Release of nitrogen and trace metal species from field stacked biosolids

Abstract

Concerns over elevated nitrate (NO3—) levels found in groundwater near former biosolid stockpiling locations resulted in the Maine Department of Environmental Protection (MDEP) imposing stricter regulations governing the stockpiling of biosolids in October 2002. The goals of this study were to measure the amount and speciation of nitrogen (N) and trace metals leaving stockpiled biosolids and travelling through the soil column. The biosolids were placed on plastic-lined cells to collect all leachate. Ammonium (NH 4+), ranging from 2000 to 4900 mg L— 1, was the dominant N species (90% of total N) in the leachate from the Class B lime-stabilized biosolids in the lined cell experiment. Nitrate (NO3—) and nitrite (NO2— ) concentrations were negligible, remaining below 0.25 and 0.1 mg L —1, respectively. Dissolved organic carbon (DOC) concentrations as high as 8900 mg L—1 and chemical oxygen demand (COD) as high as 37 000 mg L—1 were measured in the leachate leaving the lined cell. Fifteen zero-tension pan lysimeters (ZTP-lysimeter) were installed in a 90 m2 plot at depth intervals of 30, 60, and 100 cm. Leachate passing through the soil column underlying the biosolids stockpile was collected in the ZTP-lysimeters. The average ZTP-lysimeter NH4+ concentrations ranged from 1400 mg L— 1 at 60 cm depth to 145 mg L—1 at 90 cm depth. The average ZTP-lysimeter DOC concentrations ranged from 2000 mg L—1 at 60 cm to 525 mg L— 1 at 90 cm. Trace metal determinations of the leachate collected from the lined cell and ZTP-lysimeters showed arsenic loading rates exceeded the state limits of 0.5 kg ha—1 year— 1 by an order of magnitude. Arsenic concentrations were in excess of several thousand milligrams per litre in the lined-cell leachate and several hundred milligrams per litre in the ZTP-lysimeters as deep as 90 cm under the biosolid stockpile. Phosphorus, iron and manganese in excess of several thousand milligrams per litre were observed in both the lined-cell leachate and ZTP-lysimeters. Significant concentrations of other trace metals were found at depth in the zero-tension ZTP-lysimeter plot. Trace metals were largely mobilized by the DOC from the biosolids and due to the presence of anaerobic environment, especially in the underlying soil.