Field-scale Variation in Colloid Dispersibility and Transport: Multiple Linear Regressions to Soil Physico-Chemical and Structural Properties.

Water-dispersible soil colloids (WDC) act as carriers for sorbing chemicals in macroporous soils and hence constitute a significant risk for the aquatic environment. The prediction of WDC readily available for facilitated chemical transport is an unsolved challenge. This study identifies key parameters and predictive indicators for assessing field-scale variation of WDC. Samples representing three measurement scales (1- to 2-mm aggregates, intact 100-cm rings, and intact 6283 cm columns) were retrieved from the topsoil of a 1.69-ha agricultural field in a 15-m by 15-m grid to determine colloid dispersibility, mobilization, and transport. The amount of WDC was determined using (i) a laser diffraction method on 1- to 2-mm aggregates and (ii) an end-over-end shaking method on 100-cm intact rings. The accumulated amount of colloids leached from 20-cm by 20-cm intact columns was determined as a measure of the integrated colloid mobilization and transport. The WDC and the accumulated colloid transport were higher in samples from the northern part of the field. Using multiple linear regression (MLR) analyses, WDC or amount of colloids transported were predicted at the three measurement scales from 24 measured, geo-referenced parameters to identify parameters that could serve as indicator parameters for screening for colloid dispersibility, mobilization, and transport. The MLR analyses were performed at each sample scale using all, only northern, and only southern field locations. Generally, the predictive power of the regression models was best on the smallest 1- to 2-mm aggregate scale. Overall, our results suggest that different drivers controlled colloid dispersibility and transport at the three measurement scales and in the two subareas of the field.

Transport of di(2-ethylhexyl)phthalate (DEHP) applied with sewage sludge to undisturbed and repacked soil columns.

Municipal sewage sludge is often used on arable soils as a source of nitrogen and phosphorus, but it also contains organic contaminants that may be leached to the ground water. Di(2-ethylhexyl)phthalate (DEHP) is a priority pollutant that is present in sewage sludge in ubiquitous amounts. Column experiments were performed on undisturbed soil cores (20-cm depth x 20-cm diameter) with three different soil types: a sand, a loamy sand, and a sandy loam soil. Dewatered sewage sludge was spiked with 14C-labeled DEHP (60 mg kg(-1)) and bromide (5 g kg(-1)). Sludge was applied to the soil columns either as five aggregates, or homogeneously mixed with the surface layer. Also, two leaching experiments were performed with repacked soil columns (loamy sand and sandy loam soil). The DEHP concentrations in the effluent did not exceed 1.0 microg L(-1), and after 200 mm of outflow less than 0.5% of the applied amount was recovered in the leachate in all soils but the sandy loam soil with homogeneous sludge application (up to 3.4% of the applied amount recovered). In the absence of macropore flow, DEHP in the leachate was primarily sorbed to mobilized dissolved organic macromolecules (DOM, 30.3 to 81.3%), while 2.4 to 23.6% was sorbed to mobilized mineral particles. When macropore flow occurred, this changed to 16.5 to 37.4% (DOM) and 36.9 to 40.6% (mineral particles), respectively. The critical combination for leaching of considerable amounts of DEHP was homogeneous sludge application and a continuous macropore structure.

Degradation of 4-nonylphenol in homogeneous and nonhomogeneous mixtures of soil and sewage sludge.

Sewage sludge is frequently applied as fertilizers to cultivated land. However, municipal sewage sludge often contains organic contaminants including nonylphenol (NP), an intermediate from nonionic surfactant degradation. Knowledge about NP degradation in sludge-amended soil is an important prerequisite for adequate risk assessments. In this study, mineralization of 14C-labeled NP in homogenized and nonhomogenized sludge-soil mixtures was investigated. NP was degraded within 38 days in aerobic homogenized mixtures. In nonhomogeneous mixtures containing sludge aggregates, the degradation of NP was retarded and was generally not completed within 3 months (119-126 days). No detectable amounts of NP were transported from the sludge aggregates to the surrounding soil (detection limit: <0.04 mg of NP/kg dw of soil). Oxygen penetration into sludge aggregates was monitored for 50 days with an oxygen microelectrode. An extrapolation of the oxygen data suggested that more than 1 year was required to obtain fully aerobic conditions in a 2-cm sludge aggregate. Since NP is considered persistent in the absence of oxygen, residual amounts of NP may be present in the anaerobic center of aggregates for prolonged periods. The results demonstrate that sludge aggregate size and thus oxygen availability will be a major controlling factor for NP degradation in soil amended with sewage sludge and that the mobility of NP from sludge aggregates to the surrounding soil is negligible.