Sampler gives cores for celebration

A new method for sampling the chemical content of ground-water could help environmental scientists track pollutants with far greater accuracy, making it easier to analyse the effects of processes on water supplies and to design clean-up operations.

The system was developed by Israeli scientists to track the chemical and biological content of the Dead Sea - an important source of inorganic chemicals - and the freshwater Sea of Galilee. It gains its accuracy from its ability to measure the concentration of colloids, ultrafine suspensions of particles which often adsorb contaminants onto their surfaces.

Most sampling techniques involve pumping the water into an analyser, explains team leader Daniel Ronen of the Weizmann Institute in Rehovot. But no matter how gentle the pumping, colloids and contaminants form which are not present in the aquifer itself.

Ronen's sampler does not pump the water. The device consists of a rod studded along its length with dialysis cells. These are closed off with membranes with large pore sizes (typically around 10 m) that allow both the water and the colloidal particles to enter (Environmental Science and Technology, 1996, 30, 2846 & 3094).

Over a period if weeks, a dynamic equilibrium is formed between the contents of the cell and the surrounding water. Because each cell fills up with the water immediately adjacent to it, the sampler effectively collects a `core' of water, in the same way that a geologist might collect a core sample of soil. When the sampler is removed from the water, surface tension keeps the water inside the cells for long enough for them to be capped and sealed, allowing it to be removed and analysed later.

The most obvious use for the sampler is to check aquifers near chemical sites, to see whether pollutants have seeped into the water and to monitor how the contaminants are distributed, says Ronen. But the system can also help design clean-up methods. For example, the interior of the dialysis cells can be coated with bacterial strains capable of decomposing the contaminants in the water. The sampler then becomes an array of mini-reactors, testing the ability of the bacteria to destroy the pollutants at various concentrations. The team has already used it to evaluate whether Pseudomonas bacteria can degrade residues of the herbicide atrazine, which contaminates groundwater in many rural areas, and believes it can also be used with any combination of bacteria and contaminant.