Water Quality at a Sludge Entrenchment Site^a

Abstract

Surface‐water and ground‐water quality were evaluated at a site before and for four years after the area was used for entrenchment of sewage sludge. The soils in the area are sandy and are underlain by a clay barrier. Depth of the water table which in most instances is above the clay barrier is from 1.0 to 13.0 m. Water samples taken from monitoring wells, two drainage tiles located along the perimeter of the sludge trenches, a catchment pond and a nearby stream were analyzed for NO3‐N, NH4‐N, and CI. Increases in Cl concentrations were detected in shallow wells within the trench site perimeter 12 months after sludge entrenchment. Chloride levels peaked approximately 18 months after entrenchment and levels declined but not to background levels four years after entrenchment. Nitrate levels increased in shallow wells located within or near the trench site perimeter at 18 months after entrenchment and peaked at 30 months. Decreases in NO3‐N occurred thereafter but had not reached background levels in some wells. Ammonium increases were also detected in wells recording Cl and NO3‐N increases but NH4‐N increases were inconsistent. Increases in Cl and NO3‐N levels were recorded in wells potentially below the trench area, but these levels were less than those recorded for wells within the trench site perimeter. The data from this study indicate that contamination of ground water by leachate from sludge trenches was within the trench site perimeter with a lesser degree of contamination recorded in wells below the trench site. The drainage tiles and clay barrier may have had a significant effect on the resulting ground‐water data, and caution is advised in extrapolating these results to other locations.

Transformations of Nitrogen and Carbon in Entrenched Biosolids at a Reclaimed Mineral Sands Mining Site

“…Leaching of NO 3 –N significantly increased in AD and LS treatments compared with the control shortly after the biosolids application in August 2006 ( P < 0.0001). This was similar to other studies on surface application of high biosolids rates and biosolids entrenchment (Huser, 1977; Sikora et al, 1978; Daniels and Haering, 1994; Stehouwer et al, 2006; Felton et al, 2008). However, the magnitude of NO 3 –N leaching was higher compared with the biosolids trenching study by Felton et al (2008), likely due to higher aeration status of the biosolids seams in the coarse‐textured soils which favored nitrification.…”

“…This indicates that increased redox potential and the aeration status of the biosolids seams is not the only condition for release of N that poses groundwater‐quality risks. Data show that dissolved organic N can leach to adjacent soil, where pH and Eh conditions promote mineralization and nitrification and pose groundwater‐quality risk (Sikora et al, 1978; van Kessel et al, 2009).…”

“…Application of high rates of biosolids to land with no actively growing vegetation typically results in high N leaching rates (Huser, 1977; Stehouwer et al, 2006). Previous experiments on the DRI of biosolids were conducted on low‐permeability soils, which reduced the transformations within the biosolids seam and lacked reliable lysimetric measurements of N leaching (Sikora et al, 1978; Felton et al, 2008).…”

“…Furthermore, no studies were conducted at the heavy mineral mining reclamation sites, mostly represented by the coarse‐textured soils with shallow groundwater (Daniels et al, 1996), which are the primary reclamation sites of interest in the mid‐Atlantic Coastal Plain region. The changes in the net amounts and forms of N and C retained in the DRI biosolids have never been quantified, and previous studies on the DRI biosolids (Sikora et al, 1978; Felix et al, 2008; Felton et al, 2008) provided much speculation about the possible benefits and environmental effects of the DRI. It is important to understand the C and N dynamics of the DRI biosolids to assess the potential of N for the groundwater quality impairment.…”