Groundwater Pollution from Dredged Material Disposal: Retardation of Pollutants in Natural Barriers

Summary

The groundwater-pollution from dredged material disposal areas of Hamburg is depending on the following processes: mobility of pollutants in the pore water of the disposed material, rate of infiltration in the aquifer through barriers (dredged sludge and marsh sediments) and retardation of pollutants within the barrier and in the aquifer. The significance of these processes was studied in a 5-year-investigation of leachate-and groundwater-composition in an area near Hamburg harbour, which is dominated with upland dredged material disposal /1/. From 1984–1989 the following data were collected:

• leachate composition from the water saturated bottom part of the dredged material: 183 samples from 67 wells.

• groundwater composition from the first aquifer (the wells were situated in different positions in regard to the disposal area: upstream, undersite, downstream): 588 samples from 149 wells.

The samples were analysed on inorganic major, minor and trace constituents of the water.

The results showed that there is a strong spatial variability in the mobility of pollutants in the leachate as well as in the intensity of groundwater pollution /2/. Therefore the groundwater analysis of each well were averaged (median) and then grouped by cluster-analysis. The pollution indicating groups are shown in Tab.1.

The analysed parameters are higher concentrated in the leachate than in upstream groundwater (exceptions nitrate: not detectable in both water typs, iron: higher concentrated in upstream groundwater). Groundwaters of groups A - D are influenced by the dredged material disposal. For these groups the degree of influence can be calculated from the concentration of chloride. Asuming that the undersite and downstream groundwater only results from mixing of leachate with natural upstream groundwater and using chloride as a tracer it is possible to calculate the mixing portion and the retardation in the natural barriers for the other parameters. Tab. 2 shows the calculated values (in % of the concentration in the leachate).

By migrating through the marsh sediments and flowing through the aquifer most constituents of the leachate are reduced in concentration: By sulfate-reduction sulfate is nearly completely (to group C) reduced. About 35% of the bicarbonate is reduced by precipitation. The fine-textured marsh sediments are strongly influencing the kation concentration by kation-adsorption: This process increases in the following list: Na⁺ < < Mg²⁺ ≈ Ca²⁺ < < NH₄ ⁺ < K⁺. While only 7% of the sodium is adsorbed in the marsh sediments, this portion is 85% with potassium. In most of the investigated groundwater positions the arsenic content of the leachate is bound in the bottom sediments of the dredged material disposal. Iron is the only parameter wich shows increasing concentrations (resulting from declining redox-potentials?) when the leachates migrate through the marsh-sediments.