31st March 2025 Environment Monitoring Results

Here are the results from the end-March sampling safari in the River Deben catchment. March has been one of the driest months on record for East Suffolk, with almost no rain in many of our sampling locations. Anyone travelling around will have seen the impact in the fields, with bone dry soil or with the need for irrigation on recently planted crops. Equally at some of our sampling locations, the weather conditions have led to dry riverbeds or to very slow flow. 

These conditions have had quite a dramatic effect on some of the results we are reporting, making interpretation rather challenging. 

We are reporting additional features this month to try to explain what seems to be happening in the catchment. So there is lot more to read and absorb in this bulletin.

1. E.coli Results - across the non-tidal catchment E.coli levels have dropped relative to February and earlier in March. The only exceptions are at or close to the Anglian Water sewage outfalls where E.coli emissions continue at high levels. It is noticeable that there are only a few “orange” indicators on the map with many locations showing no E.coli presence at all. One possible explanation is that with no significant rain, ditches and small streams have run dry and so there is no run-off into the river. Hence almost the only E.coli sources are from the AW STW’s or from private outfalls.

In the tidal section, there was a spring high tide on the day of sampling and E.coli levels were very low (except at Martlesham Creek). Some observers we spoke to had expected high levels due to wash from river banks but there is no evidence of this at all. The one outlier is a newly discovered outfall into Woodbridge houseboats are where E.coli levels were well above “safe”; the local harbourmaster had reported a sewage smell so we are going to monitor this pipe in future.

2. Phosphate - across the catchment levels of phosphate pollution were generally about the same as those for the previous samplings in February and March. The Anglian Water STWs with phosphate stripping installed all showed levels within 0.5 ppm and are well within the Environment Agency permit limits. The outliers at Rendlesham and Easton on the River Deben, at Henley on the River Fynn and at Martlesham Creek in the tidal section all  emit phosphate at 30 to almost 70 times the “healthy river” guideline. A meeting with Anglian Water about the Rendlesham phosphate issue is expected soon.

3. Nitrate - this month’s results show quite a dramatic drop in the level of nitrates in much of the River Deben, compared to February and March. The majority of results are within the 25 ppm limit set by the Water Framework Directive with many being at 10 ppm or less. One explanation is that nitrate leaching from agricultural sources and outflow from ditches has been negligible due to the lack of rain; the spikes on the chart attached are all from AW STW outfalls. Nitrate levels in the Rivers Lark and Fynn are by contrast relatively similar to earlier numbers.

4. Electrical Conductivity - measures the presence of inorganic dissolved solids (salts) in water. Conductivity is a good indicator of water quality and is measured in milliSiemens (mS). For normal river water the expected range is 0.3 to 1.0 mS. However for streams coming from clay soils, levels are higher due to the presence of materials that ionise when washed out by water. Similarly water from chalk aquifers have higher conductivity. Waste water from sewage treatment works also shows high electrical conductivity due to the presence of inorganic compounds (e.g chlorides, sulphates and nitrates) in emissions. Sea water with its high sodium choline (salt) content registers very high conductivity (over 10 mS).

For the River Deben, most samples are within the expected range with STW emissions being significantly higher. The samples from the source streams at Debenham and Aspall all show elevated electrical conductivity with many being over 2.0 mS. This may be explained by the clay soils and by the static or very low flows observed; the latter leading to concentration of minerals in the water. The samples for the Rivers Fynn and Lark are broadly within expected levels, although those closest to the Martlesham sluice show elevated numbers probably due to the intrusion of saline water at high tide.

Our evidence to date suggests these results can help us to quickly assess the river water quality on-the-spot without resort to detailed tests. Broadly the higher the electrical conductivity the more likely there is to be a pollution incident or other feature worth investigating. But care is needed to take account of the local geology and spoil types.

5. Total Dissolved Solids (TDS) - this measure is somewhat similar to electrical conductivity and measures the presence of dissolved organic and inorganic materials in the water. The primary sources of TDS in rivers are from farm and residential run-off, from soil leaching contamination and from sewage treatment plants. The expected range for rivers in good health is 300 - 600 ppm of dissolved solids; 900 - 1200 ppm is rated as poor and any reading over 1200 ppm is deemed unacceptable.

Our analysis suggests that almost all the River Deben catchment samples show TDS levels at or above those for a “health river” (so in the range 600 to 100) with some well above this. The numbers for earlier in March are somewhat lower suggesting that the lack of rain and the low flow have contributed to a concentration of these materials in the water. However, the lower nitrate levels run counter to these results, so more data is needed from future months to understand the trends.