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Updating the User Manual (1) Modular approaches to the control of diffuse agricultural pollution: buffer zones, bioreactors, ditches and ponds - WQ0126

In order to reach the target ecological status for UK waters as outlined in the Water Framework Directive (WFD) (2000/60/EC), a major requirement is the reduction or mitigation of diffuse water pollution from agriculture (DWPA). In order to assist with this Defra has established a Catchment Sensitive Farming (CSF) programme of research. The CSF commissioned project ES0203 has identified 44 mitigation methods to control DWPA. One such method suggests the establishment of riparian buffer strips or zones. Despite the abundance of literature reporting both riparian and non-riparian buffer strip performance, the spatial and temporal mechanistics of the processes involved are poorly understood. For this reason, buffer zones have often been implemented incorrectly and failed to deliver the benefits expected of them. One of the main challenges with buffer zone effectiveness has been found to be their by-passing by the passage of polluted water through sub-surface drains which in many regions of the UK has regularly been shown to greatly limit their value with regard to nutrient control [MAFF report, Leeds-Harrison 1996].
This proposed study will fill many gaps in the knowledge of the biotic/abiotic factors and processes contributing to, and determining the long-term effectiveness of diffuse pollution control by buffer strips, and additionally address the issue of buffer zone bypassing. The work will complement other Defra funded projects such as PE0205 and PE0206. We aim to do this by undertaking a programme of research which will investigate various factors including the effect of hydraulic conductivity, loading, soil structure and texture, slope, vegetation, and wetting and drying cycles on the buffering capabilities of filter strips. Additionaly, we propose to investigate alternative 'mid to end of pipe' complementary structural methods such as ditches and ponds which may be used to enhance water quality and reduce peak water flows, as well as subsurface bioreactors which may be used to enhance buffer strip performance.
Little work has been carried out on the role of additional, complementary structural buffering methods such as managed ditches and ponds, which may provide more benefits, especially if used either alongside or instead of the more conventional use of buffer strips. Ditches are the principal means of land drainage on hydromorphic soils throughout the UK, and their prime purpose is to remove excess water from agricultural land. In so doing they constitute one of the main hydrological pathways linking agricultural land and surface water bodies. By their very nature ditches are well placed to interact with a large proportion of water moving from agricultural land to rivers and lakes, but to date ditch management has focused on maintenance of water transference capacity together with wildlife and conservation values. Simple techniques can be applied that not only will enable ditches to provide these benefits, but also effectively to act as linear wetlands, providing the capability of water quality improvement/pollution control together with hydrological regulation (flood control). The former could help improve water quality through the creation of wetland habitat supporting processes such as denitrification, sedimentation and plant nutrient uptake, while the latter could help reduce flooding through managing ditches so that they act as multiple reservoirs, storing water at critical times, and consequently attenuating downstream flood peaks. Management of ditches for these combined purposes may prove highly effective because treatment areas would be located in the hydrological conduits themselves.
Farm ponds could be effective at reducing nutrient loading of surface waters draining from agricultural land. Ponds also help reduce the velocity of runoff waters and therefore help to reduce downstream sediment losses. Widespread re-establishment of farm ponds alone will not solve the diffuse pollution problem posed by intensively managed agricultural catchments, but if integrated with a suite of other measures, could be an ‘end of pipe’ component of an effective mitigation strategy. In addition, ponds can have high biodiversity value, and can be linked to other forms of economic output from the farm such as leisure or aquaculture. They also add some amenity value to the landscape [ESO109 Final Report to Defra].
Subsurface bioreactors or barriers are effectively ditches into which reactive materials (such as those with a high carbon content) are placed below the soil surface in such a way as to intercept contaminated groundwater, provide a preferential flow path through the reactive media, and transform contaminants into environmentally benign forms. If combined with conventional buffer strips that are subject to by-passing by subsurface drainage, they could potentially enhance the nutrient removal capacity of a buffer zone.
Each of these components will be investigated both individually, and as part of a strategic network of mitigation methods. We will compare and demonstrate the impact that the individual mitigation methods can have on diffuse pollution, but also how such methods may operate in combination to provide a modular approach to the mitigation of diffuse agricultural pollution. The potential value of such an approach is that they can be easily established within an existing farm layout, with minimal impact to land use, and maximum benefit to water quality and catchment hydrology. We will deliver replicated, scientific evidence, at high temporal resolution, elucidating the efficiency, resilience and robustness of the proposed methods. The evidence will help to support the establishment of clear policy and guidelines on how such methods can be used and implemented in the agricultural landscape and contribute towards meeting the objectives of the WFD.
Our overall hypothesis is that effective diffuse agricultural pollution mitigation requires not only mitigation methods based on land and livestock management, but additionally the implementation of a suite of structural measures, such as buffer strips, managed ditches, ponds, and subsurface bioreactors. Furthermore, the performance of these structural measures requires management to optimise their efficacy, giving careful consideration to site-specific characteristics, compatibility of processes, multifunctionality and in particular to hydrological pathways. To test this hypothesis we propose to investigate and compare the short-term and long-term effectiveness of conventional grassed buffer strips, buffer strips enhanced with subsurface bioreactors, managed ditches and ponds to control water quality and more widely, catchment hydrology. A co-ordinated, integrated series of objectives (1-7) will be met through a programme of field, laboratory and desk studies (Fig 1). These objectives will culminate in the delivery of tools and information contributing towards the development of a diffuse pollution mitigation strategy. Specifically our objectives are:

1. To conduct an in-depth and critical review of the application, management, efficiencies and process compatibility of multi-functional buffer strips and other state of the art structural diffuse pollution mitigation options. A synthesis of the literature will provide guidance in objectives 5 and 6.

2. To host an international workshop on buffer zones and other structural diffuse pollution mitigation options to examine the current state of the art and implementation strategies employed in other countries.

3. To conduct mechanistic experiments at the laboratory and mesocosm scale to examine the key factors affecting the processes and mechanisms that contribute to buffer zone functioning, and to determine limitations and thresholds to performance.

4. To measure and compare the performance of a range of mid- to end-of-pipe structural mitigation methods at the field scale through the construction and instrumentation of large, field-scale lysimeter-type plots. Specifically, conventional buffer zones, modified buffer zones (incorporating bioreactors), ditches and ponds will be investigated.

5. To build on the tool developed in Defra project PE0205 and develop it into a practical, generic, decision support tool enabling the identification of suitable multi-functional buffer zone locations and other structural mitigation methods at a farm scale, and further develop this into a sub-catchment scale tool.

6. To apply the findings of our experimental work and DSS at a farm scale and assess the efficacy of measures both environmentally and economically. In doing this, we will work closely with agricultural advisory NGOs on the best way to encourage farmers to implement these measures.

7. To write reports, peer reviewed papers and to disseminate information to relevant stakeholders.
Project Documents
• FRP - Final Report : Evid4 Defra WQ0126 FinalReport   (2937k)
• ANX - Annex : Appendix 1 LiteratureReview   (165k)
• ANX - Annex : Appendix 3 Workshop Report   (48k)
• ANX - Annex : Appendix 4 KnowledgeExchange   (24k)
Time-Scale and Cost
From: 2008

To: 2013

Cost: £931,359
Contractor / Funded Organisations
North Wyke Research
Diffuse Pollution              
Sustainable Farming and Food Science              
Water Framework Directive              
Water Pollution              
Water Quality              
Water Quality and Use