Scientific Investigations Report 2006-5030
U.S. GEOLOGICAL SURVEY
Scientific Investigations Report 2006-5030
Water-level measurements and sample collection, processing, and field analyses were made in accordance with applicable USGS procedures (U.S. Geological Survey, 1997 to 2004) except samples were collected using a peristaltic pump. The geochemical measurements and concentrations determined for water samples from wells included dissolved hydrogen (H2), dissolved oxygen (DO), filtered organic carbon, filtered nitrate plus nitrite, filtered manganese, filtered iron (II), filtered sulfate, unfiltered sulfide, dissolved methane, dissolved carbon dioxide, pH, specific conductance, and filtered chloride. Concentrations of 64 volatile organic compounds (VOCs), including the chloroethenes of interest, were analyzed by the USGS for water samples from five of the wells, and chloroethene concentrations were analyzed by the Navy for samples from three other wells. Monitoring wells sampled were different depths and diameters (table 1), but all were constructed with PVC casings and screens with sand filter packs.
[Well No.: MW, monitoring well. USGS site No.: Unique number for each site based on latitude and longitude of the site. First six digits are latitude, next seven digits are longitude, and final two digits are a sequence number to uniquely identify each site. Altitudes of water levels and measuring points: Feet above or below (-) NAVD 88. Depth of well and screened interval: Feet below land surface. Measuring point: Water levels in wells usually are reported as depths below land surface, although the measuring point can be any convenient fixed place near the top of well. For these wells and piezometers, measuring points are marked points on the tops of well casings and vary from being near the land surface to a few feet above land surface. Altitude of the measuring point commonly is recorded so static water levels also can be reported as altitudes. Abbreviations: ft, foot; in., inch; –, no data]
| Well No. | USGS site No. | Time | Water level altitude (ft) | Water level (ft below measuring point) | Altitude of measuring point (ft) | Depth of well (ft) | Casing diameter (in.) | Screened interval (ft) |
|---|---|---|---|---|---|---|---|---|
| 2MW–1 | 474134122372601 | 1145 | 18.71 | 4.69 | 23.40 | 19.0 | 2 | 3–18 |
| 2MW–2 | 474136122372501 | 1335 | 14.29 | 5.81 | 20.10 | 21.0 | 2 | 5–20 |
| 2MW–3 | 474136122372301 | 1415 | 12.32 | 6.78 | 19.10 | 20.0 | 2 | 4.5–19.5 |
| 2MW–4 | 474132122372601 | 1710 | 24.41 | 7.25 | 31.66 | 18.5 | 2 | 7.5–17.5 |
| 2MW–5 | – | – | – | – | – | – | – | – |
| 2MW–6 | 474136122372401 | 1530 | 11.17 | 7.15 | 18.32 | 19.0 | 2 | 7–14 |
| MW2–5 | 474136122372601 | 1115 | 16.43 | 4.73 | 21.16 | 16.0 | 4 | 5–15 |
| MW2–6 | – | – | – | – | – | 37.0 | 4 | 26–36 |
| MW2–7 | – | – | – | – | – | 32.0 | 4 | 21–31 |
| MW2–8 | 474134122372301 | 1245 | 14.19 | 4.70 | 18.89 | 13.0 | 4 | 7–12 |
| MW2–10 | 474137122372301 | 1615 | 11.20 | 6.70 | 17.90 | 12.8 | 4 | 6.8–11.8 |
After measuring depth to water, all samples were collected with a peristaltic pump and single-use polyurethane tubing. A stainless-steel weight was attached to the bottom of the tubing to accurately collect the sample from the mid-screen elevation in each well. Samples were collected after approximately three casing-volumes of water were purged from the wells and after allowing pH, specific conductance, and DO to stabilize (within 0.1 units, 5 percent, and 0.3 mg/L, respectively). The three analytes were measured in a flow-through chamber using temperature-compensated sensors from a YSITM data sonde. The specific conductance sensor was checked daily with standard reference solutions; the pH sensor was calibrated daily with two pH standards; and the dissolved-oxygen sensor was calibrated daily with water-saturated air and occasionally verified with zero dissolved-oxygen solution. Dissolved-oxygen analyses were confirmed for most water samples using 0 to 1 mg/L CHEMets Rhodazine-DTM colorimetric ampoules (manufactured by CHEMetrics, Inc., Calverton, Va.). The ampoules were filled directly from the sampling tube after well purging was complete.
Iron (II) concentrations were measured in the field in a sample filtered through a 0.45‑µm membrane filter using a colorimetric 1,10 phenanthroline indicator method and a Hach Model 2010 spectrophotometer according to Hach analytical method number 8146 (Hach Company, 1998). Sulfide concentrations were measured in the field using a colorimetric methylene-blue indicator method using the same spectrophotometer according to analytical method number 8131 (Hach Company, 1998). Information about the methodologies used to determine iron and sulfide concentrations is described at http://www.hach.com. Dissolved carbon dioxide (CO2) concentrations were measured in the field using Titret®-Sodium hydroxide tirtrant with a pH indicator (manufactured by CHEMetrics, Inc., Calverton, Va.).
Dissolved H2 in ground water was sampled using the bubble-strip method of Chapelle and others (1997). Concentrations were measured in the field using a gas chromatograph equipped with a reduction gas detector. Initial gas samples from each well were collected and analyzed after at least 20 minutes of stripping; subsequent samples were collected and analyzed at about 5-minute intervals until consecutive H2 concentrations stabilized to within 10 percent, a process that often required 1 hour or more.
Samples collected to determine nitrate, manganese, sulfate, 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. The manganese sample was acidified in the field with nitric acid to a pH less than 2, and then analyzed at NWQL by inductively coupled plasma as described by Fishman (1993). Chloride and sulfate concentrations were analyzed using ion chromatography as described by Fishman and Friedman (1989). Nitrate plus nitrite concentrations (reported as nitrite plus nitrate as N) were analyzed colorimetrically by cadmium reduction and diazotization as described by Fishman (1993). Results for the nitrate plus nitrite analyses are referred to simply as nitrate in this report because of the expected small nitrite contribution to the total concentration.
Samples for dissolved organic carbon analysis were filtered through a 0.45-µm filter, collected in amber glass bottles, acidified in the field with sulfuric acid to a pH less than 2, chilled to less than 4°C, and shipped to the USGS laboratory in Ocala, Fla., for analysis. Organic carbon concentrations were measured using high temperature combustion according to Standard Method 5310B as described by Franson (1992).
Samples for dissolved methane analysis were collected in 150-mL glass bottles, sealed with rubber stoppers, chilled to less than 4°C, and analyzed at the USGS Field Services Unit in Tacoma, Wash. Samples were analyzed using a gas chromatograph with a flame-ionization detector as described by Kampbell and others (1989). Samples for VOC analysis were collected in pre-acidified 40-mL glass vials, placed on ice, and shipped to the Severn Trent Laboratories in Denver, Colo., for analysis using purge and trap capillary-column gas chromatography/mass spectrometry (Method 8260B, U.S. Environmental Protection Agency, 1996).
For more information about USGS activities in Washington, visit the USGS Washington Water Science Center home page .
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