What information do you need to make a decision that enhanced reductive dechlorination is applicable to your site and how can you tell that you are progressing towards your goals?
Natural systems contaminated with organics are dynamic systems striving to reach equilibrium and recognizing both the temporal and spatial changes can help the practitioner better understand the and manipulate the natural system. The proper evaluation of the geochemistry and contaminant profile of a site prior to the injection of a substrate to enhance reductive dechlorination is critical in selecting the proper substrate and application method for that particular situation. Likewise, a proper evaluation is critical after substrate injection to determine if the site conceptual model is correct and if remediation is progressing as planned. There are four major types of analytical parameters that are usually monitored for the design and evaluation of an enhanced reductive dechlorination project. These can be divided into geochemical field and laboratory parameters, and contaminant profile. In addition, specialized analysis can be conducted for different substrates and processes. These may include specialized molecular screening techniques and carbon isotope chemistry. Combined with hydrogeological data and a historical understanding of the release, a practitioner can design a more effective system and predict future trends in the remedial process. As with the collection of all data, the project goals must be understood and taken into account throughout the sampling program.
Field Geochemistry: Field geochemical parameters can be considered to include data that can degrade quickly after the sample is collected. The most common field analysis conducted on enhanced reductive dechlorination sites includes the collection and analysis of groundwater for dissolved oxygen, pH, temperature, and oxidation-reduction potential (ORP). Since these parameters can change dramatically after sample collection these parameters are usuallycollected by field staff. Although a number of sampling methods can be employed, care should be taken to select a sampling method that will provide a sample that is representative of the formation water as reasonably possible. Sampling methods such as bailing or high-volume purging are generally not conducive to providing a representative sample of the formation water and should be avoided, if possible. Field geochemical parameters are most accurately collected through down-hole sensors or through in-line probes. Low flow sampling
techniques as described by USEPA (Robert W. Puls and Michael J. Barcelona, EPA/540/S-95/504, April 1996 and updates) can also provide consistent and accurate results. In all cases, care should be exercised to follow the equipment manufactures instructions for care, maintenance, calibration, and operations of any analytical equipment used.
Geochemical Laboratory Analysis: Geochemical laboratory analysis can be viewed as including all parameters done by a laboratory other than the constituents of concern. These typically include at least an evaluation of the concentration of competing electron acceptors such as nitrates, iron, manganese, and sulfate, and biological indicators such as alkalinity and gases like methane, ethene, and ethane. Although not as sensitive to changes as many of the field geochemical analytes, attention to proper sampling, preservation, shipping, and laboratory quality assurance and quality control is essential to ensuring consistent, accurate data. In most cases, the sampling strategy employed for the collection of the field geochemical parameters is adequate for the laboratory geochemical analysis. It is therefore important to work closely with the analytical laboratory to prepare an adequate sampling and analysis plan prior to mobilization to the site.
Contaminant Profile: As with the samples collected for geochemical laboratory analysis, the contaminant profile samples must be c ollected and analyzed using proper methods. Most chlorinated solvents degrade following predictable pathways. The initial contaminant profile can be compared to post- substrate application profiles to help evaluate the progress of biological activity. Because compounds such as butanone and acetone can be biologically produced under certain conditions, data reporting should include all analytes detected by the laboratory. Attention should be given to determining the appropriate detection limits for each analyte to ensure that the data can be useful for both evaluation and regulatory purposes.
Biological Parameters: Advanced analytical methods have been developed to profile the type and number of microbial populations present at a site. These tools were initially generally used to help evaluate underperforming sites but have become an important part of the site characterization process. These methods typically require unique sampling procedures not generally employed by most practitioners therefore it is imperative that the practitioner carefully coordinate the collection and shipping of all samples for biological parameters with the respective laboratory.