Dissolved organic carbon in the unsaturated zone under land irrigated by wastewater effluent

Field evidence indicates that high concentrations of dissolved organic carbon (DOC > 20 mg/L) are found throughout the 30 m thick unsaturated zone of a phreatic aquifer under land irrigated with wastewater effluent. The DOC is not correlated with the sedimentary organic carbon and therefore is related to the liquid phase. The latter is divided into two fractions: one is associated with clay and has a constant ratio to the sediment surface area (1 X 10^-3 mg/m²) and the second is suggested to be particulate organic matter found in the sands. The source of DOC can be either the irrigation water or a result of biochemical interactions in the sediments. The DOC is mobile through the unsaturated zone and is oxidized when it reaches the water table. Field data showing DOC persistance under unsaturated conditions and laboratory BOD tests of sediments from the unsaturated zone suggest that moisture content may be a major controlling factor in biodegradation. The findings question the suitability of irrigation with water containing a high DOC on the replenishment area of phreatic aquifers.