|Van Genuchten, Martinus|
Submitted to: World Fertilizer Congress of International Scientific Centre of Fertilizers
Publication Type: Proceedings
Publication Acceptance Date: 7/13/1997
Publication Date: N/A
Citation: Interpretive Summary: The amount of wastes, wastewaters, and sewage sludges produced by agricultural, industrial, and municipal activities is rapidly increasing worldwide. One reasonable and economic way to dispose of wastewaters and sludges is to apply them to agricultural fields, thereby exploiting their water and nutrient content. This practice is becoming increasingly important in many countries. Unfortunately, excessive application of wastewater and sewage sludge to agricultural land may cause soil and groundwater pollution problems by heavy metals and other potentially toxic elements. These elements often occur in large amounts in the wastes, thus limiting their applicability in agriculture. In this study we measured metal concentrations in soil columns contaminated with sewage sludge spiked with cadmium, chromium, nickel, lead and zinc compounds. The proportion of directly plant-available amounts of cadmium, nickel and zinc (as measured in the soil solution) increased sharply at the highest application rate, also in deeper soil layers. This indicates downward transport as well as increased bioavailability. In contrast, chromium and lead entered the liquid phase (i.e., were directly available for the plants) in negligible amounts, even for extreme loadings. Metals extracted from plant tissue indicated much greater availability of the metals than was estimated from the soil solution concentrations. Results are important for assessing the long-term environmental effects (including soil and groundwater pollution) of sewage sludge disposal on agricultural fields.
Technical Abstract: Deposition of waste waters and sludges on agricultural fields is one of the most rational and economic ways to dispose of liquid wastes. However, heavy metals are characteristic contaminants in these materials, limiting their applicability. To have a better understanding of the fate and effects of metal contamination, several aspects of a soil-plant experimental system were studied in laboratory experiments on large, undisturbed soil monoliths. Communal sewage sludge or aliquots enriched with Cd, Cr, Ni, Pb, and Zn nitrates were applied to the top 10 cm to give final metal loading rates in soil equivalent to the limits specified in the Hungarian Technical Directive (Anon, 1990), or 10-, 30-,and 100-fold this amount. Maize and tomato were grown as test plants till maturity. The gas phases in the soil and in the plants' stem were sampled by capillary microsensors directly attached to the inlet system of a quadrupole mass spectrometer (QMS). Lead, ,and especially Cr entered the soil's liquid phase in negligible amounts. Release of Cd, Ni, and Zn into the soil solution as compared to their applied amounts was much higher than of Cr and Pb, and increased substantially at the higher metal application rates in the top 10 cm and somewhat also below the original application zone. The QMS technique was shown to be suitable to follow changes in the soil gas phase under the influence of physical (soil water content) and chemical factors (sludge). Increasing amounts of metals had no significant effect on the soil gas phase in the contaminated soil layer, but presented a stress situation for the plants. The onset and dynamics of plant stress were established on the basis of CO2/O2 ratios in the plants during their life-cycle.