Identifying a Spectral Signature of Untreated Wastewater from the Tijuana River

Persistent cross-border flow of untreated sewage in the U.S.-Mexico border region from two main point sources, the Tijuana River and Punta Bandera, has resulted in an environmental and public health crisis for coastal residents on both sides of the border. Satellite remote sensing has the potential to be a cost-effective approach for filling spatial and temporal gaps in field sampling campaigns which seek to identify environmental contaminants and wastewater indicators. However, more research is needed to validate satellite-based estimates of water quality parameters like CDOM (chromophoric dissolved organic matter), tryptophan, and fecal indiciator bacteria (ie. E. coli, enterococcus). Identifying spectral signatures of water quality parameters associated with untreated wastewater could allow for the direct detection of wastewater from remote sensing technologies, but has been historically difficult, and it is still unclear whether a direct spectral signature even exists. This project analyzed 147 water samples collected on 11 sampling dates between February 19th, 2025 and May 7th, 2025 from the Tijuana River, Tijuana Estuary, and coastal Pacific Ocean in Imperial Beach, California for water quality indicators and spectral features in the visible range (400-700 nanometers). Results show that Tijuana River samples are characterized by chromophoric dissolved organic matter (CDOM) and tryptophan concentrations that are very similar to samples of untreated wastewater taken from the South Bay International Wastewater Treatment Plant, indicating high wastewater content, and that CDOM and tryptophan levels decline by almost two orders of magnitude as water moves through the system from the international border, through the river, to the Pacific Ocean and is transported offshore. Using a Partial Least Squares Regression model, a spectral feature at 620nm was found to correlate with tryptophan and CDOM concentrations in unfiltered samples, which is consistent with previous findings by Scrivner et al. (2025). However, only the CDOM correlation was significant (R2 = 0.66; tryptophan R2 = 0.23). A spectral feature at 610nm was also correlated with both tryptophan (R2 = 0.52) and CDOM (R2 = 0.64) concentrations for all samples and is not well explained by previous literature. Studies such as these may nevertheless be useful in validating existing satellite-based monitoring of coastal wastewater pollution, and can help prepare for hyperspectral satellites currently in development, by contributing to the search for a unique spectral signature of untreated wastewater.