PHREEQC in Practice: Modeling Where Contaminants Actually Go

Fate & Transport Modeling for Heavy Metals: Predicting What Moves, What Sticks, and What Will Cost You Later

The Big Picture

When metals are detected in soil, sediment, or water, the most expensive decisions rarely hinge on a single concentration value. They hinge on a forward-looking question:

Where will the contamination go, how fast, and in what chemical form?

Fate & Transport (F\&T) modeling is the framework used to answer that question in a way that is defensible to regulators, useful to project managers, and practical for remedy selection.

For heavy metals, “transport” is almost never just groundwater flow or sediment movement. Metals are chemically reactive. They adsorb, precipitate, dissolve, complex with organic matter, change oxidation state, and—under certain conditions—become more toxic through transformation (for example, mercury methylation).

At ENV Water Chemistry Solutions, our Heavy Metals & Geochemistry division develops fate and transport models that reflect real geochemical behavior, not just hydraulics. The goal is simple: reduce uncertainty, avoid over-remediation, and prevent remedies that look good on paper but fail in the field.

What Fate & Transport Means (for Metals, Not Dyes)

In many engineering workflows, contaminant transport is treated like a conservative tracer moving from Point A to Point B. That assumption breaks down for metals because the same mass of metal can behave like multiple contaminants depending on speciation and binding phase.

A practical heavy-metals fate & transport model must account for:

• Hydrology & hydraulics
  Groundwater gradients, recharge, seepage, surface water mixing, storm events

• Partitioning
  Dissolved vs particulate vs colloid-facilitated transport

• Geochemistry
  pH/Eh controls, sorption, precipitation/dissolution, complexation (especially DOC), sulfide formation

• Biogeochemistry
  Microbial redox cycling (critical for arsenic, selenium, and mercury systems)

The output is not just a plume map—it is a decision-support tool that helps answer:

• What is the plausible footprint over time?

• What concentrations are likely at compliance points?

• What conditions could remobilize metals that appear “stable” today?

The Geochemical Mechanisms That Control Mobility

(And Why Total Metals Isn’t Enough)

1. Redox-Driven Mobility: The “It Changes Seasonally” Problem

Many metals and metalloids are governed by redox boundaries that shift with depth, season, and organic loading.

• Arsenic can be released during Fe(III) oxide reduction and may form thio-arsenic species under sulfidic conditions.

• Selenium mobility depends strongly on oxidation state; reduced forms may be stable only as long as reducing conditions persist.

• Iron and manganese act as master variables, controlling sorption capacity and co-precipitation behavior.

A model that includes only flow will often miss the real driver: the redox engine that turns mobility on and off.

2. Sorption and Competitive Desorption: Why “Stable” Plumes Move

Sorption is not a constant. It varies with:

• pH (often the dominant control)

• Competing ions (phosphate–arsenate competition is a classic example)

• Mineralogy (oxide-rich vs carbonate-rich vs organic-rich matrices)

• Ionic strength and salinity (important in estuarine and ASR-influenced systems)

This is why sediments that appear stable today can become active sources tomorrow—exactly the scenario regulators and Prime consultants must evaluate during RI/FS and remedy selection.

3. Precipitation and Dissolution: When Mineral Stability Is the Remedy

Many metals are controlled by mineral phases—carbonates, oxides, sulfides, or secondary weathering products.

This distinction has direct cost implications:

• If the controlling phase is stable under site conditions, aggressive removal may be unnecessary.

• If the phase is metastable (e.g., sensitive to pH or oxidation), remedies relying on “natural stability” may fail.

This issue is especially critical at mining-impacted sites and AMD-influenced systems, where small chemical shifts can change solubility by orders of magnitude.

4. Colloids and Particulates: Transport That Dissolved Data Misses

Filtered “dissolved” results often underestimate mobility. Colloid-facilitated transport can move metals farther than expected, particularly during high-flow events or in organic-rich waters.

Good fate & transport modeling asks early: what phase is actually moving?
That answer should drive both sampling design and model structure.

Modeling Approaches We Use: Fit-for-Purpose and Regulator-Ready

There is no single “best” model—only the right model for the decision being made.

Screening and Bounding Models

Used early to constrain plausible ranges, identify key drivers, and guide data collection. Often the highest ROI step.

Geochemical Speciation Models (PHREEQC-based)

Used to evaluate:

• Dominant aqueous species

• Saturation indices

• Sensitivity to pH/Eh, sulfate, sulfide, DOC, and alkalinity

• Mobility shifts under realistic scenarios (seasonal anoxia, operational changes)

For many metals projects, speciation modeling is the difference between “we think” and “we can show.”

Integrated Fate & Transport Support

When decisions require predicting trends at compliance points or evaluating remedy performance, we integrate hydrology, geochemistry, and scenario testing to directly support the Conceptual Site Model (CSM).

Who This Helps Most

• Prime engineering firms
  On-demand geochemical depth to support RI/FS conclusions, remedy selection, and technical defensibility.

• Agencies and regulators
  Transparent, chemistry-based interpretations aligned with monitoring objectives and QAPP/SAP requirements.

• Industrial operators
  Compliance planning, permit support, and cost avoidance by distinguishing background, legacy, and operational sources—and stress-testing whether controls will actually hold.

The Site, Not the Software

Metals in groundwater, surface water, sediment, or mine waste do not become defensible because a model ran—they become defensible when the model is constrained by site data and stress-tested against the conditions a regulator will challenge. An unconstrained PHREEQC run is a confident answer to the wrong question, and remedy decisions made on it are paid for at the compliance point.

If a remedy or permit case rests on a fate-and-transport model no one has stress-tested against site data, that is a conversation worth having before the model is filed.

PHREEQC does not make decisions defensible. The geochemist constraining it does.