Why Spain's beach-water tests can miss the dirty days: lessons from Herradura Bay

The study, in one paragraph

Granados-Fernández and colleagues spent seven months in 2019 tracking phytoplankton and bacteria along the shore of La Herradura Bay, then ran two intensive surveys — one the week before Easter (low tourism) and one during Easter (high tourism). They paddled a kayak rigged with a submersible fluorometer through the bay to map algae patches, and took bacterial samples at the shoreline, at the wastewater treatment plant's discharge into the Jate river, and out at the submarine outfall. The full paper is open access in Sustainability (Granados-Fernández et al., 2025).

The headline finding

La Herradura is a village of about 4,200 people. In summer — and at Easter — its population roughly triples. The local wastewater treatment plant is sized for a peak of 20,000 inhabitants, so on paper it should cope. In practice, the researchers found that during the high-tourism week:

• At the river mouth on the beach itself, fecal streptococci jumped 33× and E. coli jumped 10× compared with the quiet week before.
• At the treatment plant's discharge into the Jate river, E. coli hit 5,466 CFU/100 mL — about 11× the legal limit for inland water.
• Fecal streptococci on the beach reached 8,966 CFU/100 mL— roughly 48× what Spanish law considers "sufficient" for bathing.

And yet, looking at the same beach in the national NÁYADE database between 2016 and 2021, you'd see no "insufficient" rating at all.

How the official programme missed it

NÁYADE is the same database AguaMálaga pulls from — it's the national bathing-water register, run under the EU Bathing Water Directive. It's a serious, audited dataset. But the authors point to three structural reasons it can underplay short, sharp contamination events:

1. 1. Where the sample is taken. NÁYADE stations sit in the middle of each bathing zone. The Herradura's NÁYADE 2 point is close to the river mouth — but not in it. Bacteria from a river spill dilute fast in seawater, so a sample 100 m away can easily come back clean while the patch right at the river mouth is many times over the limit.
2. 2. When the sample is taken. Official sampling follows a fixed seasonal calendar and time-of-day window. The researchers point out — politely — that a plant operator who knows when inspectors call has every incentive to retain spills during those windows and release them outside.
3. 3. What's in the treatment plant. La Herradura's WWTP only has primary and secondary treatment. It removes solids and digests dissolved organic matter, but it has no tertiary stage — no UV, no chlorination, no ozone — to actually kill bacteria before discharge. So even when it's working, peak loads push fecal organisms straight through.

The clever bit: a fluorometer on a kayak

Most coastal water monitoring sits at one of two extremes: a handful of fixed shoreline points (cheap, sparse) or oceanographic cruises in >20 m of water (precise, expensive, far offshore). That leaves the slice of sea between roughly 1 m and 50 m from the beach — the bit people actually swim in — almost unmeasured.

The team filled that gap with a kayak towing a submersible fluorometer that fingerprints algae groups by their fluorescence, paired with grab samples for FlowCAM imaging and bacteria culture. The result was a high-resolution, cheap-to-run map of phytoplankton and bacteria across the whole bay. It revealed a patchy chlorophyll-a distribution (peaks of 5.5 µg/L — bordering on eutrophic), and crucially showed that the visible "floating organic patches" people often blame for dirty-looking water were, in fact, bacteria-free. The contamination came from somewhere else: the river.

What this means if you're reading AguaMálaga

AguaMálaga's water-quality readings come from NÁYADE. They are the official, lab-grade numbers — and for most of the season, on most beaches, they are a good guide to whether the water is clean. But this study is a useful reminder of what those numbers do and don't cover:

• They are periodic, not continuous. A "good" rating reflects samples taken on specific dates, not the day you visit.
• They sample at fixed points. Conditions a few dozen metres away — especially near a river or stream mouth — can be very different.
• They tend to be cleaner during sample windows than during peak holiday loads.

A practical takeaway: on big-influx weekends — Easter, Feria, August — and especially after heavy rain that flushes streams to the sea, give river mouths and stormwater outlets a wide berth, regardless of what the most recent published rating says.

What the authors recommend

The paper closes with two concrete asks. First, every river that flows into the sea should get a NÁYADE sampling point right at the mouth, not just along the bathing zone — because that's where inland spills actually arrive. Second, coastal towns whose summer population balloons past their treatment capacity need tertiary treatment (UV, chlorination, ozone, advanced filtration) added to their WWTPs. The economics, they argue, are obvious: in La Herradura, tourism is 57% of local income — the cost of upgrading the plant is a rounding error against the value of the beach staying clean.