Remediation of a Salt-Affected, Macroporous Soil in Central Alberta, Canada
Date
2014-09-12
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Abstract
Salt-affected soil may be characterized by both elevated salinity and sodicity. High salinity levels impair plant growth and result in reduced plant yield. Leaching of salt from affected soil may be utilized to improve plant growth or to meet regulatory soil quality guidelines. As salinity levels decline during the leaching process recalcitrant elevated sodicity levels may result in clay swelling and disruption of soil structure. The accompanying reduction in soil permeability can produce poor soil drainage and reduced plant growth. The focus of this thesis is the remediation of salt-affected soil resulting from produced water releases associated with oil and gas production. Laboratory and field characterization and experiments were conducted on salt-affected soil at a former production facility in central AB, 35 km southwest of Edmonton. The objective of this thesis is to improve remediation outcomes using salt leaching in the relatively dry climate and fine-grained soil conditions typical of the Canadian prairies (CP). In addition, the effect of soil mineralogy on remediation of salt-affected soils is examined by quantifying the effect of gypsum and calcite dissolution on soil saturated paste extract (SPE) salinity and sodicity, as well as saline-sodic effects on hydraulic conductivity due to clay swelling.
Gypsum (CaSO4•2H2O) was added to the soil as a Ca-amendment to ameliorate elevated Na levels. Calcite (CaCO3) occurs naturally in this soil. Sulphate and HCO3 mass in the SPE was compared to that in soil water samples collected from the field using suction lysimeters. Excess sulphate mass attributed to gypsum dissolution was measured in the SPE, while negligible excess calcite dissolution was determined. Excess gypsum dissolution occurred in 30 % of the soil samples. Gypsum dissolution in the saturated paste resulted in an overestimation of electrical conductivity (EC) by 22 % on average and an underestimation of sodium adsorption ration (SAR) by 7.6 % on average. The relative error increased as soil salinity and sodicity approached regulatory guidelines of 3 dS/m for EC and 4 for SAR. Following application of the correction for excess gypsum dissolution, the number of samples that met regulatory guidelines for both EC and SAR increased from 3 to 8, a 167 % increase.
Smectite, a swelling clay, is present in the soil under study, comprising 48 % of the clay-sized (<2 µm) fraction. Laboratory permeameter testing on intact soil cores demonstrated that 46 % of hydraulic conductivity reduction induced by clay swelling during leaching of moderately sodic conditions (SAR = 14) occurs following stabilization of effluent soil salinity. This time-dependent effect is attributed to slow diffusion of a small fraction of salt from smectite-bound pores. Additionally, variable degrees of recovery of hydraulic conductivity were observed upon application of high salinity permeant. This implies that saline-sodic swelling effects on pore structure are not fully reversible. Confined and intact subsoil shows greater sensitivity to saline-sodic induced reduction in hydraulic conductivity than repacked soils used in other research studies. Saline-sodic swelling effects represent a greater hazard to hydraulic conductivity to saline-sodic subsoils than surface soils for a similar swelling clay content.
A field irrigation experiment was conducted to increase leaching rates under naturally dry climate conditions. Supplementing natural precipitation with irrigation resulted in varying degrees of leaching from good to negligible, depending on the soil moisture condition. Efficient and timely leaching in fine-grained, macroporous soil is complicated by complex interaction between preferential flow in macropores and saline pore water stored within the soil matrix. Infiltration and drainage result in mixing of fresh water infiltrated through macropores with saline pore water within the aggregated soil matrix. Good leaching efficiency was achieved using repeated cycles of irrigation and drainage. In contrast, under continuous matrix-saturated moisture conditions, persistent fresh water flow in macropores bypassed the saline soil matrix resulting in negligible leaching efficiency. Control of infiltration and drainage cycles to produce good leaching efficiency are well suited to automated irrigation. Soil moisture conditions should be cycled between field capacity and saturation for optimal salt leaching.
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Keywords
Hydrology, Environmental Sciences
Citation
Callaghan, M. V. (2014). Remediation of a Salt-Affected, Macroporous Soil in Central Alberta, Canada (Doctoral thesis, University of Calgary, Calgary, Canada). Retrieved from https://prism.ucalgary.ca. doi:10.11575/PRISM/27620