Sedimentation Patterns and Mechanisms of Contaminant Transport in Lough Neagh, Northern Ireland

Abstract

The total sedimentation rate was measured in Lough Neagh, Northern Ireland, by a series of ten sediment traps over one year. The total sedimentation rate included both primary and secondary sedimentation processes. The annual average sedimentation rate measured was 53.2 ± 44.1 g m-2 d-1, more than 20 times the estimate inferred from dated sediment cores. The depth of the wave mixed layer, a function of wind speed, wind direction and effective fetch, is the strongest influence on the amount of resuspension in the Lough. South westerly winds caused the largest sedimentation rates. The spring algal bloom increases sedimentation significantly, but this is small compared to storm events. A timeseries interval trap showed that short duration wind events cause high sedimentation rates (maximum 275 g m-2 d-1) which return to normal values within twelve hours of the wind speed dropping.

The importance of contamination and performance testing is important when measuring the trace metal concentrations in the water column. The annual average concentration of Total Cu, Pb and Cd in the water column of Lough Neagh was 2.68 ± 1.07, 0.450 ± 0.420 and 0.011 ± 0.011 μg/1 respectively.

Contaminant flux measured by the sediment traps was closely related to the sedimentation rate (r2 ~-alues of 0.95, 0.67, 0.96 and 0.65 for Cu, Pb, Zn and Cdrespectively). Though contaminant concentrations in the sediment varied seasonally, overall average concentrations were almost identical to those in sediment cores. Average concentrations in the sediment from the continuous sediment trap study for Cu, Pb, Zn and Cd were 80 ± 17, 46 ± 42,240 ± 42 and 2.13 ± 1.00 μg g-1 dry weight. Average concentrations of Fe, Mn and P were 58 ± 13, 9.95 ± 10.30 and 4.1 ± 1.6 mg g-1 d ry weight respectively. River discharge does not affect contaminant nux. Dia tom blooms were shown to be capable of incorporating large amounts of trace metals and could be responsible for variations in the trace metal concentrations. Redox reactions involving Fe and Mn also affected trace metal concentrations in the sediment, especially during the summer months. Average distribution coefficients for Cu, Pb and Cd were 34 ± 15, 156 ± 177 and 101 ± 66 g/1 respectively. Pb has the shortest residence time in Lough Neagh calculated with respect to sedimentation from dated sediment cores.

Date of Award	1997
Original language	English
Supervisor	Brian Rippey (Supervisor)