Internal Nutrient Loading in the Crystal Springs Reservoirs
Internal loading is a process that occurs in lakes and reservoirs when nutrients are introduced into the water from the lake sediment. Along with external loading, internal loading can lead to lake eutrophication and cause chemicals to enter the water, causing taste, odor, and color problems. Internal loading can also begin a positive feedback loop that intensifies eutrophication. These problems are especially important if the lake is used as a drinking water resource. This experiment studied the Crystal Springs reservoirs in San Francisco, a pair of reservoirs that have the potential of becoming eutrophic from proposed chloramine disinfection that would increase nitrogen loading. A chamber study was developed that determined the internal loading potential of the two reservoirs and the effectiveness of hypolimnetic aeration as a solution. Two samples of undisturbed sediment and hypolimnetic water were taken from each reservoir and stored in study chambers. The redox potential and concentrations of phosphate, ammonia, and nitrate were monitored in these chambers through oxic, anoxic, and restored oxic conditions. Redox levels dropped in the chambers during anoxia to levels as low as -430 mV in the upper reservoir and -635 mV in the lower reservoir, while phosphate and ammonia increased to peaks of 661 ug/l and 430 ug/l in the upper reservoir and 714 ug/l and 906 ug/l in the lower reservoir. The increase of phosphate and ammonia in the chambers indicated that internal loading occurred in anoxic conditions. Nutrient concentrations decreased with aeration below detectable limits of 50 ug/l, suggesting that hypolimnetic aeration could be a good technique to eliminate future internal loading that could occur with increased productivity from chloramine based nitrogen loading.
Lake eutrophication is marked by an increase in productivity caused by an excess of nutrients. The process begins when nutrients that limit growth, most frequently phosphorous and nitrogen, enter the system through natural or human processes and stimulate algal blooms (Horne and Goldman 1994). When algae populations increase, many problems can occur. Lake water loses its natural clear blue color and becomes green, causing a loss of transparency. Intense eutrophication can produce large floating mats of algae, commonly cyanobacteria, that can produce odors and appear unpleasant to lakeside residents. The increased populations of algae lead to larger amounts of algal biomass that sink to the lake bottom and consume oxygen during decomposition. In the hypolimnion, or the lower level of the lake, oxygen levels can be depleted by decomposing biomass resulting in anoxia.
An anoxic hypolimnion can pose many problems. Fish cannot breathe without oxygen in the water, therefore anoxia can cause significant fishkills in a lake. Zooplankton populations can also decline when a hypolimnion becomes anoxic. To avoid predation by fish, zooplankton hide in lower waters during the day and come up to the surface waters to feed at night (Horne and Goldman 1994). When the hypolimnion becomes anoxic, the zooplankton lose their refuge in the hypolimnion, beginning a positive feedback loop. Anoxia caused by increased algae leads to a decrease in population of zooplankton, an algal predator. As conditions become more reduced in the hypolimnion, bacteria begin producing hydrogen sulfide, using sulfate instead of oxygen as a terminal electron acceptor (Beutel 1999, pers. comm.). Hydrogen sulfide is poisonous to fish, and can become an odor problem for people who live near a lake or use its waters as a drinking water resource. Manganese and iron are also released from the sediments in reduced conditions, and along with hydrogen sulfide can cause taste, odor, and color problems.
Two sources of nutrients can lead to eutrophication in lakes and reservoirs. External loading occurs when runoff from the watershed brings in nutrients from anthropogenic sources, such as sewage plants and agricultural runoff, and from natural sources. Frequently, external loading is the main determining factor in a lake’s trophic status because of its large scale (Horne 1998).