The influence of width and vegetation characteristics on the effectiveness of buffer strips at mitigating diffuse water pollution from agriculture

Transcription

1 The influence of width and vegetation characteristics on the effectiveness of buffer strips at mitigating diffuse water pollution from agriculture Heather Woodhead MEnvSci MCIWEM (lead author) Dr Helen Moggridge (co-author)

2 Introduction Background Objectives Study Catchment & Methodology Results Summary

3 Background Diffuse Water Pollution Agriculture UK agri-environment initiatives typically 6 or 12 m wide. Water Quality Water Framework Directive Good Status Most research focused on buffers v s no buffers. Lack of research on buffer function when > 12 m wide, and effect of vegetation diversity Sedimentation Adsorption Infiltration Dilution Nutrients / Pesticides Soil Erosion / Sediment Catchment Sensitive Farming Results reported in the literature poses a challenge for recommendations to farmers What did I want to understand from this project

4 Objectives To assess the effect of buffer width on buffer strip function To determine what extent the vegetation diversity of buffer strips affects buffer strip function To determine to what extent buffer strip vegetation height affects buffer strip function

5 Study Area - Yorkshire Derwent Catchment Catchment Sensitive Farming (CSF) priority catchment 13 sites - headwater streams or drainage ditches to the River Derwent Range of buffer widths / vegetation characteristics, similarities in other physical variables (soil type, slope)

6 Buffer Strip Characteristics Permanent for > 6 years Annual management (grazing or mowing) Dominant vegetation type grass, meadow fescue (Fescue pratense) and timothy (Phleum pratense) Measurements every 10 m intervals Vegetation survey (1m x 1m quadrats across transects) Simpsons Index of Diversity Height measurements max. (July / Aug) and min (Oct/Nov)

7 Water Quality Sampling Sampled monthly over a period to capture seasonal variation Total Dissolved Solids Conductivity Phosphate Nitrate (lab - ion chromatography) Suspended solids (lab membrane filtration) Invertebrates (July/Aug) kick sampling, Average Score Per Taxon (ASPT), Biological Monitoring Working Party (BMWP)

8 Results Width: Nitrates Buffers 6-12 m mean of mg/l Buffers > 12 m mean of mg/l. Statistically significant difference E.A nitrate waterbody classification as 'high' if 30 mg/l Rearrangement of the regression equation developed from multiple regression analysis found a minimum width of 10 m was required for nitrate concentration to be < 30 mg/l. Flow reduction Absorption / plant uptake Infiltration Denitrification

9 Results Width: Phosphates Buffers 6-12 m mean of 46 µg/l Buffers > 12 m mean of 35 µg/l. Not statistically significant difference WFD sets a target of 50 µg/l phosphate Buffered watercourses advantageous over non buffered sites Rearrangement of the regression equation developed from multiple regression analysis found a minimum width of 4.5 m was required for phosphate concentration to be < 50 µg/l.

10 Results Width: Sediment (1) Suspended solids Significant difference 2.4 mg/l reduction with wider buffer Total Dissolved Solids Significant difference 29% reduction with wider buffer

11 Results Width: Sediment (2) Conductivity Significant difference 30% reduction with wider buffer Sedimentation Infiltration Adsorption Flow reduction Buffer location and design is a key factor in the effectiveness to capture sediment

12 Results Width: Biological Quality BMWP Not significant between width groups Families with the highest BMWP score of 10 (indicators of high water quality e.g. Ephemeridae, Phyrganeidae and Leptoceridae) were present at sites >12 m width buffer and absent at sites 6 12 m. ASPT Significant difference between buffer width ASPT of 5.43 indicating 'excellent' water quality for buffers > 12 m wide ASPT of 4.01 indicating 'moderate' water quality for buffers 6 12 m wide

13 Results Vegetation Diversity / Height Diversity Sites grouped where width similar (3m range) Trends seen with improving water quality Nitrates showed a significant relationship nitrate uptake by plants is a key process in nitrate removal in buffer strips Height Summer height 33 to 131 cm Winter height 14 to 61 cm Sediment concentrations showed a significant increase with vegetation height greater barrier for reducing flow and entrapping sediment

14 Summary Width significant characteristic influencing buffer function Significant reduction in nitrate (55%), suspended sediment (35%), TDS (29%) and conductivity (30%) when buffers 12 m wide. Biological indicators improve from moderate to excellent Vegetation diversity and vegetation height No significant trends across the chemical and biological water quality indicators. Limitations Concentrations measured as opposed to loads No flow recording Future Looking beyond buffer width to mitigate diffuse water pollution from agriculture... Importance of research and monitoring the impact of policy driven management