The river is not a pipe

The conventional mental model in early ERW science treated rivers as passive conveyors: dissolved carbon enters upstream, carbon exits at the coast. The emerging science says this is wrong in important ways.

A 2026 paper in Frontiers in Climate quantified what river biology alone does to your DIC budget: aquatic photosynthesis, operating within individual stream segments, can take up between 1 and 30 percent of dissolved inorganic carbon delivered by flow. The mechanism matters as much as the magnitude. If aquatic plants take up bicarbonate via anion exchange, they simultaneously strip alkalinity from the water. If they use dissolved CO₂, the alkalinity signal is preserved but DIC decreases. The difference is not cosmetic — it changes whether the carbon that arrives at the coast carries the permanence characteristics that justify a multi-millennia credit.

Beyond photosynthesis, calcite precipitation is a risk in high-pH rivers. Where calcium concentrations are elevated and pH rises — both of which happen when ERW applications scale — CaCO₃ can precipitate from solution in the water column or in estuarine sediments. Precipitation sequesters carbon, yes, but in a form that is soluble over much shorter timescales than oceanic bicarbonate storage. Whether that counts as durable removal is an open and commercially significant question.

CO₂ degassing adds another layer. Where rivers are supersaturated with dissolved CO₂ — which is common, because soil water often enters streams at high partial pressure — some of that carbon returns to the atmosphere before reaching the ocean. For ERW projects working at tropical latitudes with high biological activity and warm temperatures, degassing can be a non-trivial efficiency loss.

What the coast actually does

Assume your bicarbonate makes it through the river network intact. The coastal zone — estuary, shelf, open ocean — is the destination, but it is not a simple terminal sink. River plumes mix into coastal waters at different rates depending on salinity gradients, tidal mixing, and stratification. In estuaries, alkalinity can be consumed by biological productivity or buffered by carbonate chemistry shifts as freshwater meets saline water.

In the open ocean, dissolved bicarbonate is stable over geological timescales — this is the 10,000-year permanence that makes ERW unique among carbon removal approaches. But the fraction of your original field-applied alkalinity that reaches this stable state is, in real projects with real catchment hydrology, meaningfully less than 100 percent.

The Puro.earth ERW standard released in 2025 now formally requires project developers to account for the expected transport losses between field application and ocean storage. A durability term must be supported by the MRV plan, and carbon release risk scenarios for both precipitated and dissolved carbon must be documented. This is no longer optional fine print.

Why this changes project design

The practical implications for ERW project developers are significant, and they flow upstream from the coast all the way back to site selection.

Catchment hydrology matters as much as soil chemistry. A site with excellent basalt geology, appropriate soil pH, and optimal rainfall can still be a poor ERW project if it sits in a catchment with high biological productivity in its downstream rivers, or where estuarine chemistry creates calcite precipitation risk. Hydrological screening — using dissolved river-network modelling to estimate bicarbonate transport efficiency from field to coast — is becoming a standard component of project feasibility assessment.

Isotopic tracers are gaining importance for a related reason. The key scientific challenge in river-network accounting is distinguishing ERW-derived bicarbonate from background geological weathering, agricultural liming inputs, and natural DIC cycling. Projects that deploy isotopic fingerprinting of their feedstock, and track that signature through soil water and stream samples downstream, can build a far more defensible chain of evidence than those relying solely on field-scale mass balances.

Far-field monitoring — river and coastal water sampling downstream of project sites — is no longer an advanced research activity. It is a credibility signal that major buyers now expect to see in project MRV plans.