Mapped: Aerial imagery shows M25 runoff ponds have not been maintained for decades

This is despite guidance recommending they be cleared every 10 to 15 years and National Highways having “a statutory duty to ensure that discharges from its network doesn’t cause pollution for the surrounding environment”, according to the government.

An investigation by NCE using data from a Freedom of Information Act response combined with historical imagery from Google Earth found that many of the 94 ponds around the M25 designed to be regularly emptied have not been maintained for decades.

The data from the FOI response showed that the roads body did not have or would not reveal information relating to the last time 87 of the 94 ponds had been maintained. This means that they are less effective in preventing floods and are likely filling up with hazardous chemicals from road runoff, which could be entering watercourses. Academics and NGOs with experience in this field told NCE that National Highways is likely not investigating them in order to avoid liabilities.

Stormwater Shepherds is a not-for-profit working to improve the health of waterways and its  director of operations UK Jo Bradley said: “The runoff ponds that were built at the time that the M25 was built [1960s to 1970s] will be flood storage ponds, built to attenuate flows off the motorway, to manage the peak flow of the runoff during rain events and to manage the volume of runoff entering local streams and rivers.

“However, their role in pollution control was recognised and well understood. The guidance published in 1993 contained design details relating to pollution capture and retention, and described the function and design of oil separators.

“They are important because the construction of a motorway changes the hydraulic behaviour of rainfall across the catchment by introducing a large impermeable area, increasing flood risk, and often draining all the rainfall to one discharge point.

“This would lead to increased flood risk, increased pollution and increased risk of erosion and physical harm to the river unless these ponds/reservoirs were installed.”

In May 2024, Stormwater Shepherds and CIWEM (the Chartered Institution of Water and Environmental Management) published a report which found National Highways and other highways authorities in England have no permits in place for managing the pollutants which are washed off their roads.

The report described road runoff as “hazardous”. It said: “Research by the Environment Agency and [National Highways] has identified a number of pollutants commonly found in highway runoff: copper, zinc, cadmium, fluoranthene, pyrene and polyaromatic hydrocarbons (PAHs).

“These are identified as specific pollutants, priority substances or priority hazardous substances (substances of major concern for European waters due to their toxicity, bio-accumulating properties and/or persistence in the environment that under the Water Framework Directive and its regulations should be progressively reduced or eliminated from surface water).”

Locations of the M25 runoff ponds

Using coordinates provided by National Highways in its FoI response, NCE can reveal the locations of the 94 attenuation ponds aroudn the M25 and through historic aerial imagery the construction dates within certain time period, and maintenance or lack thereof, can be identified.

Below is a map showing the locations of all the ponds listed by National Highways. The full location dataset is at the bottom of this article.

It’s unclear why there is a gap in the north part of the highway and there are some ponds visible on Google Earth around the M25 which look similar to the ones labelled which were not included in the list sent to NCE.

In her analysis of the state of the ponds based on historic satellite imagery, Bradley highlighted the Ciria (Construction Industry Research and Information Association) B14 document: Design of Flood Storage Reservoirs.

The document states: “Sediment removal from within the reservoir or adjacent to inlet zones should be timed so as to cause minimum disturbance to wildlife, and ideally should be undertaken in stages over a few years to allow recolonisation from the undisturbed areas; plants removed during maintenance should be transplanted whenever possible.

“Sediment dredging should take place every 10-15 years.”

Bradley said the Ciria document “was the Bible for designing flood management ponds”.

“That’s what [National Highways] should have been working to; 10 to 15 years maintenance intervals,” she added.

Bradley said features like sediment forebays in or near ponds, and the ponds themselves “would have been constructed and designed with a calculated volume”.

“An engineer would have said ‘Okay, this is the flow rate. This is the residence time that we need behind the sediment forebay, or this is the volume that we need in the sediment forebay’ and they would have worked that out,” she explained.

However, the images of overgrowth on the ponds suggests that these calculations have not been used to inform their monitoring or maintenance requirements.

Beggars Lane pond

The pond at Beggars Lane, which is between junctions 5 and 6 near Westerham in Kent, starts in 1960 as a triangular shaped body but vegetation starts to encroach over the years because of the ingress of sediment from the outfall from the M25.

“That pond is no longer functioning as it was designed to function,” Bradley said.

“Because it had a functional volume that was calculated by the engineer and it no longer functions with that functional value. Now, you could argue that it still did a good job because the vegetation will capture and treat a lot of the pollution and that’s true.

“But that’s not how it was designed to function and nobody’s monitoring it to see if it’s still functioning.

“They’re not being maintained in accordance with their design, [Beggars Lane pond has] not been maintained for years, and they’re supposed to be maintained every 10 to 15 years. And so, although they are still functioning to an extent, there’s no evidence [regarding] whether or not they’re functioning to an acceptable extent. They are not functioning as they were designed to function.

“There’s hundreds and hundreds of tons of [unwanted material] in there that somebody should be taking away and treating, but they’re just not doing it.”

New Pond

New Pond is between junction 13 and 14, near Heathrow Airport.

Judging from the historic imagery, Bradley said the pond “seems to have just completely filled with vegetation”.

There are several different features visible at and near the pond, one of which appears to be a sediment trap, but Bradley said “if that’s supposed to be sediment trap that isn’t working anymore”.

More historic images available on Google Earth of New Pond show activities taking place around the feature, including some clearances, but in the present day, vegetation appears to be thriving, which indicates the presence of large volumes of sediment.

Explaining why some but not all of the vegetation could have been removed, Bradley said: “Sometimes [National Highways commissions] condition surveys, more from an ecological point of view than anything else.

“Sometimes they might go in and go ‘There’s an invasive weed we need to clean the invasive weed’. Or they might go in and say, ‘We need to take this vegetation back to make it a better habitat’.

“The contractor who does the condition survey might come back and say ‘We need to do this piece of work and it’ll cost you £50,000’,” and Bradley said National Highways was likely to commission some work but not others, due to costs.

Blackdale Fann Pond

The images show the construction and use of Blackdale Fann Pond, which is near Blackdale Farm and sits between junctions 1b and 2 near Dartford, south of the River Thames.

Asked why the pond looks black next to other bodies of water nearby which are a different colour, Bradley said it’s because of “contaminated sediments”.

“It’s just full because the separators haven’t been emptied and they’re huge,” she added. “So they’re going to cost a lot of money to empty and the pond is full of motorway sediment. That’s why it’s that colour.”

Response from public bodies

National Highways repeated the response it had provided for the story NCE published earlier in July 2024.

National Highways director of environmental sustainability Stephen Elderkin said: “We are committed to addressing the risk of pollution from road runoff. Our Water Quality Plan 2030 sets out a high-level programme of work to achieve this. We’ve also invested in a programme of research to understand the risk of pollution from microplastics in road runoff.

“High quality road infrastructure can support positive environmental outcomes, and that is at the heart of our work in this area.”

National Highways is a executive non-departmental public body of the Department for Transport.

A Department for Transport spokesperson said National Highways “has a statutory duty to ensure that discharges from its network doesn’t cause pollution for the surrounding environment”.

The spokesperson also pointed to the fact that monitoring of environmental quality standards (EQS) and decisions on permitting are a matter for the Environment Agency.

EQS are limit values set by regulation to manage the concentrations of defined harmful substances, and for water quality, these are set by the Water Framework Directive which is a piece of EU legislation transposed into UK law.

Samples collected from three strategic road network outfalls and six local authority highway outfalls and analysed by CIWEM and Stormwater Shepherds found that discharges from the outfalls “fail the EQS for several PAHs (polyaromatic hydrocarbons)” which “consistently exceeded their maximum allowable concentrations”.

In one instance it was found that PAH concentrations were 730 times higher than the allowed limit.

PAHs are “priority hazardous substances under the Water Framework Directive, which identifies them as substances shown to be of major concern for European waters due to their toxicity, bio-accumulating properties and/or persistence in the environment,” according to the report.

The Environment Agency comes under the remit of the Department for Environment, Farming and Rural Affairs (Defra).

Defra did not respond to a request for comment.

Other ponds showing a lack of vegetation clearance

Below you can find a series of images of other ponds which have clearly not been emptied of sediment because of the presence of vegetation.

The Ciria document and Bradley’s analysis indicate that they are all not being maintained according to the relevant guidance, instead being left to fill up with potentially hazardous waste.

Spur Road Pond

Spur Road Pond is between Junction 4 and the junction with the A21 near the village of Badgers Mount in Kent.

Godstone Roundabout, Godstone Pond No. 1 and Godstone Pond No. 2

The Godstone ponds are all located on or just east of Junction 6 near the village of Godstone in Surrey.

Pavilion Pond and Flooding Pond

They are between Junctions 10 and 11 near the village of Byfleet in Surrey

Santerbury Offslip Pond and Santerbury Onslip Pond

These are at Junction 1A south of the Dartford Crossing in Kent.

Alderbourne Pond

Alderbourne Pond is near Junction 16 near Pinewood Studios which is in Buckinghamshire.

Interceptor 78/0A and Interceptor 78/0B

The Interceptor 78/0A and Interceptor 78/0B ponds are between junctions 10 and 11 in Surrey.

London Road

London Road pond is near the village of Otford in Kent, between junctions 4 and 5.

Pavillion Pond and Flooding Pond

The ponds are between junctions 10 and 11 in Surrey.

Full location data regarding the ponds sent to NCE by National Highways under the Freedom of Information Act:

Asset name (Pond)	Pond Asset Reference	Pond Condition Inspection Date	Easting (region node)	Northing (region node)	Last Sediment Removal * Maintenance Year (or not yet required
Catherine Borne	TL2101_6154a	31/03/2023	521614.84	201542	01/03/2023
Warwick Wold Pond	TQ3152_4297b	22/06/2022	531423.71	152970.3	2022
River Mole East	TQ1458_0438a	29/06/2022	514040.58	158380.6	2022
Flint Cottages No. 1	TL0701_7432a	21/08/2023	507746.12	201322.1	2023
Godstone Roundabout	TQ3552_1777a	07/06/2023	535172.8	152771.6	2024
River Mole West	TQ1358_8835b	29/06/2022	513880.75	158356	2024
Stubbs Pond	TQ0798_0395b	21/08/2023	507034.47	198950.5	Due 2023, delayed
Pond 1835	TQ5781_5895a	01/06/2022	557576.08	181950.6	Not yet required
Interceptor 78/0B	TQ0463_9216c	05/02/2023	504921.38	163163.7	Not yet required
New Pond	TQ0374_3696b	05/03/2023	503359.58	174962.4	Not yet required
Santerbury Onslip Pond	TQ5574_8291b	05/06/2022	555826.43	174914.7	Not yet required
Pond 1812	TQ5884_2912a	05/06/2022	558292.52	184125.6	Not yet required
Summit Farm 1	TQ5471_9816b	05/11/2022	554986.37	171161.1	Not yet required
Hawley Road Infiltration Pond	TQ5471_9495a	05/11/2022	554939.35	171952	Not yet required
IUS Pond 4B	TQ0291_1718a	08/04/2022	502168.37	191182.6	Not yet required
IUS Pond 5	TQ0292_4794a	08/04/2022	502471.77	192946.7	Not yet required
IUS Pond 1698	TQ5494_8504a	08/07/2023	554862.48	194036.1	Not yet required
IUS Pond 1683	TQ5395_8617a	08/07/2023	553860.34	195169.3	Not yet required
IUS Pond 1727	TQ5692_8711a	08/07/2023	556874.76	192118.5	Not yet required
IUS Basin 1605	TQ4799_6768a	08/07/2023	547666.51	199689.3	Not yet required
IUS Basin 1609	TQ4799_9447a	08/07/2023	547953.68	199466.3	Not yet required
Pond 1824	TQ5783_8821a	08/07/2023	557893.51	183214.6	Not yet required
IUS Pond 8	TQ0497_2344a	08/11/2022	504234.46	197440.9	Not yet required
IUS Pond 31A	TL2101_7125a	08/11/2022	521711	201252.3	Not yet required
IUS Pond 31B	TL2101_6835c	08/11/2022	521682.32	201351.2	Not yet required
Black Horse Lane	TL2102_0717a	08/11/2022	521068.88	202177.7	Not yet required
London Road	TQ5058_6653b	13/05/2022	550664.16	158536.9	Not yet required
IUS Pond 22 (NEW)	TL1402_9246a	15/08/2022	514923.49	202466.7	Not yet required
IUS Pond 21	TL1402_4442a	15/08/2022	514445.27	202412.5	Not yet required
IUS Pond 25 - NEW	TL1602_6769a	15/08/2022	516671.39	202697.9	Not yet required
Soakaway Six (The Stacks)	TL1103_5112c	15/08/2022	511528.98	203130.4	Not yet required
Soakaway Three (The Stacks)	TL1103_7917e	15/08/2022	511793.61	203173.1	Not yet required
Soakaway One (The Stacks)	TL1103_8817f	15/08/2022	511889.13	203175.9	Not yet required
Woodys Dell	TQ0598_8318a	16/08/2022	505832.75	198185.9	Not yet required
Quarry Pond	TQ0799_3017a	16/08/2022	507302.59	199175.8	Not yet required
Amersham Road Pond	TQ0187_6253a	16/08/2022	501633.6	187529.4	Not yet required
Chevening Lagoon	TQ4856_9453a	17/06/2022	548938.49	156534.8	Not yet required
IUS Pond 1791	TQ5886_7937a	17/08/2023	558785.99	186373.5	Not yet required
IUS Pond 1763	TQ5888_3993a	18/07/2023	558395.28	188934.2	Not yet required
IUS Pond 1776	TQ5887_8666a	18/07/2023	558862.35	187665.2	Not yet required
Mill Pond	TQ4655_4759n	19/05/2023	546471.36	155589.6	Not yet required
King George Field	TQ5571_4057d	21/06/2023	555403.45	171570.9	Not yet required
Summit Farm 2	TQ5471_8807b	21/06/2023	554880.83	171072.6	Not yet required
IUS Pond 16	TL1002_5286a	21/08/2023	510511.57	202857.2	Not yet required
IUS Pond 15	TL0802_9876a	21/08/2023	508990.72	202769.2	Not yet required
Flint Cottages No. 2	TL0701_7320b	21/08/2023	507731.4	201200.4	Not yet required
Brasted Gate	TQ4655_4471a	22/06/2022	546447.5	155714	Not yet required
Beggars Lane	TQ4555_3912a	22/06/2022	545392.67	155125.8	Not yet required
Santerbury Offslip Pond	TQ5575_8105c	24/08/2023	555814.91	175057.8	Not yet required
IUS Pond 3	TQ0188_5522a	25/08/2023	501554.15	188214.9	Not yet required
Blackdale Fann Pond	TQ5572_4245a	27/05/2022	555413.43	172452.1	Not yet required
Interceptor 78/0A	TQ0463_8813d	27/06/2022	504879.58	163132.8	Not yet required
Interceptor 78/2B	TQ0463_8723b	27/06/2022	504875.81	163238.4	Not yet required
Allotment Gardens	TQ0562_1861c	27/06/2022	505180.12	162620.4	Not yet required
QE2 Pond	TQ5777_5246d	29/08/2023	557524.91	177467	Not yet required
Brook Pond	TQ5782_9295b	29/08/2023	557919.56	182953.4	Not yet required
IUS Pond23	TL1502_2553b	First visit due 2024	515261.01	202536.8	Not yet required
IUS Pond 27B	TL1803_7719c	First visit due 2024	518773.05	203191.6	Not yet required
IUS Pond 27A	TL1903_1937a	Unknown	519193.93	203383.1	Not yet required
M25 84/4A	TQ0168_5853a	05/02/2023	501588.4	168533.7	Not yet required.
86/5A	TQ0170_5359a	05/02/2023	501529.67	170601.6	Not yet required.
Pavillion Pond	TQ0561_5459b	05/02/2023	505547.66	161592.6	Not yet required.
Flooding Pond	TQ0561_5537b	05/02/2023	505553.25	161377.6	Not yet required.
Interceptor 76/6A	TQ0561_5393b	05/02/2023	505534.47	161926.2	Not yet required.
66/0	TQ1358_8529a	05/02/2023	513859.37	158294.1	Not yet required.
86/9A	TQ0171_5106a	05/03/2023	501516.21	171061.2	Not yet required.
Godstone Pond No. 2	TQ3552_3380b	07/06/2023	535335.82	152802.5	Not yet required.
Interceptor 85/3A	TQ0169_5440a	08/04/2022	501547.73	169405.6	Not yet required.
Spur Road Pond	TQ4962_0492b	08/11/2023	549041.84	162922	Not yet required.
Godstone Pond No. 1	TQ3552_3377b	13/05/2022	535333.63	152776.5	Not yet required.
TQ3753_9261a	TQ3753_9261a	1st visit due 2024	537940.67	153578.3	Not yet required.
TQ3753_9260a	TQ3753_9260a	1st visit due 2024	537925.09	153603.5	Not yet required.
TQ0167_8969b	TQ0167_8969b	1st visit due 2024	501897.89	167692.9	Not yet required.
M20 Fast Entry Link	TQ5368_3693b	21/06/2023	553359.97	168936.3	Not yet required.
Redhill Bottom	TQ2853_9470b	22/06/2022	528943.91	153709	Not yet required.
Gatton Bottom	TQ2753_9268b	22/06/2022	527929	153684.8	Not yet required.
Pointers Pond	TQ0859_2920b	25/08/2023	508294.73	159206.9	Not yet required.
Costal Pond	TQ2154_3069b	25/08/2023	521302.61	154696.7	Not yet required.
Alderbourne Pond	TQ0185_8755a	25/08/2023	501876.78	185552.1	Not yet required.
TQ0659_3386a	TQ0659_3386a	25/08/2023	506335.72	159861.4	Not yet required.
Swanley Pond 1	TQ5267_8788b	27/05/2022	552870.25	167886.5	Not yet required.
Interceptor 74/4A	TQ0659_1988b	27/06/2022	506195.13	159885.7	Not yet required.
Interceptor 65/8A	TQ1458_0432c	27/06/2022	514048.35	158322.4	Not yet required.
Interceptor 84/5B	TQ0168_6655a	28/06/2022	501659.51	168556	Not yet required.
Interceptor 86/4B	TQ0170_6050a	28/06/2022	501582.3	170651.7	Not yet required.
Interceptor 87/4A	TQ0171_6843c	28/06/2022	501687.03	171438.7	Not yet required.
87/0B	TQ0171_5809b	28/06/2022	501587.36	171096.3	Not yet required.
West Hall Pond	TQ0560_9411a	28/06/2022	505939.58	160119.1	Not yet required.
Leatherhead Lagoon	TQ1658_6939a	29/06/2022	516690.37	158398.4	Not yet required.
Leatherhead Lagoon (second	TQ1658_6547b	29/06/2022	516657.06	158477.4	Not yet required.
M25 86/3A	TQ0170_5829a	29/06/2022	501580.38	170305.6	Not yet required.
Interceptor 73/6B	TQ0659_8261a	29/06/2022	506819.88	159610.1	Not yet required.
Muggeridge Wood	TQ1257_7297a	29/06/2022	512720.54	157973.1	Not yet required.
Pond 50/5A	TQ2652_7497a	First visit due 2024	526738.89	152967.9	Not yet required.
*Not all ponds require sediment removal due to the design of upstream silt capture devices. Sediment removal is based on condition inspection results.

 

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