Can contaminated groundwater be treated at the same time as soil?

Stijn Boeren · 28 Jul 2026

Yes, contaminated soil and groundwater can be treated simultaneously, and in many cases this is the more effective approach. When contamination has already migrated from the soil into the groundwater, treating one matrix while ignoring the other often leads to recontamination. Biological remediation methods, particularly in situ approaches, are well suited to addressing both at the same time. The sections below walk through how contamination spreads, how simultaneous treatment works, and how to assess whether it is feasible for a specific site.

How does contamination spread from soil into groundwater?

Contamination spreads from soil into groundwater through a process called leaching, where dissolved or mobile pollutants migrate downward through the unsaturated soil zone until they reach the water table. Once there, they dissolve into the groundwater and can travel significant distances with the natural flow. The speed and extent of this migration depend on the type of contaminant, soil permeability, and local hydrogeology.

Chlorinated solvents such as trichloroethylene (TCE) and perchloroethylene (PCE) are among the most problematic contaminants in this regard. These volatile organochlorine compounds (VOCl) are denser than water, which means they sink through the saturated zone and can accumulate at depth as dense non-aqueous phase liquids (DNAPLs). From there, they act as a persistent source of dissolved contamination that continuously feeds into the surrounding groundwater.

This is why, when contamination is discovered on an industrial site, a thorough assessment of both matrices is essential. Finding contamination in the soil does not automatically tell you how far the groundwater plume has extended, and the reverse is equally true. Understanding this connection is the first step toward designing a remediation strategy that actually resolves the problem rather than managing it in one location while it persists in another.

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What is in situ bioremediation and how does it work in both matrices?

In situ bioremediation is a remediation method that treats contamination directly in the ground, without excavating soil or extracting groundwater. It works by stimulating or introducing microorganisms capable of breaking down target contaminants within the subsurface environment. Because the treatment happens underground, it can reach both the soil matrix and the saturated zone where groundwater is present, often at the same time.

For chlorinated solvents specifically, the relevant biological process is reductive dechlorination. Specialized bacteria, such as those from the Dehalococcoides group, use the chlorinated compounds as electron acceptors and progressively strip chlorine atoms from the molecule, ultimately converting them to harmless end products like ethylene and chloride. This process occurs naturally in some subsurface environments, but at rates too slow to meet remediation targets. The goal of bioremediation is to accelerate and sustain this activity.

Biostimulation: feeding what is already there

Biostimulation involves adding electron donors, typically fermentable carbon substrates, to create the anaerobic conditions that dechlorinating bacteria require. If the right organisms are already present in the subsurface, this approach can be sufficient to drive the process forward.

Bioaugmentation: introducing the right organisms

Where the native microbial community lacks the capacity for complete dechlorination, bioaugmentation introduces specialized microbial consortia directly into the contaminated zone. These cultures are injected through wells and distribute themselves through the subsurface, colonizing both the soil pores and the saturated zone. Avecom’s biological soil remediation approach combines both strategies, tailored to the specific conditions of each site.

What’s the difference between treating soil and groundwater separately versus together?

Treating soil and groundwater separately typically means using different techniques for each matrix, often at different times and with different contractors. Treating them together, using a single in situ biological approach, addresses the contamination as a connected system rather than two isolated problems. The integrated approach is generally more efficient and reduces the risk of one matrix recontaminating the other after treatment.

When soil and groundwater are treated in isolation, a common outcome is that groundwater extraction and treatment removes the dissolved plume, but the residual contamination in the soil continues to dissolve and replenish it. Similarly, soil treatment that does not account for the saturated zone may leave a persistent source at depth. This cycle can extend remediation timelines considerably and drive up costs.

In situ bioremediation avoids this problem because the microbial activity is not confined to a single layer. When electron donors and microbial consortia are distributed through injection wells, they reach both the capillary fringe at the base of the unsaturated zone and the groundwater below. The biological processes that break down contaminants operate across both environments, treating the site as what it actually is: a continuous subsurface system.

From a planning and budget perspective, this matters significantly. A single integrated treatment design, with one set of monitoring wells and one reporting framework, is typically less resource-intensive than running parallel remediation programmes. For project managers dealing with OVAM reporting obligations and tight timelines, this consolidation has practical value beyond the technical argument.

How do you know if simultaneous biological treatment is feasible for your site?

The most reliable way to determine whether biological treatment is feasible for a specific site is through a microcosm test. This is a controlled laboratory experiment using actual soil and groundwater samples from the contaminated site. The test evaluates whether the native microbial community can degrade the target contaminants, and under what conditions, before any field-scale intervention is committed to.

A microcosm test answers several critical questions at once: Are the right degrading organisms present? Is complete dechlorination occurring, or does it stall at an intermediate product? What amendments are needed to stimulate the process? How quickly does degradation proceed under site-representative conditions? These answers allow a remediation strategy to be designed with confidence rather than assumption.

If the microcosm results show limited native activity, this does not automatically rule out biological treatment. It may indicate that bioaugmentation is the appropriate route, introducing a defined microbial consortium capable of completing the degradation pathway. The feasibility assessment therefore informs not just whether biological treatment is possible, but which specific approach is warranted.

For sites where classical excavation is not viable, whether due to existing buildings, deep contamination, or cost constraints, this kind of screening is a logical first step. Avecom offers microcosm-based feasibility testing as a standalone service, providing a cost-efficient way to evaluate biological remediation potential before committing to a full site programme.

How is progress monitored when treating soil and groundwater at the same time?

Progress during simultaneous soil and groundwater bioremediation is monitored through a combination of chemical analysis and molecular biological tools. Chemical monitoring tracks the concentration of target contaminants and their degradation products over time. Molecular monitoring goes deeper, quantifying the actual microorganisms responsible for degradation and measuring their activity directly in the subsurface.

The most commonly used molecular tool is quantitative PCR (qPCR), which can detect and quantify specific functional genes associated with dechlorinating bacteria. This tells you not just whether contamination concentrations are declining, but whether the biological mechanism driving that decline is active and healthy. If the microbial population is growing and expressing the right genes, the process is working. If it is stalling, the data point to where intervention is needed.

This level of diagnostic detail has regulatory value as well as operational value. OVAM reporting requirements demand evidence of remediation progress, and molecular data provides a mechanistic explanation for what the chemical data shows. Rather than presenting a decline in contaminant concentration and hoping it continues, you can demonstrate the biological basis for that decline and project its trajectory with greater confidence.

Avecom’s molecular monitoring approach integrates qPCR and amplicon sequencing to track both the presence and activity of relevant microbial communities throughout the remediation process. This continuous insight reduces the risk of unexpected stalls and supports both internal project management and regulatory submissions. For project managers under pressure to demonstrate progress, having data-backed milestones rather than open-ended timelines makes a significant practical difference.

For sites where contamination has proven resistant to conventional approaches, or where the scale and depth of the problem makes excavation impractical, Avecom’s team of environmental engineers and microbiologists can guide the process from initial feasibility screening through to full field remediation, with documentation aligned to OVAM and VLAREBO requirements at every stage.