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Most coral reefs around the world are declining due to a myriad of external stressors, including climate change impacts such as rising sea-surface temperatures and ocean acidification. An additional threat may come from coastal groundwater, which flows over reefs and can carry excess nutrients, toxins, pathogens, and other pollutants from land.

A new study from USGS researchers at the Pacific Coastal and Marine Science Center demonstrates how passive-sampling techniques can be used to detect pharmaceuticals, pesticides, and other organic compounds in coastal groundwater discharge near coral reefs. 

When precipitation falls on land, some of it infiltrates through rock and soil to join the saturated groundwater system, which may eventually discharge into streams, lakes, or the ocean. Sampling and analyzing groundwater discharge is an important tool to determine whether contaminants are present in the groundwater, and at what concentration.

A map showing study sites at West Maui, Hawai'i, where passive samplers were deployed to detect groundwater contaminants
A map showing study sites at West Maui, Hawai'i, where passive samplers were deployed to detect groundwater contaminants such as pesticides, pharmaceutical compounds, and personal care products that can harm coral reefs.

As fresh groundwater flows toward the coastal sea and coral reefs, it rises over salty and denser water. The salty and fresh water mix along the interface, and the resulting fluid often discharges near the shoreline where it can flow into coral reef environments. Any potential contaminants in this discharging groundwater are thoroughly mixed at this interface, making them more difficult to detect. 

“Traditional surface and groundwater sampling is sometimes called ‘grab sampling’ and usually involves taking a one- to two-liter sample at strategic locations,” said Pamela Swarzenski, a USGS Chemist and lead author of the study. “This provides an important snapshot of what’s in the groundwater at that moment, but it doesn’t allow us to detect contaminants that may be more diffuse in the groundwater. Passive samplers can be deployed for days or weeks, acquiring higher concentrations of contaminants of concern across targeted periods of time, such as seasons or high-runoff events.” 

In the study, passive membrane samplers (semipermeable membrane devices and polar organic chemical integrative samplers) developed by USGS scientists were deployed for 22 continuous days at 7 coastal sites in West Maui, Hawai’i, to test for a suite of organic compounds including pesticides, polychlorinated biphenyls (PCBs), pharmaceutical and personal care products (PPCPs), and flame retardants. The use of passive samplers allowed for almost one month of continuous, integrated sampling to accommodate fluctuations in daily and weekly inputs that would not be detected by grab water samples. 

The researchers found that flame retardant chemicals, often used in pharmaceutical and personal care products, were widespread in coastal groundwater discharge at West Maui. Also widespread was galaxolide, a synthetic compound used to bind fragrance in cosmetics and other products, and oxybenzone, an active ingredient in many sunscreens that is known to cause coral bleaching. 

The authors note that this study provides a baseline for groundwater contaminants found at the West Maui nearshore during a drier-than-expected wet season, which can be useful for future evaluations and comparisons. The next step would be to deploy samplers during and after a wet season, to capture runoff that potentially contains so-called legacy contaminants that remain in the environment long after their use. 

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