Sediment, nutrient and runoff management and mitigation in rural catchments

Abstract

This Thesis is concerned with the quality of surface waters in rural catchments across northern England and the mitigation of Diffuse Water Pollution from Agriculture (DWPA). Runoff Attenuation Features (RAFs) are a range of soft--‐engineered DWPA transport management options, which target hydrological flow pathways for the purpose of slowing, storing and filtering water. This study demonstrates the potential of RAFs to significantly reduce losses of suspended sediment (SS), phosphorus (P) and nitrate (NO3) in agricultural runoff. To implement RAFs effectively it is vital to understand how, where and when to best target mitigation efforts. This relies on knowledge of the sediment and nutrient regime and hydrological functioning of a catchment. In response to this a stratified, synchronous grab sampling programme was implemented over two consecutive years in the upper Eden catchment (334 km2), Cumbria, covering thirteen sub--‐catchments of multiple scales. No relationship was found between sedimentutrient yield and catchment area but it was recognised that certain lowland sub--‐catchments deliver a disproportionate amount of the pollutant load, particularly SS and P, due to increased agricultural activity, and that there were large variations in flux affected by season and hydrological conditions. One particular sub--‐catchment dominated by improved grassland, Blind Beck (9 km2), exhibited both higher nutrient and SS concentrations per unit runoff and higher yields compared with any other sub--‐catchment. The Blind Beck sub--‐catchment was selected in which to implement a more detailed investigation of SS and nutrient delivery, which included event sampling. High flows (accounting for 10% of flow duration) contributed 84% of the annual SS load, 76% of the total P and 68% of the soluble reactive P, but just 32% of the NO3 load. This highlights the acute nature of the SS and P diffuse pollution problem and demonstrates the need to target storm events for effective mitigation. A number of RAFs were constructed in two established research catchments in Northumberland with a similar mixed land use to the Eden: Belford (15 ha) and Netherton (80 ha). Synchronous inlet and outlet water samples were collected during storm events. Results demonstrate that relatively small RAFs, principally sediment traps, constructed in farm ditches (<1 km2 catchment area) can reduce mean SS, TP, SRP and NO3 loads during storm events by 30--‐49%, 23--‐37%, 12--‐27% and 8--‐14%, respectively. The potential of RAFs designed to reduce DWPA in key locations and at certain scales will be proposed based on the findings of the PhD study.