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1. An investigation was undertaken to determine the loadings of nitrate, ammonium and dissolved reactive phosphorus (DRP) to the following tidal and sea Loughs: River Foyle, Lough Foyle, Lower Bann, tidal River Lagan, Inner Belfast Lough, Outer Belfast Lough, Quoile Pondage, Strangford Lough and the tidal Newry River.
2. The primary focus of the study was on nitrate loadings, which were partitioned between the following sources: agricultural land, uplands, forest land, other land, waste water treatment works (WWTWs) and industrial inputs. This division was primarily to determine whether the contribution of agricultural land to the total nitrate loading was sufficiently high as to merit designating the catchments of eutrophic or potentially eutrophic waters as Nitrate Vulnerable Zones in compliance with the European Union Nitrates Directive (91/676/EEC).
3. River loads were based on Environment and Heritage Service (EHS) nitrate and ammonium monitoring data for period October 1994 to September 2000. This time-span ensured that data from a mix of high and low nitrate loss years were used. Due to a change in the detection limit, dissolved reactive phosphorus loads are the average of the period October 1998 to September 2002. Concentration data for ammonium and nitrate were available for three of the rivers that drain into Lough Foyle from County Donegal for the year of 1997.
4. Loads were calculated as the product of average flow weighted mean concentration and the annual runoff. Where a water is part of a larger system, the loadings are cumulative i.e. Lough Foyle is sum of inputs to Lough Foyle and the River Foyle.
5. Nutrient loadings from individual WWTWs were calculated as the product of average concentration and annual flows from WWTWs. The latter was based on a statistical relationship with the population equivalent organic loadings to WWTWs.
6. Industry inputs were based on data supplied by EHS. The major industrial input to the Tidal Lagan was the Irish Fertiliser Industries (IFI) factory but this has closed down and may be regarded as a historic rather than current nutrient source. For Foyle system, the largest single nitrogen input was from Du Pont on the River Foyle but loads from this source have been scaled back by 93% since 1993 and loading contributions are based on these lower loads.
7. In all catchments nitrate from agriculture formed the dominant proportion of the annual nitrate loading. The contributions from the catchments were as follows: River Foyle (92%), Lough Foyle (90%), Lower Bann (92%), tidal River Lagan (78%), Inner Belfast Lough (73%), Outer Belfast Lough (76%), Quoile Pondage (94%), Strangford Lough (90%) and tidal Newry River (96%).
8. The loss rates of nitrate from agricultural land in individual river catchments were close to close to a rate of 2.0 tonnes N km-2 yr-1. This rate is consistent with a) the use of fertiliser and imported foodstuffs by agriculture within Northern Ireland and b) estimates of nitrate loss from agricultural land based on nutrient export coefficients and the areas of agricultural land in each river catchment.
9. Intensification of agricultural production in Northern Ireland, with its increasing reliance on imported fertilisers and foodstuffs, commenced around 1940. The current agricultural land contributions to nitrate loads calculated in this study therefore include a component that may be considered a natural or background nitrate loading from agricultural land that occurred prior to 1940. The nitrogen output in agricultural produce from Northern Ireland in the period 1925-1940 was only 25% of that in the years 1999 to 2000. If this represents the scale of lower nitrogen inputs to agriculture in the 1925-1940 period, and assuming that nitrate losses to water increase linearly with nitrogen inputs, which is the case for grassland, then approximately 75% of the current agricultural loading can be assigned to an intensification factor. After discounting this background agricultural loading, the agricultural intensification component remained the largest contributor to nitrate loads, representing more than 50% of current catchment nitrate loadings: River Foyle, 69.5%; Lough Foyle, 67.7%; Newry River 72.2%; Quoile, 70.6%; Strangford Lough, 67.5%; Lower Bann, 69.2%; Tidal Lagan, 58.5%; Inner Belfast Lough, 55.2% and Outer Belfast Lough, 57.5%.
10. The contribution of WWTWs to nitrate loadings was less than 10% except in the catchments of the tidal River Lagan (20%), Inner Belfast Lough (24%) and Outer Belfast Lough (21%).
11. WWTWs were the largest source of ammonium in seven catchments: River Foyle (52%), Lough Foyle (55%), tidal River Lagan (42%), Inner Belfast Lough (90%), Outer Belfast Lough (94%), Strangford Lough (75%) and tidal Newry River (94%). In the Quoile catchment agriculture was the largest source. This is reflects the good.
12. treatment of WWTWs in this catchment, which results in comparatively little ammonium in effluent from WWTWs. In the Lower Bann ammonium loss rates were approximately 100% greater than expected.
13. Although WWTWs were a significant source of ammonium, losses of ammonium in rivers were much lower than nitrate losses. Agriculture therefore remained the largest component source of the combined nitrate and ammonium loading or dissolved inorganic nitrogen (DIN) loading as follows: River Foyle (81%), Lough Foyle (81%), Lower Bann (84%), tidal River Lagan (75%), Inner Belfast Lough (57%), Outer Belfast Lough (51%), Quoile Pondage (91%), Strangford Lough (82%) and tidal Newry River (82%).
14. Nitrate losses were highest in the winter reflecting both higher river flows and higher concentrations of nitrate in winter. Loadings of nitrogen from WWTWs remain fairly constant throughout the year. Therefore, if the ecological response of specific water is more sensitive to elevated inputs of nitrogen in the summer, when biological productivity is potentially high, an annual budget may overestimate the ecological importance of nitrate inputs from agriculture. However a summer (April-October) breakdown of DIN inputs found that agriculture was the largest DIN source in the following catchments: River Foyle (72%), Lough Foyle (71%), Lower Bann (70%), tidal River Lagan (53%), Quoile Pondage (80%), Strangford Lough (65%) and tidal Newry River (52%).
15. In the two Belfast Lough catchments, WWTWs were the largest summer source of DIN: Inner Belfast Lough (67%), Outer Belfast Lough (72%).
16. The analysis given above excluded the contribution form the former Irish Fertiliser Industries factory in Belfast. Historically this was the largest source of nitrate and ammonium to the tidal Lagan and Belfast Lough. When the factory was operational the agricultural component to the annual nitrate loading was still comparatively large: tidal River Lagan (42%), Inner Belfast Lough (41%), Outer Belfast Lough (46%).
17. Industrial inputs formed only a small proportion of the DIN load to the River Foyle (0.7%) and Lough Foyle (1.3%)
18. WWTWs represented a significant (>33%) source of DRP in all catchment. The contributions were as follows: River Foyle (42%), Lough Foyle (44%), Lower Bann (89%), tidal River Lagan (54%), Inner Belfast Lough (72%), Outer Belfast Lough (73%), Quoile Pondage (34%), Strangford Lough (60%) and tidal Newry River (66%).
19. Agricultural land was also generally a large DRP source: River Foyle (51%), Lough Foyle (48%), tidal River Lagan (44%), Inner Belfast Lough (28%), Outer Belfast Lough (26%), Quoile Pondage (65%), Strangford Lough (37%) and tidal Newry River (33%).
20. The exception was the Lower Bann were the contribution was exceptionally small at 4% of the DRP load. The DRP loss rate from the Lower Bann was anomalously low and was considered to reflect the uptake of DRP by the large amounts of algae entering this river from Lough Neagh.

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