BACKGROUND AND LITERATURE REVIEW .1 Salinization

Soil salinization can cause a significant decrease in agricultural productivity and in the economic viability of agriculture. It is important that the hydrologic conditions leading to salinization be well understood so that remediation and reclamation of affected lands can be effective. This paper provides both a literature review of the subject of soil salinization in the prairies and in Australia, and also some of the results obtained from the modeling research.

2.1.1 Canadian Prairies

Mechanisms of soil salinization are evidently very complex. Probably the most commonly understood mechanism is that resulting from upward flow from an aquifer due to either over- pressurization of the aquifer or high evapotranspiration (ET) in zones with high piezometric surfaces (Henry et al., 1985; Sharma, 1984; Stein & Schwartz, 1990). Another mechanism of salinization involves horizontal movement of water from irrigation water sources (Sommerfeldt & Chang, 1980).

Stolte et al. (1992), using finite element modelling of soil moisture flow and salt transport, found that the saline ring around a slough in Manitoba, Canada, was likely the result of a series of changes in the hydrologic regime caused by agricultural practices. The conversion of grassland to cropland with frequent fallowing of land probably caused decreased snow accumulation in the upland and increased snow accumulation in the slough.

The evapotranspiration in the zones surrounding the slough, along with the higher water levels in the slough, resulted in increased water flow from the slough through the highly permeable topsoil layer. This movement of water carried the salts with it, leading to leaching of the zones near the slough and the concentration of salinity in a ring further up the slope.

This effect was likely enhanced by elimination of the dense vegetation ring often associated with prairie wetlands.

2.1.2 West Australia

Both the waterlogging and the salinization of dryland landscapes associated with fresh/shallow and deep/saline groundwaters are major environmental problems in south- western Australia. Currently, over 1.8 Mha of agricultural land are affected and potentially 3 - 6 Mha are at risk within the 18 Mha agricultural region (Ferdowsian et al., 1996). The problems, as described by Peck (1978), stem from the replacement of the native perennial and deep-rooted Eucalypt dominant forest and woodlands with exotic shallow-rooted annual pastures. In this Mediterranean climate, the native vegetation has adapted to the temporal disjunction between the availability of water and the availability of energy by developing root systems which allow them to access the surplus water stored during the wet winter for transpiration during the hot, dry summer. On the other hand, the introduced annual pastures and cereals on which agriculture is based are very shallow rooted and only utilise part of the rain when it falls during the winter, dying during the hot summer season. This is shown in the relative amount of evapotranspiration from pasture compared to that of native vegetation.

Greenwood et al. (1985) found that, in a location where annual rainfall was 680 mm, evapotranspiration (ET) from annual pastures (390 mm&r) was less than rainfall whereas three common Eucalypt species were capable of transpiration rates much higher (up to 2600 mm@) than rainfall under adequate soil moisture conditions. They found that the annual pastures used the water during the winter and spring whereas the Eucalypts used it throughout the year, but mainly during the non-winter seasons.

The consequence of the change from natural vegetation and its high water use to annual pastures and their low water use is a surplus of water during the winter. This water percolates below the root zone of the annual crops and pasture and enters the groundwater system, only to reappear where the water table rises near the ground surface. During its travel through the regolith, the water mobilizes some of the massive quantities of salt stored there through immobilization by the deep tree root system prior to agriculture. The water

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transports the salts to the soil surface in the valley bottoms, or wherever a pattern of geologic structures impedes or accelerates flow (Lewis, 1993).

Given the economic losses that salinization and waterlogging entail, it is not surprising that much study has been made of the matter. One of the most thoroughly instrumented sites studied in the agricultural area of Western Australia is that reported on by George (1991) and George & Conacher (1993a, b). This site provides a substantive set of field based observations from which groundwater and surface water interactions can be further studied.

However, even here there remain important unanswered questions regarding quantification of the flow mechanisms occurring in both the unsaturated and the saturated zone, as described from field observation. As in most field-based groundwater studies, the saturated zone receives the greatest attention while the role of the unsaturated zone remains somewhat obscure. The deficiency in observations of unsaturated flow processes may prevent a full understanding of the role of perched water and its source.

2.2 Wetland hydrology

The prairies of western Canada have numerous small wetlands. They are commonly referred to as prairie potholes or sloughs and are a major component of the water resources and ecology of the region (Murkin, 1998). The water levels in a wetland fluctuate as a result of the interactions of various surface and subsurface hydrologic processes. Climatic conditions and land use strongly influence water levels in wetlands (Woo, 1992; van der Ramp, et al.,

1998).

Several models have been proposed for calculating components of the water balance of prairie wetlands (Woo & Rowsell, 1993; Willems, 1995; Poiani et al., 1996), but the hydrology of large river basins such as the Mackenzie River basin. However, the structure of the model is sufficiently deterministic that it can be adapted with minor modifications to

at present mined by an open pit process, since in the Fort McMurray area the deposits come sufficiently near the surface. The development of the oil sands has resulted in the stripping of overburden from about 440 krn2 to date. The overlying stratigraphy includes a shallow soil system that is highly peaty in nature, underlain by glacial deposits, which in turn overlay a highly saline-sodic geologic member called the Clear-water formation. Underlying this Clear-water formation is the oil sands formation. Thus, open pit mining entails the removal and stockpiling of the soil, the chemically neutral glacial deposits, and the Clearwater formation.

After mining, the deep, some 30-40 meters, hole resulting from the mining is filled again with first the Clearwater material, then a soil cover composed of the chemically neutral glacial deposit and a layer of peaty soil. The soil cover is intended to return the landscape to near the state of nature that originally obtained at that location. But the Clearwater formation is over-consolidated, meaning that its present compression is a result of the weight of a

The nearness of the saline-sodic overburden to the surface creates a significant problem for reclamation of the disturbed area. The problem stems from three factors: the dispersivity of the clays when subject to freshwater infiltration, subsidence due to the method of placement of the overburden, and salinization. Following is some information on the details of the problem. In further sections, this paper reports on some of the preliminary results obtained to date on the soil moisture movement within the soil cover and below as related to the measured climatic inputs.

(i) Dispersivity

The saline-sodic Clearwater shale formation is composed of a highly dispersive clay. Under natural conditions, it is kept intact by the chemical interaction the clay has with the brine, the natural pore water. When the clay is exposed to non-saline water, the water enters and disrupts the clay structure, causing the clay particles to lose their bonding with each other and to separate and disperse. In so doing, this soil loses most of its structural strength and its natural angle of repose reduces to 7’, or a slope of 12 % or less.

The reclaimed material is laid at slopes considerably in excess of 12%. It is obvious that these slopes will not remain stable if significant quantities of fresh water enter the matrix of the saline-sodic material. Thus, it is important from this point of view that the soil cover design be such that very minimal quantities of precipitation percolate below the root zone and into this material.

(ii) Subsidence

There is also a problem with subsidence within the reworked Clearwater material. Because the clay is so dispersive, it becomes greasy when wet and thus is totally unworkable. For this reason, the material is replaced during the winter when the soil is frozen. Because it is frozen, the soil is laid in a non-homogeneous state, with many large voids remaining in the deposited material. When this material thaws, the structure will give way causing the cavities to be

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filled with the material above it, and so on up to the surface where subsidence might even cause changes in the drainage patterns at the surface.

(iii) Soil salinization

Another characteristic of the Clear-water shale is that it is very saline. It would be expected that any precipitation percolating below the root zone into its matrix would soon become very saline. If any of this water then reappeared anywhere in the landscape, salt scalds would become immediately evident. Any extensive regions of saline seepage would likely result in zones where no vegetation could exist, and also zones where saline runoff would be

Two thermoelectric generating plants, the Shand and Boundary, are located near Estevan and- use water from the Souris River for cooling. The Boundary plant is sited on water quality at the international border is necessarily of considerable importance in fulfilling the major purpose of satisfying the Boundary Waters Treaty.

The next priority for the project was the provision, from the Rafferty reservoir, of a water supply for the Shand coal-powered thermo-electric plant. Following in priority was the need to provide water to satisfy existing municipal, irrigation, domestic and industrial water users downstream of the Rafferty dam. Enhancement of waterfowl and fish habitat was the next priority, followed by supplying new downstream water uses, including irrigation and recreation. Flood protection within Saskatchewan was mainly satisfied by meeting the flood protection needs of North Dakota.

Stolte & Sadar (1993) also dealt with some of the history of the project as it affected the development of the EIA process in Canada. Prior to the project, EIAs at the federal level were at the Minister of the Environment’s discretion. After this project and the nearly concurrent Oldman Dam development in Alberta (FEARO, 1992), there was considerably less discretion available to the minister as to the degree of assessment required for a project that had some component under federal control. A major part of the EIA process ruled by the courts to be mandatory was a federal panel appointed to review the Rafferty-Alameda project and to report to the minister (FEARO, 1991). The writer was involved in producing that report, which is now over 8 years old. It includes a summary of the expected impacts of the project, the monitoring required to ascertain the extent of these impacts, and recommended mitigation. His research in this area involves a follow-up on the EIA process. In this paper, the impacts as predicted in the panel report are summarised, followed by a comparison of the monitoring, mitigation and compensation measures recommended by the panel to those actually put in place. The degree to which the predicted impacts have occurred is related to the climatic and flow regime in existence since 1991.