Looking beyond monitoring wells to understand remediation progress

Why groundwater isn’t telling you the whole story

When evaluating remediation performance, it's natural to focus on groundwater data. Groundwater concentrations are relatively easy to collect, can be monitored repeatedly over time, and are often directly tied to regulatory compliance. As a result, groundwater trends frequently become the primary measure of remediation success.

However, groundwater represents only a small fraction of the contaminant inventory present in the subsurface.

For many petroleum hydrocarbon sites, and many other contaminated sites, the majority of contaminant mass resides in soil, either sorbed to soil particles or present as residual non-aqueous phase liquid (NAPL). Groundwater simply reflects the small dissolved fraction that has partitioned from that larger contaminant reservoir.

This distinction has important implications for interpreting remediation performance.

A Conceptual Example

Consider one cubic metre of saturated soil with:

• a bulk density of 2,000 kg/m3 and
• a water-filled porosity of 30%.

Consider a petroleum-contaminated site, such as a former fuel storage or industrial facility. Now imagine a one-metre cube of soil below the water table within the contaminated area. That cube contains roughly 2,000 kilograms of soil, but only about 300 litres of groundwater within the spaces between the soil particles.

Suppose remediation reduces the petroleum hydrocarbon concentration in the soil by 300 milligrams per kilogram. Across that one cubic metre of soil, this represents approximately 600 grams—or about 1.3 pounds—of contaminant mass destroyed. By comparison, removing the same 600 grams of contaminant from the groundwater contained within that cube would require the groundwater concentration to decrease by approximately 2,000 milligrams per litre. That is far higher than the dissolved concentrations typically measured at most remediation sites.

This does not mean that soil containing 300 mg/kg of petroleum hydrocarbons would produce groundwater concentrations of 2,000 mg/L. Groundwater concentrations are controlled by factors such as contaminant solubility, how strongly contaminants bind to soil, biodegradation, and groundwater movement.

The point is simply that much more contaminant mass can be stored in the soil than in the groundwater occupying the same volume of ground.

The comparison works in the other direction as well. A groundwater concentration decrease of 10 mg/L (10,000 µg/L) would generally look like a substantial improvement in monitoring results. But within our one-metre cube, that decrease represents only about 3 grams of dissolved contaminant.

Groundwater concentrations can therefore change dramatically while representing a relatively small change in the total contaminant mass remaining below ground.

What This Means for Remediation

This does not diminish the importance of groundwater monitoring. Groundwater data remain essential for

• demonstrating protection of receptors,
• evaluating plume stability,
• verifying compliance,
• and assessing exposure pathways.

However, groundwater alone rarely tells the entire story.

A declining groundwater trend may indicate that remediation is effectively reducing contaminant mass. It may also reflect seasonal variability, changing groundwater flow conditions, dilution, or temporary reductions in contaminant flux. Likewise, groundwater concentrations may remain relatively unchanged even after substantial contaminant destruction has occurred if residual contaminant mass continues to slowly partition into groundwater.

Without understanding the contaminant inventory remaining in the soil, it can be difficult to distinguish between these scenarios.

Measuring the Right Things

Effective remediation performance evaluations combine multiple lines of evidence, including groundwater chemistry, soil concentrations, geochemical indicators, amendment distribution, and site-specific conceptual understanding.

• No single dataset provides a complete picture.
• Groundwater monitoring tells us what is moving through the aquifer today.
• Soil data often tell us what may continue feeding the groundwater tomorrow.

Understanding both is essential for accurately evaluating remediation progress, optimizing treatment strategies, and making informed decisions about when remediation objectives have truly been achieved.