Min-Trap® Samplers to Passively Monitor In-Situ Iron Sulfide Mineral Formation for Chlorinated Solvent Treatment

Abstract

The objective of this work was to field-demonstrate the Min-Trap® sampler technology, a new in-situ passive monitoring tool that offers distinct advantages for collecting mineralogical data. The Min-Trap consists of a solid porous medium (e.g., silica sand) contained within a water-permeable mesh and support housing that is deployed inside a monitoring well. The porous medium serves as a solid substrate upon which target minerals passively form. Analysis of the sample medium using chemical, microscopic, spectroscopic, or other techniques can provide direct evidence of geochemical conditions and the formation of target minerals in-situ. The abiotic degradation of chlorinated volatile organic compounds via the reducing power stored in reactive minerals (e.g., iron sulfides) is a recognized important process, and Min-Traps are well suited to document the formation of these minerals during remediation. Min-Traps were tested in multiple wells at two field sites undergoing active remediation where reactive iron sulfide minerals were expected to be actively forming. Min-Trap samples were analyzed by a variety of analytical techniques, including total iron, acid volatile sulfide (AVS), chromium extractable sulfide (CrES; also known as chromium reducible sulfur; CRS), weak acid soluble (WAS) iron, strong acid soluble (SAS) iron, and scanning electron microscopy–energy dispersive X-ray spectroscopy (SEM-EDS). The results provided direct verification of iron sulfide minerals in all cases (n = 9) where they were expected or interpreted to be possibly present based on groundwater data. Min-Traps offer the potential for lifecycle cost savings because they do not require the collection of soil core samples, they can aid in better remedy implementation and performance, and they can potentially support earlier transition from active to passive remedy phases.