Permits: Permission to Pollute? A local case study on the River Mole of how regulation is failing one small stream.
- 2 days ago
- 4 min read
This month, we're highlighting the parlous state of one small stream in our catchment: Hookwood Common Brook near Horley. The likely cause is effluent discharging from a private sewage treatment plant which holds a legal permit to discharge into the stream. When water levels fall low, the concentration of pollutants rises to toxic levels. It is only through our testing that this has been uncovered.
Troubled Waters: Hookwood Common Brook and the Permitted Outfall
Over the past two years, as across the wider Mole catchment, we have been testing water quality in Hookwood Common Brook – measuring phosphate, ammonia and nitrogen alongside conductivity and temperature. High summer pollution spikes are unfortunately common across the catchment, but our latest July results from Hookwood Common Brook are among the worst we’ve seen.
Results for Hookwood Common Brook have shown particularly alarming spikes in pollution levels when compared to neighbouring catchments and the catchment average.
This small stream, a valuable habitat in its own right and a tributary to the River Mole, predominantly drains woodland and grazing land. A number of upstream sites hold Environment Agency permits to discharge effluent into the brook from septic tanks and a small package sewage treatment plant. Permits should mean the stream is able to achieve "Good" water quality status. Yet our results show that during dry weather, the brook is under severe pollution pressure, especially when natural flows shrink to a trickle.
During dry periods, what remains of the brook is dominated by effluent. Upstream of identified sewage treatment assets the brook was reduced to a trickle. Downstream of the sewage processing assets the brook was flowing, albeit at very low flow.
Our findings are stark, and they raise serious questions about how pollution permits are issued and enforced.
Alarming results
Our monthly results, show particularly large spikes in ammonia levels during dry months in Hookwood Common Brook. On our most recent survey in July, we recorded ammoniacal nitrogen levels of 2.54 mg/l upstream of the discharge asset and a staggering 10.11 mg/l downstream. Phosphate levels were also alarming, exceeding the maximum 2.50ppm that our Hanna meters can record. These figures are not just poor – they are classed as “Bad” water quality under the Water Framework Directive. For context, healthy streams in the "Good" water quality category, typically sit below 0.20ppm phosphate and below 1 mg/l ammonia.
A Thames Water independent test, taken after we raised the alarm, recorded 7.49 mg/l downstream, confirming the problem. Dissolved oxygen levels were also low – another sign of stress to aquatic life.
At these concentrations, ammonia is toxic to fish and invertebrates, causing lethal and sub-lethal effects. In simple terms: Hookwood Common Brook is being poisoned.
Permitted Pollution?
Our research shows that the package treatment plant identified upstream is operating under an Environment Agency permit that allows it to discharge effluent containing up to 20 mg/l ammoniacal nitrogen, with a maximum volume of 20m³ per day. On paper, the plant is legal and compliant. In practice, however, the permit effectively licenses pollution to levels that destroy river health.
The fundamental flaw is that the EA permit takes no account of seasonal low flows. In summer, Hookwood Common Brook shrinks to a trickle. When effluent makes up most of the flow, even “permitted” levels create hazardous concentrations downstream. In such conditions, the stream stands no chance of achieving “Good” ecological status, as required by law.
A wider problem
This is not an isolated case in Hookwood. Since 2010, funding cuts have stripped the Environment Agency of the staff and resources needed to carry out routine monitoring and inspections. Permits may have been handed out under “standard rules” that may be wholly unsuitable for the receiving waterbody, and once issued they are rarely revisited, particularly in recent times. In these circumstances it falls to dedicated citizen science groups like ours to discover the shortfalls of current permitting and endeavour to raise the alarm.
The EA has told us they cannot devote further resources to this incident unless the treatment plant is proven to be malfunctioning. In other words: if the system is working “as designed”, then turning a brook into an effluent channel is considered acceptable.
Why it matters
Hookwood Common Brook is a small stream – but small streams matter. They are the capillaries of our river system, supporting wildlife and feeding into larger rivers downstream.
Their smaller volumes make them far more vulnerable to pollution spikes, and when permits allow them to be reduced to drains for poorly treated sewage, both local biodiversity and national Water Framework Directive targets are undermined.
What needs to change
Permits must be fit for purpose. They should be based on the capacity of the receiving waterbody, not on arbitrary “standard rules” that ignore low flow conditions.
Monitoring must be restored. Without regular independent checks, permits become licences to pollute.
Treatment plants, just like larger sewage works, must be maintained and upgraded if necessary. Package plants and septic tanks may meet permit limits, but these are sometimes inadequate for sensitive streams.
As it stands, the permitted sewage treatment asset upstream in Hookwood Common Brook is legally allowed to turn the stream into a lifeless ditch. That cannot be right.
We will continue to monitor the brook and press for urgent reform of a permitting system that, at present, gives permission to pollute.
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