RESEARCH METHODOLOGY AND RESULTS

In this section, the research methodology and results obtained with respect to the hydrology of salinization, of wetlands, and of oil-sands waste and overburden dumps and their soil covers will be presented. Some information will also be given on the post-project analysis of the monitoring and mitigation program associated with the Rafferty-Alameda project.

3.1 Salinization

The salinization work mainly included studies done on sites in the Canadian prairies, but also a study of two sites in West Australia.

3.1 .I Canadian Prairies

The work on the hydrology of salinization was done with the support of federal and provincial agencies and was part of a larger study that included intensive monitoring of seven sites throughout the Canadian prairies. There was, thus, an extensive data base on which to base the analysis of the hydrology of these sites. The method of analysis was to use two finite element models, SEEP/W and CTRAN/W (Geoslope, 1999), both calibrated to the recorded conditions at the site. The seepage model, SEEP/W, generated the subsurface flow patterns within the site, thereby indicating the flow mechanism responsible for the salinization. The model, CTIUUUW, then used the seepage patterns generated by SEEP/W to determine the salinization patterns that would result. Results are shown for two sites, the first a saline ring adjacent to a wetland near St. Denis, SK., and the second, salinity occurring in a barely undulating landscape near Cory, SK, just outside Saskatoon. Comments will be made on the other sites analysed but no results will be shown.

(i) St. Denis wetland

Figure 1, below, shows the seepage patterns occurring within the soil profile generally throughout the summer. The slough, or wetland, extends from the centreline at zero distance to about the location of N2. Nl through N5 represent piezometer nests; the water levels recorded in these piezometers were the main data used for calibration. The subsurface is

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composed of several layers of till of different hydraulic characteristics. The floral till and the layer above it are very impermeable. The vertical boundaries are assumed to have no flow crossing them. The bottom boundary is set at a head recorded in a piezometer that was finished at that level nearby. The surface boundary was subject to precipitation in the form of both rain and snowmelt, and also to evaporation. The evaporation was reduced as the suctions in the soil at the surface increased. The surface fluxes were mean monthly values. The lines and numbers indicate contours of total head, or energy. Flow is from zones of high head to zones of low head.

It can be seen from the figure that the highest heads occur in the upper soil layers in the uplands. The lowest heads occur just upslope from the wetland near N2. This zone is created by evapotranspiration maintained at high levels here by the availability of water from the wetland. If this water were not available, the soil would dry out, reducing permeability to extremely low values, and thus preventing any significant flow.

The low flow zone becomes the sink for flow from the zones upslope from N2 and also from the slough. The main flow is in the topsoil unit. Although the same head gradients occur in the tills as in the topsoil layer, the very low permeability of the till prevent any appreciable flow from occurring through it.

E I repeatedly for 3 consecutive years, generated the salinization patterns shown in Figure 2,. To do the modelling, it was assumed that there was no salinity crossing the vertical boundaries, that the flow into the profile from the bottom boundary would have a concentration of 1 .O, that the water within the profile at the beginning of.simulations had a concentration of 1.0, and that the incoming precipitation or outgoing evapotranspiration had a concentration of 0.

The model concentrations are thus relative values.

The bottom part of Figure 2 gives the results from the modelling while the top gives the results obtained in the field by using the EM 38 electrical conductivity measurement device. The numbers included directly above the simulated concentration contours are maximum values of salinity concentration found at that location. It can be seen that there is a very good correspondence between the modelled patterns of salinity concentration and those measured.

The only divergence is that shown at the right where the surface slopes into the uplands. The high concentrations predicted there by the simulation process are not shown by

the measurements. This discrepancy results from the fact that the particular conditions being modelled included a substantial snowbank further up the slope, something not likely to occur every year. In any event, it is reasonable to conclude that the salinity rings found around so many wetlands in the Canadian prairies are due to a saturated-unsaturated flow pattern out of the wetland upslope to adjacent zones where high evapotranspiration produces low heads.

EM38 Measured Data along North Transect (May 20, 1993) 300 .

100 150 200

Distance X (m)

Nl N2 N3

1.95 7

**x---r.= .:. .- ’ *c. .__ ~~ i

i,, - ; _ _‘,,I’ \ ,: -- --;-- 1. .: ” ::,,;>$y: : , / .,

i 0.95 ..

--1-w---- e---7- ~

~ ~ ‘-

Figure 2. Salinity patterns at St. Denis, recorded versus simulated.

(ii) Cory site

The seepage and salinization patterns occurring at the Cory sites are shown in Figures 3 and 4. The profile and the hydrology at this site are considerably different than they are at the St.

Denis site. The Cory site has a barely undulating surface that results in a pond on either side of a very minor hill in the centre. The site also has a fairly permeable till below the topsoil and an aquifer at the bottom of the profile that has a head that is just below the elevation of the ground surface.

The simulations were done in the same manner as they were at the St. Denis site, using SEEP/W and CTRAN/W. The two vertical sides were assumed to be no-flow boundaries, the bottom was assumed to be a constant head boundary, and the surface boundary was assumed to be subject to climatic fluxes, expressed as monthly values. The concentrations of the pore fluids were also the, namely 1.0 for initial pore fluid and for that entering and leaving the bottom boundary, and 0 for that entering or leaving the surface.

The contours of seepage head at the end of May are shown in Figure 3. (Note that the contour values must be increased by a constant value of 80 to match the elevations.) At this time of the year, the head in the aquifer is just slightly above that at the surface and there is very gradual flow upwards. More importantly, the depressions at D3 and D4 on either side of the hill Dl - D2, are still wet from snowmelt accumulating there. The wetness results in high heads at these locations compared to the heads under the raised portion. A result is flow from

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the depressions to the zone under the hill. According to seepage results (not shown) for the middle to end of summer, evapotranspiration has dried out the profile to the point that there is significant flow upward from the aquifer to the surface, uniformly distributed throughout the whole width of the profile.

Elevation (m)

100 ^D-4-l n-d-2 r “-4-L

Distance - X (m)

Figure 3. Head contours and flow patterns within the Cory site profile.

The results of the modelling of the salt movement are shown in Figure 4. It can be seen that salt has accumulated under the raised portion of the profile while it has remained relative unchanged under the left depression but strongly leached out of the right depression.

There is a obviously a net increase in salt content within the profile. The modelled salt concentrations match, with some differences, the EM38 measurements, lending credence to the results of the modelling.

It is apparent that the hydrologic mechanisms driving salinization in this location are more complex than those at the St. Denis site. At St. Denis in the spring there is very minor flow toward the wetland, shifting to flow outward from the wetland during the high evapotranspiration season of mid to late summer. There is negligible interaction of the aquifer with the wetland. The flow systems are merely rearranging the salt that is already within the profile.

At the Cory site, the flow outward from the depression, or wetland, occurs in spring and early summer. In addition, at the Cory site, the high evapotranspiration during the summer causes flow upward from the aquifer, carrying with it the salts within the aquifer.

These accumulate near the surface at the end of the year. The flow from the wetland depressions to the zone under the hill during the spring and early summer serve to cause the concentration of salt observed and modelled under the hill. The total salt within the profile is increasing because of the flow from the aquifer.

3.1.2 West Australia

The research into West Australian salinization mechanisms had two thrusts. The first was an analysis of the monitored piezometric and salinity levels in a small catchment, near Boundain about 2 hours SW of Perth, W.A. The second thrust was the use of SEEP/W to model the hydrology of salinization in a nearby small cat&n-tent, the Hardie site.

EC EM38 Measurements Along Profile 300 ;

200 I 100 :

0 : - .~ ~~~ ~~. ~~..~_

0 50 100 I50 200

Distance X (m)

/ ,11-7---;-- ~~~~~.~~~ -~~ ~~

0 50 100 150 200

Distance - X (m)

Figure 4. Recorded and simulated salinity concentrations within the Cory site . profile.

(i) Boundain site

The research at the Boundain site was intended to define the effect of tree plantations located in the lower zones of a salinized catchment. The trees were planted in order to decrease the degree of waterlogging that was occurring at this site. The question was whether the density of the tree plantation had an effect on the piezometric levels below, and whether the removal of soil moisture led to an increase in salinity. The data shown in Figures 5 and 6 allow some conclusions to be drawn in this matter (Stolte et al., 1997).

Figure 5 shows the change in piezometric levels from the end of summer, 1983, to the end of summer, 1994. It can be seen that the levels dropped everywhere trees were planted, but most significantly, up to 2.5 m, in the larger high-density tree zone within the right block.

Figure 6 shows the electrical conductivity of the water in 4 of the piezometers located in the high density treed area. It is evident that the salinity was relatively constant for the first several years after the trees were planted. There is a long hiatus in the data but the last two measurements show that the salinity had been rising rapidly at the end of the measurement period. This is significant evidence that the reduction of the piezometric water levels caused by the high-density tree plantation, as shown in Figure 5, is being accompanied by an increase in salinity concentration. It is doubtful that these increases could continue indefinitely without eventually causing the destruction of the trees, most of which are not highly tolerant of salinity. The loss of the trees would, of course, once again allow the piezometric levels to increase and waterlogging would again join with salinity in destroying the land for agricultural purposes.

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Electrical Conductivity - mS/m

10/6/60 2116162 7/3/63 ltH4/64 3/29/66 6111167 12/23/66 517190 g/19/91 1131193 6/15/94 Date

Figure 6. Groundwater salinity in the high density treed area as a function of time.

Distance - m I

250.00

1

200.00

150.00 c

100.0d

50.00-

Zone where piezometric heads measured

o.o&

0.00 50.00 100.00 150.00 200.00 250.00 300.00 350.00 400.00 450.00 500.00 550.00 Distance - m

Figure 5. Piezometric levels in May, 1994, minus those in May, 1983

(Outline indicates region over which piezometric levels recorded. Vertical hatching indicates low densi@ tree plantation; horizontal hatching indicates high dens@ tree plantation.)

Electrical Conductivity -

10/6/60 2118l62 713103 11114l64 3l29l06 6llll67 12/23/66 517190 9/19/91 ll3ll93 6115194

Date

010 30 50 70 90 1101301501701902102302502702903103303503703904104304504704905105305

Distance from Creek -

The profile analysed is shown in Figure 7 (Stolte et al., 1999). The profile is throughout the bottom two-thirds of the profile is a B-horizon made very impermeable by the addition of the clay from the surface layer. In the upper one-third of the profile, the B-horizon is more permeable; it is called the gradational soil zone.

The salt is contained within the profile, deposited there from several millions of years of rainfall with minuscule concentrations of salt in it. The salt, once immobilised throughout the profile a few meters below the surface, has been remobilized by significant recharge caused by rainfall no longer being balanced by evapotranspiration. The excess escapes below the root zone of the crops and recharges the profile. A good portion of this water enters the saprolite aquifer at the bottom of the weathered regolith and travels downslope.

Figure 7 and other results from the SEEP/W analysis show that the flow system is relatively complex. In the upper part of the basin, the gradational soils allow the recharge of all excess rainfall. In the bottom part of the basin, the very low permeability B-horizon blocks the downward flow of water and diverts it downslope through the highly permeable A horizon. The flow through most of the A-horizon is unsaturated. It serves to feed the saturated wedge at the bottom of the slope. This is the zone of waterlogging.

The waterlogging is also fed by another flow system. The recharge that occurs through the gradational soil zone upslope eventually resurfaces in the bottom of the profile.

The resultant seepage zone then becomes a discharge zone of rainfall that has been salinized by the salt store through which it passed. The waterlogging and the salinization of the bottom of the profile make it extremely inhospitable to vegetation of any sort. Slightly further up the slope, salt- and waterlogging-tolerant vegetation of little agricultural value does grow.

Part of the question raised about the hydrologic system was the role of macropores.

The modelling showed that in the lower part of the profile, the hydraulic heads are so high that macropore flow there is relatively limited. At this particular site, modelling indicated the existence of a geologic barrier, shown in Figure 7 as a dolerite dyke. This dyke tended to create high hydraulic heads upstream of it. These high heads made saturation of the surface horizon there fairly frequent, whereas saturation would never have occurred without the dyke.

When the zone behind the dyke saturates, the potential for macropore recharge becomes quite high. This is another potential source of the saline discharge in the lower part of the profile.

3.2 Wetland hydrology

The research being done in wetland hydrology has two components. The first is the analysis of data gathered from many monitored wetlands in the St. Denis National Wildlife Refuge (SDNWR). This data shows the effects of a land-use change on water level variability. The second component is the modelling of the variability of the wetland water levels.

3.2.1 Effects of land use on wetland water level variability

The refuge was established in 1969, and the wetland water levels have been monitored on a regular basis since then. The catchments of all the wetlands in the refuge were cropped at the

time of its establishment. In 1985, the catchments of some of the wetlands were converted to grasses, with the intent of improving waterfowl breeding habitat.

A comparison of the water level regime for the entire period of record is given in Figure 8 for wetlands with no change in catchment land use, 109 and 120, and wetlands with catchments converted from cropping to grasses, 92 and 130. The differences between the two classes of wetlands are very pronounced. It is obvious that the conversion of the land use from crop to grassland resulted in the drying out of wetlands 92 and 130. Not only was this true for these two wetlands, it was true for all the wetlands in the SDNWR that had their catchments converted to grasses.

It is not altogether obvious why this change occurred. It can be stated with certainty that spring runoff into the grassed wetlands nearly ceased; however, why this occurred is open to some conjecture. It appears likely that the macropore structure developed under grasses (Hino et al., 1987) has led to increased permeability and less overland flow. This is reasonable given that grasses have a longer transpiration season than do crops and thus the soil profile could be expected to be drier in the fall under grasses than crops. In the spring the snowmelt infiltration would be higher and runoff lower in the grasses, since both storage and permeability are higher.

Another factor must be the difference in snow redistribution for grassland compared to crops, particularly summer fallow, i.e. holding the land fallow for one year out of two or three in order to conserve moisture. Figure 9 shows, for one year, the difference in snow accumulation between adjacent fields of summer fallow and grassland. Several features are immediately apparent. First of all, the snow accumulation on the fallow land is highly

CROPPED 1969 - 1997 CROPPED 1969-85, GRASSED 85-97

Pond #92 3; 75

;i 50 25 Bi =! 0

Pond #I 30

variable, with significant snowdrifts in low areas, and almost no snow elsewhere. On the Figure 8. A comparison of wetland water levels with cropped

and grassed catchments.

other hand, the snow accumulation in the grassed area is very uniform at about 80 mm water equivalent, about the amount of winter snowfall. There is a significant snow accumulation at the boundary between the two fields, i.e. the location of a major change in aerodynamic roughness. This implies that the snow that falls within the grassed area stays where it falls, whereas the snow that falls on the fallow land blows around, accumulating where there is a change in aerodynamic roughness. It has been noted that the vegetation rings around sloughs

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in cropped areas represent one such region of roughness change and so sloughs tend to trap a lot of snow. The transference of snow from fallow land to wetland vegetation rings could be looked at as solid runoff, rather than the liquid runoff of spring snowmelt.

It is highly likely that the wetlands with cropped and fallow catchments gained both additional liquid runoff due to reduced infiltration and also additional solid runoff because of the greater accumulations of snow in the vegetation rings surrounding wetlands in