MNDOT during two heavy rain events, one on June 6 and the second June 24 – 25. Water going into the wetland (Curve and data B in Figures 2 and 3) had inlet total phosphorus 2.78 and 2.97 times greater than the wetland outlet (curve and data A in Figures 2 and 3) The reason that much more water flows out of the wetland at the weir than flows into the wetland at the B where it was measured is because there is a large storm sewer inlet near the outlet of the wetland. This comes from a large residential area in Golden Valley. From May 20 – September 12, total phosphorus into the wetland averaged 0.521 mg/l and total phosphorus out of the wetland into Sweeney Lake averaged 0.176 mg/l.

During the June 25 – 26, 2003 rain event, the lake level rose three to four feet, submerging the CLEAN-FLO compressors and destroying them. With the entire system inoperable, bottom dissolved oxygen dropped from 7.85 mg/l to 1 mg/l on June 10 and then to 0.22 mg/l by July 15. This caused a release of sediment phosphorus (Figure 4). Bottom phosphorus increased from 0.044 mg/l to 0.362 by July 15, an 823 percent increase.

The inversion and oxygenation system was repaired and bottom dissolved oxygen increased from 2.86 mg/l back up to 7.36 mg/l by September 9. By August 12, bottom oxygen had reached 2.86 mg/l, a level at which phosphorus begins to bind to bottom sediment and bottom total phosphorus had dropped back down to 0.041 mg/l. During this entire equipment down time, surface phosphate did not rise above 0.062 mg/l, and by August 8, was down to the critical algal growth level of 0.030 mg/l.

Phosphorus in the water coming into the lake from the artificial wetland was 7.3 times greater than the bottom water (0.044 mg/l) in Sweeney Lake and 5.6 times higher than the surface water (0.057 mg/l) during the storm. Even so, the CLEAN-FLO inversion system reduced bottom phosphorus to 0.023 – 0.035 mg/l and surface phosphorus back down to 0.030 – 0.024 mg/l as the equipment was being replaced during the three months following the storm. During the three months following the storm, wetland outlet water to the lake averaged 6.3 times greater than the level in the lake.

Many other studies have revealed similar results. These projects were all done with the diffusers 2.5 – 7 inches above the surface of the sediment to prevent any possible resuspension of sediments. Tests were then made to determine that no sediment was suspended.

3. The Helpe Minuere River

In Formies, France, all influent nutrient abatement possible was enacted. Two years later, the Helpe Mineure River was still dead from the city of Formies for 43 kilometers downstream. There were no fish or insects in this portion of the river. Within five months after the CLEAN-FLO system was installed at Fourmies, the river went from Class 3 – 4, where Class 4 is “unusable for any purpose” to Class 1 – 2 where Class 1 is “useable for every purpose”. Fish and insects returned in abundance (Laing and Rausch, 1993; Batteux and Laing, 1993; Mitterrand and Barnier, 1994). The mayor of Fourmies received the two highest environmental awards from the French government for the work done by CLEAN-FLO and French engineers.

Conclusion

The experience at Sweeney Lake clearly shows the great benefit of wet landing and the extreme importance of keeping bottom water oxygenated above about 2 mg/l. It also demonstrates that non-turbulent, laminar flow lake inversion does not cause phosphorus to be released from the sediments and does not degrade water quality. Indeed, the exact opposite occurred.

Eurasian watermilfoil gets most of its nutrients directly out of the water column, attaching itself to the sediment by a thread-like root. When this plant is cut and free-floating, it will continue to rapidly grow and eventually attach itself to the bottom. Eurasian watermilfoil then is an excellent indicator of poor water quality, while most other rooted plants derive their nutrients from the sediment.

There are many cases showing milfoil reduction after implementing Clean-Flo inversion and oxygenation.  Hundreds of acres of milfoil have been eliminated in some of these cases producing a much more pleasurable experience for recreational users of the lake.  As a result of reducing the milfoil and improving the water quality, property values around these water bodies have also increased.

Lake Weston, Sweeney Lake, and the Helpe Minuere River required both wet landing or nutrient abatement and continuous laminar flow inversion and oxygenation before they could be fully restored.

It has been our experience that in some cases watershed management and nutrient abatement was necessary before the CLEAN-FLO system could complete the restoration of the lakes, ponds, rivers or reservoirs. In other cases, no treatment other than the CLEAN-FLO in-situ treatment was necessary. While this paper does not investigate watershed management as a sole restoration method, the same might be said: In some cases, watershed management alone brought the water bodies to the desired performance goals. In other cases, the need for non-turbulent oxygenation of the interstitial water is indicated. Thus, the CLEAN-FLO Process should become a welcome addition to any limnologist’s toolbox.

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