📄 earth_science

Pollutant export under a new regime of continuous transboundary sewage flow in the Tijuana River

The 2022 shift from intermittent to perennial transboundary sewage flow in the Tijuana River has caused a six-fold increase in water volume and a ten-fold surge in pollutant export to the Pacific Ocean, resulting in continuous beach closures and a near-total lack of natural attenuation for contaminants over the final 12 kilometers to the coast.

Original authors: Alexandra Grant, Trent Biggs, Daniel Sousa, Matthew E. Verbyla, Stephany Garcia, Sneha Korat, Alina Alex, Elise Piazza, Jeffrey Crooks, Douglas Liden, Yongping Yuan, Natalie Mladenov

Published 2026-07-07

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CC BY 4.0

Original authors: Alexandra Grant, Trent Biggs, Daniel Sousa, Matthew E. Verbyla, Stephany Garcia, Sneha Korat, Alina Alex, Elise Piazza, Jeffrey Crooks, Douglas Liden, Yongping Yuan, Natalie Mladenov

Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

The Big Picture: A River That Never Sleeps

Imagine the Tijuana River, which flows between Mexico and the United States, as a giant, open sewer pipe that used to run dry for months at a time. For years, it was like a seasonal faucet: it would gush with dirty water during rainy storms, but then turn off completely during the dry summer months.

However, starting in December 2022, that faucet broke open and never turned off again. Due to damaged pipes and overwhelmed treatment plants in Tijuana, the river shifted from an "intermittent" flow (sometimes on, sometimes off) to a "perennial" flow (always on). This means the river is now a constant, unbroken stream of untreated sewage flowing toward the Pacific Ocean.

The "Before and After" Comparison

The researchers looked at the river before this change (2021–2022) and after (2023–2024).

The Old Days: The river flowed about 31,800 cubic meters of water a day on average during dry spells.
The New Reality: That number jumped to 191,000 cubic meters a day.
The Analogy: Think of it like a garden hose. Before, you were dripping a cup of dirty water every day. Now, you are blasting a firehose of dirty water, 6.3 times stronger than before.

What's in the Water? (The "Soup" Ingredients)

The team took over 280 samples to see what was in this new, constant flow. They found that during dry weather, the river water is essentially untreated wastewater.

The "Fecal" Factor: The water is teeming with E. coli bacteria. The levels are so high that they are equivalent to raw sewage straight out of a pipe. This is why beaches in San Diego and Imperial Beach have been closed for over 1,000 consecutive days since 2023.
The Chemical Load: The water is loaded with nitrogen and organic carbon. The amount of nitrogen flowing into the ocean has increased five-fold.
The Fluorescence: The researchers used a special "glow-in-the-dark" test (fluorescence) to detect sewage. They found that the water glows with the same intensity as raw sewage, confirming that the river is acting as a direct pipeline for human waste.

The Mystery of the "Magic Filter" That Didn't Work

Usually, when a river flows toward the ocean, nature acts like a giant filter. As the water travels, sunlight kills bacteria, plants eat nutrients, and sediment settles down. Scientists expected that by the time the river reached the ocean (about 12 kilometers away), the water would be cleaner than it was at the border.

But that didn't happen.

The Analogy: Imagine a dirty sponge moving through a cleaning station. Usually, the sponge gets cleaner as it moves through. In this case, the sponge stayed just as dirty from start to finish.
Why? The water is so dark and "colored" by all the dissolved waste that it blocks out the sunlight. Without sunlight, the natural "sunlight sanitizer" (photodegradation) can't work. The river is so full of pollution that it shields the germs and chemicals from the sun, allowing them to survive the entire journey to the ocean without dying off.

The Storm vs. The Sewage

A surprising finding was that dry weather is actually worse than rainy weather.

The Analogy: Think of a cup of strong coffee. If you add a little water (rain), it gets weaker. If you have a cup of pure coffee (sewage) and it keeps flowing without rain, it stays strong.
The Reality: When it rains, the stormwater dilutes the sewage, making the concentration of bacteria slightly lower. But when it's dry, the river is just a steady stream of concentrated raw sewage. The study found that the dry-weather water was significantly more polluted than the wet-weather water.

The Bottom Line

The Tijuana River has transformed from a seasonal river into a continuous, high-volume conveyor belt of untreated sewage.

Volume: The amount of pollution hitting the ocean has increased by a factor of five.
Persistence: The river no longer cleans itself; it acts as a direct pipe, delivering pollutants to the ocean with almost no natural reduction.
Impact: This creates a massive public health risk, leading to beach closures and threatening the local ecosystem, which is now suffering from low oxygen levels and a lack of fish diversity.

The paper concludes that this new "baseline" of pollution is a critical warning. It shows that without major infrastructure repairs upstream to stop the flow of raw sewage, the river will continue to act as a direct pipeline of pollution to the Pacific Ocean.

Technical Summary: Pollutant Export under a New Regime of Continuous Transboundary Sewage Flow in the Tijuana River

Problem Statement
The Tijuana River, a transboundary waterway shared by Mexico and the United States, has historically suffered from severe pollution due to untreated wastewater, industrial waste, and inadequate infrastructure. A critical shift occurred in December 2022, when damaged wastewater infrastructure and wet weather conditions transformed the river's flow regime from intermittent (seasonally dry) to perennial (continuous daily flow). This transition created a new baseline of chronic cross-border contamination, yet significant knowledge gaps remained regarding the magnitude of pollutant export under this new regime. The study addresses the urgent need to quantify chemical and microbial loads to inform multinational management strategies, protect public health in coastal communities (including Imperial Beach and San Diego), and mitigate ecosystem degradation in the Tijuana River Estuary (TRE), the largest remaining coastal wetland in Southern California.

Methodology
The research team conducted a comprehensive monitoring campaign from September 2023 to April 2025, collecting 280 distinct grab samples across sixteen events. Sampling sites spanned the river from the US-Mexico border to the Pacific Ocean, including the South Bay International Wastewater Treatment Plant (SBIWTP) for untreated influent comparisons.

Key analytical approaches included:

Hydrologic Analysis: Utilization of IBWC discharge data and NOAA precipitation records to characterize flow duration curves and calculate runoff coefficients. Statistical tests (Mann-Kendall, linear regression) were applied to identify trends in discharge and flow regimes between water years (WYs) 2021–2022 and 2023–2024.
Chemical and Optical Analysis: Samples were analyzed for Chemical Oxygen Demand (COD), Dissolved Organic Carbon (DOC), Total Dissolved Nitrogen (TDN), and Total Dissolved Solids (TDS). Spectroscopic methods (UV-vis absorbance and 3D fluorescence) measured Chromophoric Dissolved Organic Matter (CDOM) and Tryptophan-like fluorescence (TRP) as proxies for wastewater-derived organic matter.
Microbiological Analysis: Quantification of fecal indicator bacteria (FIB) including E. coli, total coliforms, and enterococci. Specific microbial source tracking markers (HF183/BacR287 for human fecal matter) and enteric viruses (Adenovirus, Norovirus, PMMoV) were assessed via qPCR and RT-qPCR.
Experimental Validation: Controlled exposure experiments (24-hour duration) were conducted on wastewater solutions to evaluate the effects of photodegradation and biodegradation on pollutant attenuation.
Load Estimation: Volume-weighted mean (VWM) concentrations were paired with flow data to estimate annual dry-weather pollutant loads delivered to the Pacific Ocean.

Key Results

Hydrologic Shift: The average daily dry-weather flow increased 6.3-fold, rising from $3.18 \times 10^4$ m³/day in WYs 2021–2022 to $19.1 \times 10^4$ m³/day in WYs 2023–2024. Zero-flow days dropped from 63% (2001–2020) to 9% (2023–2024), confirming a transition to a perennial flow regime.
Pollutant Concentrations: Dry-weather river concentrations of DOC, TDN, TRP, and E. coli were statistically indistinguishable from untreated wastewater and significantly higher than wet-weather storm runoff. E. coli concentrations averaged $7.8 \times 10^6$ MPN/100mL, exceeding US water quality objectives by more than three orders of magnitude.
Lack of Attenuation: Contrary to typical riverine expectations, there was essentially no attenuation of chemical constituents, microbial markers, or pathogens over the ~12 km stretch from the border to the ocean. While COD showed slight decrease, TDN and TRP actually increased with distance.
Mechanisms for Persistence: Experimental data indicated that while sunlight can degrade pollutants, the highly colored river water (high CDOM) likely shields contaminants from photodegradation. Additionally, the "plug flow" nature of the river, with peak discharge traveling from the border to the mouth in approximately 12.4 hours, limits the time available for natural biogeochemical processing.
Pollutant Loads: Annual dry-weather exports to the Pacific Ocean increased by an order of magnitude. The river now exports approximately 1.06 million kg of Total Dissolved Nitrogen (TDN) and 1.15 million kg of Dissolved Organic Carbon (DOC) annually. This TDN load alone exceeds the historical average annual nitrogen load from all Southern San Diego rivers combined (1997–2017) by approximately 16%.

Significance and Claims
The authors establish this dataset as a critical "existing water quality baseline" for the Tijuana River under a continuous cross-border flow regime. The study claims that the river has transitioned from functioning as a biogeochemical filter to acting primarily as a conduit, transporting pollutants directly to coastal waters with minimal natural attenuation.

The findings underscore that the current pollution levels are driven by chronic, untreated wastewater rather than episodic storm events. The authors assert that these results provide essential data for:

Binational Management: Supporting US and Mexico efforts to prioritize infrastructure investments and reduce source pollution.
Public Health Protection: Explaining the persistence of beach closures and health risks in Imperial Beach and surrounding communities.
Modeling and Monitoring: Providing a baseline for future remote sensing of sewage plumes and coastal pollutant transport modeling.

The paper concludes that without upstream interventions to reduce pollutant loads at the source, the natural capacity of the estuary to buffer pollution is overwhelmed, necessitating sustained binational monitoring and data sharing to mitigate ecosystem and human health risks.