Disclaimer & Copyright Notices; Optimized for the MS Internet Explorer

WATERSHEDS/COWBAYR/SETTLE/CLEANUP/PIC/diver.jpg

Lake Restoration, the Canadian experience

Summary of in-lake methodology for both culturally and naturally eutrophic lakes

Soil & Water Conservation Society of Metro Halifax (SWCSMH)

Updated: Septewmber 19, 2013      Restoration

Contents:

"The lakes we remember once drew us to swim, to picnic, to canoe. We found in them beauty, mystery, and tranquillity. Lakes continue to inspire our daily lives, while at the same time providing transportation, an abundant food supply, and water for drinking and bathing. By serving as a critical link in the ecosystem, lakes also are important sanctuaries for our fish and wildlife. But for some of us, the lakes of our childhood- and of our nation's economic strength- are rapidly changing. While we all share responsibility for the cause, we also share responsibility for restoring our deteriorating lakes to their former beauty". ............ Anon


Introduction

Several procedures to reduce the level of nutrients within lakes without changing loading rates have been devised (see our Synopsis titled "Lake Restoration/Management", Feb. 93). Dredging may be appropriate for lakes whose sediments are high in nutrients. Chemical precipitation is possible. Dilution or flushing is possible if large quantities of low nutrient water are available. Harvesting of both algae and macrophytes can remove nutrients. Lake sediments can be sealed to prevent nutrient release. Artificial aeration techniques can be used to improve water quality, and provide suitable cold water habitat during periods of restricted circulation.

The different pathways of phosphorus and nitrogen in lake metabolism make phosphorus the obvious choice for eutrophication control. A certain reduction of phosphorus input will generally result in a greater reduction in algal biomass compared with the same reduction of nitrogen. Furthermore, the reduction of nitrogen input without a proportional reduction in phosphorus, creates low N/P ratio which favors nitrogen fixing nuisance algae, without any reduction in algal biomass.

Lime Treatment

(1) While lime treatment has been extensively used to mitigate acidification effects, several studies of calcium carbonate precipitation led to the hypothesis that the addition of lime to lakes can also reduce eutrophication. Lime has been added to several lakes and dugouts in Western Canada (Frisken, Figure Eight, Andorra, Beaumaris, Valencia, Halfmoon, Gour, Monnette, Desrosier, Frey, Fedora, Pederson, Sullivan, Schreger, Limno) to improve water quality. These hardwater lakes are eutrophic due to high natural, agricultural, or urban loadings of phosphorus. Source control of phosphorus loadings would be extremely difficult at all sites. Most of the lakes are primarily used for recreation but the dugouts have been used for human and agricultural water supplies. In two of the study sites, Figure Eight Lake and Frisken Lake, most of the sediment iron is converted into pyrite. These lakes have little reactive iron and presumably phosphorus biogeochemistry is not controlled by iron reactions.

(2) The usual method for algal control is application of copper sulfate or alum. But copper sulfate is toxic to nontarget organisms, and its use can upset the ecostructure of lakes. The long term adverse effect of alum in the natural environment is unknown. Proper application of lime (specifically calcium hydroxide) reduces chlorophyll a levels. Calcium hydroxide dissociates and forms calcium carbonate per
Ca(OH)2 + CO2 --> CaCO3 + H2O

These newly formed calcite crystals are small and present a relatively large surface area for adsorption. Associated with phosphate adsorption onto calcite is the molecular exchange of CO3-2 and PO4-3 on the surface of growing calcite crystals as follows;
3CO3 - 2(S) + 2PO4 - 3(L) <--> 3CO3 - 2(L)+ 2PO4 - 3(S)
where S and L denote calcite and aqueous phases, respectively.

(3) Although biological reactions must influence phosphorus biogeochemistry, the effect of lime treatment on phosphorus biogeochemistry can be easily explained via apatite for mation. The generally accepted model for apatite formation is that phosphorus initially adsorbs to calcite and then a surface rearrangement produces phosphate heteronuclei that ultimately form the stable mineral apatite. If the surface application of calcium hydroxide was repeated for a number of years, the titration should exceed an end point, phosphorus and calcium should not redissolve, and phosphorus could be converted into apatite. The optimal method of enhancing calcite and apatite formation in lakes is not obvious, but several recommendations are possible. Lakes with rapid hydraulic flushing or high and continuous nutrient loading are less appropriate for lime treatment; however, lakes with a high short-term spring loading of nutrients respond well to lime treatment. In lakes without fish, a large dose of Ca(OH)2 should be used. In lakes with valuable fisheries, alternative approaches to enhance apatite formation could include hypolimnetic injection of Ca(OH)2 or larger surface applications of CaCO3.

Lake Aeration

(1) The applicability of aeration techniques to improve water quality is briefly discussed below. (2) Many deep lakes situated in the prairie provinces are eutrophic. In May, 1988, a system was installed to inject pure oxygen into the bottom of the hypolimnium of the north basin of Amisk Lake (mean depth= 14.4 m). The south basin was not treated and served, along with 7 yrs of background data, as the control. During summer 1988 (Prepas et al, 1990), the hypolimnetic oxygen depletion and TP accumulation rates in the treated basin were only 42.5 % of historic averages; during winter 1988-89, under-ice DO concs. were maintained at 5 mg/l. In contrast, the rates in the untreated south basin remained near or above pretreatment rates.

(3) Experimental aeration systems have been installed in three British Columbia lakes (Ashley, 1988; Ashley et al, 1987). The lakes were Black (naturally eutrophic, surface area= 4.0 ha, mean depth= 4.5 m), Glen (culturally eutrophic, surface area= 16.0 ha, mean depth= 7.2 m), and St. Mary (culturally eutrophic, surface area= 182 ha, mean depth= 9.1 m). Full scale systems were installed in the last two lakes, and the results were promising.

References

Restoration                            Soil & Water Conservation Society of Metro Halifax (SWCSMH) Master Homepage


We salute the Chebucto Community Net (CCN) of Halifax, Nova Scotia, Canada for hosting our web site, and we applaud its volunteers for their devotion in making `CCN' the best community net in the world


Google