The diffuse pollution challenge

Diffuse sources of water pollution are becoming increasingly significant, thanks to success in tackling the traditionally dominant sources such as industrial and sewage effluent. The new EC water framework Directive, with its target of good ecological quality, will add to the pressure. A change of regulatory focus will be needed if diffuse sources are to be controlled effectively, and the new culture will have to extend to water companies, Ofwat and local authorities.

The water industry has spent some £18 billion since privatisation on reducing the impact of sewage effluent discharges and sewer overflows on water courses. Other industries have also spent substantial sums - and the result is that the quality of rivers is now better than ever.

In England and Wales, some 91.8% of rivers were classified as "good" or "fair" in 1999, compared with only 85% in 1990 (ENDS Report 308, p 12 ). Scottish rivers have shown a similar improvement, so that some 97.5% of rivers are now of good or fair quality.

While the environment agencies expect further improvements through continuing investment by industry, they are increasingly aware that some improvements will be achieved only by making new efforts to deal with more elusive pollution sources.

In Scotland, where investment in sewage treatment has lagged behind that south of the border, sewage discharges still account for 34% of polluted river lengths, but only 2.1% are due to industrial discharges. However, the Scottish Environment Protection Agency (SEPA) ascribes a massive 46% of polluted river lengths to diffuse sources, including agricultural runoff, urban drainage and forestry inputs.

Comprehensive review
Three years ago, SEPA and the Environment Agency agreed to commission a review of diffuse pollution, under the auspices of the Chartered Institution of Water and Environmental Management and the International Water Association.

The resulting report was launched at a conference in London in October. 1 It underlines the increasing importance of diffuse pollution. In part, this reflects the successful control of point sources - such as unsatisfactory sewer overflows, sewage effluents or industrial discharges - which has exposed previously undetected pollution from less obvious sources.

But the report also suggests that diffuse pollution is a growing problem because of increasing agricultural intensification and sprawling urban and industrial development.

The report gives a comprehensive overview of diffuse pollution issues and impacts in the UK. Although these can broadly be considered to come from agricultural, forestry and urban land uses, it takes a more detailed look at sources of individual pollutant types (see table

).

Widespread agricultural inputs
The diversity of sources is bewildering and one of the report's achievements is to catalogue these and describe their impacts. The most widespread effects are from agricultural inputs of nutrients, pesticides and silt. Phosphorus inputs - often in soil particles from arable land or erosion of river banks - cause eutrophication and the attendant problems of algal blooms, declines in oxygen levels and reduced biodiversity. Nitrate from fertilisers pollutes ground and drinking waters and adds to the enrichment of coastal waters.

The report notes that nitrate levels in rivers have increased markedly since the 1930s. Although this is sometimes cited as evidence of increasing diffuse pollution, there is also evidence of a stabilisation of levels since 1990.

Water pollution with soil particles and phosphorus from agriculture increase water companies' treatment and filtration costs. One study quoted in the report estimated the capital cost to the industry at some £69 million between 1992 and 1997, with a further £4 million in operating costs. Remedial costs in reservoirs were estimated to cost an additional £4 million.

The costs of eutrophication are not limited to water companies. One case study showed that the cost of an algal bloom in Loch Leven, Fife, in 1992 amounted to £673,000 for local tourist businesses and £110,000 for a local trout fishery. Local industries, particularly paper mills abstracting from the river Leven, reported costs of £160,000 due to abstraction problems and blocked intakes.

Industrial wastes, farm manures and sewage sludges spread on land are also a source of diffuse pollution, particularly if spreading is carried out when the soil is waterlogged or prior to heavy rain. When washed into watercourses the biochemical oxygen demand of such wastes may kill aquatic life and the nutrient content can contribute to eutrophication.

The report expresses concern about the increasing use of systems such as tractors with low pressure tyres and umbilical spreaders that overcome the problems of taking heavy machinery on to waterlogged soils. The authors fear that these systems will increase the capacity for spreading when soil and weather conditions are likely to be unsuitable.

Pesticide inputs
Diffuse pesticide inputs are a major concern for water companies. At the launch of the report, Dr Peter Spillett, Thames Water's environmental and quality manager, told delegates that removing pesticides was "one of the most expensive" water treatments the company had to deliver.

Although amenity uses of herbicides such as diuron pose problems for drinking water suppliers, Thames' biggest issue at present is the agricultural herbicide isoproturon. The product, which controls weeds in winter-sown cereal crops, occurs more frequently than any other pesticide above the 0.1 microgram per litre drinking water limit.

A stewardship programme for isoproturon backed by its manufacturers Aventis and farming organisations in recent years has promoted voluntary restrictions on application rates and tried to avoid applications late in the autumn when rainfall is likely to wash the compound into drains and watercourses. However, Dr Spillett said that the programme had failed to deliver any long-term improvement in pollution levels.

Increasingly, wash-off from farm spray equipment, surfaces where products are mixed and empty containers are also being blamed for pesticide pollution. Research suggests that up to 50% of pesticide in streams could come from such farmyard sources (ENDS Report 290, pp 9-10 ). The report suggests that this could translate to 30-40% of pesticide detections above 0.1 micrograms per litre.

Pesticides also have major effects on aquatic life. A survey of wildlife in Welsh rivers in 1997 suggested that 5% of streams in sheep farming areas - 700 kilometres in Wales - may be impacted by sheep dip pollution.

The Environment Agency's diffuse sources manager, David Griffiths, believes that "tougher binding rules" will be needed to stem diffuse pollution from agriculture. He told ENDS that only a tiny minority of farmers could lay their hands on a copy of the Ministry of Agriculture's non-statutory code of good agricultural practice for the protection of water. One way to go, he suggested, would be to link the payment of agricultural subsidies to a test of environmental compliance.

Urban sources
Drainage from industrial estates is one of the most common and pernicious causes of diffuse urban pollution, according to Brian D'Arcy of SEPA, one of the report's authors. Such estates are a major feature of new towns and new industrial development. They accommodate a great variety of small and medium businesses. However, planners typically provide foul sewer connections only for toilet and canteen facilities.

"All yard areas where firms carry out activities like materials handling and steam cleaning drain directly to surface waters", Mr D'Arcy observed. He said one solution would be to designate wash-bays with foul sewer connections, but re-routing drains was an expensive and disruptive business. Areas of hard standing draining to sewer also risk overloading the hydraulic capacity of sewage treatment works because of their ability to collect rainfall.

A study of streams draining 22 industrial estates in Merseyside in 1984 found almost all caused major deterioration in the biological quality in watercourses, Mr D'Arcy said. Another study of three estates in Scotland in 1996 using the Microtox bacterial toxicity test found high or moderate toxicity in drainage in 58%, 22% and 30% of samples.

"One solution is pond systems for surface water at the bottom end of [industrial estate] sites - this allows remedial measures such as booms to be used in case of an accident," Mr D'Arcy suggested. Another option - widely adopted by companies such as Tesco and Ford - was the use of permeable block paving, which allows rainwater to filter through to the soil, minimising both runoff and puddles.

Sustainable drainage
The use of such novel solutions to drainage issues is part of a growing movement towards sustainable urban drainage, being promoted to planners and local authorities by more forward-looking regulators such as SEPA. SEPA's east region is currently trying to interest Edinburgh City Council in sustainable urban drainage systems for a major new development to the south east of the city.

Mr D'Arcy said there were already about 100 sites in Scotland with various kinds of filter drains, such as gravel trenches alongside trunk roads, which provide some filtration and biodegradation potential for runoff before it enters surface waters or infiltrates into groundwaters.

Swales - gently sloping grass areas designed to perform similar functions - are an alternative which has been little used to date because of their larger land take, he said. Swales need to be several metres wide to be effective, he added, and some constructed in the UK had been poorly designed.

Water companies, which are responsible for drainage except for highways, should accept the need for systems for treating low-level contamination, Mr D'Arcy believes. However, it is not a view widely shared in the water industry. Howard Brett, a principal scientist at Thames Water who attended the launch of the report, expressed a personal view that the answer lay in more separation of contaminated sources of runoff rather than novel pollution attenuation systems.

"We don't provide any treatment for surface waters which should therefore be clean.?When we charge for treatment then it becomes trade effluent," Mr Brett told ENDS. The idea of water companies providing systems for dealing with low levels of contamination in urban runoff raises many questions, he said: "How could we design systems for that? What can we realistically do? And what can we charge for?"

Clearly, if water companies are to get involved with systems to deal with polluted runoff, then some new thinking will be needed. One crucial issue for companies is whether industry regulator Ofwat will accept that sustainable drainage systems could be considered as assets and be funded under the industry's asset management review process. The issue is a key one because the systems may not be particularly expensive to build, but will certainly require regular maintenance.

An Ofwat spokesman said that it acknowledged the need to discuss new and innovative drainage systems, not only with water companies but with the Agency and the Department of the Environment, Transport and the Regions. However, such discussions were still "at an early stage".

Organisational issues
The new EC water framework Directive, with its target of "good environmental quality" (ENDS Report 305, pp 50-52 ), will provide an imperative to address diffuse pollution sources. Some regulators believe that this will mean a major shift of focus away from consenting and compliance activities, traditionally the bread and butter of water pollution control. They believe this will be a major challenge and involve a change in culture of the environment agencies.

"You can't resolve [diffuse pollution] by sending an inspector out to say 'stop it'," Mr D'Arcy observed. "You can't stop a farmer ploughing his field."

The options for the control of diffuse pollution are three-fold:

lTighter regulation, for example in planning and farming.

  • Economic instruments, such as taxes on fertilisers and pesticides.

  • Public persuasion.

    Examples of the latter include the Oil Care public education campaign launched by the environment agencies and oil companies on the safe disposal and recycling of waste motor oils, and the water industry's Bag It And Bin It campaign which seeks to prevent nappies, condoms and other non-degradable wastes being flushed into the sewers.

    One of the prime movers behind SEPA's strong focus on diffuse pollution is the East Region Director, Willie Halcrow. He explained how he thought diffuse pollution control should be brought within the Agency's regulatory activities: "I'm a great believer in measurement of results. What do you call core business? It's about improving the environment. Pollution control officers should have a dual role. Effective environmental improvement is about regulating those things worthy of regulation while promoting those things that are good and giving advice on best practice."

    Uphill battle
    But not everyone is convinced that diffuse pollution is a significant issue. One omission from the report is strong quantitative evidence of exactly how important diffuse pollution is. The 46% of polluted Scottish river lengths affected by diffuse sources may be unrepresentative of the situation in England and Wales.

    The Environment Agency told ENDS that it intended to release detailed figures on diffuse pollution next year - complete with explanations of how the data were derived and caveats about how they should be interpreted. However, at the launch of the report the Agency's head of water quality, Dr Martin Griffiths, revealed that only some 12.5% of English and Welsh rivers were downgraded due to diffuse inputs. Some 80% of these were affected by pollution of agricultural rather than urban origin.

    However, David Griffiths (no relation), the Agency's diffuse pollution manager - a relatively recently created post - explained that these figures were flawed and were likely to underestimate diffuse pollution: "They're based on a spot sampling regime designed to look at compliance - mostly between 10am and 3pm. Diffuse sources are intermittent and are not amenable to being treated in that way. We can tease out [pollution] due to known point sources and look at what remains, but it will take a major shift in the type of monitoring we do to fully reflect diffuse sources."

    A recent paper by researchers from the University of Hertford supports Mr Griffiths' viewpoint. 2 The researchers made a detailed study of Pymmes Brook, an urban watercourse in north London whose quality is classified under the Environment Agency's chemical general quality assessment as "fairly good", class C.

    However, they found that the Agency's practice of monitoring during low-flow conditions, when point source pollution is likely to be most acute, underestimated sources such as sewer overflows and diffuse pollution from surface water drains. These are likely to be worst during heavy rain, and be intermittent depending on events in the catchment.

    The researchers concluded that the monitoring system was "seriously flawed" in its application to urban watercourses, resulting in the problems of such waters being "greatly under-estimated". The Agency's biological assessment showed that the urban stretches of the brook were significantly worse than expected, being "poor" or even "very poor".

    The report's authors believe urban diffuse sources will become "increasingly prominent" as the current round of water industry investment proceeds. And the Agency told the recent House of Commons Environmental Audit Committee inquiry (see pp 30-31 ) that it was changing the shape of its regulatory departments to focus more on diffuse sources - a policy which the DETR supports, particularly where bathing water quality or river quality objectives are threatened.

    However, the Environment Agency still appears to be divided over how significant diffuse urban impacts really are. Ashley Holt, an Agency water quality planning advisor, commented that the picture varied across the regions of England and Wales, but that diffuse urban impacts were often "vanishingly small". The largest effects were likely to be from inland cities in the north, where investment in improving point source discharges was still having a major impact on water quality.

    The Agency has been in the process of a fundamental review of water monitoring for two years (ENDS Report 295, p 5 ) and part of that process will consider diffuse pollution, Mr Griffiths says. However, he admits the debate is in the early stages and there are some in the Agency who are not yet ready to accept the importance of diffuse pollution.

    "Some in the Agency might hold the view that all the watery issues are covered and they are all passé. We are trying to convince our colleagues of the importance of diffuse sources but sometimes it is like rolling a pebble uphill."

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