Industrial Mercury in Combination with Natural Pb210 as Time-dependent Tracers of Sedimentation and Mercury Removal from Haifa Bay, Israel

The input rate of a particle reactive pollutant, mercury, from a chloralkali plant, which was known in detail, was used in combination with Pb210 to determine the sedimentation and mixing rates in several locations in the area of a factory outfall. Sediment profiles were modelled using a numerical solution to the diagenetic equation. The calculated sedimentation rate of 0•5 cm year-1 was close to values based on estimates of regional sand transport. Surface mixing rates of 7-8 cm2 year-1 with an exponential decrease with depth are similar to rates estimated in other near-shore areas. An atmospheric flux of excess Pb210 to the area of 0•41 dpm cm-2 year-1 was estimated from the inventory in the sediment from two nearby fishponds. The model predicted that despite the input of 0•5 cm year-1 of clean sand into the area, the level of mercury in surface sediments would remain 0•3 ppm above background (precontamination) levels for more than 50 years after input controls were introduced due to particle mixing. Unlike most previous studies on the discharge of mercury into the near-shore environment, this receiving area is one of relatively coarse sandy sediment and particle poor oceanic water. The majority of the mercury in the sediment was not attached to fine-grained particles. Only 10% of the mercury supplied in the effluent, was found in the sediments within a 150 km2area adjacent to the outfall. It was hypothesized that there were insufficient fine-grained particles to remove dissolved mercury from the water column. Thus the principal pathway of even particle reactive pollutants to the biosphere in this coastal area may be via the planktonic food-chain, with the benthic food-web being important only locally.