Lab Science News

Nuisance odors are a complex and subjective issue, often resulting in odor complaints directed at industrial or agricultural facilities such as wastewater treatment plants, landfills, large scale composting facilities, or animal feed operations. At these types of facilities, most odorous chemical compounds are produced under anaerobic conditions. Contrary to popular belief, nuisance odors themselves do not generally cause long term illness or any direct health effect.  In other words, if the source of the odor is taken away, any associated illness symptoms (e.g. nausea) will also go away. Therefore, unlike investigations centered on human health risk, investigations involving nuisance odor are governed by the perception of the receptor. A person’s perception of odor is related to the human olfactory system, which can vary widely from person to person; what smells bad to one person might not have an odor at all to someone else. To further confuse the issue, there is a distinct lack of odor regulations, and those that exist are extremely vague. The EPA defaults to the state level for nuisance odors, and most states defer to the county or local level.

Odor is a parameter which may be measured unto itself, following established ASTM and/or European Standards. This approach will quantify how odorous a sample is, ranking it on a relative scale with units of dilution to threshold (D/T).

Knowing the magnitude of an odor problem is useful, but often more detailed chemical information is necessary when odor control engineering solutions are being evaluated.  When a detailed chemical analysis of odorous compounds is needed, there are several analytical options:

1. Produced during the acidogenesis stage of anaerobic digestion, reduced sulfur compounds have a very characteristic odor of rotten eggs, rotten garlic/cabbage, skunk or natural gas. In fact, the human nose is sometimes more sensitive than the most current analytical instrumentation used to detect these compounds. An example of these compounds is methyl mercaptan, which has an extremely low odor threshold (this is why mercaptans are used as natural gas odorants). The most popular analytical option for reduced sulfur compounds is ASTM Method D5504. This method quantifies a list of 20 speciated reduced sulfur compounds (such as hydrogen sulfide, mercaptans, thiophenes) using gas chromatography with a sulfur chemiluminescence detector (GC/SCD).

2. With a characteristic fishy/fertilizer or putrid/sour/pungent odor, amines are the result of the biological breakdown of amino acids and are produced at various stages of anaerobic digestion. Columbia Analytical has developed a comprehensive amine sampling and analytical method that reports a list of 13 amine compounds with reporting limits at or below published odor threshold concentrations. A sample is collected on an in-house designed sorbent tube using a personal sampling pump. Due to their unique chemical characteristics, amines will not always be detected in any of the other tests described here (e.g. VOC test). Analysis of the samples is via a specially modified gas chromatography with nitrogen phosphorous detection (GC/NPD).

3. Ammonia, which is produced by microbial decomposition of animal waste, has a characteristic odor most people will recognize due to the compound’s use in window cleaners. At higher concentrations, ammonia can cause serious health damage, irritating and/or burning nasal passages and lungs. Collection of airborne ammonia may follow the OSHA ID-188 method, which uses sulfuric acid-coated Anasorb-747 (carbon bead) tubes and a personal sampling pump for collection. This means of sample collection is much easier and safer than the traditional collection technique of sulfuric acid solution impingers. Analysis may follow the OSHA-ID 164 analysis, which utilizes an ion-specific electrode (ISE) to detect ammonia.

4. Carboxylic (volatile fatty) acids are produced as a result of the biological anaerobic breakdown of proteins, with typical odor characteristics including a rancid, fecal, vomitous, or sweaty gym sock smell. Columbia Analytical has developed a comprehensive sampling and analytical method that reports a list of 15 carboxylic acid compounds with reporting limits at or below published odor threshold concentrations. The sample is collected on a sodium hydroxide-treated silica gel tube using a personal sampling pump; the subsequent sample is then analyzed via gas chromatography/mass spectrometry (GC/MS).

5. Several other analytical methods may be used to quantify levels of aldehydes and other miscellaneous volatile organic compounds (VOCs). EPA Method TO-11A (silica gel tubes coated with acidified 2,4-dinitrophenylhydrazine (DNPH) ) is an appropriate method for sampling of aldehydes (carbonyl compounds).  EPA Methods TO-15 (stainless steel canisters) and TO-17 (thermal desorption tubes) are appropriate methods for sampling of volatile organic compounds.  Polar volatile compounds such as alcohols, aldehydes, esters, ketones, ethers, phenols and cresols are often contributors to nuisance odors.

Due to their complex nature, there is no “one size fits all” approach for evaluating the chemical composition of odors. Odorous compounds may have additive, synergistic or antagonistic effects, all contributing to odor perception. Multiple analytical methods or evaluation approaches may be required to address a single source.

Soil gas sampling is increasing in frequency across the country as vapor intrusion continues to gain regulatory attention. When evaluating the potential for vapor intrusion at a particular site, it is useful to collect soil gas samples to find out how vapors and contaminants of concern are migrating in the subsurface, and whether or not those vapors are migrating indoors. Soil gas sampling, used in conjunction with state specific screening criteria and/or modeling, is often an intermediate step between screening based on groundwater concentration and collecting indoor air samples.

The goal of soil gas sampling is to collect a sample of the vapor that resides in the interstitial soil pores near a source of contamination and/or near a potential receptor structure. To sample soil gas, a temporary or permanent soil vapor probe is installed. If the well is installed incorrectly or is not sealed properly, leaks to the ambient air may occur. This can dilute or otherwise influence the concentrations seen, potentially leading to incorrect decision making.

Using a tracer gas can give quantitative proof that the sampling system was installed and sealed correctly. The tracer compound is placed around the soil gas probe at the ground surface, so that if the well is installed correctly and everything is sealed properly, no tracer compound will be seen in the sample. The soil gas sample is then collected. If the tracer compound is detected in the soil gas sample, it is an indicator that some amount of leaking has taken place and the sample may be deemed unrepresentative or even invalid.

Several state vapor intrusion guidance documents make recommendations about which soil gas tracers to use, but most states leave room for professional judgment by the environmental professional to use other compounds. Each potential tracer compound has its benefits and its drawbacks from a sampling and analytical viewpoint.

Many professionals have successfully used helium as a tracer compound for soil gas surveys. As opposed to other tracers, such as isopropyl alcohol, helium will not interfere with the TO-15 analysis even if there is a small leak. Another unique benefit is that helium may be monitored and evaluated onsite so leaks can be proactively fixed in the field prior to sampling.

In addition to choosing an appropriate tracer compound, when collecting a representative and defensible soil gas sample, it is also important to follow thorough quality control and quality assurance practices, and to have several lines of evidence to support your conceptual site model.

Measuring the levels of oxygen, carbon dioxide, and methane present in the soil gas (indicators of biological activity) can also prove useful. Measurement of these indicator compounds can be done onsite with a multigas meter and/or at the analytical laboratory via EPA Method 3C modified (GC/TCD). Interpretation of the fixed gas data can provide a secondary line of evidence to support the conceptual site model. For instance, if the oxygen profile is decreasing with depth and suddenly a deeper soil gas sample shows an increased concentration (or near ambient levels) of oxygen, it is possible that a leak occurred, letting in ambient air.

It is important to note that leak testing of the sampling train can also be done with the use of a vacuum pump and a magnahelic gauge. Evacuate the sampling line, close off the soil gas point and the canister with a gauge in line, and observe whether the gauge needle returns to zero. If the needle moves back to zero, a leak is present somewhere in the system.

The field of vapor intrusion is constantly evolving, and accepted sampling procedures may change. Always check for any applicable state or Federal regulatory guidance prior to conducting sampling.