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Scientific Investigations Report 2007–5075

Scientific Investigations Report 2007–5075

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Summary and Conclusions

The potential for contamination of ground water from residual sewage sludge in re-graded sediments following deconstruction and site restoration of a sewage lagoon was evaluated. Composite soil and sediment samples were collected and analyzed for organic carbon and nitrogen species, ground-water levels were measured in temporary drive-point wells, and ground-water samples were collected and analyzed for nutrients and other water-quality characteristics. Wells were located in three areas based on the anticipated direction of ground-water flow toward the beach. Resulting water-level measurements indicate the direction of ground-water flow is toward the Puget Sound. Prior to the installation of temporary wells, composite soil samples were collected by the U.S. Navy and analyzed to determine the dry-weight concentration of organic carbon and nitrogen species.

The amount of total nitrogen and organic carbon present in the sediments at the former sewage lagoon was only about 25 percent of the amount typically present in developed agricultural soils, indicating that the sediment of the former lagoon area is essentially devoid of organic matter that would be present in residual sewage sludge. The largest concentration of total nitrogen measured in soil samples from the former lagoon (330 milligrams per kilogram) was used to develop an upper-bound estimate of the amount of nitrogen that might be leached by recharge and mixed with ground water. Nitrate concentrations measured in ground-water samples were smaller than the upper-bound estimate and did not exceed established U.S. Environmental Protection Agency’s maximum contaminant level of 10 milligrams per liter. Seventy to 90 percent of the total nitrogen present in ground water was in the ammonia form with a maximum concentration of 7.67 milligrams per liter.

Overall, concentrations of nitrogen species in ground water beneath the lagoon were small with respect to the U.S. Environmental Protection Agency maximum contaminant level for nitrate in ground water (10 milligrams per liter). Concentrations of total nitrogen in ground water beneath the lagoon, which is the sum of all forms of nitrogen including nitrate, nitrite, ammonia, and organic nitrogen, ranged from 1.15 to 8.44 milligrams per liter as N. Thus, even if all forms of nitrogen were converted to nitrate, the combined mass of nitrogen would be less than referenced as the maximum contaminant level. Given the abundant supply of organic carbon in ambient ground water at the site, any nitrate that may leach from residual sludge at the former sewage lagoon is expected to quickly transform to nitrite and ammonia under the strongly reducing geochemical conditions that are present.

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