Environmental Geochemistry

Core Idea

Environmental geochemistry applies geochemical principles to understand the sources, transport, fate, and remediation of contaminants in natural systems. Contaminant behavior is controlled by the same thermodynamic and kinetic processes that govern natural geochemistry: speciation (which determines toxicity and mobility), adsorption (which retards transport), precipitation/dissolution (which creates sinks and sources), and redox transformations (which change mobility and toxicity). Key contaminant classes include heavy metals (As, Pb, Cd, Cr, Hg), radionuclides (U, Cs, Sr), organic pollutants, and excess nutrients. Understanding the geochemical controls on contaminant behavior enables prediction of plume migration, risk assessment, and design of remediation strategies that work with natural processes rather than against them.

Explainer

Environmental geochemistry is applied aqueous and redox geochemistry in the service of environmental protection. The same principles that govern natural water-rock interaction also control contaminant behavior -- speciation, sorption, precipitation, redox transformation -- but with the added complexity of anthropogenic source terms and regulatory thresholds.

Metal contaminant mobility is controlled by speciation and sorption. Arsenic illustrates the complexity: As(V) (arsenate) adsorbs strongly onto iron oxyhydroxides at near-neutral pH, providing a natural attenuation mechanism. But if redox conditions become reducing, the iron oxyhydroxides dissolve (reductive dissolution), releasing both iron and adsorbed arsenic. Simultaneously, As(V) is reduced to As(III), which adsorbs less strongly. The result is arsenic mobilization under reducing conditions -- the mechanism responsible for the arsenic crisis in South and Southeast Asian aquifers. Understanding these coupled redox-sorption processes is essential for predicting contaminant behavior.

Organic contaminant fate is governed by biodegradation, sorption to organic matter, and volatilization. Chlorinated solvents (TCE, PCE) are denser than water (DNAPLs) and sink to the bottom of aquifers, creating persistent source zones that dissolve slowly over decades. Biodegradation under anaerobic conditions can transform TCE to less-chlorinated products (reductive dechlorination), but incomplete dechlorination can produce vinyl chloride -- more toxic than the parent compound. This is why understanding the geochemical and microbiological conditions along a plume is critical: the wrong conditions produce worse contaminants.

Remediation design leverages geochemical processes. Permeable reactive barriers use zero-valent iron to reductively dechlorinate solvents or precipitate metals as the groundwater flows through. In-situ bioremediation stimulates microbial degradation by adding electron donors or acceptors. Monitored natural attenuation relies on demonstrating that natural processes (biodegradation, dispersion, sorption) are reducing contaminant concentrations at rates sufficient to protect receptors. In each case, the remediation strategy must be grounded in site-specific geochemical characterization to succeed.