Pharmaceuticals and Pesticides in Coastal Waters: Why Sediment Monitoring Matters
Finding a chemical in coastal water does not automatically mean that it is causing ecological damage. It does, however, identify an exposure pathway that may require closer investigation.
That distinction is central to new research from South Africa’s False Bay. Scientists detected selected pharmaceuticals, herbicides and metals across the bay, with concentrations of the organic contaminants generally higher in marine sediments than in seawater. Yet their assessment found little evidence of immediate ecological damage at the concentrations measured.
The study offers a valuable lesson for coastal pharmaceutical pollution monitoring: chemical detection, ecological exposure and demonstrated harm are related, but they are not the same thing.
For Caribbean environmental management, the research should not be interpreted as proof that the same contaminant pattern exists at any particular beach, bay or harbour. It should be treated as evidence for designing stronger monitoring programmes that examine water, sediment, pollution sources and biological condition together.
What the False Bay study investigated
The researchers sampled 19 locations in False Bay between April and June 2021. They examined seawater and marine sediments for selected contaminants, including:
- The pharmaceuticals acetaminophen, also known as paracetamol; carbamazepine; and diclofenac
- The herbicides atrazine and metolachlor
- Copper, iron, manganese, lead and zinc in marine sediment
They also studied organisms living on and within the seabed. These included larger benthic animals, known as macrofauna, and microscopic nematodes commonly used as indicators of sediment and ecosystem condition.
The study therefore went beyond simply reporting whether chemicals were present. It attempted to connect measured concentrations with ecological risk benchmarks and biological community data.
According to the original paper in Marine Pollution Bulletin, the selected pharmaceuticals and herbicides were detected in both seawater and sediment. The University of Cape Town’s research summary reports that concentrations were generally higher in sediment, suggesting that the seabed was retaining contaminants entering the bay.
Higher contaminant loads in seawater were recorded in the northeastern part of the bay near Gordon’s Bay, while comparatively high sediment accumulation occurred near Muizenberg and sections of the northeastern coastline.
The spatial patterns were associated with possible influences from wastewater, urban drainage, rivers, stormwater systems and maritime activity. However, spatial association alone does not conclusively identify the source of each chemical. Confirming individual sources would require targeted source sampling, chemical fingerprinting or other forms of pathway analysis.
Detection is not the same as demonstrated toxicity
The word “toxic” in a headline can create the impression that ecological injury was observed. That is not what this field study established.
Risk quotients for the evaluated organic contaminants were below 0.1. A risk quotient generally compares an environmental concentration with a concentration expected to produce no adverse effect. A value below the selected concern threshold indicates low predicted risk under the assumptions and benchmarks used in the calculation.
Measured metals were also generally below effects range-low thresholds, which are sediment-quality benchmarks used to identify concentrations below which adverse biological effects are less likely.
The biological evidence supported this low immediate-risk interpretation:
- Benthic macrofaunal communities remained diverse.
- Nematode indicators suggested good to high ecological quality at most stations.
- Researchers found no significant relationship between contaminant concentrations and the measured abundance, diversity or structure of the benthic communities.
These findings indicate that the measured concentrations were not associated with consistent, community-level ecological disruption during the study.
They do not prove that the chemicals were harmless, that no individual organism experienced an effect or that future conditions will remain unchanged.
Why the sediment results matter
Water samples frequently provide a snapshot of conditions at a particular place and time. Concentrations can change with tides, rainfall, wastewater discharge cycles, river flow and coastal circulation.
Sediments can provide a different type of information.
Depending on their physical and chemical properties, contaminants may associate with suspended particles or organic material and settle onto the seabed. Fine-grained sediment and sediment with higher organic-carbon content can retain some substances more effectively than coarse sand.
This means a contaminant may be difficult to detect in a short-lived water sample while remaining measurable in sediment.
The seabed is not necessarily a permanent or inactive sink. Contaminants may be:
- Exchanged between sediment and porewater
- Taken up by sediment-dwelling organisms
- Ingested with contaminated particles
- Disturbed by dredging, vessel activity or construction
- Resuspended during storms and strong currents
- Transferred through food webs under suitable exposure conditions
The behaviour differs among pharmaceuticals, pesticides and metals. Sediment results must therefore be interpreted using chemical properties, grain size, organic carbon, local hydrodynamics and biological exposure pathways.
This is why water-only monitoring can provide an incomplete picture of coastal contamination.
What the study could not determine
The False Bay research provides an important field baseline, but it also has clear limitations.
Sampling took place during one period between April and June 2021. It cannot by itself establish long-term trends, seasonal variability or how conditions change after intense rainfall, sewage releases, floods or other episodic events.
The researchers assessed selected chemicals rather than every pharmaceutical, pesticide, industrial compound or transformation product that might be present. A low risk estimate for the analysed substances cannot be applied automatically to chemicals that were not measured.
The study also evaluated many biological organisms at family-level taxonomic resolution. This is useful for broad community assessment, but it may be less sensitive to species-specific or subtle ecological changes.
Other unresolved questions include:
- Long-term and repeated exposure
- Sublethal effects on behaviour, reproduction or development
- Combined exposure to multiple contaminants
- Additive, antagonistic or synergistic mixture effects
- Chemical transformation products
- Uptake by edible marine organisms
- Changes in contaminant availability under different sediment conditions
The study was an ecological field assessment. It was not a human-health or seafood-safety assessment. Its results should not be used to claim that seafood from the area is either unsafe or proven safe.
Why chemical mixtures complicate coastal risk assessment
A receiving environment rarely contains one pollutant in isolation.
Wastewater, stormwater, rivers, agricultural runoff, harbour operations and urban drainage can introduce different combinations of nutrients, pathogens, hydrocarbons, metals, pesticides, pharmaceuticals and other contaminants.
Substance-by-substance risk quotients remain useful screening tools, but they may not fully represent cumulative exposure. Chemicals with similar modes of action may produce additive effects, while other combinations may alter chemical availability or biological response.
A stronger coastal assessment should therefore consider three connected questions:
- What is present?
This requires chemical analysis with suitable detection limits. - Is it biologically available?
Total concentration does not always equal the concentration capable of entering an organism. - Is the ecosystem responding?
Biological indicators, toxicity testing and ecological community assessment can help answer this question.
The False Bay study is particularly valuable because it attempted to integrate these lines of evidence rather than relying on contaminant detection alone.
Caribbean relevance: a reason to measure, not assume
The South African results cannot be transferred directly to Trinidad and Tobago or another Caribbean jurisdiction.
However, the pathways investigated—domestic wastewater, stormwater, rivers, agricultural inputs and coastal development—are also relevant to environmental management in the Wider Caribbean.
The United Nations Environment Programme’s Protocol Concerning Pollution from Land-Based Sources and Activities establishes a regional framework for reducing land-based marine pollution. It includes regional requirements concerning domestic wastewater and calls for measures addressing agricultural non-point pollution.
There is also Caribbean-specific evidence that contaminants of emerging concern can enter wastewater systems. A study of two wastewater-treatment plants in Barbados detected caffeine, ibuprofen, steroid hormones and several prescription pharmaceuticals. That study concerned wastewater—not the general condition of Caribbean coastal waters—but it demonstrates a plausible regional source pathway that may warrant receiving-environment investigation. The research is available through PubMed.
In Trinidad and Tobago, the Institute of Marine Affairs describes an environmental-quality programme that examines water, sediment and biota in coastal and marine environments. This multi-media approach reflects an important principle: coastal condition cannot always be characterized adequately through water samples alone.
What an effective coastal monitoring programme should include
A coastal pharmaceutical pollution or emerging-contaminant investigation should begin with a conceptual source–pathway–receptor model. This identifies potential inputs, how contaminants could move and which habitats or organisms could be exposed.
| Monitoring component | Main question |
|---|---|
| Source and pathway sampling | Are contaminants entering through wastewater, stormwater, rivers, drains, marinas or industrial discharges? |
| Seawater sampling | What is present in the water column under current tidal and weather conditions? |
| Sediment sampling | What has accumulated on the seabed, and where are depositional zones located? |
| Biological assessment | Are benthic communities, indicator organisms or tissues showing evidence of exposure or ecological response? |
| Seasonal and event-based monitoring | How do concentrations change between wet and dry periods or after rainfall and discharge events? |
| Laboratory and field quality control | Are the results sufficiently reliable and sensitive for environmental decisions? |
The programme should also define appropriate reference sites, source-proximal stations and downstream or depositional locations.
For trace pharmaceuticals and pesticides, planning must address analytical detection limits, sample-container compatibility, preservation, holding times, field blanks, duplicate samples and laboratory quality assurance before fieldwork begins.
Sediment sampling should consider grain size, organic carbon, redox conditions and other characteristics that influence contaminant retention. Where biological monitoring is included, the organisms and taxonomic resolution should match the management question.
Repeated sampling is essential for distinguishing isolated detections from persistent contamination and for evaluating whether pollution-control measures are working.
How Ecotox can support stronger coastal evidence
Ecotox Environmental Services can support the design and implementation of integrated coastal investigations through:
- Marine water and sediment sampling
- Specialized environmental sampling
- Stormwater and riverine pathway assessment
- Environmental baseline studies
- Field water-quality measurements
- Sediment-quality investigations
- Environmental analytical-testing programmes
- Ecological risk assessment
- Environmental impact assessment
- Construction and operational compliance monitoring
- Emerging-contaminant investigations
A defensible programme connects sampling locations, analytical methods and ecological indicators to a clear environmental decision. That decision may involve establishing a baseline, investigating a discharge, assessing a development, evaluating dredging risks, tracing land-based inputs or measuring long-term change.
Learn more about Ecotox’s marine water, sediment and specialized sampling services.
Evidence before conclusions
The False Bay research demonstrates why environmental interpretation must go beyond both alarmism and reassurance.
The contaminants were real and geographically widespread within the study area. Sediment retention and continuing inputs justify attention. At the same time, the measured risk quotients and benthic-community evidence indicated low immediate ecological risk during the study.
The correct response is neither “pollutants were detected, therefore the ecosystem is being poisoned” nor “no immediate damage was observed, therefore monitoring is unnecessary.”
The evidence supports a more disciplined conclusion: coastal contamination should be evaluated across water, sediment and biological systems, using repeated measurements capable of distinguishing detection, exposure and actual ecological response.
Sources
- Mazeka, B., et al. Field-based assessment of selected pharmaceuticals, pesticides and sediment metals using macrofauna and nematode communities in False Bay, South Africa. Marine Pollution Bulletin, 2026.
- PubMed record and study abstract
- University of Cape Town. UCT study finds toxic pollutants in False Bay waters
- University of Cape Town Faculty of Science. UCT study finds toxic pollutants in False Bay waters
- United Nations Environment Programme. Protocol Concerning Pollution from Land-Based Sources and Activities
- Institute of Marine Affairs. Environmental Quality Research Programme
- Edwards, Q. A., et al. Contaminants of Emerging Concern in Wastewaters in Barbados, West Indies. Bulletin of Environmental Contamination and Toxicology, 2018.