Journal of Hydrology 499 (2013) 188–199
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Journal of Hydrology
journal homepage: www.elsevier.com/locate/jhydrol
Review papers
Sulfate salt dynamics in the glaciated plains of North America
Uri Nachshon a,⇑, Andrew Ireson a, Garth van der Kamp b, Howard Wheater a
a
School of Environment and Sustainability and the Global Institute for Water Security, University of Saskatchewan, 11 Innovation Boulevard, Saskatoon, Saskatchewan S7N
3H5, Canada
b
Environment Canada, 11 Innovation Boulevard, Saskatoon, Saskatchewan S7N 3H5, Canada
a r t i c l e
i n f o
Article history:
Received 7 February 2013
Received in revised form 28 June 2013
Accepted 1 July 2013
Available online 10 July 2013
This manuscript was handled by Corrado
Corradini, Editor-in-Chief, with the
assistance of B. Mahler, Associate Editor
Keywords:
Prairie
Salinity
Evaporation
Wetland
Sulfate
s u m m a r y
The semiarid glaciated plains of the North American continent, known as the prairies, are characterized
by an undulating terrain rich in sulfate salts in the subsurface, with ephemeral streams and large numbers of wetlands containing seasonal or semi-permanent ponds. Salinization is a potential concern for the
diverse community of vegetation, aquatic ecosystems, wildlife and agricultural production supported by
the prairies, especially as a result of land use changes and climate change. In this paper, a literature
review of prairie salt dynamics and distribution is presented. On the basis of observations from past field
studies, a conceptual model describing prairie sulfate salt dynamics is proposed, which identifies a number of important zones of salt accumulation in the subsurface and in surface water. As is the case in any
other environment, the distribution of salts is determined by the hydrological conditions, in particular
subsurface flow pathways and evapotranspiration front locations. However, the semi-arid climate and
glacial geology of the region result in unique and characteristic hydrological conditions and distributions
of accumulated salts. The hydrology of the prairies is sensitive to land use, with the major changes over
the past 100 years or so being conversion of prairie grasslands to annual dryland crops and drainage of
wetlands. Moreover, in semi-arid environments the hydrological system is highly sensitive to climate
variability and change. Hence, even small hydrological changes may result in mobilization of salts concentrated in the shallow subsurface, and, if sustained, may generate ground surface, wetland and surface
water salinization. However, these changes are difficult to predict, involving multiple interacting processes, and it is therefore necessary to develop an improved, quantitative understanding of the coupled
hydrological and geochemical processes in order to manage or adapt to future changes.
Ó 2013 Elsevier B.V. All rights reserved.
Contents
1.
2.
3.
4.
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Prairie hydrology and hydrogeology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.1.
Wetland hydrology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.2.
Hydrogeology. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.3.
Groundwater surface water interactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.4.
Streams and rivers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Salt dynamics in the prairies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.1.
Origins of salts in the prairies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.2.
Geochemical processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.3.
Salt transport . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.4.
Salt accumulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.5.
Seasonal salt dynamics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.5.1.
Observed salt distribution in the subsurface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Synthesis of a conceptual model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.1.
Region 1 – Saline Ring (SR). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.2.
Region 2 – Surface Salt Belt (SSB) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.3.
Region 3 – Deep Salt Belt (DSB) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
⇑ Corresponding author. Tel.: +1 306 966 2585.
E-mail address: [email protected] (U. Nachshon).
0022-1694/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.jhydrol.2013.07.001
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5.
6.
4.4.
Region 4 – Unsaturated fractured till (UT) beneath the upland . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.5.
Region 5 – Saturated unfractured till (ST) beneath the upland . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.6.
Region 6 – Recharge Pond (RP) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.7.
Region 7 – Discharge Pond (DP) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.8.
Region 8 – Flow-Through Pond (FTP). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Potential environmental changes on salt dynamics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.1.
Precipitation shift to more snowfall. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.2.
Precipitation shift to more rainfall . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.3.
Wetter conditions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.4.
Drier conditions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.5.
Managing the risk of salinization with land use . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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1. Introduction
‘The Prairies’ – The semiarid glaciated plains of the North
American continent (Fig. 1) are characterized by undulating terrain, rich in sulfate salts, with ephemeral streams and a large
number of wetlands marked by seasonal or semi-permanent
ponds (Winter, 1989; Hayashi et al., 1998a; van der Kamp and
Hayashi, 2009). The prairie region supports a diverse community
of wildlife species, as well as a major agricultural industry, which
are both highly sensitive to the hydrological conditions of the
wetlands (Wienhold and van der Valk, 1989; Batt et al., 1989;
Winter, 1989). Climate variability and land use changes associated
with agricultural activities have had major impacts on prairie
hydrological processes (Leitch, 1989; Poiani and Carter, 1991; Euliss and Mushet, 1996; Mitch and Hernandez, 2012; Todhunter
and Rundquist, 2008; Winter and Rosenberry, 1998; Shook and
Pomeroy, 2011).
High salt concentrations in subsurface amplify the potential
hazard associated with changes in the hydrology, as these
changes may result in salinization of the soils, surface and subsurface water (Eilers et al., 1997; Florinsky et al., 2000; Wiebe
et al., 2006). It is well known from other places in the world that
changes in the hydrological regime can result in severe environmental and economic problems associated with salt dissolution,
migration and crystallization. Examples include soil salinization
in Australia (Dehaan and Taylor, 2002) and sink-hole formation
in the Dead Sea region (Shalev et al., 2006). Fig. 2, taken near
Kindersley (KN in Fig. 1), shows efflorescent salt accumulation
in a typical cultivated prairie landscape, away from any pond.
Fig. 1. Map of the prairie region, indicated by the gray area. Nine sites that are
referred to in this paper include: Vauxhall (VA); Kindersley (KN); Dalmeny (DL);
Warman (WR); St. Denis (SD); Moose mountain (MM); Smith Creek (SC); Nelson
County (NC); Cottonwood Lake (CL); and Orchid Meadow (OM).
Fig. 2. Efflorescent salt accumulation (white material) in an agricultural field and
an unaffected adjacent natural grassland. The dashed white line indicates the
location of the division between cultivated land and natural grassland (lower part of
the picture). Vegetation is restricted in the saline area.
Such efflorescence salt formations are not rare in the prairies
(Timpson et al., 1986). A large scale survey of soil salinity in
the Canadian prairies was carried out by Agriculture and AgriFood Canada (http://sis2.agr.gc.ca/cansis/publications/surveys/sk/).
Pham et al. (2009) reported on electrical conductivity (EC) from
70 closed-basin lakes in southern Saskatchewan. The measured
values varied from 285 lS/cm (fresh) to 137,800 lS/cm (saline)
with a mean of 17,201 lS/cm (subsaline). For perspective, sea
water EC is in the order of 33,000 lS/cm (saline) (Stewart
and Harold, 1972; Wynn and Fleming, 2012). Therefore, understanding the coupled hydrological, hydrogeological and geochemical system of the prairies under various climatic conditions and
land use scenarios and the consequences for salt migration and
accumulation is an important aspect of water and land
management.
Here, a review of prairie hydrology and hydrogeology (Section 2)
and geochemistry (Section 3) is given, drawing heavily on past field
studies. A conceptual model of prairie salt dynamics, taking as a
specific example the distribution of sulfate salts (the most abundant salts in the prairies), is presented in Section 4. This model
identifies the unique and characteristic salt accumulation and
redistribution processes attributable largely to the climatological
and geological conditions of the region. In Section 5 the potential
impacts of environmental changes on salt dynamics and distributions, based on the conceptual model, are discussed and gaps in
understanding are identified. Finally, Section 6 provides a
summary.
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U. Nachshon et al. / Journal of Hydrology 499 (2013) 188–199
2. Prairie hydrology and hydrogeology
A large number of hydrological and hydrogeological studies
have been conducted within the glaciated prairies in Canada
(e.g., Hayashi et al., 1998a, 2003; Millar, 1971; Shaw et al., 2012;
Spence and Woo, 2003; van der Kamp and Hayashi, 2009) and
the United States (e.g., LaBaugh and Swanson, 1992; Winter and
Rosenberry, 1995; Winter, 2003; Winter and LaBaugh, 2003; Mills
et al., 2011; Eisenlohr, 1972; Johnson et al., 2004; Euliss et al.,
2004). The geology, topography and climate of the prairies result
in a unique hydrological environment. The geology of the northern
prairie region is dominated by glacial deposits resulting from the
Pleistocene continental glaciers which repeatedly covered the region. These deposits are tens to hundreds of meters thick and consist mostly of clay-rich glacial tills, with interspersed deposits of
glacio-lacustrine sand and gravel (Lennox et al., 1988). The topography consists of extensive areas of low relief hummocky terrain
(Hayashi et al., 1997; van der Kamp and Hayashi, 2009). The climate is variable from south (warmer, wetter) to north (colder,
drier), but is generally semiarid, with potential evaporation (the
order of 700–800 mm/year) exceeding precipitation (300–
500 mm/year) (Akinremi et al., 1999; van der Kamp et al., 2003).
The annual mean air temperature in center of the region is about
2 °C with monthly means of 19 °C in January and 18 °C in July.
Approximately 30% of annual precipitation occurs in winter
(November–March) as snow, whereas more than 60% of evaporation occurs over the summer Hayashi et al. (1998a). Consequently,
the effective precipitation (precipitation minus evaporation) may
be positive or negative at different parts of the landscape and at
different times.
are the main sources of pond water (Woo and Rowsell, 1993; Su
et al., 2000; Heagle et al., 2013). Since the amount of summer precipitation is exceeded by potential evaporation, the water input by
snowmelt water is critical to the existence of the wetlands. In early
spring the ground is frozen during the snowmelt period (Gray and
Male, 1981; Ireson et al., 2013), thus infiltration through the uplands is limited, and snowmelt water runs off into depressions, filling up the wetlands (depression-focused infiltration). In summer
the unfrozen soil has a high infiltration capacity, and high evapotranspiration demands, thus resulting in relatively dry soil in the
uplands. Consequently, summer overland flow is generated in the
uplands only during intense rainfall events (Hayashi et al.,
1998a). In very wet seasons pond water may be routed from higher
to lower-lying wetlands and to streams as surface runoff by the
‘‘fill and spill’’ mechanism (Shaw et al., 2012; Spence and Woo,
2003). Dominant losses from the ponds are direct evaporation
(Millar, 1971) and lateral subsurface flows toward the upland
and the riparian zone, where most of the transpiration losses occur
(Millar, 1971; Hayashi et al., 1998a; Berthold et al., 2004). In addition, smaller vertical exchanges of water between the ponds and
the subsurface occur (discussed in Section 2.3). Ponds may be classified as ‘‘terminal’’ or ‘‘non-terminal’’, based on the dominant surface water drainage patterns. Terminal ponds may be completely
isolated topographically, or may be at the end of a sequence of
ponds. The definition is implicitly based on the historical spill record, with terminal ponds being those which have rarely, if ever,
spilled during the period of observation (van der Kamp and
Hayashi, 2009).
2.2. Hydrogeology
2.1. Wetland hydrology
van der Kamp and Hayashi (2009) defined a number of surface
hydrological units, each of which plays a distinct role in the hydrological processes in the prairies, as shown in Fig. 3. A ‘‘wetland’’ refers to a topographic depression having saturated or nearly
saturated soil most of the year. This includes the ‘‘pond’’, defined
as the flooded portion of the wetland, and the ‘‘riparian zone’’
which is occupied by dense vegetation (grass and trees). The rest
of the drainage basin is considered the ‘‘upland’’. Wetlands collect
runoff, snowmelt and wind-blown snow from the uplands which
This review will focus on those parts of the prairies dominated
by clay-rich glacial tills (most of the prairies, but there are exceptions – see Lennox et al., 1988). Here, three hydro-geologically distinct components can be considered: shallow till, deep till, and
sand/gravel aquifers. In the top 5–15 m, the tills are fractured
and oxidized and form zones of surficial active groundwater storage and transmission (van der Kamp and Hayashi, 2009). Underlying this are thick, slightly fractured or un-fractured, unoxidized
tills, forming extensive aquitards which occur mostly as continuous layers that extend over tens to hundreds of kilometers
(Christiansen, 1992; van der Kamp and Hayashi, 1998). Sand and
Fig. 3. Schematic of the major hydrological units and their hydrological functioning (after van der Kamp and Hayashi (2009)).
U. Nachshon et al. / Journal of Hydrology 499 (2013) 188–199
gravel deposits are interspersed throughout the tills and form generally isolated aquifers. The aquifers’ dimensions are diverse, from
extensive sheets to long narrow channel deposits, and numerous
small deposits of local extent (van der Kamp and Hayashi, 1998).
The distinction between deep aquitards and the shallow active
zone is due to the dependence of the hydraulic conductivity of
the tills on the degree of fracturing. The saturated hydraulic conductivity of the top 5–6 m of the fractured till is relatively high,
of the order of 10 2–1 m/d. The deeper till layer conductivity decreases to 10 6–10 2 m/d (Hayashi et al., 1998a; van der Kamp
and Hayashi, 2009).
2.3. Groundwater surface water interactions
Two important groundwater–surface water interactions occur
between the ponds and subsurface: shallow lateral fluxes into/
out of active storage in the shallow tills; and deep vertical fluxes
into/out of the sand/gravel aquifers (if they are present), through
the deep till aquitards. The deep vertical fluxes are small, typically
of the order of several cm per year or less, yet the dominant direction of exchange at a particular point in the landscape has a strong
impact on the salinity of the pond. Where the dominant vertical
flow direction is downward, from the wetland into the deep aquifer (Fortin et al., 1991), the wetland is known as a ‘‘recharge wetland’’. Conversely, where water from the aquifer feeds the
wetland with very slow fluxes, the pond is known as a ‘‘discharge
wetland’’ (Heagle et al., 2013; van der Kamp and Hayashi, 1998).
In contrast to the low permeability of the deep unfractured till,
the shallow till has a high saturated hydraulic conductivity, but under unsaturated conditions the hydraulic conductivity decreases
dramatically, as the fractures that give the clay till its high bulk
conductivity are dewatered (Hayashi et al., 1997). Therefore, active
lateral groundwater flow is essentially confined within the saturated zone between the water table and the bottom of the highly
fractured till. This zone can be referred as the effective transmission zone, ETZ (light gray zone in Fig. 3) (van der Kamp and
Hayashi, 2009). If the ETZs from two or more adjacent wetlands
are connected, then there can be sub-surface transmission of water
between the wetlands, and the receiving wetland is called a ‘‘flowthrough’’ wetland if the water is in turn passed on by subsurface
flow (Winter, 1989) or by surface flow to a lower-lying wetland.
Flow-through wetlands are generally recharge wetlands with
respect to the deep aquifer (Arndt and Richardson, 1989).
2.4. Streams and rivers
Due to the low effective precipitation in the prairies, the
amount of runoff is not normally large enough to form regionally
integrated drainage networks. Local runoff is dominated by the
seasonal snowmelt over frozen soils, and streams that do exist typically only flow for brief periods of time in the spring (Shook and
Pomeroy, 2011). Therefore, the area is dominated by small drainage basins with temporally dynamic patterns of connectivity,
which are generally hydrologically isolated from one another and
from the regional drainage networks that carry streamflow to terminal lakes or to the continental drainage network.
3. Salt dynamics in the prairies
3.1. Origins of salts in the prairies
The glaciated prairie region is rich in sulfate in particular, but
also calcium and magnesium, which originate from glacial deposits
(van Stempvoort et al., 1994). The glaciers pulverized large masses
of fresh bedrock, rich in pyrite (FeS2), clay minerals, calcite
191
(CaCO3), and dolomite (CaMg[CO3]2), over the course of the Pleistocene, which were subsequently exposed to weathering and dissolution by shallow groundwater flow (van der Kamp and Hayashi,
2009; Wallick, 1981). In the upper part of the glacial till, i.e. the
fractured zone, generally extending down to somewhat below
the present-day deepest position of the water table (Hendry
et al., 1986; Keller et al., 1988), sulfate (SO4) is produced by the oxidation of Pyrite. Oxidation processes within the saturated portion
of the till are very slow since the transport of O2 is controlled by
slow diffusive processes through the saturated media (Goldhaber
et al., 2011; Hillel, 1998). Hendry et al. (1986) and Remenda and
Birks (1999) explained the deep oxidation below the current water
table depths as evidence for drier conditions in the Holocene
(Altithermal period), when the water tables were deeper than their
current location. Oxidized zones, some with gypsum crystals, also
occur at the top of earlier glacial till deposits below the Holocene
weathering zone (Christiansen, 1992).
Sulfate together with calcium (produced from the dissolution of
calcite and dolomite) form gypsum (CaSO42H2O) (Keller et al.,
1991). Beside gypsum, other sulfate salts can be found (Last,
1984) including mirabilite (Na2SO410H2O), bloedite (Na2Mg[SO4]24H2O), and epsomite (MgSO47H2O), where the Na is derive from the clay by cation exchange processes (Mermut and
Arshad, 1987; Keller et al., 1991). Other non-sulfate salts that have
been observed in the prairies include halite (NaCl) in minor
amounts, originating from the parent material, as well as from
atmospheric deposition (Wardlaw and Schwerdtner, 1966; Hayashi et al., 1998b). Secondary carbonate minerals are also present
in the soil zone (Pennock et al., 2011), although these are not normally thought of as ‘‘salts’’.
3.2. Geochemical processes
General salt migration processes that determine which salts
form, where they form in the landscape and in what quantities,
are widely discussed in the literature (e.g., Appelo and Postma,
2005; Hillel, 1998). Different salts have different equilibrium concentrations in aqueous solutions, above which crystallization occurs, and below which dissolution occurs (Appelo and Postma,
2005; Skarie et al., 1987; Timpson et al., 1986). Salts are dissolved
by water into their composing ions, and the ions migrate by advection, diffusion and dispersion (Hillel, 1998). While flowing through
a porous medium, the ions might be attracted to the particles of
the porous media in an ion exchange process that affects the solute
migration and the relative ion concentrations (Harrington et al.,
2001; Hillel, 1998). Crystallization leads to an accumulation of salt
in the vadose zone (Appelo and Postma, 2005). Usually the crystallization process is associated with evaporation of the pore water
(Gran et al., 2011; Nachshon et al., 2011; Timpson et al., 1986).
In cold regions, such as the Canadian prairies, ground freezing also
affects the salt distribution as during freezing salts are rejected
into the unfrozen part of the solution, while the frozen ice in the
ground is relatively free of salts (Gray and Grnager, 1986; Ireson
et al., 2013).
3.3. Salt transport
Salts are transported by surface and subsurface fluxes of water,
mainly associated with snowmelt and rainfall-runoff (Hammer,
1978; Hammer et al., 1990; Heagle et al., 2013; van der Kamp and
Hayashi, 2009; Waiser and Robarts, 1995). Within closed basins,
under normal conditions, salts are internally redistributed and
transformed, but do not leave the system. Hence many of the lakes
and wetlands on the prairies have high salinities (>3000 mg/L)
(Hammer et al., 1990; Labaugh, 1989; Pham et al., 2009; Waiser
and Robarts, 1995). The dominant salt transport processes are likely
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to occur in the sub-surface at the scale of a wetland (Euliss et al.,
2004). Subsurface transport of salts over greater distances can occur
through underlying aquifers if they are present and continuous (e.g.
Henry et al., 1985; Fortin et al., 1991). However, surface fluxes associated with the characteristic fill and spill behavior of the prairies
may in some cases transfer salts between ponds in an episodic manner. If the salts are transported into a local stream by surface or subsurface flow they are likely to be washed out of the watershed into
the regional river systems (van Stempvoort et al., 1994). In areas
where connections with the regional drainage system do occur,
the wetland-upland complexes can play an important role in regional salinization as shown by Strobel and Haffield (1995). The
authors reported that an area of wetlands in North Dakota supplied
only about 1.2% of the flow in the Red River, but 17% of dissolved
solids. In addition surface winds also may play a role in removing
salt from a watershed as it may blow away the crystalline salt from
the ground surface (Last and Slezak, 1988; LaBaugh et al., 1996; Last
and Ginn, 2005).
These two basic principles essentially explain the distributions
of salts in the prairie landscape, and the specific manner in which
they are found to operate are described in detail in Section 4.
3.5. Seasonal salt dynamics
For all ponds in the prairies (recharge, flow-through and discharge) a seasonal cycle of salinity is observed (Hayashi et al.,
1998b; Heagle et al., 2013). Lowest concentrations occur in early
spring, due to the inflow of fresh water from snowmelt. Over the
summer, evaporation (which exceeds precipitation) consumes
the fresh water, leaving the salts behind in the pond water and
in the adjacent till, resulting in increasing salinity. The following
year, spring snowmelt again dilutes the pond water and the cycle
continues such that, under stable climatic conditions and stable
land use (i.e. vegetation), the average salinity of each pond remains
relatively constant. Seasonal salt dynamics can be important, but
are not the central focus of this paper, where we are more interested in longer term changes.
3.4. Salt accumulation
Salt accumulation in the subsurface is associated with areas
where evaporation and transpiration take place (Nachshon et al.,
2011; Tyler et al., 2006). Water is removed from the soil either
as vapor at the evaporation front (Nachshon et al., 2011), or by root
uptake, leaving most of the salts in the ground (van Stempvoort
et al., 1994). The depth of salt accumulation and crystallization is
thus determined by the location of the evaporation front and root
zone – termed here the evapotranspiration front. This is in turn
determined by the amount of precipitation, the shallow groundwater, the potential evaporation, the soil properties and the root distribution (Fisher, 1923; Federer et al., 2003; Tyler et al., 2006).
Different salts have different solubilities, and thus accumulate
at different points along a flow path, according to the Hardie-Eugster model (Miller et al., 1989; Miller and Brierley, 2011; Skarie
et al., 1987; Timpson et al., 1986). Slightly soluble salts tend to
crystalize and accumulate close to their origin, whilst highly
soluble salts undergo longer migration in solution before reaching
saturation and crystalizing further away (Last, 1990, 1999).
3.5.1. Observed salt distribution in the subsurface
Various field studies have been undertaken to measure the distribution of salts in specific parts of the prairie landscape. Keller
et al. (1991) explored salt dynamics in a recharge pond at the Dalmeny (DL in Fig. 1) and Warman (WR in Fig. 1) sites. Van Stempvoort
et al. (1994) and Berthold et al. (2004) looked at SO4 concentrations
in and around recharge ponds and uplands at the Warman and St.
Denis sites, respectively (WR and SD in Fig. 1). Rozkowski (1967) reported on salt properties in the uplands in the area of Moose Mountain (MM in Fig. 1). Heagle et al. (2013) looked at salt processes
under a discharge pond at St. Denis. Hendry et al. (1986) explored
salt processes at the uplands close to Vauxhall (VA in Fig. 1) and
Arndt and Richardson (1989) explored soil and ground water salinity under recharge, discharge and flow-through ponds in the Nelson
County area (NC in Fig. 1). Wallick (1981) discussed the deep aquifer
geochemistry in the central prairies. All of these works and others
(e.g., Arndt and Richardson, 1993; Hayashi et al., 1998b; Mills and
Zwarich, 1986; Steinwand and Richardson, 1989) have shown consistent patterns in the salt behavior. Observations from Berthold
Fig. 4. SO4 concentrations in the main Prairie hydrological units, as reported by B (Berthold et al., 2004), K (Keller et al., 1991), V (van Stempvoort et al., 1994), R (Rozkowski,
1967), and H (Heagle et al., 2013), at sites shown in Fig. 1. Solid lines indicate solid sulfate mass per mass of soil; broken lines indicate dissolved SO4 mass per mass of pore
water. Black arrows indicate the accumulation of salt close to the ground surface.
U. Nachshon et al. / Journal of Hydrology 499 (2013) 188–199
et al. (2004), Keller et al. (1991), van Stempvoort et al. (1994), Rozkowski (1967), and Heagle et al. (2013) from various field sites in
Saskatchewan, Canada, were used here to show typical SO4 concentrations for each of the different prairie hydrological units: (1) recharge pond; (2) riparian zone; (3) uplands away from the ponds;
and (4) discharge pond (Fig. 4). Keller et al. (1991), Van Stempvoort
et al. (1994) and Rozkowski (1967) reported SO4 concentrations in
units of mass of SO4 per mass of soil. Berthold et al. (2004) reported
SO4 concentrations in units of mass of SO4 per volume of water and
Heagle et al. (2013) used both sets of units. Here the units of mass of
SO4 per mass of soil were kept as reported in the original papers.
However, the units of mass of SO4 per volume of water were converted to units of mass of SO4 per mass of water. This was done by
multiplying the volumetric concentration by the solution density,
assumed to be equal to that of fresh water (1000 kg/m3), due to
the low solubility of the common SO4 salt solutions (Lide and Bruno,
2012).
A detailed study of salinity under flow-through ponds has not
been reported in the literature, although several works found the
flow-through wetlands (whether by surface or subsurface flow)
to be more saline than the recharge wetlands, yet not as saline as
the discharge wetlands (e.g., Arndt and Richardson, 1989; Euliss
et al., 2004). For example, Arndt and Richardson (1989) reported
on pond water EC in the order of 500, 4500, and 17,000 lS/cm
for recharge, flow-through, and discharge ponds, respectively.
The distribution of salts as shown in Fig. 4 will be described in detail in Section 4, and forms the basis for a conceptual model of salt
dynamics in the prairies.
4. Synthesis of a conceptual model
The discrete hydrological units with the associated observed
SO4 distributions presented in Fig. 4, were used as the basis for
an integrated conceptual model that explains SO4 dynamics for
the till portions of the prairie landscape. Our working hypothesis
is that the controls on the distribution of salts at any given location
in the prairies can be established as a function of the arrangement
of a number of discrete hydrological units. Analogous approaches
have been applied in physical hydrology within models that apply
the grouped or hydrological response unit, GRU or HRU, concept
(Kouwen et al., 1993), such as the Cold Regions Hydrological Model
(CRHM) which has been applied in the prairies (Pomeroy et al.,
2010). Fig. 5 presents a conceptual prairie landscape that includes
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all the hydrological units (pond, wetland, upland) and hydrogeological units (shallow till, deep till, sand/gravel aquifer) that were
identified in Section 2. This includes recharge ponds (a, c), a
flow-through pond (b) and a discharge pond (d). The ETZ is also
shown, which produces a continuous pathway between ponds (a)
and (b). Ponds (c) and (d) are isolated from one another and from
(a) and (b), except for the slow flux through the aquitard (vertical)
and deep aquifer (lateral), as well as episodic surface fill and spill
events. Superimposed over this are the regions of SO4 accumulation, transformation and transport, based on the SO4 concentrations presented in Fig. 4. Under the flow-through ponds salinity
was assumed to be in between that beneath the recharge and discharge ponds. Lateral distances in Fig. 5 are not given because the
distances between neighboring ponds can vary from tens to hundreds of meters. The distance may affect the duration and magnitude of the processes, but conceptually the processes are the
same. Since the main appearance of solid SO4 in the prairies is as
the anion component of the salt gypsum, Figs. 4 and 5 also indicate
the regions of gypsum accumulation in the prairie till.
In this conceptual model, salt accumulation, transport and
transformation processes are described by the interactions between 8 distinct regions of the subsurface, shown in Fig. 5. The
functional characteristics of each region are described in the
remainder of this section, making extensive reference to relevant
field studies.
4.1. Region 1 – Saline Ring (SR)
The saline ring (SR) (Fig. 4B) is a region of saline pore water and
high concentrations of crystalline salts that accumulate around all
ponds in the prairies (recharge, flow-through and discharge). The
SR is caused by an evapotranspiration front that is supplied with
water by lateral flows from the ponds through the high permeability fractured till (Heagle et al., 2007; Keller et al., 1988; Keller and
van der Kamp, 1988; van der Kamp and van Stempvoort, 1992). A
large number of studies have reported consistently high salinity in
the SR, with pore water SO4 concentrations of the order of 2.5%,
high Cl concentrations and EC of the order of 20,000 lS/cm (Arndt
and Richardson, 1989, 1993; Berthold et al., 2004; Hayashi et al.,
1998b; Mills and Zwarich, 1986; Steinwand and Richardson, 1989).
Slightly soluble carbonates accumulate close to the pond edges,
up to 10 m from the shore (Arndt and Richardson, 1993; Miller
et al., 1985; Pennock et al., 2011). Gypsum, which has a higher solubility, is usually found further away, of the order of 20 m off the
Fig. 5. Conceptual model of water flow and SO4 transport in the prairie landscape. The discrete regions with respect to salt accumulation, are marked by the abbreviations
presented in in Section 4. Subscripts ‘r’, ‘t’, and ‘d’ indicate recharge through-flow, and discharge wetlands, respectively. Arrow lengths are not related to flow rates.
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pond shore (Berthold et al., 2004; Steinwand and Richardson,
1989). Even more soluble salts, such as mirabilite and halite, can
be found further away, representing longer migration with the
solution before reaching saturation, as reported by Last (1990,
1999).
Berthold et al. (2004) conducted detailed salt measurements for
a single recharge pond. The authors used electrical resistivity imaging and ground water and till sampling in order to quantify the SO4
and Cl distribution in the vicinity of a recharge pond (Pond 109 at
St Denis; SD in Fig. 1). More than 50 point measurements of Cl and
SO4 (Table 1 in Berthold et al., 2004) along a 44 m length and 12 m
deep transect from the pond to the saline ring, were used to produce the distribution shown in Fig. 6. The Cl and SO4 concentrations between the discrete measurement points were estimated
by spline interpolation. It is assumed that the SO4 concentrations
coincide with the locations of gypsum and other sulfate salts crystallization and accumulation (Keller and van der Kamp, 1988).
Fig. 6 shows clearly the low salt concentration below the pond
due to dissolution and flushing of the solutes by the infiltrating
fresh water, and the high salt accumulation in the saline ring.
The relatively large depth (2 m) of the salt in the SR is likely
due to the deep and developed root systems of the vegetation
(including trees) in the riparian zone (van der Kamp and Hayashi,
2009; Canadell et al., 1996), which would lower the evapotranspiration front.
An interesting phenomenon, which was not previously discussed in the literature, is the presence of a secondary zone of salt
accumulation between the main salt body of the saline ring and
ground surface, visible in Figs. 4B and 6. This phenomenon is further discussed in Section 4.2.
4.2. Region 2 – Surface Salt Belt (SSB)
The surface salt belt is a region of salt accumulation that can be
observed just below ground surface beneath the uplands (Fig. 4C)
and the edges of the wetlands (Fig. 4B, located above the SR, as described above). Measurements from Fig. 4 are consistent with other
field measurements that indicate high salt concentrations in the
top 0.5 m of prairie soils (Mermut and Arshad, 1987; Wiebe
et al., 2006). It is suggested that, unlike the SR where evaporation
is supplied by pond water, evaporation from the SSB is supplied by
infiltrated rainfall and snow melt water. Infiltrating rainfall tends
to leach salts downwards following rainfall events, whilst between
rainfall events, capillary flows tend to draw saline water upwards
to satisfy evapotranspiration (Fig. 7). The SSB can therefore be
formed with no hydrological connection to any pond on the uplands. Because rainfall is highly variable, salt accumulation in the
SSB is likely to be dynamic on short (<1 yr) time scales, and can
also account for the recycling of salts to depressions and wetlands
with surface runoff (as documented by Hayashi et al., 1998b).
Another mechanism that may affect the location of the SSB is
ion rejection from a freezing solution that can happen during the
freezing of the top 1 m of the prairies till during winter (Granger
et al., 1984; Gray and Grnager, 1986; Hayashi et al., 2003; Willis
et al., 1961; Zhang et al., 2008; Ireson et al., 2013; Vrbka and Jungwirth, 2005).
4.3. Region 3 – Deep Salt Belt (DSB)
The deep salt belt (DSB) is a zone of salt accumulation beneath
the uplands which reflects the in situ SO4 concentration in the oxidized till (Keller et al., 1991), which has not been leached. This
zone can be seen in Fig. 4C. In this region the vertical and lateral
subsurface flow rates are minimal and the salts are therefore relatively immobile.
4.4. Region 4 – Unsaturated fractured till (UT) beneath the upland
Beneath uplands there is a portion of the unsaturated zone between the SSB and the DSB where the salinity is reduced. Here SO4
originating from the oxidation of the parent material has been leached away by subsurface fluxes (Keller and van der Kamp, 1988).
Salts may be flushed downwards into this region with infiltration,
but they are removed again by capillary fluxes upwards towards
the evapotranspiration front in the SSB.
4.5. Region 5 – Saturated unfractured till (ST) beneath the upland
Below the DSB, SO4 concentrations are low, due to a combination of the low oxidation rates and the low flow rates (Berthold
et al., 2004). Therefore, this region is effectively excluded from
both the flow processes and the salt dynamics in the surrounding
regions. The major solute transport in this region is dominated
by diffusive processes (Remenda et al., 1996; Hendry and
Fig. 6. Measured Cl and SO4 concentrations (mass% of SO4 in pore water) along a cross section from the pond to the saline ring, produced from the measurements of Berthold
et al. (2004). Straight dashed lines indicate the locations of vertical transects measurements.
U. Nachshon et al. / Journal of Hydrology 499 (2013) 188–199
Wassenaar, 1999), except for some deep tills that have significant
fracture permeability (Keller and van der Kamp, 1988; Zebarth
et al., 1989; Hayashi et al., 1998b).
4.6. Region 6 – Recharge Pond (RP)
Recharge wetlands represent the beginning of the subsurface
flow paths and are freshwater environments. The pond freshwater, originating from snow melt and rainfall-runoff, infiltrates
and flushes any solutes laterally to the SR and vertically to the
underlying aquifers if they are present (Fortin et al., 1991).
Available EC measurements in recharge ponds from the Canadian
prairies (Driver and Peden, 1977; Keller and van der Kamp, 1988;
van der Kamp and Hayashi, 2009), as well as from North Dakota
(Arndt and Richardson, 1989) are of the order of 400 lS/cm.
Steinwand and Richardson (1989) have shown that evaporative
mineral and salt concentrations around recharge ponds are notably lower compared to discharge wetlands, though the SR is still
present, if the rate of lateral flows towards the riparian zone is
much larger than vertical flows to the deep aquifer (Hayashi
et al., 1998a). Keller and van der Kamp (1988) presented field
measurements from the Warman (WR in Fig. 1) and Dalmeny
(DL in Fig. 1) sites, where there was strong downward flow
beneath the recharge ponds, and the SO4 concentration in the
upper till, including the saline ring (Section 4.4), was minimal
(of the order of 100 mg/L). Similar observations were reported
by Zebarth and de Jong (1989).
4.7. Region 7 – Discharge Pond (DP)
Discharge ponds represent the termination of subsurface flow
paths and are highly saline environments because there is an upward hydraulic head gradient between the deep sand/gravel aquifer and the pond and they only lose water to evaporation (Heagle
et al., 2013). This results in conditions of slow, steady transport
of solutes into the discharge pond, where they are enriched by
evaporation. The major input of water to the discharge ponds is
still relatively fresh rainfall-runoff and snowmelt, which results
in seasonal dilution of the pond water (Birks and Remenda,
1999; Heagle et al., 2013). Discharge ponds are well known to be
in the most saline and gypsum rich wetlands in the prairies (Arndt
and Richardson, 1989; Euliss et al., 2004; Heagle et al., 2007; van
der Kamp and Hayashi, 2009; Goldhaber et al., 2011).
Several studies have found discharge ponds to be less saline
than the immediately underlying pore water and sediments (e.g.,
Eisenlohr, 1972; Birks and Remenda, 1999; Zlotnik et al., 2010;
Heagle et al., 2013). The reason for the solute accumulation below
the pond, in the sediment and the pore water, is not completely
understood and could be related to periodic drying of the pond;
differences in densities between the deeper groundwater and pond
water; or the nature (rate and chemical compositions) of the discharge water (Birks and Remenda, 1999).
In conditions where drainage networks are well developed, discharge from the deep aquifer may occur directly to streams and
rivers (Fortin et al., 1991; Grasby and Betcher, 2002), resulting in
salt migration into the regional drainage systems (Strobel and Gerla, 1992; Strobel and Haffield, 1995; van Everdingen, 1971). In
these cases the salts are transported through the regional rivers
(Fortin et al., 1991; Strobel and Haffield, 1995) towards terminal
lakes or the ocean.
and upslope ponds. The shallow ground water that flows into the
flow-through pond through the ETZ may dissolve salts from the
SR and DSB and transport them into the pond. Consequently, the
flow-through ponds are typically more saline than other recharge
ponds, but not as saline as discharge ponds. In general the salinity
of flow-through pond depends on the balance between the inflow
of shallow ground water (a source of salinity), surface runoff
(source of fresh water), episodic surface outflow, and recharge into
the deep aquifer (a sink for the salts) (Euliss et al., 2004; Heagle
et al., 2007). The relatively high salinity of the flow-through pond
water, results in higher salinities of the saline ring around the
pond, compared to pure recharge ponds (Steinwand and Richardson, 1989).
5. Potential environmental changes on salt dynamics
In Section 4 a conceptual model of the salt dynamics in the
prairies, under typical current climatic and land-use conditions
was presented. Fortunately, the salt concentrations in the near surface are usually low enough to enable prosperous agricultural
activities and a flourishing natural wildlife environment. However,
climate change may result in significant changes to the prairie
hydrological and geochemical system. In this section, a qualitative
assessment of the impacts of a variety of possible future scenarios
is presented. Future climate scenarios may include changes in the
proportion of rainfall versus snowfall, as well as changes in the
total amount of precipitation (Shook and Pomeroy, 2012). Specific
scenarios that are considered here include: a shift in precipitation
towards more snowfall; a shift in precipitation towards more
rainfall; wetter conditions associated with an overall increase in
precipitation; and drier conditions associated with an overall
reduction in precipitation and/or increases in evaporation. In all
cases we are referring to long term changes in the climate – rather
than temporal wet or dry periods, which have occurred in the
prairies throughout the period of record (Sauchyn and Beaudoin,
1998). At the end of this section, the impacts of land use changes
are discussed, which could potentially be tailored to minimize the
risk of salinization, subject, of course, to other considerations that
may be relevant.
4.8. Region 8 – Flow-Through Pond (FTP)
Flow-through ponds are usually recharge ponds (i.e. deep vertical flows are in a net downward direction) that in addition receive
lateral flow from shallow groundwater from adjacent hillslopes
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Fig. 7. Conceptual mechanism of SSB formation above the SR and DSB.
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5.1. Precipitation shift to more snowfall
5.3. Wetter conditions
If a larger proportion of precipitation falls as snow, this is expected to result in an increased risk of surface salinization in the
riparian zone particularly, but also in the uplands. An increase in
snowfall would lead to an increase in spring snowmelt runoff, largely over frozen soils, which results in an increase in depressionfocused recharge and rising water tables beneath the depressions.
The rising water tables bring the evapotranspiration fronts, and
hence the SSB and SR, closer to the ground surface (LaBaugh
et al., 1996; Rozkowski, 1967; Stolte et al., 1992). At the same time,
infiltration beneath the uplands will be reduced (since a greater
proportion of precipitation now runs off over frozen soils) so that
leaching of the SSB, DSB, and SR will be reduced.
A second outcome that could result from water tables rising up
is that hydraulic connections between neighboring ponds through
the ETZ (van der Kamp and Hayashi, 2009; Winter and Labaugh,
2003) may be enhanced and downslope recharge ponds may turn
into flow-through ponds, with elevated salinity. The effect of both
of these outcomes is illustrated in Fig. 8, in particular between
ponds a, b and c.
The scenarios in Sections 5.1 and 5.2 above do not consider a
change in the total amount of precipitation, only a change in the
proportions of snow and rainfall. If the total precipitation increases
then it is expected that some combination of these effects would
occur simultaneously, and thus the outcome, in terms of the risk
of salinization, is not easy to predict.
On the one hand, a rise of the water tables would increase the
risk of surface salinization. The time needed for elevated water tables to salinize the upper till profile is dependent on the rate of
water table elevation changes and the hydro-geochemical process
rates, that mainly include ion exchange, dissolution and crystallization processes. In general, for a rising water table the time scale
for ground surface salinization is of the order of a few tens of years
(Lambert and Shiati, 2002). Changes will affect the SR faster than
other locations, since a large amount of the salts here are dissolved
in the pore water, and hence no dissolution processes are needed
to mobilize them.
On the other hand, two processes may decrease the salinity
close to ground surface if wetter conditions persist over time scales
of hundreds to thousands of years. The first process is development
of surface drainage network, leading to regional surface runoff that
will carry the salts out of the basin. This has occurred already in the
Red River basin in North Dakota, where wetter conditions have led
to a more developed drainage system compared to the central prairies, and significant migration of salts from the wetlands to the regional drainage system has been observed (Strobel and Haffield,
1995).The second process is leaching of the salts downward toward the deep aquifer, caused by increased infiltration (Miller
and Brierley, 2011). An existing example where this may have happened is in the Boreal forest, to the immediate north of the prairies.
Here the geological conditions are similar to the prairies (Florinsky
et al., 2000; Schoenau and Bettany, 1989), but conditions are
wetter and there is a surplus of precipitation over evaporation
(Florinsky et al., 2000; Hogg, 1997). As a result salts in the Boreal
forest have been flushed to the regional river system or washed
down to greater depths in the till, and the risk of salinization is
much lower than in the prairies (Eilers et al., 1997; Florinsky
et al., 2000).
It is important to emphasize that while these flushing scenarios
will reduce the risk of salinization over time scales of the order of
5.2. Precipitation shift to more rainfall
If a larger proportion of precipitation falls as rain, this is expected to result in a reduced risk of surface salinization in the uplands. More rainfall onto unfrozen soils is likely to lead to more
infiltration, less runoff and hence a more even spatial distribution
of groundwater recharge. This would result in enhanced leaching
of the salts in the SSB.
Another possible outcome of more evenly distributed recharge
is for higher water tables to develop beneath the uplands, compared to the ponds. In this case the flow directions might reverse
and flow from the uplands to the ponds, resulting in migration of
salts from the SR to the ponds. Such conditions have been observed
for short periods after strong rainfall events in central Saskatchewan (Berthold et al., 2004), as well as in the Cottonwood Lake site
in North Dakota (Rosenberry and Winter, 1997). However, these
events were of short duration, and as the groundwater levels
dropped, evapotranspiration from the uplands resumed pumping
water out of the ponds.
Fig. 8. Conceptual model of hydrological and salt dynamics in the prairies for wet conditions. All symbols correspond to Fig. 5. The major change compared to dry conditions
(Fig. 5) is the elevated water tables that expose larger parts of the uplands to salinization by the SR and the SSB. In addition, ponds that were isolated under dry conditions are
now connected through the ETZ, resulting in dissolution of salts from the SSB, DSB, and SR and transport of the dissolved salts into the ponds. The most prominent salinization
occurs at ponds that were recharge ponds under dry conditions (pond c) and turn into flow-through ponds under wetter conditions. Elevated pond levels may result in fill and
spill events as demonstrated between ponds ‘a’ and ‘b’ by the curved dashed arrow.
U. Nachshon et al. / Journal of Hydrology 499 (2013) 188–199
hundreds and thousands of years, on shorter time scales, of the order of tens of years (Lambert and Shiati, 2002), the risk of salinization may increase, as salts in the till become mobilized and slowly
leach into the ponds, rivers and aquifers.
5.4. Drier conditions
For periods drier than the present, ponds are expected to dry
and water tables depths are expected to fall. Consequently, deep
till that was previously unoxidized due to the low aeration and oxidation rates below the water table (Goldhaber et al., 2011; Hillel,
1998) will be exposed to oxidation. Remenda and Birks (1999)
noted a general trend of increasing depth to the oxidation front
from the eastern prairies (high precipitation, shallow water tables)
to the western prairies (less precipitation, deeper water tables).
The authors also observed that in many parts of the prairies the
oxidation depth is a few meters deeper than the modern water table depth, and suggest that this is due to lower water tables during
the warmer and drier climate conditions of the mid-Holocene,
7500–4000 years ago (Grasby et al., 2010).Oxidation processes will
turn the unoxidized S into SO4, increasing the concentration of SO4
and gypsum in the region. These processes are very slow, of the order of 1000 years (Goldhaber et al., 2011); therefore, a long term
climatic change is needed in order to make this scenario relevant.
Moreover, due to the low water flow rates under dry conditions,
SO4 and associated salts are not expected to move and to salinize
the soil surface or groundwater, as long as the dry conditions remain. Therefore, under drier conditions, salinization is not going
to be as serious a problem as the lack of water for agricultural, ecosystems and water resources.
5.5. Managing the risk of salinization with land use
Anthropogenic land use changes in the prairies mainly include
various agricultural activities. Traditional tilling of the land for cultivation tends to decrease the hydraulic conductivity of the upper
soil by almost an order of magnitude compared to natural grassland (van der Kamp and Hayashi, 2009). Consequently, this land
use may significantly reduce infiltration, enhance surface runoff
and enhance depression focused recharge – the implications of
which were discussed in Section 5.1, and include an increased risk
of salinization in the riparian zones. It follows, therefore, that
returning land use to native grassland, or perhaps implementing
zero-till cultivation, may reduce the risk of salinization.
Differences in vegetation have a strong influence on the location
of the SSB. In the riparian zone there tend to be trees with deep and
developed root systems, which would result in deeper SSB (possibly indistinct from the SR). The uplands are characterized by natural grass or agricultural crops, both having shallower root systems
(van der Kamp and Hayashi, 2009; Canadell et al., 1996), shallower
evapotranspiration fronts, and salt accumulation closer to the
ground surface. The transition from natural grassland to cultivated
agricultural land use may also affect the evapotranspiration front
location and the SSB depth, due to differences in the root systems
and transpiration rates associate with each vegetation type (Christie et al., 1985). An example of this can be seen in Fig. 2, where the
SSB in the cultivated zone is at ground surface, while for the uncultivated zone, on the other side of the fence, thriving natural vegetation is observed without any evidence of salinization. This
suggests that, unlike the cultivated area, the natural grass root system is deep enough to push down the evapotranspiration front,
thus minimizing the SSB effect. This also demonstrates the vulnerability of the prairies to changes in the location of the SSB. Salt
buildup and efflorescent crusts have been documented in the
upper unsaturated zone in many arid regions, attributed to the
closed cycles of wetting (infiltration) and drying (evaporation)
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(Amit and Gerson, 1986; Drever and Smith, 1978; Gee and Hillel,
1988; Weisbrod et al., 2000).
Some agricultural activities result in lowering, or emptying, of
ponds. These include pumping pond water for irrigation, and digging ditches to drain ponds and wetland areas to expand cultivation. The reduced water levels may result in reversing of shallow
groundwater fluxes from the uplands toward the ponds as described in Section 5.2. This may result in the slow dissolution
and migration of salts from the SR toward the pond. In the drainage
scenario the salts are likely to be transported away to the regional
drainage system, potentially causing problems downstream.
6. Summary
As shown in this paper, the general processes and mechanisms
related to subsurface salt dynamics in the prairies are relatively
well understood, especially for conditions of stable climatic and
environmental conditions. Under such circumstances, a conceptual
model is proposed here that explains, qualitatively, the spatial distribution of sulfate salts. These salts migrate with the flow of water
and for the most part are internally redistributed within the surface and subsurface environment of the closed prairie basins. Areas
of accumulation include flow-through ponds, discharge ponds and
evapotranspiration fronts within the till. The properties associated
with dissolution, crystallization, and ion exchange affect the rates
of the processes and the location of their accumulation along the
flow paths. For other salts the geochemical parameters differ, but
the processes are the same and accumulation patterns are similar,
and therefore this model is still relevant.
Currently over most of the prairies area, the salts are at depth of
a few meters below ground surface, thus allowing agricultural
activities to prosper, as well as a rich natural wild life environment.
Yet, small changes in the delicate balance of the hydrological processes may increase the risk of salinization in a manner which may
not be trivial to predict, and may result in severe environmental,
social, and economical problems. For conditions of long term environmental change (e.g. climate change, land use changes), salt distributions can change due to modified flow directions, changing
water table elevations, and moving evapotranspiration fronts. On
the uplands, if the salts move deeper, the risk of salinization is reduced, whilst if the salts move shallower, the risk is increased. The
ponds, riparian zones and aquifers can also be affected in a variety
of ways. In many cases (e.g. see Section 5.3) the specific outcome of
a particular change (for example in precipitation) depends on the
balance of two or more counteracting impacts. In such a case, a
qualitative understanding, as presented here, is insufficient. A full,
quantitative understanding of the implications of such a scenario is
necessary, which requires the use of detailed physically-based
models that couple the geochemical and hydrological processes.
In particular, there is a crucial need for a model which is able to
quantify the depth of salt accumulation in the subsurface beneath
the uplands and riparian zone. This will require improved process
models that can simulate groundwater-surface water interactions
with complex topography, unsaturated zone flow processes, and
reactive transport, and the models must be combined with well
designed, long term, comprehensive field observations.
Acknowledgements
This research was supported by the University of Saskatchewan
Global Institute for Water Security. Any opinions, findings, and
conclusions or recommendations expressed in this material are
those of the authors and do not necessarily reflect the views of
the sponsoring agencies.
The authors would like to thank Kent Keller and two anonymous reviewers for their helpful reviews of the paper.
198
U. Nachshon et al. / Journal of Hydrology 499 (2013) 188–199
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