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Figure 1. Fluctuations of the

water level of Great Salt Lake.

The Continued Rise of Great Salt Lake, Utah

The Great Salt Lake rose 5.0 feet from September 25, 1983, to July 1, 1984, the second largest seasonal rise on record since 1847. The maximum seasonal rise was observed the previous year when the lake rose 5.1 feet from September 18, 1982, to June 30, 1983. The lake declined only 0.5 foot during summer 1983; thus, the net rise from September 18, 1982, to July 1, 1984, was 9.6 feet. By comparison, the previously recorded maximum net rise over a similar period of time was 4.75 feet from 1970 to 1972.

The lake has a yearly cycle (fig. 1). The level begins to decline in the spring or summer when the weather is hot enough so that the loss of water by evaporation from the lake surface is greater than the inflow from surface streams, ground water, and precipitation directly on the lake. It begins to rise in the autumn when the temperature decreases, and the loss of water by evaporation is exceeded by the inflow. According to past records, the rise can begin at any time between September and December, and the decline, at any time between March and July.

The level of the lake surface and the volume of the lake reflect an instantaneous equilibrium between the inflow and the loss by evaporation. The surface area and brine concentration are the

major aspects of the lake that affect the volume of evaporation. During dry years, the water level declines, causing a decrease in surface area; consequently, the volume of evaporation decreases. As the lake declines, the brine generally becomes more concentrated, which decreases the rate of evaporation. During wet years, the water level rises, causing an increase of surface area; consequently, the volume of evaporation increases. As the lake rises, the brine generally becomes less concentrated, which increases the rate of evaporation.

Precipitation was above average during summer 1983, and evaporation was relatively small because of greaterthan-normal cloud cover. This resulted in an unusually small decline of lake level during the summer. By September 25, when the seasonal rise began, the lake level had declined only 0.5 foot. The excessive precipitation continued throughout the autumn and culminated in the wettest December ever recorded in Salt Lake City. By New Year's Day, Salt Lake City had recorded 24.26 inches of precipitation during the past year-about 1.6 times the average.

The cumulative precipitation during the first one-half of 1984 also was above average, and, from October 1983 through June 1984, the precipitation at

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the Salt Lake City Airport was about 1.5 times greater than average. Much of the precipitation fell in the form of snow on the mountains in the drainage basin. The snowmelt began soon after May 1, at which time the snow cover ranged from about 1.2 to 1.5 times greater than average in the Bear River basin, about 1.5 times greater than average in the Weber River basin, and from about 1.3 to 1.8 times greater than average in the Jordan-Provo River basin.

The lake rose steadily from October 1983 through June 1984, and one of the major contributing factors that resulted from the excessive precipitation was the inflow in three major surface tributaries. The measured inflow during that period, which accounts for more than 90 percent of the total from the three tributaries, was about 3.1 million acrefeet in the Bear River, 923,000 acre-feet in the Weber River, and over 1.2 million acre-feet in the Jordan River. The flow

in the Bear River was about 2.7 times greater than average; in the Weber River, about 2.1 times greater than average; and in the Jordan River, about 5.2 times greater than average.

When the lake peaked on July 1, 1984, it was at a level of 4,209.25 feet above sea level. The increase in area covered by the lake during its 5-foot rise since September 25, 1983, was about 210,000 acres (330 square miles). This resulted in continuing damage to the roads, railroads, wildfowl-management areas, recreational facilities (fig. 2), and industrial installations that had been established on the exposed lakebed. The Utah Division of Water Resources estimates that the capital damages at these facilities as the lake rose the 5 feet was about $150 million. The Geological Survey continues to work with State and local authorities to provide information that can be used to mitigate damages from rising lake levels.

Figure 2. Saltair, a huge dance and recreational pavillion, being submerged by the Great Salt Lake. Water covered the dance floor to a depth of more than 1 foot on April 11, 1984, as the lake level reached 4,207.7 feet. The lake peaked at 4,209.25 feet on July 1, 1984. (Photograph by Ted Arnow, Water Resources Division, U.S. Geological Survey.)

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Movement and Fate of Contaminants From Treated Sewage Infiltrated to Ground Water, Cape Cod, Massachusetts

Understanding the movement of contaminants from waste-disposal sites through a ground-water system is important to the protection of water supplies. A contaminant introduced to ground water generally will move with it but may adsorb onto the aquifer materials or may react, either chemically or biologically, to form other, possibly less harmful, products. A contaminant moving with ground water also tends to disperse and becomes less concentrated due to nonuniform flow velocities and to the complex branching of flow in an aquifer. Dispersion will reduce the maximum concentration of a contaminant but will increase the volume of contaminated water.

Since 1936, secondarily treated domestic sewage has been discharged to the ground on surface sand beds at a sewage-treatment plant at Otis Air Force Base, Massachusetts. Infiltration of the sewage through sand beds to an underlying sand and gravel aquifer and the subsequent movement of the sewagecontaminated ground water in a southerly direction have caused a plume of contamination to form that is 3,000 feet wide, 75 feet thick, and more than 11,000 feet long.

The extent of the plume can be defined by chemically analyzing water sampled from the aquifer or by geophysical techniques. Ions (charged particles) of boron, chloride, and sodium and detergents that are known to occur at much higher concentrations in sewage than are found normally in the local ground water make good indicators of plume location. Increasing the dissolved ions in water increases its electrical conductivity. Because of the elevated level of dissolved ions, the plume is detectable, from instrumentation on the land surface, by a number of methods for measuring electrical conductivity of the soil, the aquifer, and the ground water.

In 1977, the U.S. Geological Survey, in cooperation with the Massachusetts Department of Environmental Quality Engineering, Division of Water Pollution Control, began a study to describe the

extent and chemical composition of the zone of ground water contaminated by the sewage disposal at Otis Air Base to better understand the potential for ground-water contamination at other similar waste-disposal sites. The field data collected from this first study, released as Open-File Report 82-274, Sewage Plume in a Sand and Gravel Aquifer, Cape Cod, Massachusetts, by D. R. LeBlanc, provided one of the best three-dimensional descriptions of a contaminant plume available at that time. That description, and the results from a subsequent study, demonstrated the potential of this site to advance scientific understanding of the processes that control convective dispersion transport and chemical change of contaminants in a plume. In March 1983, the Otis site was proposed, and later accepted, for a major research study of these topics. Some results from the first year of the research study follow.

Most contaminants move readily through the sand and gravel that make up the aquifer on Cape Cod, and little discernible attenuation by sorption of solutes on the aquifer material is evident. Transverse and longitudinal spreading of the plume is significant, but little vertical mixing occurs. After 10,000 feet of movement, the plume is only about 75 feet thick, with a core zone 20 to 30 feet thick where the contaminants remain at concentrations more than one-fourth of what they are in the treated sewage.

Microbiological reactions affect some contaminants in the plume. For example, a high concentration of nitrogen in the form of nitrate is present in the treated sewage. Within a few tens of feet in the direction of ground-water flow from the infiltration beds, nitrate is found at a much lower concentration, but ammonia is present. At 1,000 feet downgradient, no nitrate is found in the core of the plume, but ammonia concentrations exceed 10 milligrams per liter. At 5,000 feet downgradient, nitrate is again found in the core but no ammonia.

Dissolved organic carbon decreases more rapidly with increasing distance

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