Leaching to the groundwater can become a problem if the groundwater is used for water supplies. Transport of the metal by the groundwater to surface waters can affect the natural environment in these or limit the use of the surface water for various purposes. It is widely reported that the treatment of soils with sewage sludge often results in increased concentrations of metals in the food chain. This is particularly true for Cd, Cu, Ni and Zn which are considered to be phytotoxic. The Tolerance level for plants is the level at which growth reduction occurs or effects can be seen on animals feeding on the plants.
Plants differ in their ability to take up, accumulate and tolerate heavy metals.
Also, a plant acts differently with respect to each heavy metal. The transfer coefficient of a heavy metal is defined as the ratio of the concentration of the metal in a plant to the concentration of the metal in the soil, the plant has been grown in. This number reflects the availability of the specific metal in a given soil-plant system. Table 2 shows transfer coefficients, soil concentrations that are considered to be critical for plant growth and animal consumption, regulatory soil concentrations and some suggested tolerance levels for some of the heavy metals. The values are not to be considered as exact, but more as an example of such values.