Chapter 4 Agricultural Catchment Restoration R. W. McDowell, R. M. Monaghan, M. E. Close, and C. C. Tanner Abstract A good understanding of the sources of contaminants and the processes that facilitate and modify their transport to and within freshwaters is the first step in the restoration of good water quality in agricultural catchments. This understanding needs to be combined with knowledge on how different agricultural systems and practices influence contaminant sources and processes, and this knowledge embed- ded within tools that enable robust decisions to be made to meet desired water quality objectives. Such information and tools will enable us to explore the conse- quences of mitigation decisions, trade-offs, and ultimately optimise where agricul- ture and desired water quality best coexist. Keywords Escherichia coli · Good management practices · Groundwater · Mitigation · Nitrogen · Phosphorus · Runoff · Sediment 4.1 Introduction The quality of water leaving agricultural catchments, as assessed by a variety of indicator analytes (nitrogen, N; phosphorus, P; suspended sediment, SS; Escherichia coli, E. coli), is often worse than in forested (native or exotic) catchments (e.g. Quinn and Stroud 2002). These analytes, hereafter called contaminants, indicate directly or indirectly the potential for a variety of detrimental effects in flowing streams, rivers, lakes, and reservoirs. Much research has quantified the factors involved in R. W. McDowell (*) AgResearch, Invermay Agricultural Centre, Mosgiel, New Zealand e-mail: richard.mcdowell@agresearch.co.nz R. M. Monaghan Faculty of Agriculture and Life Sciences, Lincoln University, Christchurch, New Zealand M. E. Close Institute of Environmental Science and Research, Christchurch, New Zealand C. C. Tanner National Institute of Water and Atmospheric Research, Hamilton, New Zealand © Springer International Publishing AG, part of Springer Nature 2018 D. P. Hamilton et al. (eds.), Lake Restoration Handbook, https://doi.org/10.1007/978-3-319-93043-5_4 107