Saturday, 15 July 2006

Assessing the Risk of Soilborne Heavy Metals Leaching in an Andosol after Sewage Sludge Spreading.

Emmanuel Doelsch1, Frédéric Feder1, Antoine Findeling1, Yves Dudal2, and Hervé Saint Macary1. (1) CIRAD (French Agricultural Research Centre for International Development), Station de la Bretagne, BP 20, F-97 408 Messagerie Cedex 9, Reunion, Saint-Denis, France, (2) INRA (French National Institute for Agricultural Research), Domaine Saint-Paul, site Agroparc, F-84 914 Cedex 9, Avignon, France

In Réunion, volcanic island in the Indian Ocean, about 70% of the arable land area is covered by andic soils. Sewage sludge spreading on these soils is generally prohibited because heavy metal content of soils were above the standard threshold values set by French legislation (Doelsch et al., In Press). To assess the risk of heavy metals leaching after sewage sludge spreading, we circumvented this restriction by working in the laboratory with an original column set-up. A Silic Andosol from Sainte-Rose (SE part of island) was packed in three large columns (0.375 m dia. × 1 m long PVC tube) according to observed in situ horizons and bulk density. Sewage sludge was spread at the surface of columns C1 and C2 (5.4 T/ha dry matter equivalent to 350 kg N/ha), while the control column C3 remained untreated. This amendment represented 8 and 6% of the total initial soil content for Zn and Cu respectively, and only ca. 1% for Ni and Cr. The three columns were maintained at 25°C over a 4-month period during which they received water applications that matched the rainfall conditions of Réunion. The columns were equipped equally to monitor water and solutes contents and fluxes. TDR probes and tensiometers were set at 15, 35 and 45 cm depth to measure soil moisture and matric potential. Tipping bucket raingauges were placed at the outlet of the column to measure drainage flow. All these instruments were connected to a datalogger which stored measurements hourly. After each watering, soil solution was collected in the columns with Rhizon soil-solution microsamplers (12, 20 and 37.5 cm depth) and in drainage water. These samples were used to measure Eh, pH, electrical conductivity (EC), dissolved O2, dissolved organic carbon (DOC), and Zn, Cu, Cr and Ni contents. The water budget terms were well estimated for the three columns. The water fluxes were essentially gravitary since the water matric potential was always close to saturation at 45 cm depth. Different hydraulic patterns appeared from day 56 to day 93: C2 and C3 experienced ponding conditions whereas C1 remained unsaturated. For the three columns the variations of pH and Eh were negligible at all depths after 20 days while dissolved O2 decreased slowly from 4 to 1 mg/L for C2 and C3 and stayed close to 3 mg/L for C1. At 12 and 20 cm depth, EC increased up to 1.1 mS/cm for C2 and C3. For the two amended columns, the peak concentration of DOC (180 mg/L) was reached after 90 days at 12 and 20 cm depth but no breakthrough was observed at 37.5 cm (DOC ca. 10 mg/L). However, in soil solutions, Cr was not mobile (Cr < 0.005 mg/L) and Ni and Zn concentrations were always very low (Ni < 0.02 mg/L and Zn < 0.1 mg/L). Total amounts of leached metals were calculated on the basis of measured water flows and concentrations. Massive input of sewage sludge induced no substantial heavy metal leaching flux at any depth, in spite of the high initial metals content of the Andosol. To conclude, leaching risk of heavy metals appeared low for this soil (short term experiment). Longer experiments and investigation on metals speciation is required though to back up this result. Acknowledgements: This work was conducted within the framework of a CIRAD-MVAD collaboration agreement, with funding support from FEOGA, ADEME, the Chambre d'Agriculture de La Réunion, the Region Réunion and the Département de La Réunion. Reference: Doelsch, E., Van de Kerchove, V., Saint Macary, H. Heavy metal content in soils of Reunion (Indian Ocean). Geoderma. In Press, Corrected Proof.

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