EVALUATION OF THE MOBILITY OF POLLUTANTS, NUTRIENTS AND PATHOGENS FROM BIOSOLIDS TO GROUNDWATER AFTER LAND APPLICATION

The mobility of constituents of land applied biosolids to groundwater was investigated to evaluate potential impacts on the groundwater quality. A literature review indicated a potential for the leaching of nitrate from biosolids because of its relatively high mobility in soil. However, the impact on groundwater is minimal because the Part 503 Biosolids Rule requires land application of biosolids at nitrogen-based agronomic rates to balance nitrogen supply with the uptake by crops. The potential for leaching of other biosolids constituents to groundwater is relatively insignificant because of immobilization in soil (phosphorus, heavy metals, some organic pollutants), biodegradation or volatilization to the atmosphere (some organic pollutants), or inactivation (pathogens). Groundwater monitoring data from several land application sites in the USA with over 10 years of land application confirmed the literature survey findings. Elevated nitrate concentrations in groundwater were sometimes observed, however, an unequivocal correlation between nitrate in groundwater and the land application of biosolids could not be established. The presence of other biosolids constituents in groundwater has never been reported.The second part of this contribution discusses the City of Los Angeles' biosolids land application program at the Green Acres Farm in Kern County, California. From 1994, the City of Los Angeles applied Class B biosolids from the Hyperion Treatment Plant (HTP). In response to Kern county regulations, HTP was converted to thermophilic anaerobic digestion to produce Exceptional Quality (EQ) biosolids since October 2002. EQ biosolids are considered to be as safe as any other fertilizer and can be land applied without site restrictions. Groundwater monitoring at the Green Acres Farm over 1990-2002 did not indicate any effect of biosolids land application on the groundwater quality. This can possibly be attributed to a combination of two factors: a) low concentrations of pollutants in HTP biosolids; b) limitation of the biosolids application rate to the agronomic rate as calculated from the nitrogen needs of the crops cultured on the Green Acres Farm. Future effects on the groundwater quality at the Green Acres Farm are considered to be very unlikely because since October 2002 only the highest quality biosolids has been applied.Overall, this study confirms the findings of the 2002 reviews of the U.S. EPA Part 503 Biosolids Rule by the National Research Council (NRC) that there is no scientific evidence that the Part 503 Biosolids Rule has failed to protect public health. Hence, the potential impact of biosolids land application on the groundwater quality is minimal if biosolids are land applied according to the requirements of the Part 503 Biosolids. However, the 2002 NRC review also identified several areas for further research to reduce uncertainties about the potential for adverse human health effects from exposure to biosolids.