Tidal river Lagan and Belfast Lough

Dissolved oxygen parameters in the water column; Concentration of polychlorobiphenyls (PCBs) in the water column; Salinity of the water column; Temperature of the water column; Concentration of other organic contaminants in the water column; Concentration of polycyclic aromatic hydrocarbons (PAHs) in the water column

Discrete water samplers; salinity sensor; dissolved gas sensors

The McConnell Weir was completed around 1937 in the River Lagan in Belfast. Before the Weir was erected there existed exceedingly unsightly mud- flats upon the banks of the river which were visible from the inner city of Belfast especially at low water. The Weir was founded basically to serve an aesthetic purpose and the environmental consequences were not adequately considered. As a result the Weir has added to a serious pollution problem within the River. Since the building of the Weir, the degree of industrial, agricultural and domestic outputs to Belfast Lough via the River Lagan has increased dramatically. The Weir has not only aided and abetted in the trapping of organic and other forms of pollution, but has compounded a stratification problem within what has become the semi-tidal impounded reach enclosed by the Weir. The hydraulic behaviour of the impounded reach results in the formation of an 'intermediate salt wedge' which forms a partition between a fresher upper layer of water and an older trapped layer of high salinity water at the bottom. The organically enriched sediments exhibit a naturally high Sediment Oxygen Demand (SOD) and soon strip the isolated bottom layers of water of oxygen; once anoxic then nitrogen compounds are utilised anaerobically, eventually sulphur compounds are utilised. The ramifications are that the impounded reach of the Lagan gives off the obnoxious smell of hydrogen sulphide especially during the summer months when low flows and high temperatures predominate. The salinity and dissolved oxygen structure of the River has attracted rather intense interest of late and will continue to do so for the foreseeable future, particularly in the light of a new weir which is being built further downstream as part of a campaign to solve some of the problems.

Northern Ireland Environment Agency, Water Management Unit
17 Antrim Road
Lisburn
County Antrim
BT28 3AL
United Kingdom