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U.S. GEOLOGICAL SURVEY
Scientific Investigations Report 2007–5075

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Assessment of Potential for Ground-Water Contamination

Concentrations of nitrogen species in ground-water samples from beneath the former sewage lagoon obtained after the first season of winter rains following deconstruction and restoration of the former sewage lagoon were less than the maximum contaminant level. Multiple lines of evidence, including (1) the lack of substantial measurable quantities of organic matter in the re-graded sediments; (2) agronomic analysis of nutrient uptake by plants and the potential sequestration of nitrogen in the developing soil at the site; and (3) the possible occurrence of peat deposits within the aquifer material, indicate that the potential for substantial additions of nitrogen to ground water beneath the former sewage lagoon resulting from its deconstruction is small.

The lack of measurable quantities of total nitrogen and organic carbon in the composite soil-sediment samples indicate the lack of substantial organic matter or residual sewage sludge that could serve as a nitrogen source in the re-graded sediments. The amount of total nitrogen in the soil samples is used as a factor in the agronomic analysis that is used to assess potential for ground-water contamination at sites where land application of biosolids is practiced (Washington State Department of Ecology, 2000). The analysis considers the conversion of nitrogen bound in organic matter from biosolids or sewage sludge to forms that are mobile and available for either plant uptake or leaching to ground water. Nitrogen is released over several years from the organic matter in biosolids or sewage sludge although the rate of release decreases with time (Gilmour and others, 2003). Using worksheets for calculating agronomic biosolid-application rates in Washington (Cogger and Sullivan, 1999), an estimated 30 percent of the nitrogen present in land-applied biosolids is converted to plant-available nitrogen (nitrate or ammonia) in the first year following application. In the second and third year, the amount of nitrogen converted to plant available forms decreases to 8 and 3 percent, respectively, and in the years following, only 1 percent of the nitrogen in original application of biosolids is converted to plant available or leachable forms. Following year five, residual organic matter is considered part of the stable soil organic matter and are not included in subsequent calculations. Thus, the potential for nitrogen contamination of ground water arising from residual sewage sludge will diminish commensurately with time.

Applying the agronomic analysis approach (Cogger and Sullivan, 1999) to determine the amount of nitrogen that might be present at the NASWI former sewage lagoon and might be released from any remaining residual sewage sludge is complicated by several factors. First, the total amount of nitrogen present in the surficial soil sediments of the former sewage lagoon is difficult to estimate quantitatively because more than one-half of the laboratory analysis results were less than the reporting limit of 100 mg/kg. And secondly, the sediments in question constitute a very poorly developed soil and may have substantially different nutrient release and uptake rates than those determined for well-developed agronomic soils. In addition, crop nutrient requirements are a component of the analysis, and the lack of established vegetation at the site further complicates the analysis. Lastly, because the former lagoon soils are not well developed, much of the nitrogen converted from any residual sludge to plant available forms likely will be sequestered in developing plant and soil bacteria, as is common in soils developed on river terraces and sand bars (Kaye and others, 2003).

The potential for leaching of nitrogen to ground water is greatest at times when the temporal and spatial vegetative cover at the former lagoon is sparse and plants are not present to utilize available forms of nitrogen. An estimation of the nitrogen concentration that might occur in recharge that has leached all available nitrogen from the soil can be calculated. The plausible estimation of maximum concentration of nitrogen (both nitrate and ammonia) in recharge water passing through and leaching all available nitrogen from the upper 6 in. portions of the re-graded sewage lagoon area can be calculated based on the maximum measured total nitrogen concentration of 330 mg/kg, a soil density of 1.5 g/cm³, a recharge rate of 11.8 in/yr (equivalent to 0.03 L/cm²), and the organic nitrogen mineralization rates provided in a worksheet for calculating bio-solids application rate (Cogger and Sullivan, 1999). The resulting combined concentration of nitrate and ammonia in the recharge water leaching all available nitrogen from the upper 6 in. of the soil zone would be:

The amount of nitrate-nitrogen that would be transported to ground water beneath the former sewage lagoon is unknown; however, concentrations would be expected to be less than the estimated concentration of the recharge leachate. The recharge water containing the leached nitrogen contains both ammonia and nitrate species. Most of the nitrogen in the recharge leachate likely was in the ammonia form as the concentration of ammonia typically was at least twice as large as nitrate in the soil sediments that were analyzed and 70 to 90 percent of the total nitrogen present in ground-water samples from the site was in the ammonia form. In addition, the recharge-leachate would mix with ambient ground water that typically contains less than 0.5 mg/L nitrate (Sapik and others, 1988). Because the area is near the shoreline of Puget Sound, tidal influence on ground water is expected to cause daily oscillation of ground-water levels that would increase mixing of local recharge with regional ground water. Mixing of the limited amount of recharge reaching the water table at any one time is likely to result in substantial dilution of concentrations of nitrogen species.

During well installation, peat material was observed in the sediments at wells at sites LSC3 and LSC4 and its presence within the aquifer material may contribute to reductions in the concentrations of nitrogen species in recharge leachate and ground water. Wetlands and peat deposits have been used in the treatment of wastewater to reduce nutrient concentrations as well as many other potential contaminant in domestic and municipal wastewaters (Couillard, 1994; Verhoevan and Meuleman, 1999). The extent of peat deposits in not known; however, soils in the vicinity of the former sewage lagoon were mapped as Rifle or Tanawax peat soils (Ness and Richins, 1958).

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