Regular Tillage Research Articles

Full‐scale in situ bioremediation of hexachlorocyclohexane‐contaminated soil

AbstractDaramend bioremediation technology was used to treat 1100 tonnes of hexachlorocyclohexane (HCH)‐contaminated soil at a former lindane manufacturing plant. Half of the site (area A) was treated using a cycled anoxic/oxic treatment, and the other half (area B) was treated under oxic conditions. Each area was divided east to west into five zones. A control area (area C) consisted of strips of soil along the north and east edges of the site. Total HCH concentrations along a west to east gradient ranged from 22 430 to 1069 mg kg−1 in area A and from 21 100 to 730 mg kg−1 in area B. Concentrations in area C ranged from 52 to 1427 mg kg−1. The soil was treated for 371 days, during which time seven anoxic/oxic cycles were completed in area A and regular tillage was performed on area B. Soil samples (one per zone) were collected after 154 and 371 days of treatment. After 371 days, total HCH concentrations were reduced in the most highly contaminated zones of areas A and B by 60% (from 22 430 to 8910 mg kg−1) and 75% (from 21 100 to 5120 mg kg−1), respectively. The average HCH reductions for all five zones of areas A and B were 40 and 47%, respectively, with the data indicating decreased concentrations of selected isomers in certain zones of both areas. Less substantial changes in HCH concentrations were observed in control area C. Elevated chloride ion concentrations were observed in zones that had demonstrated HCH removal. This full scale project demonstrated the potential for solid phase bioremediation treatment of soil containing high HCH concentrations. Copyright © 2005 Society of Chemical Industry

Effect of cropping and tillage on the dissipation of PAH contamination in soil

This study examined the effect of regular tillage and cropping on the dissipation rate of PAHs in contaminated soil. Lysimeters were placed under natural climatic conditions for 2 years and designed to measure the concentration of PAHs in soil and leachates and their toxicity. The soil initially contained 2077 μg PAHs g −1. The largest decrease in PAHs concentration occurred during the first 6 months. No further significant decrease was observed after this time. The surface soil layer always contained significantly less PAHs than the deeper layer, regardless of the treatments. Less than 8.4×10 −8% of the PAH initially present in the soil (e.g. less or equal to 33 μg PAHs per lysimeter) were leached from the soils during the experiment and the leachates presented no toxicity (as measured by the Microtox® test). The toxicity of the soils decreased with time and was significantly lower on the cropped soil compared to the other treatments, despite the residual concentration of PAHs being the highest in this soil. This study demonstrated that the dissipation rates of PAHs were slow after using natural attenuation even when tillage and cropping were performed at the soil surface.

Restoration of Petroleum-Contaminated Soil Using Phased Bioremediation

A conceptual approach is presented for the restoration of petroleum-contaminated sites by combining bioremediation with revegetation using native plants. Phased bioremediation includes active and passive treatment options for soil containing greater than 1% total petroleum hydrocarbons (TPHs). Phase I is used when initial soil TPH exceeds 1%. Phase I utilizes either active land treatment, with regular soil tillage, or passive bioremediation to attain a treatment endpoint of 1% soil TPH. Passive treatment utilizes static soil and TPH-tolerant plants. Phase II is utilized when soil contains 1% TPH or less. It combines passive bioremediation with revegetation using native plants to complete the site restoration process. The phased approach to bioremediation was developed from results of full-scale field bioremediation and laboratory treatability studies. This approach assumes that the kinetics of TPH biodegradation are initially rapid, followed by a much slower second stage. It provides active initial treatment, followed by lower-cost passive treatment. The selection of either active or passive treatment in Phase I depends on whether total cost or time of treatment is more important. Passive treatment, although less costly than active treatment, generally requires more time. Phased bioremediation may provide a flexible, cost-effective, and technically sound approach for restoration of petroleum-contaminated sites.Vegetation used with passive bioremediation has several benefits. Plants stabilize soil, preventing erosion and thereby minimizing exposure to soil contaminants. Phytoremediation may also occur within the rhizosphere. The use of native plants has a strong ecological basis. They provide ecological diversity, are aesthetically pleasing and beneficial to wildlife, while requiring little maintenance. Phased bioremediation can provide a flexible, cost-effective, and technically sound approach for the restoration of petroleum-contaminated sites.

Decomposition of α- and β-hexachlorocyclohexane in soil under field conditions in a temperate climate

A field experiment was made to establish whether in temperate climates the microbial degradation of hexachlorocyclohexane (HCH) occurs at a rate suitable for soil sanitation. Contaminated sandy loam soil (6 m 3) was collected sieved and homogenized. The initial pollution levels were 367 ± 60 mg α-HCH kg −1 and 280 ± 25 mg β-HCH kg −1. After 23 weeks of field exposure α-HCH in moist soil (water content 6–18%) decreased to 107 ± 8 mg kg −1, and in slurry (water content 50%) to 62 ± 11 mg kg −1. In anaerobic slurry, α-HCH was not biodegraded. β-HCH concentration was not affected under aerobic or anaerobic conditions. Regular tillage of soil, covering with black PVC film or extra aeration did not affect degradation in moist soil. However regular stirring slightly accelerated degradation in the aerated slurry. The total number of microorganisms remained more or less constant but the number of α-HCH degrading organisms in moist soils and slurry increased from 10 3 to 10 7 g −1 dry soil. Basal respiration increased in moist soil but was constant in aerated slurry. The decomposition rate of glutamic acid increased with a decreasing concentration of α-HCH. It was concluded that degradation of α-HCH in moist soil (land-farming) and aerated slurry was possible in a temperate climate and that cultivation or covering with black PVC film were not useful.

Tillage effects on root development, water uptake and growth of oats

The experiment was carried out in 1976 on a well-drained, loess-derived soil. The general objective was to study the interrelation between root development, water uptake and shoot growth of oats ( Avena sativa L.) under field conditions during one growing season. A specific purpose was to determine if regular tillage induces differences in rooting pattern, water uptake and plant growth as compared to untilled soil. In tilled soil a plough-sole layer at 20–30-cm depth induced higher rooting densities within the 10–20-cm layer, but restricted proliferation of roots in deeper layers. Accordingly, total water uptake from the 10–20-cm layer was greater and from the 20–60-cm layer it was less than from untilled soil. Water uptake was particularly limited in the plough-sole layer. The water uptake rate was functionally related to rooting density and soil water potential. Relative growth rate of root length decreased with increasing soil water tension and ended at approximately 19 bar. Tillage favored initial shoot growth, but in June accelerated shoot growth on untilled soil was associated with higher evapotranspiration and a deeper soil exploration by roots. Shoot growth rate was linearly related to transpiration rate. One mm of water use corresponded to a production of 40 kg/ha dry matter.