Groundwater Contamination: How Toxins Move Underground and Into Homes

Groundwater is invisible, slow-moving, and widely misunderstood. Most people have a mental model of groundwater as underground lakes or rivers — discrete bodies of water that sit in aquifer caves and flow through well-defined channels. The reality is more complex and more difficult to remediate: groundwater is water that fills the pore spaces between soil particles and fractures in rock, moving slowly in directions determined by geology, gradient, and the properties of the porous medium it flows through.

This complexity matters for understanding contamination. A chemical spilled on the ground doesn't behave like water poured into a jar. It moves through the subsurface in ways determined by whether it dissolves in water, how strongly it sorbs to soil particles, whether it is denser or lighter than water, and how the geology channels flow. Understanding these properties helps explain why some contaminated sites — particularly those with dense non-aqueous phase liquids — remain challenging to remediate decades after the source was removed.

Dense non-aqueous phase liquids — DNAPLs — are organic solvents that are both immiscible with water (they don't dissolve readily, existing as separate-phase liquid) and denser than water (they sink through the water table rather than floating).

The most important DNAPLs in the Superfund context are chlorinated solvents: trichloroethylene (TCE), perchloroethylene (PCE), carbon tetrachloride, and 1,1,1-trichloroethane (TCA). These were used extensively in industrial degreasing and dry cleaning and were commonly disposed of or spilled in ways that reached groundwater.

Why DNAPLs are so difficult to remediate When DNAPL enters the subsurface, it sinks through the saturated zone (where all pore spaces are filled with water) until it reaches an impermeable layer or loses momentum. Along the way, it leaves behind residual DNAPL in the pore spaces and pools in depressions on impermeable layers. These residual and pooled masses serve as long-term sources of dissolved-phase contamination — slowly dissolving into the groundwater passing by and creating dissolved-phase plumes that can extend for miles.

The source removal problem Removing the dissolved-phase plume without addressing the DNAPL source is like mopping a floor while a pipe continues leaking — you reduce the contamination temporarily but the source continues re-contaminating the groundwater. Identifying and addressing DNAPL sources requires characterisation techniques (partitioning tracer tests, high-resolution site characterisation) that are technically demanding and expensive.

Pump-and-treat limitations The most common groundwater remediation technology — extracting contaminated groundwater, treating it, and discharging it — reduces contaminant concentrations in the plume but rarely achieves cleanup goals at DNAPL-impacted sites. The dissolved-phase contamination continues to be replenished from residual DNAPL. Many contaminated sites with DNAPL sources have been operating pump-and-treat systems for decades without reaching groundwater restoration goals.

Vapour intrusion is the exposure pathway that most directly affects homeowners near contaminated groundwater sites — and it's among the least understood by the public.

When volatile organic compounds in contaminated groundwater evaporate and migrate upward through unsaturated soil, they can enter building foundations through cracks, utility penetrations, and other openings. Once inside, they can accumulate to concentrations significantly higher than outdoor air — in some cases far exceeding health benchmarks — without any visible sign or odour.

The health concern TCE, PCE, benzene, and other volatile chlorinated solvents are carcinogens. TCE is specifically associated with kidney cancer, non-Hodgkin's lymphoma, and liver cancer at occupational exposure levels. The question of what concentration is dangerous for residential vapour intrusion exposure has driven significant research and regulatory debate, as residential exposures are chronic (24-hour daily) in settings (sleeping areas, children's bedrooms) where ventilation is often lower than workplaces.

Detection Vapour intrusion is invisible and frequently odourless at relevant concentrations. Detection requires air sampling — either sub-slab sampling (to detect vapours below the foundation before they enter the building) or indoor air sampling. Both can be conducted by environmental consultants; in areas with documented groundwater contamination, the EPA or state agency may provide free testing.

Mitigation The primary mitigation for vapour intrusion — when confirmed — is sub-slab depressurisation: a system of pipes inserted through the foundation that draws soil gas from below the slab and exhausts it outside, preventing vapour migration into the building interior. This technology is the same used for radon mitigation and is similarly effective for chlorinated solvent vapour intrusion.

Some contaminated groundwater sites present technical challenges so profound that complete restoration to drinking water standards is not achievable with current or foreseeable technology — a reality that the EPA acknowledges but that is difficult for affected communities to accept.

The "technical impracticability" waiver CERCLA allows the EPA to waive the requirement to restore groundwater to drinking water standards when it is technically impracticable — meaning the contamination cannot be meaningfully reduced to MCL levels despite implementation of all reasonable technologies. This waiver applies primarily at sites with extensive DNAPL contamination in complex geology.

When a technical impracticability waiver is issued, the remedy typically shifts to long-term containment and monitoring: preventing the plume from spreading, preventing exposure through institutional controls (water supply restrictions, land use restrictions), and monitoring to verify that containment is maintained.

Monitored natural attenuation At some sites, natural processes — dilution, dispersion, sorption, and biodegradation — are sufficient to reduce contamination to acceptable levels without active intervention. Monitored natural attenuation (MNA) uses regular monitoring to verify that natural degradation processes are proceeding at sufficient rates to be protective. It is appropriate for some sites but is often proposed by responsible parties as a lower-cost alternative to active remediation when it may not be sufficiently protective.

What this means for residents If you live near a contaminated site where the EPA has issued a technical impracticability waiver or selected MNA as the remedy, the key questions are: Are all exposure pathways controlled? Is the monitoring programme adequate to detect any change in contaminant conditions? What are the institutional controls that prevent future exposure? Are those controls enforceable and enforced?

PollutionProfile's Historical Exposure Recorder flags proximity to contaminated groundwater sites and links to the relevant ATSDR health consultations — which include pathway-specific exposure assessments that help residents understand which exposure routes are controlled and which warrant personal protective action.