Spatial and temporal variations in trace metal concentrations in sediments, pore water of the Forth estuary, and their potential impact on water quality
The concentrations of trace metals: cadmium, copper,
nickel, lead and mercury, in addition to aluminium, iron and
manganese were determined in the sediments and pore water
(except aluminium) of the Forth estuary. Organic matter (as
loss on ignition), total organic carbon (TOe), total organic
nitrogen (TON), particle size and redox potential were also
determined in the sediments. In addition, chloride, sulphate,
conductivity, alkalinity, pH, dissolved organic
carbon, phosphate and nitrate were determined in the
sediments pore water. Trace metals in the pore water were
determined using a modified preconcentration technique and
measurements were carried out by graphite furnace. Mercury
was measured using the p.s.a mercury specific fluorescence
detector. organic matter (TOC & TON) in the sediments were
measured using CHNS/O analyzer.
Samples were collected at seasonal intervals for a
period of 18 months on board the survey vessel "Forth
Ranger". Six stations were selected to represent upper,
middle and lower estuary. Sediment cores collected at each
station were subjected to centrifugation under nitrogen gas
atmosphere to separate the pore water after being sectioned
at 2.5 cm intervals for a depth of 10 centimetres.
Concentrations of trace metals in the Forth estuary
varied from station to station and from one season to
another. Mean concentrations for sediments were as follows
(mg\kg): lead, 56.9; cadmium, 0.22; copper, 38.9; nickel,
33.7 and mercury, 1.86. For pore water, the mean concentraxxi
tions (Jlg\l) were as follows: lead, 3.14; cadmium, 1.86;
copper, 31.9; nickel, 24.7 and mercury, 0.075.
Measurements of sulphates, nitrates, phosphate,
conductivity and dissolved organic carbon were used to
examine the processes controlling their distributions in the
pore water. The results indicated that the distributions of
nutrients (phosphate and nitrate) in the upper 10 cm seemed
to be controlled by macro-infaunal irrigation activities and
their values varied according to the overlying water
conditions.
The distributions of trace metals in the sediments were
found to be controlled by particle size and organic matter
both spatially and temporally. Vertical distribution of
trace metals in the pore water were found to be controlled
by redox potential variations especially for iron and
manganese. Concentrations of trace metals (Pb, Hg, Ni, Cd
and CU) in the pore water were found to be higher than that
in the overlying water. Enrichment factors were limited to
10-50 fold increase. Lead and mercury were 10 fold higher
while 50 fold higher were found for nickel, copper and
cadmium.
Benthic flux calculations showed a significant flux of
trace metals both in the summer and winter. However, flux of
copper, nickel and lead tended to increase in the summer.
Spatially, the lower estuary showed a higher flux of trace
metals than the upper and middle reaches of the estuary
