The Invisible Killers in Our Groundwater

How Science and Policy Are Tackling Chlorinated Ethenes

Beneath our feet, a silent chemical crisis unfolds—one that links industrial progress to environmental vulnerability and human health.

Introduction The Problem Bioremediation EVO Experiment Policy

Introduction: The Unseen Threat

Chlorinated ethenes—industrial workhorses like tetrachloroethene (PCE) and trichloroethene (TCE)—have seeped into groundwater worldwide through leaks, spills, and improper disposal. These solvents, once celebrated for their degreasing prowess in dry cleaning and manufacturing, transform into carcinogenic threats like vinyl chloride when they break down. With over 113,000 contaminated U.S. sites and cleanup costs exceeding $200 billion, the quest to neutralize these toxins blends cutting-edge microbiology with evolving regulatory policy. Here's how scientists are turning microbes into cleanup crews and how policymakers are rewriting rules to protect our future 1 5 .

The Chlorinated Ethene Problem: From Industry to Toxicity

Chlorinated ethenes include PCE, TCE, dichloroethene isomers (DCEs), and vinyl chloride. Their chemical stability—once ideal for industrial use—makes them persistent environmental pollutants. As dense non-aqueous phase liquids (DNAPLs), they sink through soil into groundwater, forming long-lasting plumes.

Health Impacts
  • Vinyl chloride is a known human carcinogen linked to liver cancer, neurotoxicity, and developmental harm 5
  • TCE is a known carcinogen associated with kidney and liver cancer
  • PCE is a probable carcinogen with neurological effects
Regulatory Turning Points
  • In 2025, EPA designated vinyl chloride a high-priority substance under TSCA 3 5
  • Risk evaluations now focus on exposure pathways and vulnerable populations
  • Cleanup costs exceed $200 billion across 113,000+ U.S. sites

Table 1: The Chlorinated Ethene Family

Compound Common Uses Carcinogenicity Key Degradation By-products
PCE (Tetrachloroethene) Dry cleaning, metal degreasing Probable carcinogen TCE, cis-DCE
TCE (Trichloroethene) Electronics cleaning, refrigerants Known carcinogen cis-DCE, vinyl chloride
Vinyl chloride PVC plastic production Known human carcinogen Ethene (nontoxic)
cis-1,2-DCE TCE/PCE breakdown product Suspected carcinogen Vinyl chloride

Bioremediation: Nature's Detox Squad

Microbes offer a powerful solution to clean up contaminated sites. Through reductive dechlorination, specialized bacteria strip chlorine atoms from molecules, converting toxins like PCE into benign ethene.

Key Microbial Players
Dehalococcoides mccartyi

The only known bacterium capable of complete dechlorination of PCE → TCE → DCE → vinyl chloride → ethene until recently. Its enzymes (e.g., VcrA, BvcA) are critical for vinyl chloride destruction 1 .

Dehalogenimonas etheniformans strain GP

A groundbreaking discovery (2022): This non-Dehalococcoides bacterium dechlorinates TCE, all DCE isomers, and vinyl chloride to ethene. Uses hydrogen or formate as electron donors, expanding options for substrate delivery in bioremediation 4 .

Aerobic Cometabolism

Bacteria like methanotrophs co-metabolize TCE/DCE using enzymes (e.g., soluble methane monooxygenase). Limited by oxygen depletion and lower efficiency compared to anaerobic pathways 1 .

Table 2: Microbial Degraders of Chlorinated Ethenes

Organism Chlorinated Ethenes Degraded End Product Electron Donors Functional Genes
Dehalococcoides mccartyi PCE, TCE, DCEs, vinyl chloride Ethene H₂ vcrA, bvcA
Dehalogenimonas etheniformans TCE, DCEs, vinyl chloride Ethene H₂, formate cerA
Methane-oxidizing bacteria TCE, DCEs, vinyl chloride CO₂ Methane sMMO
Dehalobacter PCE, TCE cis-DCE H₂ pceA

In-Depth: The EVO Biostimulation Experiment

A 2024 study tested Emulsified Vegetable Oil (EVO) to boost microbial dechlorination in a vinyl chloride-contaminated aquifer. Here's how science validated this approach:

Methodology:

  1. Site Selection: A source zone with high vinyl chloride levels (∼2,000 μg/L)
  2. EVO Injection: EVO was injected into groundwater wells to stimulate native microbes
  3. Monitoring: Used 2D Compound-Specific Isotope Analysis (2D-CSIA) to track carbon and chlorine isotope shifts in vinyl chloride—a fingerprint of biodegradation
  4. Molecular Tools: Quantified Dehalococcoides and functional genes (vcrA, bvcA) via qPCR 2
Results and Analysis
  • 90% vinyl chloride reduction within 6 months, with ethene as the end product
  • Isotopic enrichment (δ¹³C and δ³⁷Cl) confirmed biological degradation, not just dilution
  • Dehalococcoides and vcrA gene counts surged 100-fold post-EVO, proving biostimulation success 2

Table 3: EVO Biostimulation Results

Parameter Pre-EVO Post-EVO (6 months) Change Interpretation
Vinyl chloride (μg/L) 1,950 195 -90% Effective degradation
Dehalococcoides (cells/mL) 10³ 10⁵ 100x Microbial growth stimulated
vcrA genes (copies/mL) 10² 10⁴ 100x VC reductase activity enhanced
δ¹³C (‰) -30 -20 +10 Biological breakdown

The Scientist's Toolkit: Key Research Solutions

Field and lab tools for chlorinated ethene analysis:

CENSUS® qPCR

Quantifies degraders (e.g., Dehalococcoides) and functional genes (vcrA, bvcA)

Predicts detoxification potential at a site
QuantArray®-Chlor

Multiplex test for 15+ microbial targets

Profiles microbial community in contaminated groundwater
2D-CSIA

Measures δ¹³C/δ³⁷Cl to confirm biodegradation

Distinguishes biological vs. abiotic breakdown 2
Emulsified Vegetable Oil (EVO)

Slow-release carbon source for anaerobes

Stimulates Dehalococcoides/Dehalogenimonas 2
Formate

Alternative electron donor to H₂

Supports Dehalogenimonas strain GP 4

Policy Meets Science: Rewriting Risk Management

The EPA's 2025 TSCA draft scope for vinyl chloride marks a shift toward exposure-specific risk evaluations:

  • Focuses on vulnerable subpopulations near manufacturing sites or contaminated aquifers
  • Requires bioremediation efficacy data to support monitored natural attenuation (MNA) as a cleanup strategy 3 5
Scientific Impact on Policy

The discovery of Dehalogenimonas strain GP further impacts policy:

  • Expands biomarker monitoring beyond Dehalococcoides (e.g., cerA gene quantification) 4
  • Promotes flexible electron donor delivery (formate/EVO) for wider site applicability
Future Timeline
2022

Discovery of Dehalogenimonas etheniformans strain GP

2024

EVO biostimulation study published

2025

EPA designates vinyl chloride as high-priority under TSCA 3 5

2026

EPA's vinyl chloride risk evaluation concludes

The Future: Rewriting the Textbook

The isolation of Dehalogenimonas etheniformans in 2022 revolutionized bioremediation, proving multiple organisms can achieve full detoxification. Future work will optimize reactive transport models to predict plume behavior and integrate abiotic degradation (e.g., iron minerals) with biological processes 1 4 .

As EPA's vinyl chloride risk evaluation concludes in 2026, its outcomes could mandate stricter containment or endorse enhanced bioremediation—proving that in the battle against invisible killers, microbes are our smallest but mightiest allies.

Science doesn't just solve problems—it reshapes policy, protects communities, and turns contamination into hope.

References