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Scientific Investigations Report 2007–5075

Scientific Investigations Report 2007–5075

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Nitrogen in Soils and Sediment

Concentrations of total nitrogen in all soil and sediment samples from the former sewage lagoon were small (<100 to 330 mg/kg; table 3) compared to concentrations (1,200 mg/kg as N or about 5,000 lb/acre) that are typical for well developed agricultural soils in the Puget-Willamette region west of the Cascade Mountains (Marx and others, 1999) and substantially less than 3 percent (30,000 mg/kg) that is the typical concentration of nitrogen in sewage sludge (National Research Council, 1996). Nitrate-nitrogen concentrations measured in soils from the former sewage lagoon were in the range considered small (less than 10 mg/kg) for typical soils west of the Cascades and ammonia-nitrogen concentrations were more in the typical range of 2–10 mg/kg. The largest nitrate and ammonia concentrations, 14 and 24 mg/kg respectively, were measured in soil samples from the two lower depth intervals at sampling site LSC1 in the former sewage lagoon and may represent the downward migration of nitrogen in that area. The lack of substantial accompanying organic carbon indicates that measured nitrogen was not due to the presence of buried organic matter. Nitrate and ammonia also were detected in the sediments from lower depth intervals beneath the upgradient locations at concentrations approaching 25 to 50 percent of those beneath the former sewage lagoon. Total nitrogen concentrations in soil samples from the top and middle depth intervals at the upgradient locations were larger than those in the samples from the former sewage lagoon.

The measured organic carbon fractions ranged from 0.2 to 1 percent (table 3), which were less than the 2 to 5 percent that is typical for soils developed on glacial outwash. There was little organic carbon or nitrogen measured in the composite soil samples from the former sewage lagoon indicating the amount of residual sewage sludge present in the sediments was small. During sample collection in June 2006, no discernible organic layer within the soil profile was observed nor was there visual evidence of sewage-sludge or residual organic material. Stands of clover, lupine, and rye-grass were established and growing during June 2006, but were withered during August of the same year. There was no apparent soil profile observed at the sampling locations within the former sewage lagoon. At the upgradient sampling locations, poorly developed profiles consisting of a layer of slightly oxidized surficial sediments were present in the upper few inches.

Concentrations of nitrogen measured in soil samples collected in June 2006 were less than the concentrations reported for the soil samples collected in August 2005. A likely cause of this difference could have resulted from differences in the depth interval over which the samples were collected. Most of the organic matter that was visible in the sediments from the site was observed near the soil surface, which is consistent with natural soils where the largest concentrations of soil nitrogen typically are found in organic matter occurring near the soil surface (Brady, 1974; Bohn and others, 1984). Information was not available on the depth interval used in the collection of soil samples in August 2005. Samples collected in June 2006 integrated soil material from intervals of 6 in. or more of the soil-sediment profile and may have included a much larger fraction of the non-organic rich soil material that essentially diluted the nutrient contribution from the more organic rich soil from near the land surface thus resulting in substantially smaller concentrations of soil nutrients. The June 2006 soil samples clearly show the absence of extensive residual sewage sludge in the sampled soils.

Based on the relatively small concentrations of organic carbon and nitrogen species measured in soil samples integrated over the approximately 30 in. profile from the former sewage lagoon (table 3), little residual sewage sludge remains in the re-graded sediments and soil overlying the area. The smallest concentrations of both total nitrogen and organic carbon were measured in the deepest depth interval sampled (19–30 in.) within the former lagoon indicating that any residual sewage sludge is expected to be above that depth.

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