The Prevention Of Resalinization The Reclamation Of Saltwater Spill Sites

Abstract

Abstract After the initial treatment of a saltwater spill site has effected a reduction of soil salinity, a resalinization due to capillary rise ofsoil water can occur in certain circumstances. As an alternative to successive and costly chemical retreatments, prevention of resalinization by means of an organic trash covering of the spill site was investigated in a field study. The results suggest that such a cover prevents serious resalinization of the topsoil, in contrast to untreated sites, although the trend in sodicity is not as clear. Uneven coverage of the soil can to some extent be tolerated. Some implications for field treatment techniques are discussed. Introduction The initial effect of a saltwater spill on a mineral soil is to exchange calcium, magnesium and other ions for sodium ions, primarily on the surfaces of clays and organic matter(l,2). A subsequent freshwater flush of the soil may then cause the swelling of montmorillonite and dispersion and transport of kaolinite, among other effects, resulting in a permeability reduction. The physical chemistry governing this process is identical to that encountered in formation damage to freshwater sensitive reservoirs. A parameter commonly in use in soil science is the sodium adsorption ratio Equation (Available In Full Paper) with aqueous concentrations of ions expressed in millimole per litre (if milliequivalents per litre are used, the mean, rather than the sum of concentrations in the denominator, needs to be used). Impairment of permeability is to be expected if the S.A.R. exceeds 15(3), although sometimes problems are encountered at values greater than 8. The standard approach to reclamation is the addition of calcium salts such as calcium sulphate(l) or the more soluble calcium nitrate (2), preferably prior to the next rain or snowfall (Organic forest soils are an exception, and freshwater flushing is in most cases indicated(4).) The intent of the calcium amendment is to reverse the ion exchange process. In many cases, however, drainage in the spill area is poor, and the installation of a drainage system, with recovery and disposal of the sodium contaminated drain water, is not always a practical alternative. In such cases, therefore, reclamation programs rely on chemical amendment of the spill site soil, with attempts at early establishment of salt-tolerant vegetation, and the expectation of relatively slow drainage of the contaminating sodium salts. Some years ago, we reported the salinity reduction of the topsoil in a spill site by calcium nitrate amendment and irrigation(2). The site was identified as 9-16-6-11 W2M, in the Midale field (southeastern Saskatchewan) operated by Shell Canada Ltd. The soil electrical conductivity data prior to this treatment ranged from 29 000 - 56 000 microsiemens per centimetre (µS.cm− I)Treatment and leaching reduced this figure to 6900 µS.cm−I. However, by the following spring, it was found that the electrical conductivity had risen to 18 200 µS,cm− I, and the very dry following summer season did not permit a reduction to less than 11 000 µS.cm−l.