Methods of Sample Collection and Analysis

Water-level measurements and sample collection, processing, and field analyses during 2004 were in accordance with applicable USGS procedures (U.S. Geological Survey, 1997–2004) except that samples were collected using a peristaltic pump. Geochemical measurements and concentrations determined for samples from 19 wells and 9 piezometers 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, oxidation-reduction potential (ORP), and filtered chloride. Concentrations of 64 volatile organic compounds (VOCs) were determined for samples from 11 of the 19 wells, from all 9 piezometers, and from 12 of the 17 passive-diffusion samplers deployed (five samplers were damaged during either deployment or retrieval). The concentrations of dissolved gasses ethane, ethene, and methane also were determined for samples from all but one well, from all piezometers, and from the 12 undamaged passive-diffusion samplers.

After measuring depth to water, all well and piezometer samples were collected with a peristaltic pump and single use polyurethane tubing. A stainless-steel weight attached to the bottom of the tubing facilitated sample collection from the mid-screen altitude in each well. Samples were collected after about three casing-volumes of water were purged from the wells and after allowing pH, specific conductance, and DO to stabilize to within 0.1 unit, 5 percent, and 0.3 mg/L, respectively. Those three analytes and ORP were measured in a flow-through chamber using temperature-compensated probes from a YSI^TM data sonde. The specific conductance probe was calibrated daily with standard reference solutions; the pH probe was calibrated daily with two pH standards; and the DO probe was calibrated daily using the water-saturated air method and occasionally verified with zero DO solution. Dissolved-oxygen analyses were confirmed for most samples using 0 to 1 mg/L CHEMets Rhodazine- D^TM colorimetric ampoules (manufactured by CHEMetrics, Inc., Calverton, Virginia.). These ampoules were filled directly from the sampling tube after well purging was complete.

Concentrations of iron (II) were measured in field samples filtered through a 0.45-µm membrane filter using a colorimetric 1,10-phenanthroline indicator method and a Hach Model 2010 spectrophotometer following Hach Method 8146 [Hach Company, 1998; adapted from American Public Health Association (1980)]. Sulfide concentrations were measured in the field with a colorimetric methylene-blue indicator method immediately using the same spectrophotometer according to Hach Method 8131 [Hach Company, 1998; procedure is equivalent to U.S. Environmental Protection Agency method 376.2 (U.S. Environmental Protection Agency, 1983)]. Dissolved carbon dioxide (CO₂) concentrations were measured in the field with Titret®-Sodium hydroxide tirtrant with a pH indicator (manufactured by CHEMetrics, Inc., Calverton, Virginia).

Dissolved H₂ in ground water was sampled using the bubble-strip method of Chapelle and others (1997), and 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 min. of stripping; subsequent samples were collected and analyzed at about 5-min. intervals until consecutive H₂ concentrations stabilized to within 10 percent, a process that often required 1 hour or longer.

Samples for determination of nitrate plus nitrite, 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. Manganese samples were acidified in the field with nitric acid to a pH of less than 2, and then analyzed at NWQL by inductively coupled plasma as described by Fishman (1993). Chloride and sulfate were analyzed using ion chromatography as described by Fishman and Friedman (1989). Nitrate plus nitrite were analyzed colorimetrically by cadmium reduction and diazotization as described by Fishman (1993). The results for the nitrate plus nitrite analyses are referred to simply as “nitrate” in this report because nitrite was not detected at the site (Dinicola and others, 2002).

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 of less than 2, chilled to less than 4°C, and shipped to the NWQL. Organic carbon concentrations were determined using persulfate oxidation as described by Brenton and Arnett (1993).

Samples for VOC analysis were collected in pre-acidified 40-mL glass vials, placed on ice, and shipped to the NWQL for subsequent analysis at Severn Trent Laboratories (STL) in Denver, Colo., using purge and trap capillary-column gas chromatography/mass spectrometry (U.S. Environmental Protection Agency Method SW846 8260B; accessed November 2005 at http://www.epa.gov/epaoswer/hazwaste/test/pdfs/8260b.pdf). Samples for analysis of ethane, ethene, and methane were collected in pre-acidified 40-mL glass vials, placed on ice, and shipped to the NWQL for subsequent analysis at STL using gas chromatography with a flame ionization detector (U.S. Environmental Protection Agency Method RSK SOP-175; Kampbell and Vandegrift, 1998).

Seventeen passive-diffusion samplers were deployed in the marsh stream and pond to collect VOC samples in shallow ground water. (fig. 2). The samplers consisted of 8-in.-long by 2-in.-diameter polyethylene lay-flat tubing filled with de ionized water and heat sealed at both ends. Samplers were built by the USGS, Tacoma, Wash. Filled bags were inserted into plastic mesh sleeves to protect them from damage. Sixteen samplers were buried by hand in about 12-in. of mud at various locations beneath the marsh creek and pond, and one sampler was buried beneath an isolated puddle. The holes were backfilled with native materials, tamped down, and left to equilibrate with the surrounding pore water for more than 2 weeks. Sampling sites were marked with wooden stakes and were approximately located on a site map. Samplers were retrieved by hand and immediately processed. A corner of each bag was cut and three 40-mL glass VOC vials were filled, acidified to a pH of less than 2 with hydrochloric acid (HCl), sealed, and kept on ice for shipment to the NWQL for VOC analysis at STL. Five of the 17 deployed samplers lacked enough water for analyses, presumably due to damage during deployment or retrieval.

Quality assurance and control of geochemical and contaminant sampling included collecting duplicate samples for selected redox-sensitive analytes and VOCs and analyzing a field blank sample for VOCs. No substantial quality issues were identified in those samples (Appendix A).

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