Summary: Final Report of the ERTDI-funded project 2000-LS-184.108.40.206-M2
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A considerable body of research already exists on establishing nitrate dynamics in agricultural systems’ subsoils. However, the capacity to predict rates of nitrogen (N) arrival at the receiving environment, namely surface or groundwaters, has been elusive in an Irish context. A farm-scale (50 ha) hydrogeological investigation was established on an intensive dairy farm, in north Cork, characterised by a freely draining limestone till which forms the subsoil overlying a karstified-limestone bedrock aquifer. The overburden depth is 2.5 m, on average, but undulates in depth from 0-4.5 m, consistent with the karst terrain. Part of the farm is located within a source protection zone delineated for a public supply borehole located 1.5 km to the northeast, in the direction of groundwater flow. This public supply borehole has demonstrated an upward trend in nitrate-nitrogen (NO3-N) concentrations over the last twenty years, with periodic breaches, in the last decade, of the EU parametric limit of 11.3 mg/l for potable water (EC, 1998). However, this research was commissioned by the EPA with regard to the contribution of dairy farming agriculture to eutrophication of water resources and not with regard drinking water legislation. The appreciably lower 2.6 mg/l N eutrophication criteria for tidal freshwaters (EPA, 2001) must be acknowledged when evaluating groundwater nitrate (NO3) concentrations beneath agricultural lands. Ireland has yet to define hydrochemical criteria for definition of groundwater ‘good status’, with respect to the Water Framework Directive (EC, 2000). However, the EPA has set 25mg/l NO3 (equivalent to 5.6mg/l NO3-N) as the Interim Guideline Value (IGV) towards protection of groundwater.
Definition of groundwater nitrate responses was the fundamental aim of the project, with objectives of measurement of the response of the groundwater system to loadings, both meteorological and agronomic, at the farm-scale. Nine specifically designed monitoring boreholes allowed bi-monthly water level- and hydrochemical-monitoring of the groundwater body. Boreholes were instrumented with piezometers to ensure that the groundwater sampled was from a specific depth, 27-30 m below ground level (bgl), that was isolated from contamination from the ground surface using bentonite and cement grout seals. The water table is 25 m bgl, on average, and has a maximum annual range of 15 m. Boreholes were positioned to target either one of four distinct dairy management zones in operation on a typical Irish dairy farm: grazing pasture, grazing and dirty water treatment, one-cut silage and grazing, and two-cut silage and grazing. The farm stocking density was ~2.4 Livestock Units (LU)/ha and all plots on the farm received 290 kg /ha, on average, as inorganic N fertiliser. There was large spatial variation in the organic N loading rates in each of the four management zones mentioned.
For the 2001-2002 hydrological year NO3-N concentrations in groundwater ranged from 3-31 mg/l, depending on the location of the borehole within the farm. The observed range in the succeeding year was 4–23 mg/l NO3-N. In the second hydrological year groundwater concentrations were significantly lower, on average, than those observed in the first year because 50% more effective rainfall (
Reff )fell in the winter of the second year, as compared to the first winter. Averaging all piezometer groundwater NO3-N concentrations over the entire farm showed that the means were 16.5 mg/l in the first year and 12.6 mg/l in the second year. Groundwater levels rise by up to 8 m within one month of the heavy block of winter recharge, observed to be 200 mm in the wettest winter month in both years. Spring and summer recharge events also increased groundwater levels. The observed groundwater nitrate response at Curtin’s farm has a clear temporal dimension. Groundwater NO3-N concentrations were observed to rise in response to significant rainfall events in spring and summer but decreased, initially, with autumn and winter recharges. However, despite the initial fall in the NO3-N concentrations caused by winter recharges, they were observed to increase again later. There was a rapid response to groundwater loadings. Moreover, the groundwater NO3-N response was discernible in correspondence to differing agricultural practices – being highest in the areas of highest organic N loading. At the field scale there was a strong relationship between grazing intensity and the following year’s average NO3-N concentration. An agricultural signature in the groundwater is evident at some locations. In addition to high NO3-N concentrations, spikes in groundwater concentrations of phosphorus (P), potassium (K), ammonium (NH4) and nitrite (NO2) were observed in response to recharge events, which again suggests a highly vulnerable hydrogeological setting. <...>