Metals in the Hydrocycle

Trace metals have been transported along the hydrological cycle since the first occurrence of water on the planet earth. Water providing the medium for weathering of the continents as well the medium for transport (together with the atmosphere) of trace metals.

A rough description of the occurrence and behaviour of trace metals in aquatic and terrestrial systems is presented in Fig. 5. In the aquatic environment four abiotic reservoirs are distinguished: the suspended matter, the sediments, the surface waters and the pore waters. These four reservoirs strongly interact with each other. Between the suspended matter and the metals in solution, adsorption/desorption and (co-)precipitation processes take place. The suspended matter and the deposited sediments are interlinked through sedimentation and erosion processes. Processes taking place after deposition (diagenesis) provide the interstitial waters with sometimes high concentrations of trace metals, which through processes such as diffusion, consolidation and bioturbation are able to influence the metal concentrations in the surface waters.

Sediments are important carriers for trace metals in the hydrological cycle, an aspect which will be discussed in chapters dealing with individual parts of the hydro-logical cycle. However, sediment research has certain aspects of its own, which warrants a separate discussion. Therefore, we will discuss those aspects of metals and sediments which are common to all parts of the hydrological cycle.

The influence of the atmospheric deposition on trace metal concentrations is found in all parts of the hydrological cycle (e.g. soils, rivers, lakes, estuaries and the ocean). In this respect the atmosphere plays a unique role. Additionally, transition metals act as catalyst in the atmosphere (e.g. the oxidation of sulfur dioxide) (Graedel and Weschler, 1981; Lindberg, 1981). Compared with other parts of the hydrological cycle, metals in the atmosphere have a short residence time, estimated to vary between days and weeks (Hidy, 1973). However, within this short time span, these trace metals are able to travel large distances.

Although the volume of continental waters is insignificant with respect to the mass of the hydrosphere, these waters have their particular importance as the basis of our freshwater supply as well as geochemically, since they are responsible for most of the weathering and erosion of the land masses. In the latter sense terrestrial waters are the major medium involved in the “exogenic cycle”, i.e. processes on the earth’s surface and uppermost lithological units, in which solid and dissolved weathering products are transported, deposited and solidified, and — after diagenesis and tectonic uplift — may be exposed to weathering and erosion again. While most of these processes of deposition and rock formation take place in the marine milieu, there are characteristic geochemical interactions in the soils and, particularly interesting, in the lacustrine environment.

Like lakes, estuaries are ephemeral in respect to geologic time. They must be regarded as dynamically evolving land-forms that go through a life cycle from valley creation, followed by the drowning phase, and ending up with the progressive backfilling with sediments (Fairbridge, 1980). The origin of most modern estuaries dates back to the rise of the sea level after the last Ice Age. The history of an estuarine system after it has been established as a geomorphological entity is largely determined by its sediment supply (Postma, 1967). Mass particulate balance is controlled by the input to the estuary from rivers, the atmosphere and sea, as well as by the output of substances to the ocean and to the bottom (Goldberg, 1978). Accumulation of sediment — even of fine-grained material — can take place due to protection from wave action and currents. Net shore-ward transport of suspended matter is influenced by waves, tidal movement and a typical estuarine circulation, the latter being caused by differences in specific gravity between river and sea water.

The oceans are the last part of the hydrological cycle and here trace metals are removed from the hydrological cycle and incorporated in the sediments. There they spend several hundred million of years before taking part in the next hydrological cycle.

In the preceding six chapters an overview of recent developments of trace metal behaviour on the earth’s surface has been given. Hydrological cycles have been used as a convenient framework, which enable the various interactions of biogeochemical factors in an integrated system — both natural as well as effected by man’s impact to be presented.