Determination of Interstitial Water Chemistry and Porosity in Consolidated Aquifer Materials by Diffusion Equilibrium-Exchange

Abstract

Diffusion equilibrium exchange (DEE) is presented as a novel, practical alternative to centrifugation for the recovery and chemical analysis of interstitial water in contaminated core samples from consolidated rocks and aquifers. The methodology is suitable for sampling organic and inorganic compounds, including redox sensitive species such as SO4(2-), NO3-, NO2-, Mn(II), Fe(II), and sulfide (HS-). DEE also permits analyte extraction from kilogram quantities of core, which avoids extended centrifugation or sample amalgamation and provides analyte masses appropriate for stable isotope analysis. The procedure involves simple and rapid on-site sectioning of representative core samples, which are preserved in the field by storage in airtight bottles filled with deoxygenated deionized water containing a conservative tracer (Br-). Equilibration times for individual solutes can be estimated in advance to reduce the need for time-series analysis; for an effective diffusion coefficient of 2.5 x 10(-10) m2 s(-1) (Br- in chalk rock) equilibration was >90% completed after 30 h, consistent with the predicted equilibration time. The DEE method presented minimizes sampling errors from temperature changes, oxidation of reduced chemical species, and loss of volatile compounds, which can occur with other interstitial water sampling techniques. It also gives superior resolution of in situ solute distributions and geochemical processes in consolidated sediments than centrifugation and can provide estimates of aquifer porosity in core samples. Laboratory experiments using chalk rock core and simulated extraction procedures confirm the superior performance of the DEE method over centrifugation for a range of solutes. The method has been used to generate detailed interstitial water profiles of electron acceptor and contaminant concentrations along the flow path of a petroleum hydrocarbon plume in the U.K. Upper Chalk aquifer as part of a natural attenuation assessment.

Lithological controls on biological activity and groundwater chemistry in Quaternary sediments

“…Alkalinity was measured by colorimetric titration (Hach Chemical Co.) on filtered samples in the field. Pore water was extracted from sediment samples by leaching in an anaerobic environment with a measured amount of distilled water (Bottrell et al, 2000b;Spence et al, 2005b); a sonic bath was used to aid disaggregation of the clay-rich samples. The leachate from this process was filtered in an anaerobic chamber and split into two aliquots for analysis of major ions and recovery of sulfate for isotopic analysis.…”

“…Major cations (Ca 2C , Mg 2C , Na C , K C ) and anions (Cl , SO 4 2 , NO 3 ) were analysed using a Dionex Chromeleon 6Ð2 ion chromatograph system. Original pore water ion concentrations were calculated using the ratio of original sediment water content (measured by drying at 110°C) to leach water added (Spence et al, 2005b). Fertilizer samples were obtained from regional suppliers and dissolved in 1 M NaCl solution and filtered before recovery of sulfate for isotopic analysis.…”

Biogeochemical Processes in the Active Layer and Permafrost of a High Arctic Fjord Valley

“…A pore water extraction method was adapted from Spence et al (2005). Vials containing samples were transferred to a Coy Vinyl Anaerobic chamber with a N 2 atmosphere (0 ppm oxygen).…”

“…The vials were crimp-capped, inverted and stored for 5 days at 4 • C whilst submerged in water (to prevent gas diffusion across the septa). This storage time enabled the de-ionised water to equilibrate with the sediment pore water (e.g., Spence et al, 2005). 7 days after first saturation of the sample, the vials were centrifuged at 7750 rpm for 5 minutes and transferred back to the anaerobic chamber.…”

Distribution of ETBE-degrading microorganisms and functional capability in groundwater, and implications for characterising aquifer ETBE biodegradation potential

“…The collection of core material is undoubtedly the gold standard for sampling the aquifer microbial community (Lehman 2007 ), but core recovery may be difficult in cohesionless sediments (e.g. sands and gravels) and is susceptible to fragmentation or contamination by the drilling methods used (Wilkins et al 2014 ; Kiaalhosseini et al 2016 ; Close et al 2020 ), or requires elaborate measures to assess and avoid contamination during sampling (Lehman et al 2004 ; Spence et al 2005 ; Wilkins et al 2014 ; Kallmeyer 2016 ; Friese et al 2017 ). The destructive nature of core sampling also means temporal sampling is not possible for the same location (Lehman 2007 ; Wu et al 2013 ).…”