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In many parts of the UK, the development sector is facing significant barriers to new proposals due to failure to demonstrate sufficient mitigation of nutrient pollution from new developments. Although critical to planning application success, progress on how to deliver nutrient neutral development seems faltering.
Through discussion with stakeholders, including Natural England (NE) and local planning authorities (LPAs), global environmental consultancy Ricardo has identified that, while there is a strong ambition to implement a range of mitigation solutions that enable new housing targets to be met, there is uncertainty around whether proposed measures will meet Habitats Regulations Assessment (HRA) in perpetuity benefit requirements. This refers to the need for UK developers to implement nutrient mitigation measures that produce demonstrable long-term nutrient load reduction. In this article Ricardo experts, Drs Gabriel Connor Streich and Jenny Mant, discuss some of these uncertainties and suggest how they might be addressed.
The Legal Requirement
Globally, ecosystems are under pressure from a range of pollutants from building developments. This article focuses on nitrogen and phosphorous – the main pollutants associated with excess nutrient load. In many countries, water bodies (rivers, lakes and coastal areas) are subject to environmental legislation with associated mitigation measures aimed at reducing pollution entering our aquatic systems. Until recently, interventions had focused on tackling the legacy nutrient load stemming from historic farming, urban land management and treated wastewater. Following the much cited 2018 ‘Dutch Case’ ruling in the European Court of Justice, Natural England has adopted nutrient neutrality as an ‘ecologically and legally acceptable’ approach to tackling the nutrient pollution burdens placed on protected aquatic ecosystems by new development.
“in England and Wales, many areas are now grappling with the concept of how to achieve ‘nutrient neutral’ development”
In England and Wales, many areas are now grappling with the concept of how to achieve ‘nutrient neutral’ development. This is especially so where wastewater nutrient outputs from new housing discharge to European designated sites (water bodies) that are currently failing, or are close to failing, their conservation targets for nitrogen and phosphorous concentrations. In such cases, increased nitrogen and/or phosphorous could lead to eutrophication: a process where nutrient overload can result in algal growth within a waterbody with significant impact on a range of aquatic organisms (see Figures 1 and 2).
Mitigation: Meeting Requirements
Where a development has been identified as needing to achieve nutrient neutrality, a mitigation plan will be required. The plan will need to demonstrate that mitigation is significantly robust to meet the stringent European Directive 92/43/ECC HRA process of ‘no likely significant adverse effect’ on natural habitats and wild fauna and flora for Natura 2000 sites.
Nature-Based Solutions (NbS) – The Way Forward?
There is a growing appetite to adopt more NbS (e.g. river and interconnected floodplain restoration, wetland creation, SuDS, riparian buffers etc.) to manage nutrients. This is not surprising: if designed appropriately NbS should be able to deliver nutrient neutrality, plus additional environmental and societal co-benefits such as ecological habitat resilience, creation and/or preservation of natural outdoor recreational spaces, and improved air quality. This is in addition to supporting carbon sequestration and low embodied carbon emissions targets.
Ricardo has previously highlighted that as part of the nutrient neutrality process, a nutrient budget is required. Nutrient budgets measure the amount of nitrogen or phosphorous generated by a new development in kilograms per year (kg/year). For new developments, where “likely significant effects” on the ecology of the site cannot be ruled out, mitigation is required that can evidence sufficient nutrient reductions in kg/yr. In the case of NbS, there is a question as to the scale of nutrient reductions such approaches can achieve and how to show this robustly?
The Uncertainties:
Lack of unified measurement standards
Theoretically, NbS provide nutrient reduction benefits, but whether they can pass the HRA test of providing nutrient reductions beyond reasonable scientific doubt remains uncertain. To date mitigation monitoring programmes have not focused on specific metrics that enable robust answers to questions such as ‘how many kg/yr of nutrients does a mitigation measure remove?’ Even in studies that do, there is variation in how mitigation is measured. River restoration, for example, tends to be much less explicit on this question than for riparian buffers and wetlands. Added to this, other variables such as local environmental influences and the spatial scale of a mitigation measure compound the issue of uncertainty.
Efficient removal of pollutants
The physical processes that remove nutrients are complex, with differing processes active for nitrogen and phosphorous, respectively. Where river restoration permanently removes nitrogen through denitrification (see Figure 3), phosphorus is generally removed by sediment deposition and can be remobilised via sediment transport during storm events. Wetland systems provide greater control over the processes that remove nutrients but even here, geology, soil types and climatic conditions add to the uncertainty surrounding precise nutrient removal efficiency.
Mitigation scheme success is also dependant on input nutrient concentrations. Existing data (e.g. Land et al. 20161) indicates that, generally, wetlands treating high nutrient concentration inflows are more efficient, whereas low nutrient concentration inflows may achieve almost no mitigation benefit. Further complexity is added by ‘legacy’ nutrients that may already be present in the environment and thus slow or reduce the benefit that a mitigation measure may deliver.
Ensuring in perpetuity benefits
Demonstrating long-term, ‘in perpetuity’ (generally seen as around 80-125 years) benefits from mitigation is also difficult to predict. While vegetated swales are accepted as beneficial for pollutant reduction, the mass and volume of sediment build-up and potential re-circulation and onward transport into the environment over time is not well defined (Doenie et al., 20153). Studies may also produce figures for the percentage removal of nutrients by a mitigation measure, for example riparian buffers have been found to remove anywhere from 21 per cent (Udawatta et al., 20024) to 97 per cent (Hefting & Klein, 19985) of the incoming nutrient load. However, questions remain about whether these benefits are retained ‘in perpetuity’.
Some solutions such as wetlands can be engineered in order to have more robust estimates of nitrogen or phosphorous removal. However, the data for many NbS remains limited and does not cover all scenarios (type, scale, geology etc.). This uncertainty is reflected in the current precautionary approach to calculating nutrient reductions which may underestimate the true benefit of NbS.
Navigating Uncertainty
Monitoring to date has simply not focused on answering questions based on achieving nutrient neutrality. To increase the certainty of achieving HRA compliance, bespoke monitoring regimes and associated guidance on what to measure are essential. Only then can we answer the right questions to increase confidence in the scale of nutrient reductions using NbS and prescribe suitable adaptive management programmes that will deliver mitigation in perpetuity. Demonstrating in perpetuity benefits requires long-term monitoring programmes, which can be financially and time intensive. To justify such investment, monitoring programmes need to be proportional to the uncertainty associated with the mitigation solution and have buy-in from LPAs, the regulators, water companies, developers and the local community.
“achieving nutrient neutral development has given a new focus to the limitations in knowledge related to nutrient mitigation using NbS”
Strategies for Deploying Mitigation
Nutrient neutrality issues are only going to grow in scale in the coming years as demand for housing increases. Wider strategies for tackling nutrient pollution from the agricultural and wastewater treatment sectors through integrated catchment management and improvements in wastewater treatment technologies alone are unlikely to bring nutrient pollution to acceptable levels for decades.
We need a strategic ‘roadmap’ to deploying NbS nutrient mitigation at scale and avoid slowing down in-demand house building. Such an approach could demonstrate:
- Effectiveness of different mitigation solutions relative to scale of deployment, based on up-to-date evidence-based calculations. This could include an updatable compendium of case studies with associated data and recommendations for further research.
- Funding sources that cover capital costs and adaptive management needs to achieve in perpetuity requirements for LPA-administered and private-run schemes.
- Partners’ engagement to support mitigation delivery and long-term adaptive management.
- Adaptive management and monitoring guidelines.
- Frameworks for LPA Local Development Plan policies and priorities which account for local needs.
The need for a roadmap has already been recognised by some councils. In Herefordshire, the LPA is developing in-house strategies with support from Dwr Cymru Welsh Water (DCWW) and the Wye and Usk Foundation. In contrast, the Solent region is developing a more market-led approach where private mitigation providers are being encouraged to develop strategic mitigation schemes for LPAs, with regional oversight from the Partnership for South Hampshire. Both models provide different solutions. It is likely that a hybrid of the two models will be needed to deliver the range of mitigation needed to support development.
“nutrient neutrality issues are only going to grow in scale in the coming years as demand for housing increases”
Long-Term Responsibility
Any robust roadmap will have to address the requirements for adaptive management as highlighted above. Key questions include:
- With whom will management responsibility sit, especially with regards to privately run mitigation schemes?
- How can robust agreements be put in place that provide certainty that a mitigation solution will be maintained to deliver over decadal timescales?
Currently, funding streams are available to design and ‘build’ NbS nutrient mitigation and, in theory, there are willing partners in the environmental NGO sector to manage delivery and maintenance. Associated strong legal and financial processes/frameworks are, however, needed to ensure ongoing maintenance continues. These are not yet fully formulated but readily replicable approaches would greatly ease the practical issues that can hamper mitigation scheme viability.
Interaction with Other Reduction Strategies
Nutrient neutrality is not the only nutrient pollution reduction driver related to aquatic ecosystems. In some Habitat sites’ catchments, nutrient management plans are in place to address the high levels of nutrient pollution coming from agriculture and wastewater treatment. Currently these plans cannot be relied upon with enough certainty to mitigate the additional nutrients from new development in affected catchments. Neither can such mitigation take the place of measures already required as part of wider nutrient management plans and strategies. Nonetheless, Catchment Sensitive Farming initiatives, upgrades of wastewater treatment works and other environmental initiatives, such as those driven through the Environment Agency’s Water Industry National Environment Programme (WINEP) and Natural Resources Wales, National Environment Programme (NEP), can support the identification of opportunity areas to reduce nutrients and help achieve nutrient neutrality.
Steps Required to Reduce Uncertainty
Home building is essential for delivering local government housing targets to support a thriving economy and societal wellbeing. Yet, this needs to be balanced against a resilient ecological environment. Achieving nutrient neutral development has given a new focus to the limitations in knowledge related to nutrient mitigation using NbS. This in turn hurts industry confidence on which mitigation measures to implement and what management may be necessary to achieve mitigation in perpetuity.
Furthermore, with the current changes in environmental policy and government ambitions to deliver on environmental and societal benefits, there is a strong local and national agenda to consider how wider actions to tackle nutrient pollution interlink with mitigation to achieve nutrient neutrality. NbS can support nutrient neutrality as mainstream mitigation solutions, but a concerted effort is needed to remove operational uncertainties related to what works well, where and when.
“strong science supported by appropriate legal frameworks is needed to allow the development industry to achieve nutrient neutrality”
With the roll out of nutrient calculators to all affected areas of England, the need for appropriate mitigation measures based on best available evidence has never been greater. Ricardo has outlined a roadmap that will show what can be achieved now and provide a focus for what may take years to understand and implement. Strong science supported by appropriate legal frameworks is needed to allow the development industry to achieve nutrient neutrality through environmentally beneficial mitigation solutions that function in perpetuity.
- Land, M., Granéli, W., Grimvall, A., Hoffmann, C.C., Mitsch, W.J., Tonderski, K.S. and Verhoeven, J.T., 2016. How effective are created or restored freshwater wetlands for nitrogen and phosphorus removal? A systematic review. Environmental Evidence, 5(1), pp.1-26.
- Yousaf, Anum, Noreen Khalid, Muhammad Aqeel, Ali Noman, Nayab Naeem, Wajiha Sarfraz, Ujala Ejaz, Zonaira Qaiser, and Arifa Khalid. 2021. “Nitrogen Dynamics in Wetland Systems and Its Impact on Biodiversity” Nitrogen 2, no. 2: 196-217. https://doi.org/10.3390/nitrogen2020013
- Allen, Deonie, Valerie Olive, Scott Arthur, and Heather Haynes. 2015. “Urban Sediment Transport through an Established Vegetated Swale: Long Term Treatment Efficiencies and Deposition” Water 7, no. 3: 1046-1067. https://doi.org/10.3390/w7031046
- Udawatta, R.P., Krstansky, J.J., Henderson, G.S. and Garrett, H.E., 2002. Agroforestry practices, runoff, and nutrient loss: a paired watershed comparison. Journal of Environmental Quality, 31(4), pp.1214-1225.
- Hefting, M.M. and de Klein, J.J., 1998. Nitrogen removal in buffer strips along a lowland stream in the Netherlands: a pilot study. Environmental Pollution, 102(1), pp.521-526.
