Nutrient loss from poorly drained grassland soils and land drainage systems, and the potential for loss mitigation

The high rainfall and low evapotranspiration of Ireland’s temperate climate cause water-saturated root zones that deplete soil oxygen for root growth and promote unhealthy crop growth for pastures on poorly drained soils. Wet soils are also subject to damage by machinery or animal traffic. These factors limit the full potential of such soils in producing optimal grass for animal production, therefore necessitating the need for the installation of drainage systems. Within drainage systems, networks of surface drains (i.e., open artificial ditches and natural drains) are linked to drain excess water from subsurface (in-field) drains and surface runoff, thereby enhancing grass production and reducing adverse field trafficability conditions in poorly drained soils. Surface drains transport nutrients from varied surface and subsurface hydrological connectivity pathways to receiving water sources. However, to date, assessment of the connectivity risk of surface drains in transporting nutrients to receiving waters has only assessed phosphorus (P) loss, neglecting nitrogen (N), and has not considered the varying risks from connecting hydrological pathways such as surface roadways and subsurface drains, springs, upwelling and seepage. In addition, the identification of surface drains which pose a high-risk to receiving waters, as well as the contributing factors to these risks (landscape nutrient content, vicinity to farmyards, etc.), which vary spatially along the nutrient transfer continuum (NTC), has remained unexplored. Lastly, farm roadway runoff is an identified nutrient and sediment contributor to connecting surface drains. Yet, farmers’ willingness to implement recommended mitigation measures such as swales, sediment ponds and bunded drains, among others, is limited as these measures have not been widely tested for efficiency. To address these knowledge gaps, the aims of this study were to: (1) create an integrated connectivity risk ranking for surface drains considering P and N simultaneously, (2) develop a semi-quantitative risk model to identify high-risk surface drains, and (3) assess the efficiency of one mitigation measure (sedimentation ponds) for roadway runoff. Across seven dairy farms, surface drains were mapped, assessed for hydrological connectivity pathway nutrient losses and reclassified to create an integrated N and P loss connectivity risk ranking for surface drains. A semi-quantitative risk assessment model identified high-risk surface drains for targeted mitigation. At three locations where farm roadway runoff was connected to surface drains, three different configurations of sediment ponds were designed and operated for a period of 6 months, to remove nutrients and sediments. Farmyard-connected drains were ranked as the riskiest due to connectivity to point source losses, whereas outlet drains had the highest risk across surface drains with diffuse sources connectivity. In surface drains associated with diffuse sources, nitrate was introduced by subsurface sources (in-field drains and groundwater interactions from springs, seepage, and upwelling) and ammonium was introduced through surface connectivity pathways (runoff from internal roadways). This study classified 23 %, 68 %, 9 % and 0 % of all surface drains across all farms studied as low, medium, high, or very high-risk class, respectively, with high or above requiring a mitigation plan. Two-thirds of high-risk surface drains were connected to farmyards, with a potential for high nutrient loss from point sources, while other factors including hydrological connectivity pathways from farm roadways contributed to the remaining one-third. A combined source management and targeted mitigation approach is recommended for high-risk or above classes. The study showed sediment ponds are efficient for reducing roadway runoff pollution to surface drains especially for removing total suspended solids and particulate nutrients but vary in their effectiveness in removing dissolved nutrients. Sediment ponds designed to incorporate segmentation, considering all site conditions and containing vegetation, may enhance nutrient and sediment removal. This may facilitate uptake from farmers. The study recommends long-term monitoring to inform maintenance procedures and scheduling.

Funder

Teagasc

Publisher

University of Galway

Rights

CC BY-NC-ND

cbn

Collections

University of Galway Theses (PhD Theses)