Permits: Permission to Pollute? A local case study on the River Mole of how regulation is failing one small stream.
- Aug 24
- 5 min read
Updated: Sep 4
This month, we're highlighting the parlous state of one small stream in our catchment: Hookwood Common Brook draining into the Mole to the south west of Horley and just north of Gatwick Airport and the realigned stretch of the River Mole.
Hookwood Common Brook is a small subcatchment of the Upper Mole draining mostly grazing pasture fields separated by woodland strips and hedgerows.
Parts of Charlwood village extend into the catchment of 4.4km2. The main suburban and urban areas are on the A217.
The likely cause of toxic ammonia levels is effluent discharging from a private sewage "package" treatment plant which holds a legal permit to discharge effluent into the stream. In a perfect world this should safeguard the stream and, according to the Water Framework Directive, allow it to achieve Good water quality status. However, when water levels fall low, which happens quite regularly due to the clay catchment providing little sustained base flow, the effluent provides the majority of stream flow. This means that there is no dilution of effluent and the concentration of pollutants rises to toxic levels. It is only through our testing across the whole Mole catchment over the last 2 years that the appalling state of this stream 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 at a test site where the stream goes under the A217.
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. Much of the catchment is not on mains sewerage and a few residential dwellings and a hospital upstream hold Environment Agency permits to discharge effluent into the brook from septic tanks and a small package sewage treatment plant. As said, an EA Permit 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 from the package sewage treatment works. Upstream of the sewage treatment asset the brook was reduced to a trickle. Downstream of the sewage processing asset the brook was flowing, albeit at very low flow.
Our findings below make for stark reading, and they raise serious questions about how EA permits, designed to ensure water quailty in streams remains healthy, are issued and enforced.
Alarming results
Our monthly results, show particularly large spikes in ammonia levels during the recent dry months in Hookwood Common Brook. On our 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 typically allows the discharge of effluent containing up to 20 mg/l ammoniacal nitrogen, with a maximum volume of 20m³ per day and with no "visual impact" on the river. On paper, this means 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 or any expansion of the hospital. 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 to safeguard the Brook
The sewage plant permit allowing discharge into Hookwood Common Brook is clearly inadequate, resulting in declining water quality and toxic levels. The EA needs sufficient funding to effectively monitor local streams like this one. Currently, it seems that only citizen science testing, such as ours, can identify where permits are failing.
Permits must be appropriate for their purpose. Comprehensive monitoring should inform permits prior to issuance. These permits should be based on the capacity of the receiving waterbody rather than arbitrary "standard rules" that overlook low flow conditions. The permitting process should consider the nature of seasonal low flows, especially in our clay streams, many of which dry up completely in most summers, such as Gad Brook and Hookwood Common Brook.
Monitoring must be reinstated. Without regular independent assessments, permits become licenses to pollute. There is a significant lack of routine monitoring to ensure compliance with permits. Regular inspections of facilities using package sewage plants must be conducted by the regulatory body to ensure proper maintenance.
Treatment plants, similar to larger sewage works, must be maintained and upgraded when necessary. Package plants and septic tanks may meet permit limits, but these limits are sometimes insufficient for sensitive streams. Ideally, large buildings/industries, such as hospitals, should be connected to the main sewerage system to manage large sewage volumes. This should be considered during the planning phase of all developments to reduce risks to small tributaries. In this case, a main sewer is located not too far from the hospital.
Most importantly, permits should be established based on recent environmental conditions and future projections due to climate change and land use changes. For instance, 600 homes are planned for construction across parts of Hookwood Common Brook, further threatening sustained baseflow with increased runoff and changing local micro-climates.
Currently, the sewage treatment asset permitted upstream in Hookwood Common Brook is legally allowed to degrade the stream into a lifeless ditch. This is unacceptable.
We will continue monitoring the brook and advocating for urgent reform of a permitting system that currently grants permission to pollute.
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