E: Impact of agronomic practices of an intensive dairy farm on nitrogen entrations in a karst aquifer in Ireland HORS:

Exploring the relationship between agricultural nitrogen (N) loading on a dairy farm and groundwater reactive N concentration such as nitrate (NO3) is particularly challenging in areas underlain by thin soils and karstified limestone aquifers. The present study on such a site aims to relate changes in detailed paddock specific agronomic N-loading, hydrogeological and geological site characteristics with N occurrences in groundwater over a 10 year period from 2001 to 2011. In addition, the concept of vertical time lag from source to receptor is considered. Statistical analysis used regression with automatic variable selection. Four scenarios were proposed to describe the relationships between paddock and groundwater wells using topographic and hydrogeological assumptions. Monitored nitrate (NO3 -) concentrations in the studied limestone aquifer showed a general decrease in the observed time period (2002 – 2011). Statistical results showed that a combination of improved agronomic practices and site specific characteristics such as thicknesses of the soil and unsaturated zone together with hydrogeological connections of wells and local weather conditions such as rainfall, sunshine and soil moisture deficit (SMD) were important explanatory variables for NO3 concentrations. Statistical results suggested that the following agronomic changes improved groundwater quality over the 10 year period: reductions in inorganic fertiliser usage, improvements in timing of slurry application, the movement of a dairy soiled water (DSW) irrigator to less karstified areas of the farm and the usage of minimum cultivation reseeding on the farm. In many cases the explanatory variables of farm management practices tended to become more important after a 1 or 2 year time lag. Results indicated that the present approach can be used to elucidate the effect of farm management changes to groundwater quality and therefore the assessment of present and future legislation implementations.