The Science of Testing For Mercury in Wildfire Impacted Soil and Groundwater

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THE SCIENCE OF TESTING FOR MERCURY IN WILDFIRE IMPACTED SOIL AND GROUNDWATER

California’s increasing wildfire frequency and intensity have raised significant environmental concerns, including the mobilization of toxic substances like mercury (Hg). Mercury, a persistent and bioaccumulative heavy metal, can be released from both natural and anthropogenic sources during wildfires, posing risks to ecosystems and human health.

Wildfires can significantly alter the chemical and physical properties of soil, leading to:

  • Thermal release of mercury from soil and vegetation.
  • Increased erosion of contaminated soils into waterways.
  • Enhanced methylation of mercury due to post-fire changes in organic matter and microbial activity.

Post-wildfire conditions facilitate mercury transport through:

  • Surface runoff during rain events, carrying ash and soil into streams and lakes.
  • Leaching into groundwater, especially in areas with permeable soils and shallow aquifers.
  • Bioaccumulation in aquatic food webs, affecting fish and wildlife.

The California Water Science Center has been modeling these processes to understand how mercury moves through watersheds post-fire

According to the State Water Resources Control Board, mercury has been detected in public water wells across California. Sixteen (16) out of 9,201 wells tested between 2007–2017 had mercury levels above the federal Maximum Contaminant Level (MCL) of 2 µg/L. The highest concentrations were found in Monterey, Kern, Los Angeles, and Napa counties.

While not all of these detections are directly linked to wildfires, the risk of wildfire-induced mobilization of mercury into groundwater is a growing concern.

Recent Studies Indicating Mercury Contamination from Wildfires

  1. A 2015 study in the Cache Creek and Putah Creek watersheds found that wildfires increased concentrations of methylmercury (MeHg), the most toxic form of mercury in sediments, plants, and aquatic organisms.
  1. Coastal Ranges Wildfires (2022–2023):

A study published in Environmental Science & Technology examined two burned watersheds and one reference site:

  • Initial storm runoff post-fire carried suspended solids with mercury levels up to 46 times higher than the reference site.
  • Mercury was predominantly in particulate form, associated with ash and eroded soil.
  • Rapid vegetation regrowth helped reduce mercury transport within months
  1. Camp Fire (2018, Butte County)

The Camp Fire, which destroyed the town of Paradise, released toxic metals including mercury:

  • Elevated levels of lead, zinc, and mercury were detected in air samples up to 150 miles away.
  • Structural fires contributed to the release of synthetic materials containing heavy metals.
  1. Dixie Fire (2021)

While specific mercury data is limited, the Dixie Fire prompted legal and environmental scrutiny:

  • Highlighted the role of airborne pollutants and post-fire runoff in contaminating watersheds.

• Ongoing studies are assessing the long-term impacts on water quality and ecosystem health

 

How Wildfires Cause Volatile Organic Compound (VOC) Contamination

VOCs are a group of chemicals that can easily become vapors or gases. VOCs like benzene, toluene, ethylbenzene, and xylene are commonly released during wildfires due to the combustion of vegetation, building materials, plastics (e.g., PVC, HDPE) and household chemicals, especially in areas with permeable soils and shallow aquifers

These VOCs can infiltrate soil and groundwater through direct deposition from smoke and ash, leaching from burned infrastructure and debris, suction into water systems and when pressure drops during firefighting efforts. Once in the environment, VOCs can persist in groundwater due to their chemical stability and low biodegradability.

Notable Locations Indicating VOC Contamination from Wildfires

Several wildfire events in the U.S. have led to documented VOC contamination:

Paradise, California (Camp Fire, 2018): Benzene and other VOCs were found in the drinking water system, traced back to melted plastic pipes and intrusion of smoke and gases.

Santa Rosa, California (Tubbs Fire, 2017): Similar contamination was observed, with VOCs detected in water mains and service lines. The contamination was linked to the degradation of plastic pipes and the intrusion of smoke and gases into water lines during pressure loss events.

Gallinas Creek, New Mexico (Calf Canyon/Hermit Peak Fires, 2022): Post-fire monitoring showed impacts on water quality, including potential VOC presence 3.

Traditional methods for mercury analysis in soil:

Cold Vapor Mercury Analysis: Overview

As discussed, the analysis of mercury in environmental samples, such as air, water, and soil, is crucial for assessing the impact of mercury pollution and for ensuring public health and safety. The most common methods for mercury detection is cold vapor atomic absorption spectroscopy (CVAAS), which relies on the reduction of mercury ions (Hg²⁺) to mercury vapor (Hg⁰) at low temperatures. Cold vapor analysis involves two primary steps:

  1. Reduction of mercury: Mercury in environmental samples, typically present as Hg²⁺ (ionic form), is reduced to its elemental form (Hg⁰), which is gaseous.
  2. Detection of mercury: The mercury vapor is then quantified using atomic absorption spectroscopy (AAS), where the absorption of light by mercury vapor at a specific wavelength is measured, providing a direct indication of the concentration of mercury in the sample.

Cold vapor analysis is valued for its sensitivity, specificity, and relatively simple sample preparation, making it ideal for low-concentration mercury detection.

The following EPA cold vapor methods have been established for the analysis of mercury in environmental samples:

  • EPA 7470A: Mercury in Liquid Waste
  • EPA 7471B: Mercury in Solid or Semi-Solid Waste

 

Limitations and Possibilities of Interference in Cold Vapor Mercury Analysis in the Presence of Volatile Organics

While this technique has proven effective for many environmental matrices, its performance can be significantly impacted by the presence of volatile organic compounds (VOCs). Interferences stated in these methods includes “certain volatile organic materials that absorb at this wavelength may also cause interference.”

Potential Interferences from Volatile Organics in the detection of Mercury

The primary limitation in the cold vapor technique arises when volatile organic compounds (VOCs) are present in the sample matrix. VOCs are a broad class of organic chemicals that have high vapor pressure at ambient temperature, including compounds such as benzene, toluene, xylene, and various chlorinated solvents. These compounds can interfere with the cold vapor analysis in the following ways:

  1. Chemical Interference During Reduction:
    • VOCs as Reducers: Some VOCs can act as reducing agents, potentially reducing not only mercury ions (Hg²⁺) but also other elements in the sample. This can lead to a false positive result, where the mercury concentration is overestimated. For instance, VOCs containing active hydrogen atoms (such as alcohols or aldehydes) may reduce mercury ions along with other metal ions, making it difficult to distinguish mercury from other reduced species.
    • Competing Reactions: In the presence of certain organic solvents or compounds, the reaction between the sample matrix and the reducing agent (commonly stannous chloride, SnCl₂) may be disrupted. This can affect the efficiency of mercury reduction and result in incomplete or uneven mercury vapor generation, reducing the accuracy of the analysis.
  2. Physical Interference from VOCs:
    • Volatility Overlap: Many VOCs have similar vapor pressures to mercury, meaning they could be co-distilled or co-evaporated during the cold vapor generation process. This can lead to erroneous readings or contamination of the mercury signal with signals from the VOCs, complicating the analysis. In some cases, VOCs could enhance or suppress the absorption signals for mercury, leading to inaccurate quantification.
    • Matrix Effects: The presence of VOCs in the sample matrix can also alter the characteristics of the cold vapor generation system, such as the temperature or the efficiency of mercury vapor collection. Organic solvents may change the viscosity of the sample or introduce bubbles, which could interfere with the atomization or detection of mercury.
  3. Instrumental Interference:
    • Spectral Interference: Some VOCs may have absorbance or emission spectra that overlap with that of mercury. This could cause spectral interference, particularly when using atomic absorption or atomic fluorescence techniques for detection. As a result, the instrument may record signals from both mercury and the VOCs, leading to inaccurate quantification of mercury.

Analysis for Mercury by Inductively Coupled Plasma Mass Spectrometry (ICP-MS)

Cold vapor mercury analysis remains a powerful and widely used technique for detecting mercury in environmental samples. However, the presence of volatile organic compounds poses significant challenges to the accuracy and reliability of this method. While there exists several strategies to help mitigate interferences in the cold vapor mercury analysis technique in the presence of VOCs, alternate analytical techniques should be considered.

Inductively Coupled Plasma Mass Spectrometry (ICP-MS) can be preferable to Cold Vapor Atomic Absorption Spectroscopy (CVAAS) for mercury analysis in environmental samples in the presence of volatile organic compounds (VOCs) for several important reasons. These reasons revolve around ICP-MS’s sensitivity, specificity, and ability to handle complex matrices, including those contaminated with VOCs.

The ICP-MS method measures ions produced by a radio-frequency inductively coupled plasma. Analyte species in liquid are nebulized and the resulting aerosol is transported by argon gas into the plasma torch. The ions produced by high temperatures are entrained in the plasma gas and introduced, by means of an interface, into a mass spectrometer. The ions produced in the plasma are sorted according to their mass-to-charge (m/z) ratios and quantified with a channel electron multiplier. Below are the key advantages of using ICP-MS in such scenarios:

  1. Reduced Interference from VOCs
  • Chemical and Physical Interferences: ICP-MS is much less affected by the presence of VOCs because it uses mass spectrometry to detect ions at specific mass-to-charge ratios (m/z). VOCs typically do not interfere with the mass spectrometry detection of mercury, as their molecular masses are quite different from that of mercury (Hg, m/z 202). This makes ICP-MS a more robust method in complex sample matrices containing VOCs.
  1. High Sensitivity and Low Detection Limits
  • Detection Sensitivity: ICP-MS offers exceptional sensitivity, with detection limits in the low parts per trillion (ppt) range, which is orders of magnitude better than Cold Vapor Atomic Absorption Spectroscopy (CVAAS). In environmental samples where mercury concentrations can be very low, ICP-MS provides the sensitivity needed to detect trace amounts of mercury even in the presence of background interferences, such as VOCs. CVAAS, while sensitive, is not as effective at detecting very low concentrations of mercury when the matrix is complex or when there is significant interference.
  1. Multi-element Capability
  • Simultaneous Detection of Other Elements: ICP-MS can simultaneously analyze multiple elements in a single sample, which is highly beneficial when mercury is one of several potential contaminants in an environmental sample. For example, ICP-MS can measure mercury alongside other toxic metals like lead, arsenic, and cadmium, providing a comprehensive analysis in a single run. This is particularly useful in environmental monitoring, where multiple contaminants may be present in a single sample. Cold vapor techniques, in contrast, are focused specifically on mercury and would require separate analyses for other elements, making them less efficient in complex environmental testing scenarios.
  1. Ability to Handle Complex Sample Matrices
  • Matrix Tolerance: ICP-MS is highly versatile and capable of analyzing samples with a wide range of matrices, including those that may contain VOCs, organic solvents, and other complex chemical species. The robustness of ICP-MS in handling difficult matrices (such as those with high organic content or high salinity) is a significant advantage over CVAAS.
  1. No Need for Reduction or Distillation
  • Sample Preparation: Cold vapor analysis typically requires a reduction step (e.g., using stannous chloride or other reducing agents) to convert mercury from its ionic form (Hg²⁺) to its elemental vapor (Hg⁰). This step can introduce potential interferences, particularly in the presence of VOCs. In contrast, ICP-MS does not require such a reduction step for mercury analysis. The sample is introduced into the plasma in its dissolved state, and mercury is ionized directly. This removes the need for complex sample preparation and makes ICP-MS more suitable for rapid, straightforward mercury analysis, particularly in environmental samples with challenging matrices.
  1. Quantitative Analysis in the Presence of Organics
  • Improved Quantification: While CVAAS can suffer from matrix effects in the presence of VOCs, ICP-MS’s mass spectrometric detection is more selective and precise. Even in the presence of interfering organic compounds, the ability of ICP-MS to resolve ions based on their mass-to-charge ratio allows for accurate quantification of mercury without the confounding effects of VOCs. This makes ICP-MS a more reliable method for accurate mercury quantification in complex environmental samples.
  1. High Throughput and Efficiency
  • Speed and Automation: ICP-MS is capable of analyzing a large number of samples quickly and with minimal operator intervention. It can be easily automated, making it a high-throughput option for environmental monitoring programs. On the other hand, CVAAS is often more time-consuming and requires more frequent calibration, especially in complex sample matrices. When VOCs are present, CVAAS analysis may require extra time to address potential interferences, making it less efficient compared to ICP-MS for large-scale environmental testing.

Conclusion

Utilizing an ICP-MS method is often preferred over cold vapor atomic absorption spectroscopy (CVAAS) for mercury analysis in fire impacted environmental samples potentially containing VOCs due to its superior ability to handle complex matrices, reduce interference, and provide high sensitivity with minimal sample preparation. The mass spectrometric detection offered by ICP-MS ensures accurate and reliable quantification of mercury, even in the presence of VOCs and other interferences. Furthermore, the ability to simultaneously analyze multiple elements makes ICP-MS a more versatile and efficient tool for environmental monitoring and contamination assessment. Finally, advancements in ICP-MS technology continues while Cold Vapor technology is more static.

BSK recommends EPA method 6020B for the analysis of Mercury in soils and groundwater impacted by wildfires. Acid digestion of the soil and groundwater is required prior to filtration and analysis. The acid digest can then be analyzed for Mercury along with the other California Title 22 metals.

 

 

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