Passive water sampling and air-water diffusive exchange of long-range transported semi-volatile organic pollutants in high-mountain lakes

The concentrations of legacy and currently emitted organic pollutants were determined in the freely dissolved phase of water from six high-mountain lakes in the Pyrenees (1619-2453 m) by passive water sampling. Low-density polyethylene (LDPE) and silicone rubber (SR) sheets were exposed for three consecutive periods lasting each one year between 2017 and 2020 for the study of polychlorinated biphenyls (PCBs), organophosphate esters (OPEs), polycyclic aromatic hydrocarbons (PAHs), and other organochlorine compounds (e.g., hexachlorobenzene, HCB). HCB concentrations (1.0-14 pg L-1) remained essentially the same as those measured with pumping systems over two decades ago in the same area. Sigma PAHs (35-920 pg L-1) were around half of those observed in the past, which agrees with reductions in European atmospheric emissions. Sigma PCB concentrations (1.2-2.2 pg L-1) were substantially lower, although unexpectedly large differences could be due to comparing yearly averages from the present study to seasonally variable (i.e., affected by snowmelt, stratification, and colloidal organic matter) episodic pumping measurements from previous studies. Sigma OPEs (139-2849 pg L-1) were measured for the first time in this area and were found at high concentrations in some sites. Concentrations of most compounds obtained with LDPE and SR samplers agreed with each other by ratios generally lower than three or four times, except for a few PAHs and OPEs. Diffusive exchange flux calculations between the atmospheric gas phase and the freely dissolved water phase revealed net deposition of pollutants from air to water, except for some OPEs and PCBs presenting equilibrium conditions, and HCB with volatilization fluxes. Atmospheric degradation fluxes of PAHs and OPEs pointed at competing removal mechanisms that support the air-to-water direction of their diffusive exchange, while PCBs and organochlorines were not affected by photodegradation. In their current state, these remote lakes accumulate many emerging and legacy pollutants subject to long-range atmospheric transport.

Automated summary

The concentrations of legacy and currently emitted organic pollutants were analyzed in the water from six high-mountain lakes in the Pyrenees. Passive water sampling using LDPE and SR sheets was conducted over a three-year period between 2017 and 2020. The results showed that HCB concentrations remained the same as those measured two decades ago, while PAHs concentrations were reduced by half. PCB concentrations were lower, but variations could be due to differences in sampling methods. OPEs were measured for the first time and found at high concentrations in some sites. The study also revealed the deposition of pollutants from air to water, except for some OPEs and PCBs that reached equilibrium conditions, and HCB with volatilization fluxes. PAHs and OPEs were affected by atmospheric degradation, supporting their diffusive exchange from air to water, while PCBs and organochlorines were not influenced by photodegradation. These remote lakes in the Pyrenees accumulate both emerging and legacy pollutants due to long-range atmospheric transport.