In 1974 about 30 percent of the nearly 600 acre-feet of water used on the Tulalip Indian Reservation was obtained from a surface-water reservoir, while nearly 70 percent was obtained from ground-water sources. Domestic use accounted for about 93 percent of total water use. Nutrient (phosphorus) concentrations measured in most surface-water samples were less than the maximum limit recommended by the U.S. Environmental Protection Agency, whereas the recommended maximum limit for total coliform bacteria was never exceeded. Ground water is withdrawn from aquifers in unconsolidated deposits. Shallow aquifers, which provide about 45 percent of the total ground-water supply, are tapped by about 250 wells and yield 5 to 20 gallons per minute to 30- to 42-inch diameter dug wells. Deeper aquifers yield about 55 percent of the ground-water supply to about 125 wells that are mostly between 100 and 150 feet deep. Yields are generally at least 20 gallons per minute to 6- to 8-inch wells, and several wells have yields exceeding 300 gallons per minute. Water in the shallow aquifers generally had an excessive concentration of dissolved iron, often exceeding the recommended maximum limit of 0.30 milligram per litre, and total coliform bacteria in water from six wells exceeded 1 colony per 100 millilitres of water. Some wells in the deeper aquifers yield water with dissolved iron and (or) manganese concentrations exceeding the recommended maximum limit of 0.30 and 0.05 milligram per litre, respectively. Although many deep coastal wells bottom far below sea level, only two wells indicated local saltwater intrusion. An aquifer underlying the central plateau and an artesian aquifer in the northeastern part of the reservation appear to offer the best potential for development of additional ground-water supplies.
