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

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Introduction

Between April 2004 and September 2005, the unused sewage-treatment lagoon at Naval Air Station Whidbey Island (NASWI), Island County, Washington, was deconstructed and the site re-graded to conditions matching the surrounding area. Sewage sludge is composed primarily of partially decomposed organic matter and can be a source of contaminants such as nitrogen as further decay releases nitrogen bound in organic forms. An evaluation of the potential for contamination of ground water from any residual sewage sludge not removed during the deconstruction and remaining in the re-graded sediments needs to be completed as part of the closure process for the former sewage lagoon.

The sewage treatment lagoon was constructed in 1972 between the beach frontage road along the Strait of Juan de Fuca and the aircraft taxiway of Ault Field (fig. 1). Four aerators were located near the southern end of the lagoon. The 17-acre lagoon measured about 1,500 × 500 ft and was constructed above grade using rip-rap faced earthen berms to a height of 6 ft. The bottom of the lagoon was lined with a synthetic liner covered with 6–24 in. of native fill, consisting mostly of sand. The lagoon was an integral part of NASWI’s wastewater-treatment facility until a more modern type of sewage-treatment plant utilizing a sequencing-batch reactor was built in 1998.

Although the lagoon was unused after 1998, it remained filled with water and the thickness of sludge at the bottom of the lagoon ranged from 6 to 33 in. until the following deconstruction steps began in April 2004:

• The lagoon was drained from about 6 ft of water to about 3 ft.
• A floating dredge removed sludge from the lagoon bottom, and the sludge was dewatered using mobile centrifuges and trucked to a landfill for disposal.
• Remaining water was drained from the lagoon.
• Residual lagoon sludge was allowed to air dry, and was removed along with the top 2 in. of sand.
• Synthetic liner was removed, berms were broken down, and the former lagoon was graded to promote drainage.
• Site was hydro-seeded with native grass seed (completed October 2005).

Initial measurements of total Kjehdahl nitrogen concentrations ranged from 177 to 8,700 mg/kg in soil samples collected from the re-graded sediments in the former sewage lagoon in August 2005. The median concentration for the 15 individual soil samples was 283 mg/kg (Matt Jabloner, Naval Engineering Facilities Command Northwest, written commun., January 4, 2006). Ancillary data such as location of sample-collection sites, sampling depth, and wet-weight or dry-weight were not reported with this data. Large concentrations, greater than 1,000 mg/kg, measured in 2 of the 15 soil samples prompted concerns regarding potential for contamination of ground water from residual sewage sludge remaining at the former lagoon.

The U.S. Environmental Protection Agency (USEPA), which has regulatory oversight at this installation, considers the former lagoon site to fall under the classification of a biosolids land-application site wherein the surficial material is considered equivalent to applied biosolids. A site-specific evaluation of the potential release and transport of nutrients and possible contamination of ground water is important for final regulatory closure of this site.

Purpose and Scope

This report describes the characterization of nitrogen concentrations in near-surface soils and shallow ground water at the former sewage lagoon at Naval Air Station Whidbey Island, Washington, and presents an evaluation of the potential for contamination of ground water from nitrogen released from residual sewage sludge. The U.S. Geological Survey (USGS) installed nine temporary direct-push wells to collect water-level data and ground-water samples for analyses of nutrient concentrations and other water-quality characteristics. The U.S. Navy collected composite soil samples for analyses of organic carbon and nitrogen species during June 2006.

Description of Study Area

Naval Air Station Whidbey Island (NASWI), is located on Whidbey Island in Island County, Washington, at the north end of the Puget Sound and on the eastern extent of the Strait of Juan de Fuca. (fig. 1). The NASWI lies in a low-gradient valley along the western shore of Whidbey Island. The altitude of land surface over most of the runway area that comprises the air station ranges from 20 to 40 ft above sea level.

Whidbey Island has a temperate marine climate characterized by warm, dry summers and cool, wet winters. Mean annual temperature is about 50°F. January is the coolest month and August the warmest. Mean annual precipitation is about 19 in/yr in the area of the NASWI. Ground-water recharge on Whidbey Island has been estimated to be 11.8 in/yr (Vaccaro and others, 1998). There are no perennial streams draining the study area and most surface-water runoff drains to wetlands near the runways at Ault Field. Wetland areas are depicted near the upgradient runway areas on 7 ½-minute topographic maps of the area. Surface indications of two buried stormwater drainage lines crossing from the taxi-way to the beach area were observed.

The hydrogeologic framework of Whidbey Island and NASWI is predominantly composed of Pleistocene age glacial drift deposits. At land surface, these deposits typically include tills consisting of a dense-compact mixture of silts, clay, sands, gravels; fluvial outwash deposits consisting of variously sorted deposits of coarse-grained sands, silts, and gravels; and interglacial deposits of silts and clay. Near Ault Field, the surficial sediments are composed of clay, silt, and silty fine sand (URS Team, 1993). Depth to ground water beneath the former sewage lagoon is approximately 12 ft, and the general direction of ground-water flow typically is towards the coastline of Puget Sound (Sapik and others, 1988).

Peat and peat-soils, while not common on Whidbey Island, are present in low-lying coastal areas including the vicinity of the former sewage lagoon. Soils in the vicinity of the former sewage lagoon were mapped as Rifle or Tanawax peat soils (Ness and Richins, 1958). The largest peat deposit on the Island is near Cranberry Lake located 1.25 mi north of Ault Field (Riggs, 1958).

The former rectangular shaped sewage lagoon was located between the Ault Field taxiway and the adjacent shoreline road along the Strait of Juan de Fuca. The altitude of land surface upgradient of the former sewage lagoon ranges from 20 to 25 ft. The altitude of land surface immediately downgradient of the former sewage lagoon typically is about 15 ft. There is about 200 ft of irregular surface between the roadway and beach area. Concrete rip-rap has been placed over portions of the upper beach area.

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