Organic Chemical Sorption Heterogeneity in a Sedimentary Framework
Abstract
Reactive mass transport models require spatial representation of both the hydraulic (permeability, k) and geochemical (sorption distribution coefficient, Kd) attributes within an aquifer. Current modeling methods and techniques adequately address spatial distribution of the former better than the latter, due to gaps in both data and knowledge of geochemical heterogeneity. In low-fraction organic carbon content aquifers, such as the well-known Borden Ont. aquifer, micro and grain-scale heterogeneity contribute to Kd spatial heterogeneity. These heterogeneities impact the fate and transport of organic contaminants at an aquifer scale. We posit here that hydrophobic organic contaminant (HOC) sorption heterogeneity can be predicted using a geologically realistic framework by evaluating the geochemical aspect of HOC sorption in terms of sedimentary architecture. Using Perchloroethene (PCE) as a probe solute, batch measurements and characterization was conducted at lithofacies, lithocomponent and grain scales for > 700 samples that were obtained from aquifer cores. Greater Kd magnitudes and variance were observed in the medium-grained, poorly sorted lithofacies compared to fine-grained, well-sorted lithofacies. Lithocomponent grains (> 2mm) were sieved from a bulk aquifer sample, characterized and analyzed for fOC and Kd. Compared to the background bulk aquifer Kd of 0.3 mL/g, very high Kd values (600 mL/g) were found for some of the darker sedimentary lithocomponents. Previous research has shown that a lower PCE Kd is associated with the sand-sized grain fraction compared to other grain-size fractions sieved from bulk aquifer samples. In this aquifer (and potentially other low-organic carbon aquifers), these highly-sorbing lithological components were identified as potential contributors to sorption heterogeneity and were subsequently investigated in core samples. Lithological analysis on grains > 0.84mm was performed prior to Kd analysis in a subsample of the 350 vertically-sampled cores. Dark carbonaceous lithocomponents (2.5Y, values 5-6 from Munsell soil color chart) were found to have Kd values ranging 2-40 mL/g and very dark carbonaceous lithocomponent (values 3-4) Kd values ranged 60-600 mL/g. Light sedimentary lithocomponents (values 7-8) were found to have Kd ~ 1 mL/g and silicate Kd was found to be negligible. Kd ‘predictions’ based on the lithologic characterization of 350 samples were demonstrable for highly sorbing core samples (≥ 0.8 mL/g) within a medium-grained lithofacies and low-sorbing core samples (≤0.3 mL/g) and is correlated (wt%) to these dark and very dark carbonaceous lithocomponents, despite their low occurrence within the aquifer. The distribution of these dark and very dark carbonaceous lithocomponents control sorption heterogeneity in our core samples. New outcrop analogue data collected during summer ’10 will provide a sedimentological context to the lithofacies identified in core samples, allowing extension and testing of these concepts at larger spatial scales. Mapping Kd to outcrop architecture will improve sorption heterogeneity predictions.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2010
- Bibcode:
- 2010AGUFM.H24E..07K
- Keywords:
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- 1000 GEOCHEMISTRY;
- 1055 GEOCHEMISTRY / Organic and biogenic geochemistry;
- 1831 HYDROLOGY / Groundwater quality