Pilcomayo Basin Contamination Study
1.5 Million People. One Industrial River.
For eight years, we tracked heavy metal contamination across 850 km of the Pilcomayo River — from Bolivia's silver-mining highlands to the floodplains of Paraguay. This is what we found.
46 Monitoring Stations
1,436 Samples
8 Years
Bolivia · Argentina · Paraguay
Why This River Matters
Mining Legacy at Basin Scale
The Pilcomayo drains 270,000 km² across three nations. Potosí — once the world's largest silver mine — has discharged heavy metals into this watershed for over 400 years. The contamination isn't historic. It's ongoing, seasonal, and expanding.
1.5M
People in the affected basin
400+
Years of industrial mining
850 km
River corridor monitored
46
Active monitoring stations
Spatial Coverage
46 Stations Across Three Nations
Sampling sites span Bolivia, Argentina, and Paraguay — from the headwaters of Potosí (where pH drops to 3–4) to the transboundary floodplains.
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Interactive map. Click stations for metadata. Color-coded by contamination intensity. Data: Trinational Commission 2016–2024.
Note: Potosí-region stations (red) reflect elevation and watershed drainage geography — these high-altitude sites sit in the upper Pilcomayo basin and feed the river system downstream.
Research Findings
Six Interconnected Analyses
Each analysis answered a different question about how metals enter, move through, and accumulate in the Pilcomayo system. Together they tell a complete story.
Sediment-Water Partitioning
KdThe ratio of a metal's concentration on sediment to its concentration in water at equilibrium. A higher Kd means more of the metal is held in the sediment and less is dissolved in the water column. Distribution Coefficients
265 matched sample pairs (per metal; 264 for Cd) quantify how strongly each metal binds to sediment. Lead holds 10× tighter than cadmium — a critical difference for remediation.
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Hypoxic Events
Oxygen Crash & Metal Release
Three oxygen-crash events (
DOThe concentration of molecular O₂ dissolved in water. Reported in mg/L. Below ~4 mg/L, anaerobic microbial processes can begin reducing iron- and manganese-oxide minerals in sediment. < 4 mg/L) caused metals locked in sediment to re-dissolve into the water column. Lead spiked 3× during the best-documented event.
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Spatiotemporal Migration
Seasonal Metal Hand-off
Eight years of quarterly sampling reveal a predictable seasonal hand-off:
wet-seasonIn the upper Pilcomayo, the wet season runs roughly November through April, the dry season May through October. Quarterly sampling captures both phases of the annual hydrological cycle. rainfall flushes metals into the water column; dry-season settling traps them in sediment.
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Sediment Texture
Grain Size vs. Mining Source
Grain size does not control contamination in the Potosí headwaters. Mining-source dominance is so extreme that it overrides the texture hierarchy entirely.
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3D Contamination Models
Interactive 3D Surfaces
Interactive three-dimensional surfaces map As, Cd, Pb, and Zn across the 850 km corridor over 8 years — revealing dilution gradients and transboundary persistence.
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Remediation Strategy
Intervention Strategy Synthesis
Evidence-based intervention modalities synthesized from the project's four analytical chapters — characterizing in-situ stabilization, phytoremediation, and ex-situ treatment by their controlling chemistry, spatial scope, and evidence base.
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Core Mechanism
The Inverse Hand-off
The Pilcomayo operates on a seasonal metal cycle. During the wet season (Nov–Mar), rainfall mobilizes metals from Potosí's tailings into the water column — spiking dissolved concentrations across the basin. As the dry season arrives (Apr–Oct), flow drops, pH rises, and metals settle into riverbed sediments. Then the cycle begins again.
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Wet Season Pulse
Rainfall mobilizes tailings. Wet-to-dry water-column concentration ratios reach 3×–30× depending on metal and region — peak amplification at distal stations for high-Kd metals like Pb and Cd.
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Dry Season Settling
Reduced flow allows sedimentation. Metals bind to Fe/Al oxides in sediment. Risk shifts to benthic organisms and future resuspension events.
Key Findings at a Glance
Median Kd — Lead
1,667
L/kg — highest sediment retention of all metals tracked
Event 2 Enrichment
3.0×
Lead increase in water column during the 2017 Naciente hypoxic event
Cadmium Mobility
1.6×
Cadmium water-column enrichment during hypoxic events — driven by acid-mediated dissolution of Ca/P binding phases (Tarapaya, Oct 2017)
