River Corran and Mackerel Lake stream-June testing results.

The Laugharne Citizen Science team completed its June survey of the River Corran and Mackerel Lake stream on 11 June 2026, testing all nine monitoring sites across both watercourses. The results contain two significant findings: a sharp pH crash across the upper and mid Corran that mirrors what we recorded on the River Taf two days earlier, and an exceptional phosphate spike on the Mackerel Lake stream that requires urgent investigation.

The pH Crash — What the Numbers Show

The most striking result from the June survey is a dramatic fall in pH across the River Corran. In May, our readings were elevated but broadly within or near the safe range. This month, five of the six Corran sites recorded pH values at or below the threshold that protects aquatic life:

Site	May 2026	June 2026
AberCorran	9.2	5.4
GlanCorran	8.6	4.3
Horsepool Tributary	8.3	5.0
Hill Farm Stables	8.3	5.1
Horsepool Rd Bridge	8.3	5.4
Newbridge Rd Bridge	7.5	8.3 ✓

GlanCorran at pH 4.3 is the lowest reading we have recorded anywhere across all three catchments since monitoring began. The safe range for aquatic life is 6.0 to 9.0; below 5.0, fish mortalities become likely. To find readings of this severity across multiple consecutive sites on the same day is unusual and, understandably, alarming at first sight.

The one encouraging exception is Newbridge Rd Bridge — the furthest downstream freshwater site before the estuary — which actually rose slightly to 8.3. This likely reflects tidal buffering from the estuary.

Is the River in Danger? Why We Think Not

The most important measurement this month is not pH but dissolved oxygen. Despite the alarming pH numbers, dissolved oxygen across all five tested Corran sites is healthy: between 9.7 and 10.2 mg/L, which falls in the very healthy range.

This matters enormously for how we interpret the data. When pollution enters a river — slurry, silage effluent, sewage — bacteria rapidly break down the organic material, consuming oxygen in the process. Dissolved oxygen crashes, often to dangerous levels within hours. That is not what we are seeing here. Oxygen levels are strong, and ammonia — another key indicator of organic pollution — is within safe limits at every site.

The Mackerel Lake Stream — A Different Problem

While the Corran pH story appears to be natural in origin, the Mackerel Lake stream is showing something that requires a different kind of investigation.

Phosphate readings at two of the three stream sites have spiked sharply this month:

Site	May 2026	June 2026
Llandawke (Rectory Farm)	0.16 mg/L	0.43 mg/L
Broadway Cav Park	0.07 mg/L	0.54 mg/L
Culvert	0.11 mg/L	0.28 mg/L

To put Broadway Caravan Park's 0.54 mg/L in context: this is the highest phosphate reading we have recorded anywhere across the River Taf, River Corran, and Mackerel Lake stream in all our monitoring to date. Readings above 0.30 mg/L are associated with algal bloom risk and are typically caused by sewage or agricultural inputs. Llandawke at 0.43 mg/L is the second highest in our dataset.

What makes this particularly interesting — and worth investigating carefully — is the pattern. The highest reading is in the middle of the stream at Broadway, not at the headwaters or the outflow (Culvert). This suggests there may be a localised input somewhere in the Broadway reach rather than a source flushing down from upstream. Possible causes include drainage infrastructure at the caravan park, surface water connections, or concentrated runoff from nearby land following recent rain.

We are not raising an alarm — ammonia and nitrate at all three Mackerel Lake stream sites are within normal bounds, and dissolved oxygen was not tested this month as our single DO meter is prioritised for the Corran. But we will be returning to this reach before the July survey to investigate, and the July results will be critical in determining whether this is a one-off spike or the beginning of a pattern.

We will publish our follow-up results as soon as they are available.

(Comment by Claude AI)