Environ. Sci. Technol. 2008, 42, 3766–3772
Passive Sampling and Analyses of Common Dissolved Fixed Gases in Groundwater BRIAN P. SPALDING* AND DAVID B. WATSON Environmental Sciences Division, Oak Ridge National Laboratory, P.O. Box 2006, Oak Ridge, Tennessee 37831-6036
Received September 24, 2007. Revised manuscript received February 19, 2008. Accepted February 20, 2008.
An in situ passive sampling and gas chromatographic protocol was developed for analysis of the major and several minor fixed gases (He, Ne, H2, N2, O2, CO, CH4, CO2, and N2O) in groundwater. Using argon carrier gas, a HayeSep DB porous polymer phase, and sequential thermal conductivity and reductive gas detectors, the protocol achieved sufficient separation and sensitivity to measure the mixing ratio of all these gases in a single 0.5 mL gas sample collected in situ, stored, transported, and injected using a gastight syringe. Within 4 days of immersion in groundwater, the simple passive in situ sampler, whether initially filled with He or air, attained an equivalent and constant mixing ratio for five of the seven detected gases. The abundant mixing ratio of N2O, averaging 2.6%, indicated that significant denitrification is likely ongoing within groundwater contaminated with uranium, acidity, nitrate, and organic carbon from a group of four closed radioactive wastewater seepage ponds at the Oak Ridge Field Research Center. Over 1000 passive gas samples from 12 monitoring wells averaged 56% CO2, 32.4% N2, 2.6% O2, 2.6% N2O, 0.21% CH4, 0.093% H2, and 0.025% CO with an average recovery of 95 ( 14% of the injected gas volume.
Introduction Although dissolved fixed gases are known to be indicators, products, and substrates for important biogeochemical processes in both natural and contaminant-impacted groundwater (1–7), their volatility and pressure-dependent solubility makes their sampling, handling, transport, and complete analyses challenging (2) compared to that of nonvolatile dissolved constituents and contaminants. Recently, three techniques, using in situ syringe-based passive gas samplers (1–3), have demonstrated their utility to collect, store, transport, and analyze dissolved gas samples. The recent approach of McLeish et al. (2) is particularly useful as the first published integrated sampling and analytical approach for many common dissolved gases in groundwater. Previously, we reported the use of a simple in situ passive dissolved gas sampler (made from a short length of silicone tubing and a gastight syringe) to collect, transport, and inject gas samples onto a gas chromatograph (GC) for limited analyses of H2, O2, He, and Ne (1). Our present investigation was undertaken with the objective to develop an integrated sampling and gas chromatographic method for most of the common dissolved gases (He, Ne, H2, N2, O2, CO, CH4, CO2, and N2O) in * Corresponding author e-mail:
[email protected]; phone: 865-574-7265; fax: 865-576-3989. 3766
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groundwater using a single chromatographic column. The potential for dissolved gases to interact through their partial pressures emphasizes a need for complete and concurrent analyses of all major dissolved gases when attempting to understand field dynamics of any single gas. Such integrated and complete protocols should prove useful to follow microbial and chemical processes which produce and consume many of these dissolved gases in groundwater. A group of groundwater monitoring wells at the Oak Ridge Field Research Center (8), used previously to test passive samplers for H2 and O2 (1), was selected to test the present integrated protocol. The test area is located near a group of four one-acre closed radioactive liquid waste ponds, and its groundwater is heavily impacted with uranium (>50 mg/L), 99Tc (>40 nCi/L), nitrate (>8000 mg/L), sulfate (>1000 mg/ L), total organic carbon (>100 mg/L), and acidity (pH < 3.5) (8, 9). The average hydraulic conductivity in the contaminantimpacted deep saprolite is wide-ranging (10-3 to 10-5 cm/s) depending on residual fractures in the saprolite; interstitial groundwater flow averages 0.1 m/day along strike with hydraulic gradients relatively flat (0.01). Both natural and induced bioremediation processes are under investigation at the site (9) including sulfate reduction, denitrification, uranium and technetium reduction, oxygen consumption, and methanogenesis. Such processes may be amenable to in situ monitoring if such dissolved gases are present at detectable mixing ratios using an integrated gas sampling and analytical protocol.
Methods and Materials Gas Chromatographic Protocol. A 30-ft long × 1/8 in. stainless-steel column, packed with 100/120 mesh HayeSep DB solid phase (Alltech Chromatography) with an Ar carrier gas pressure of 55 psi was maintained at a flow rate of 10 cm3/min. After an initial 4 min at 84 °C, the column oven was programmed at 2.65 °C/min for 14 min and held at 123 °C until a total run time of 22 min; this column heating regime yielded adequate separation of the major fixed gases for quantification by the thermal conductivity detector (TCD) and reductive gas detector (RGD; Figure 1). High carrier gas pressure (55 psi), required to elute all gases in a 22 min run, made on-column needle injection difficult to replicate; a fixed-volume (0.5 mL) gas sampling loop, plumbed to a programmable rotary valve of a SRI model 8610-0072 GC, was used for sample injection at standard atmospheric pressure (uncorrected for (2% normal barometric variations) and constant laboratory temperature (25 °C). The valve loading connections employed minimal lengths of smallbore tubing (0.01 in. i.d.) so that only 0.7 mL of gas sample was required to flush completely and fill the 0.5 mL sampling loop. The on-column injector port was also plumbed into the carrier gas line so that occasional samples with
