The Pacific Northwest Forum
Volume 8, Number 4, Pages 14-16
Raymond A. Soltero
Raymond Soltero is in the Biology Department at Eastern Washington University.
Man, being the curious creature he is, often becomes engrossed in studying various aspects of his environment. Most phenomena are mixtures of physical, chemical and biological events. Understanding what takes place only comes from dissection, analysis, synthesis and integration of all events in one's mind.
A phenomenon that intrigues me is the aging process of lakes. Like people, lakes come in all sizes and shapes. Some lakes are man-made. A well-known example would be a reservoir (an impoundment of a stream). Lakes of natural origin may come about because of glacial action. This was true for many of Washington's lakes. As glaciers advanced and retreated, approximately 20,000 years ago, elongated troughs were cut into the landscape by the passing sheet of ice. With time, the basins filled with water, marking the lakes' beginnings.
The ultimate fate of any lake is eventually to revert to land as the basin fills with sediment. Filling is primarily due to the introduction of wind-blown materials, sediment from inflowing streams, wave action cutting away shorelines, and accumulation of plant and animal remains. Thus, over a period of hundreds to tens of thousands of years, a lake becomes a pond, then a marsh or swamp, and eventually dryland.
As a lake ages it tends to become fertile. That is, nutrients (like nitrogen and phosphorus) accumulate and enhance lake productivity. The term "eutrophic" means well-nourished; thus, eutrophication is an aspect of aging. As a lake ages it becomes more eutrophic, increasing the rate at which it will disappear.
While lakes naturally become eutrophic, man has accelerated the aging process of many lakes by disposing of domestic, agricultural and industrial wastes into them. This widespread impact has been termed cultural eutrophication and results in excessive growth of microscopic plants (algae) and larger aquatic plants. This increased plant growth, particularly algae, is so profuse that it cannot be harvested to any great extent by aquatic herbivores. The increased algal populations cloud the water, decreasing its clarity. As the algae die, they sink to the bottom and decompose, thereby depleting oxygen needed for the support of fish and other animal life.
Medical Lake Salts (primarily sodium bicarbonate)
Shore's edge prior to the alum treatment. Note alum accumulation.
Aerial photograph during alum application showing "floc" formation.
Shore's edge following alum treatment. Note the enhanced water quality.
Many lakes serve as recreational and economic bases for certain regions. Such a lake at the turn of the century was glacially-formed Medical Lake, located in eastern Washington within the corporate boundaries of the City of Medical Lake. People traveled great distances to bathe in its healing waters. Thousands of people rented bathing suits on weekends; others paid for excursion boat rides. Salts (primarily sodium bicarbonate) derived from evaporating lake water were marketed, as were soaps and ointments containing the salts.
Medical Lake's popularity was not long-lived. It soon began to exhibit signs of eutrophication, probably the result of both cultural and natural influences. For the last several decades, high phosphorus concentrations have contributed to the recurrence of noxious algal "blooms,” forming odoriferous scums which frequently curtail swimming.
In our studies of the lake, my students and I have ruled out external sources of phosphorus as being significant because the lake receives no sewage effluent or agricultural runoff and has no surface inlets or outlets. We determined the major source of phosphorus within the lake is decomposing algae and the bottom sediments. Since phosphorus was controlling algal growth, its elimination or reduction should improve water quality.
The most feasible approach to minimizing phosphorus levels in Medical Lake is to chemically precipitate it by adding aluminum sulfate, commonly known as alum. The alum reacts with the water to form an aluminum hydroxide complex called "floc." The floc chemically binds phosphorus and, being heavier than water, settles out to the lake bottom. In addition to removing phosphorus from the water, the "blanket" of floc on the bottom forms a chemical barrier, preventing further phosphorus release from the sediments.
Citizens of Medical Lake had frequently expressed interest in improving the lake's water quality, making it once again a beneficial resource. The city fathers, in response to their constituents, were successful in obtaining funds for a restorative project. In the summer of 1977, a number of alum applications were made dispensing a total of 1031 tons of liquid chemical throughout the lake to achieve the desired phosphorus reduction.
Water quality was monitored before, during and after the alum application, The results to date have shown the treatment to be highly successful. Briefly, phosphorus concentrations were reduced more than 90 percent, algal populations declined more than ten-fold, and water clarity increased. One can now see five feet deeper into the water.
The improvements in Medical Lake's water quality have renewed it as a recreational resource. Summer weekend use has increased substantially. Pre-treatment conditions of the lake precluded the survival of trout because of extensive oxygen depletion and the buildup of other toxicants in its bottom waters during the summer. However, a trout fishery now exists in the lake.
As long as phosphorus inputs remain low, the alum treatment should be long-lasting along with the other benefits associated with the restoration. But it should be remembered that this artificial limitation of fertility will only be for a short period when compared to the lake's lifespan.