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Scientific Investigations Report 2007–5075

Scientific Investigations Report 2007–5075

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Sample and Data Collection and Analysis

Water-level measurements and water-sample collection, processing, and field analysis were performed in accordance with applicable USGS procedures (U.S. Geological Survey, 1997–2006). Soil samples were collected by the U.S. Navy in June 2006 and analyzed to determine the dry-weight concentration of organic carbon and nitrogen species. Soil samples were collected using non-contaminating materials, and surficial plant litter material was removed from the samples. Additional details concerning soil sampling are available from the U.S. Navy (Matt Jabloner, written commun., September 8, 2006).

Nine temporary drive-point wells (fig. 2) were installed at the study site to measure ground-water levels and collect water-quality samples in June–August 2006. Wells were located along three transects that were approximately perpendicular to the anticipated direction of ground-water flow toward the beach. Four wells were installed along the approximate center line of the former lagoon, three wells were installed upgradient of the lagoon, and two wells were installed downgradient of the lagoon. Buried rip-rap did not allow installation of additional downgradient wells near the beach area.

The drive-point wells were constructed using Schedule 80 0.5 in black-iron pipe. The lower ends of the pipe were crimped closed and a series of 20 to 25 1/8th in. diameter holes were drilled to allow ground water entry into the pipe. The pipes were manually driven into the ground to the depth just below the water table using a slide hammer. Wells were developed using a surge block and peristaltic pump. After a drive-point well was driven into position, the casing was filled with clean water and a surge-block was used to move water through the well perforations. A peristaltic pump was then used to extract more than 10 times the volume of water placed in the well during the surge-block development process.

Depth to ground water below a measuring point at the top of the exposed casing was measured in the wells using a steel tape. The relative altitude of water-level measuring points for each well were determined using a Surveyors level and a reference altitude obtained from Light Detection and Ranging (LiDAR) imaging of land-surface altitude at well LSD2. The LiDAR data were obtained in August 2001 and shows the general outline of the former sewage lagoon area. Figure 2 shows the LiDAR image of the former sewage lagoon and the locations of the drive-point wells and soil and sediment sampling sites. The surveying closure error, taken as the difference in altitude of the starting reference point following seven changes of instrument location, was 0.03 ft.

After measuring depth to water, ground-water samples were collected with a low-flow peristaltic pump and single-use polyethylene tubing. Samples were collected after approximately three casing-volumes of water were purged from the wells and after allowing specific conductance and dissolved oxygen (DO) to stabilize to within 5 percent, and 0.3 mg/L, respectively. The geochemical measurements and concentrations analyzed for water samples collected from wells included nutrients (filtered total nitrogen, filtered nitrite plus nitrate, filtered nitrite, filtered orthophosphate), organic carbon, filtered concentrations of chloride, ferrous iron (II), and carbon dioxide. A 0.45 µm membrane filter was used to prepare filtered samples. Specific conductance was measured using a sensor that was checked with a standard reference solution. Dissolved-oxygen concentrations were measured using a 0–1 mg/L CHEMets Rhodazine-DTM colorimeteric ampoules (manufactured by CHEMetrics, Inc., Calverton, Va.). The ampoules were filled directly from the sampling tube after well purging was complete. Ferrous iron (II) concentrations were measured in the field using a colorimetric 1,10 phenanthroline indicator method and Hach Model 2010 spectrophotometer according to Hach analytical number 8146 (Hach Company, 1998). Dissolved carbon dioxide concentrations were measured in the field using Titret-Sodium hydroxide titrant with a pH indicator (manufactured by CHEMetrics, Inc., Calverton, Va.).

Samples collected for analysis of nutrients and chloride concentrations were filtered through a 0.45-µm membrane filter into polyethylene bottles, chilled, and sent to the USGS National Water Quality Laboratory (NWQL) in Lakewood, Colo. Chloride concentrations were analyzed using ion chromatography as described by Fishman and Friedman (1989). Nutrient concentrations were analyzed following procedures described by Fishman (1993) using colorimetric techniques with cadmium reduction and diazotization as appropriate. Concentrations of total nitrogen were determined colorimetrically using alkaline-persulfate digestion as described by Patton and Kryskalla (2003). Samples for total organic carbon analysis were collected in an amber glass bottle, chilled to less than 4°C and sent to the NWQL. Organic carbon concentrations were determined using high temperature combustion according to Standard Method 5310B as described by Franson (1992).

Soil samples were collected from four areas in the former lagoon (LSC1, LSC2, LSC3, and LSC4; fig. 2). In each sampling area, soil samples were collected from three depth intervals to determine the vertical extent of residual sewage sludge that might be present and potentially a source of nitrate contamination of ground water. The three depth intervals were 0–6, 7–18, and 19–30 in. Based on the size of the berms surrounding the former sewage lagoon, the re-graded sediment material was not expected to provide more than 6 in. of additional mineral material to the land surface. The re-graded sediment would likely be thinnest along the central north-south axis of the former sewage lagoon.

Composite sampling is used to evaluate soil nutrient concentrations (Mahler and Tindall, 1994; Jacobson, 1999) because of the large degree of spatial variability that is present in soil nutrient concentrations (Stark 1994; Cain and others, 1999). For each of the three sampling depth intervals, a composite sample was collected by combining soil material obtained from five individual subsample cores. Subsamples were collected for compositing soil from five randomly selected locations within a 50-ft diameter area of the center mark. A total of 12 composite samples were analyzed from the former sewage lagoon area.

For comparison to nitrogen concentrations in soil samples from the adjacent upgradient area of the former sewage lagoon, an additional set of composite samples were collected from the same three depth intervals. The upgradient composite sample consisted of soils collected from single subsample cores collected at soil sampling sites LSU1, LSU3, and LSU5 (fig. 2).

Quality assurance and control included field and laboratory procedures. Analyzing laboratories follow standard laboratory and quality-assurance procedures and participate in National proficiency testing programs. An inorganic blind sample monitoring program also is used at the USGS National Water Quality Laboratory to provide ongoing monitoring of laboratory generated water-quality data. Quality-control samples including a field blank and a field sample replicate also were submitted. Results from the field quality-assurance samples are shown in table 1. No substantial quality issues were identified in the results of those samples.

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