Phosphorus Inactivation A significant reduction in nutrient loading to a eutrophic lake is a necessary but sometimes insufficient step in order to decrease water column phosphorus concentrations enough to reduce the amount of algae. Phosphorus release from lake sediments at high pH, or when dissolved oxygen in overlying water is low or zero, can be a major source of phosphorus to the water column. Under certain conditions, phosphorus released from lake sediments will be transported to the upper layers of a lake and stimulate an algal bloom. This process, in which sediments enriched in organic and inorganic matter from external loading and in-lake production cause dissolved oxygen consumption and phosphorus release, is known as internal loading. It can be great enough to delay or prevent a lake's recovery from nutrient diversion or interception (see Box 4.3).
Phosphorus inactivation reduces the rate of phosphorus release from lake sediments by the addition of aluminum salts (sodium aluminate, aluminum sulfate) to them (Cooke et al., 1986). Aluminum hydroxide is formed and appears as a visible floc that settles to the sediment and binds with phosphate ions to form a solid that is insoluble under low or zero dissolved oxygen. Phosphate ions diffusing from the sediment are trapped by the floc. The process has proved to be effective and long-lasting. Several Wisconsin lakes treated in the early 1970s exhibited improved conditions 10 years later (Garrison and Knauer, 1984; see Box 4.5). Treatment of shallow, well-mixed lakes can also be effective but appears not to have the longevity found with deep, thermally stratified lakes. A representative case history is Long Lake, Kitsap County Washington (Welch et al., 1988).
In contrast, Eau Galle Reservoir, a flood control impoundment in Wisconsin, illustrates the ineffectiveness of phosphorus inactivation when nutrient loading is not reduced significantly (Kennedy et al., 1987). The effects of treatment on the quality of this water body were overwhelmed in a few months by continued nutrient loading. Because reservoirs are difficult to protect from nutrient loading, this technique is not considered widely applicable to this type of water body (see Box 4.5).
Aluminum is a potentially toxic metal. At naturally occurring pH (6 to 8) in waters with carbonate alkalinity, nearly all aluminum is found as nontoxic aluminum hydroxide. If the pH falls much below 6, toxic forms of soluble aluminum will increase. Several observations of treated lakes with normal pH have failed to demonstrate any