Upward trend in soluble phosphorus loadings to Lough Neagh despite phosphorus reduction at sewage treatment works

After allowing for changes in point source phosphorus (P) loadings from sewage treatment works (STWs) and creameries, an upward trend in the soluble reactive phosphorus (SRP) loadings in the six major rivers draining into Lough Neagh was established over the period 1974–1991. Urban SRP loadings were reduced by a P reduction programme at nine major STWs, which commenced in 1981, while the loading from other STWs was considered to have been stable as population growth was balanced by reduced P per capita loadings reflecting reductions in the P content of domestic detergents. Post 1987, creamery loadings were reduced following closures and a switch away from phosphoric acid as a cleaning agent. STWs were subjected to P reduction in four river catchments but reductions in SRP loads of these rivers were only comparable to the reduction in STWs loadings of SRP for a period of up to four years. Of the two remaining rivers, one experienced no major change in point source P loadings from 1974 to 1991, over which time a steady increase in SRP loadings occurred, while a similar SRP loading increase was also detected in the remaining river over a 12 year period when point sources were quite constant. In all river catchments, long-term changes in loadings or the short-term response to point source reductions could only be made after taking into account the variation in river SRP loads due to annual flow variations. The rates at which SRP loadings were increasing was in the range 1.30 to 1.45 kg P km−2 yr−1.