Identifying wastewater chemicals in coastal aerosols

Abstract

The Tijuana River, at the US-Mexico border, discharges millions of gallons of wastewater daily-sewage, industrial waste, and runoff-into the Pacific Ocean, making it the dominant source of coastal pollution in this region. This study examines how such wastewater influences coastal aerosols by tracking spatial gradients from near the border northward. Using benzoylecgonine (a nonvolatile cocaine metabolite) as a sewage tracer, we find that wastewater compounds-including a mixture of illicit drugs, drug metabolites, and chemicals from tires and personal care products-become aerosolized and are detectable in both water and air. Spatial analyses confirm that most measured chemicals concentrate in aerosols near the Tijuana River, potentially exposing local populations to tens of nanograms per hour (e.g., octinoxate and methamphetamine) via inhalation. This airborne pathway highlights a largely overlooked source of atmospheric pollution, emphasizing the need to reassess health risks in coastal regions as global water contamination continues to escalate.

Fig. 1.. Selected wastewater chemical concentrations and spatial distributions.

(A) River and ocean water. (B) Aerosol. Box plots are positioned so the vertical line indicates the median, the boxes indicate the 25th and 75th percentiles, and the whiskers represent the 5th and 95th percentiles. Compounds are organized from top to bottom based on their highest-to-lowest median concentrations in the water samples from the TJR site. Estimated inhalation exposures of aerosols on a daily basis are shown in the top horizontal axis of (B). The sampling sites are TJR, BF, IB, SS, and SIO (SIO pier). ppt, parts per trillion; ppb, parts per billion; ppq, parts per quadrillion.

Fig. 2.. Temporal profiles of benzoylecgonine concentrations in water and aerosols at IB and their links to TJR flow and sea spray generation.

(A) Example temporal profile of benzoylecgonine concentrations in IB water and TJR flow rates surrounding the precipitation event on 10 February. Boxes indicate the timeframe before and after the precipitation event used to calculate Δ[benzoylecgonine]_IB in water and the ΔTJR flow rate shown in (B), representing a dry period followed by a wet period. (B) The correlation between the increase in TJR flow and benzoylecgonine in IB water across all precipitation events indicates river-to-ocean transfer dependent on river flow. (C) Example temporal profile of benzoylecgonine concentrations in IB aerosol, precipitation rate, and predicted SSA mass flux surrounding the precipitation event on 10 February. The box indicates the accumulation period following the precipitation event used to calculate Δ[benzoylecgonine]_IB in aerosols and integrated predicted SSA mass flux, ∑ Flux_{SSA OC}, in (D). (D) The correlation between the integrated modeled SSA organic carbon mass flux and the increase in benzoylecgonine in IB aerosols over the same accumulation period in (C) for all precipitation events indicates the ocean-to-air transfer of the sewage relates to SSA generation. The precipitation event surrounding 11 March was excluded from this correlation analysis because no air measurements were available to determine the concentration after precipitation ended. Error bars represent the propagated SDs in the concentrations for those days. MGD, millions of gallons per day.

Fig. 3.. Linear regressions between the quantified pollutants and benzoylecgonine in aerosols from the BF, IB, and SS measurement sites.

(A) caffeine, (B) carbamazepine, (C) cocaine, (D) diazinon, (E) dibenzylamine, (F) erythromycin, (G) heroin, (H) imazapyr, (I) isoxaben, (J) methamphetamine, and (K) octinoxate. Error bars reflect the relative uncertainty in the calibrated sensitivity for each compound. Horizontal error bars that extend to the left axis were below the quantification limit of benzoylecgonine and extended to its detection limit. Vertical error bars that extend to the bottom axis were below the quantification limit of the pollutant and extended to their detection limits. The intercept of the line of best fit is set to zero.

Fig. 4.. Aerosol-to-water ratios of pollutants and dependence on hydrophobicity and bubble scavenging efficiency.

Log average AFs for each pollutant are shown as a function of their octanol-water partitioning coefficient, K_ow. (98). Data points are colored by their scavenging coefficient, K_s, defined as the ratio between the aqueous adsorption coefficient (K_aq) and air adsorption coefficient (K_a). BEN, benzoylecgonine; CAF, caffein; CAR, carbamazepine; COC, cocaine; DIA, diazinon; DIB, dibenzylamine; ERY, erythromycin; HER, heroin; IMA, imazapyr; ISO, isoxaben; MET, methamphetamine; OCT, octinoxate. The AF for OCT is not included in the linear fit due to its relatively poor scavenging by bubble entrainment. Only days at BF, IB, and SS are included in the average.

Fig. 5.. Exploratory estimated transfer of the pollutants between major environmental compartments and humans in an urban coastal environment.

Reported are daily median-to-max mass transfer rates of the three most correlated pollutants (dibenzylamine, methamphetamine, and octinoxate) with benzoylecgonine, a tracer of sewage from the TJR, including river-to-ocean transfer, ocean-to-air transfer, and air inhalation. The black arrows refer to mass transfer processes that can be determined directly from the measurement data. The dark gray arrow refers to the estimated ocean-to-air flux based on IB’s measured ocean water concentrations using a predictive model relying on additional assumptions as described in the text. The light gray arrows refer to other likely intercompartmental transfer scenarios, yet the fluxes are unknown, requiring further study. DBZ, dibenzylamine; METH, methamphetamine; OMC, octyl methoxycinnamate, also known as octinoxate.

Fig. 6.. Map of sampling locations and environmental conditions.

(A) Map of the San Diego coastal region and the border with Mexico. Field sites are labeled from south to north: BF, TJR, IB, SS, and SIO. (B) Hourly flow rate of the TJR measured at the international boundary (77). (C) Hourly precipitation measured at the international boundary (77). Sampling dates are designated as red markers.