Wednesday, 29 November 2017 10:14

Urban runoff – the forgotten polluter?

Jo Bradley, Market Development Manager, SDS Limited, takes a look at a neglected issue..... Rainwater that runs off impervious surfaces such as roads, roofs and car parks can carry with it hazardous pollutants.  Heavy storms pose not only a risk of flooding but the danger of delivering poisonous metals and hydrocarbons to sensitive rivers, streams and groundwaters.

flood - High risk discharges need extra treatment webMost environmental professionals are aware of the problem. Yet, while air pollution in our towns and cities is now firmly in the grip of the public consciousness, surface water pollution is frequently overlooked and inconsistently monitored, as well as being poorly controlled in the UK.  

Furthermore, infiltration of polluted water to the ground is considered acceptable for higher risk locations, in cases where such an approach is questionable at least; and certainly should be subject to closer scrutiny and monitoring.

Pounding an Environmental Beat
My career began more than 25 years ago and I earned my stripes pounding an environmental beat in Lancashire, investigating pollution incidents for what was originally the National Rivers Authority, later becoming the Environment Agency.  There can be little doubt that regulation has become more complex since those days and less consistently enforced.

During my career, I’ve become convinced that Sustainable Drainage Systems (SuDS) offer the best approaches to treating surface water runoff.  What’s more, the necessary professional knowledge, regulatory tools, drainage devices and technologies are already available; we just need to apply them.   

A year ago, I left the Environment Agency for a new life working for SDS, a manufacturer of proprietary surface water drainage. To some it might have seemed an odd move, but for me it was a logical way of encouraging practical and pragmatic delivery of SuDS for environmental protection.

floods feature Jo Bradley webRegulation
Much of the debate, policymaking and guidance on SuDS has focused on flood control in new development, particularly in England, and SuDS schemes tend to focus on quantities of water, runoff rates and volumes.
Meanwhile, a patchwork of regulations covering water pollution across the devolved regions of the UK is in place to reflect the European Water Framework Directive’s goal to achieve ‘good’ ecological status for water bodies. Scotland is the most advanced and successful, operating under the Water Environment (Controlled Activities) Regulations (2011).  
In England and Wales, the Environmental Permitting Regulations (2016) give the regulators powers to require operators to apply for a permit where there is a risk of water or groundwater pollution.  In practice, the regulators mostly rely on voluntary improvements and, while it’s encouraging that treatment schemes are being introduced without permits, by Highways England for example, such interventions are limited and inconsistent and little monitoring is taking place to see if they are adequate.

Low-Risk Sites
On low-risk sites, such as residential developments and pedestrian areas, surface water pollutants such as oil residues and faecal matter from dogs and birds can be readily broken down in vegetative SuDS like swales and basins, where micro-organisms thrive in the upper layers of soil.  In a well-designed pond, the mechanisms which break down pollutants range from nutrient uptake in plants to breakdown by the sun’s rays, so that the capacity to mitigate pollution is impressive.
On a low-risk development with an effective stormwater management system in place that includes attenuation, in order to manage flow rates, and storage, to manage discharge volumes, any lingering pollutants will be treated and broken down in the drainage components that are included to manage water quantity.    
Slowing the flow of the water and holding it in ponds, basins, below-ground tanks or swales, will allow time for pollutants to be broken down.  
On such low-risk sites, pollution prevention is not a key concern unless the receiving environment is very sensitive or the discharge poses a risk to a protected species.  

flood - Highway in rain image webHigh-Risk Sites
On higher-risk sites such as motorways, trunk roads, retail car parks, commercial facilities, freight and logistics centres, the nature of pollutants is very different and, even at very low levels, the persistent, toxic and bioaccumulative pollutants in runoff can cause harm to the aquatic environment.
Pollutants from vehicle exhausts, brake pads, atmospheric deposition and fuel spills cannot be dealt with simply by slowing the flow of the water, allowing the sediment to settle out and relying on micro-organisms to do the rest.  
Of most concern for the aquatic environment, and for human health, are metals, particularly copper and zinc, and a spectrum of chemicals known as polyaromatic hydrocarbons (PAHs), including benzo[a]pyrene.  
Not all pollutants can be broken down biologically.  Dissolved metals cannot be broken down at all and copper and zinc are directly toxic, so will affect aquatic species as soon as they enter the watercourse.  
Metals in runoff have been shown to reduce species numbers of both macroinvertebrates and photosynthesising algae in rivers and streams, which in turn affects fish and other aquatic species by reducing the amount of food available to them. Fish spawning grounds can also be affected by the sedimentation of contaminated solids.

Stormwater Treatment
A drainage design should maximise the opportunities for vegetative features that provide habitats and havens for native species and for the local community to enjoy.  However, using vegetative components alone could be counterproductive on high-risk sites; designing a vegetative treatment device that can be a habitat for wildlife, then contaminating it with toxic metals and hydrocarbons, makes no sense at all.
A SuDS management train of apparatus, that incorporates vegetative and manufactured devices, is often the best way to manage pollutants effectively in higher risk locations and keep persistent bioaccumulative chemicals out of the environment.  This will also ensure that a vegetative SuDS feature operates effectively for the maximum length of time between essential maintenance, for example by preventing a pond from silting up too quickly.
A hydrodynamic vortex separator such as the SDS Aqua-Swirl™ can remove pollution carried in sediment where high pollutant loads are expected. For even more challenging sites, the SDS Aqua-FilterTM combines vortex separation and filtration of stormwater to deliver reliable removal of suspended solids and soluble pollutants together.
Especially as the Government prepares for environmental legislation post Brexit, it’s essential that the public and politicians are aware of the threats to our water quality.  As professionals, we need to work to ensure current regulation is applied consistently, as well as to explore and embrace a full range of treatment technologies to protect sensitive environmental locations effectively.