M.E.F. van Mensvoort

Contamination of surface water as affected by land use in acid sulfate soils in the Mekong River Delta, Vietnam.

Leaching toxic substances out of the rootzone is an effective measure for improving soil quality and crop yield in acid sulfate soils (ASS). However, leaching of ASS may pollute the environment. We quantified the concentration and amount of pollution from leaching of ASS for rice, pineapple and yam cultivation in a Typic Sulfaquept in the Mekong River Delta, Vietnam. The fields studied were originally uncultivated and had been reclaimed 2 months, and 1 and 2 years prior to the initiation of the study. Pineapple and yam were cultivated on raised beds constructed using soil excavated from adjacent lateral ditches. The pH of the drainage water ranged from 2.9 to 3.9 and aluminum concentration from 3 to 13 mmol(+) 1−1. The mean monthly aluminum concentration in the leachate from pineapple and yam raised beds was about three times greater than that from rice fields. The monthly total amount of aluminum released by the upland raised beds could be as high as 16 690 mol ha−1, and was three to five times greater than that from rice fields. Consolidation and crust forming in pineapple raised beds reduced the concentration and amount of aluminum released with respect to the age of the raised beds. Pollution from ASS leaching was probably greatest in June due to a combination of the highest total amount of aluminum released to the canal network and low river discharge. Environmental hazards make it imperative to plan carefully the reclamation of ASS, such that the toxicity carrying capacity of the surface water is not exceeded.

Soil and water table management effects on aluminum dynamics in an acid sulphate soil in Vietnam

Abstract Understanding the process of toxicity accumulation in the root zone is important in improving the quality of acid sulphate soils. The effect of straw mulching, soil surface ploughing and water table depths on aluminum dynamics during the dry season and the first three weeks of the rainy season 1994 were studied in field and lysimeter conditions in an acid sulphate soil, in Mekong Delta, Vietnam. More acidity was produced in the subsoil when the water table was maintained at 60 and 90 cm compared with 30 cm deep. The amount of aluminum that accumulated in the topsoil during the dry season was linearly proportional to the cumulated evaporation. Straw mulching significantly lowered the dry season aluminum accumulation (0.6 cmol(+) kg−1) in comparison with nonmulching treatment (2.0 cmol(+) kg−1). Surface ploughing also reduced aluminum accumulation, but the reduction was not statistically significant compared with nonploughed plots. Rains at the beginning of the rainy season raised the water table rapidly to the topsoil layer. Aluminum brought up with the rising water table greatly increased the toxicity level in the topsoil and nullified the positive effects of dry season soil treatments. Controlling groundwater level to below the topsoil and enhancing the leaching effects at the beginning of the rainy season are important for retaining the effects of the dry season treatments.

Environmental impacts of abandoned dredged soils and sediments; available options for their handling, restoration and rehabilitation

Aim and backgroundIn the process of creating safe navigable waterways for oil exploitation, the companies operating in the Niger Delta generate tons of sulfidic spoils. These are often deposited over bank, mostly upon fringing mangroves, and abandoned. This leads to a myriad of environmental problems. The extent of these impacts is not exactly known, but was inferred from the activities of oil companies operating in the area. This paper describes the impacts following the disturbance of sulfidic sediment through dredging and by subsequent poor spoil management practices.Environmental impacts of exposed and abandoned sulfidic sediments. The practice of dumping and abandoning sulfidic dredged spoils along canal banks triggers a series of environmental problems leading to extreme acidification, heavy metal pollution, and general habitat degradation which prevent the re-colonization of the sites by native species. The resultant spoil dumps remain bare for several years and they become colonized by invasive species later. Later still they may become attractive to the local population as sites for houses, fishing camps and home gardens, which is nevertheless regarded as a positive impact.Management of abandoned sulfidic spoil dumps. As a panacea to environmental problems caused by the sulfidic spoils, they need to be properly handled to prevent their acidification. This should be followed by mangrove restoration to pristine conditions or rehabilitation to other beneficial uses.Conclusion and Recommendation. The abandoned sulfidic spoils in the Niger Delta have resulted in the death of several thousands of hectares of mangrove habitat and their associated biota. Despite the increasing scale of dredging and spoil abandonment, the accompanying problems have been recognized by neither the oil companies nor by national government. This paper attempts to highlight these problems and concludes by recommending sound spoil management strategies involving the handling, restoration and rehabilitation of impacted areas. Furthermore, possible beneficial use of dredged spoils for civil construction, beach nourishment / shoreline protection is identified. The Nigerian Government needs to promulgate and enforce laws that will permit the sustainable use of mangrove resources.

Decision trees for farm management on acid sulfate soils, Mekong Delta, Viet Nam

Our study shows how farmers in the Mekong Delta, Viet Nam, have developed new cropping systems and management practices to overcome the constraints of their land. Decision trees for land evaluation at farm level, combining farmer and expert knowledge, were developed to support management decisions and to indicate possibilities to improve land suitability. Our study concentrates on 2 districts, Tan Thanh (fresh water, deep flood, acid soils) and Hong Dan (acid soils, shallow flood, salt surface water). We recommend double rice, yam, sugarcane, and Melaleuca cajiputi for Tan Thanh, and double rice, rice combined with shrimp, single shrimp or crab, sugarcane, bamboo, and Melaleuca for Hong Dan. We show the land-related constraints and how they can be lifted.