Determination of hexavalent chromium concentrations in matrix porewater from a contaminated aquifer in fractured sedimentary bedrock

• We developed a new method for determining Cr(VI) concentrations in rock porewater.
• The method allows for detailed investigations of Cr(VI) distributions in porous bedrock.
• Results reveal the heterogeneous distribution of Cr(VI) through a plume in fractured porous bedrock.
• Detailed porewater concentration distributions are compared to data from a multi-level groundwater sampling device.

A new method for quantification of hexavalent chromium (Cr(VI)) in the porewater of rock core samples from contaminated sedimentary bedrock has been developed here. The method combines alkaline extraction with cation exchange column separation followed by determination of Cr concentrations by inductively coupled plasma mass spectrometry (ICP-MS). A porewater detection limit of 45 μg/L was determined by performing extractions on uncontaminated samples, and accounts for dilution of porewater volumes by the extraction solution. Recoveries of Cr(VI) in quality control (QC) samples were greater than 90% and there was no significant interference from Cr(III). Relative standard deviations (RSD) were less than 10% for QC samples spiked with Cr(VI), and 2 to 47% (average of 21%) for replicate analyses of core samples. Cr(VI) analyses were conducted on depth-discrete core samples collected at intervals of < 0.3 m from sandstone and siltstone bedrock within a contaminated groundwater plume. Groundwater samples were collected using multilevel well ports and were also analyzed for Cr(VI) concentrations. Significant Cr(VI) anomalies were observed in the rock matrix of the core samples. Overall, we observe general agreement in the Cr(VI) concentrations between the samples of immobile rock-matrix porewater and the samples of groundwater which is mobile in rock fractures. This method provides a viable procedure for determination of Cr(VI) concentration in bedrock porewater, and these datasets are valuable for developing conceptual models, assessing plume transport and fate, and for considering remedial options.