HESSHydrology and Earth System SciencesHESSHydrol. Earth Syst. Sci.1607-7938Copernicus PublicationsGöttingen, Germany10.5194/hess-20-1-2016Soil–aquifer phenomena affecting groundwater under vertisols: a reviewKurtzmanD.daniel@volcani.agri.gov.ilBaramS.DahanO.Institute of Soil, Water and Environmental Sciences, The Volcani
Center, Agricultural Research Organization, P.O. Box 6, Bet Dagan 50250,
IsraelDept. of Land, Air and Water Resources, University of California
Davis, CA 95616, USADept. of Hydrology and Microbiology, Zuckerberg Institute for Water
Research, Blaustein Institutes for Desert Research, Ben Gurion University of
the Negev, Sde Boker Campus, Negev 84990, IsraelD. Kurtzman (daniel@volcani.agri.gov.il)15January201620111230July201521September201513December201515December2015This work is licensed under a Creative Commons Attribution 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by/3.0/This article is available from https://hess.copernicus.org/articles/20/1/2016/hess-20-1-2016.htmlThe full text article is available as a PDF file from https://hess.copernicus.org/articles/20/1/2016/hess-20-1-2016.pdf
Vertisols are cracking clayey soils that (i) usually form in alluvial
lowlands where, normally, groundwater pools into aquifers; (ii) have different
types of voids (due to cracking), which make flow and transport of water,
solutes and gas complex; and (iii) are regarded as fertile soils in many
areas. The combination of these characteristics results in the unique
soil–aquifer phenomena that are highlighted and summarized in this review.
The review is divided into the following four sections: (1) soil cracks as
preferential pathways for water and contaminants: in this section lysimeter-to basin-scale observations that show the significance of cracks as
preferential-flow paths in vertisols, which bypass matrix blocks in the
unsaturated zone, are summarized. Relatively fresh-water recharge and
groundwater contamination from these fluxes and their modeling are reviewed;
(2) soil cracks as deep evaporators and unsaturated-zone salinity: deep
sediment samples under uncultivated vertisols in semiarid regions reveal a
dry (immobile), saline matrix, partly due to enhanced evaporation through
soil cracks. Observations of this phenomenon are compiled in this section
and the mechanism of evapoconcentration due to air flow in the cracks is
discussed; (3) impact of cultivation on flushing of the unsaturated zone and
aquifer salinization: the third section examines studies reporting that
land-use change of vertisols from native land to cropland promotes greater
fluxes through the saline unsaturated-zone matrix, eventually flushing salts
to the aquifer. Different degrees of salt flushing are assessed as well as
aquifer salinization on different scales, and a comparison is made with
aquifers under other soils; (4) relatively little nitrate contamination in
aquifers under vertisols: in this section we turn the light on observations
showing that aquifers under cultivated vertisols are somewhat resistant to
groundwater contamination by nitrate (the major agriculturally related
groundwater problem). Denitrification is probably the main mechanism
supporting this resistance, whereas a certain degree of anion-exchange
capacity may have a retarding effect as well.
Introduction
Vertisols can be briefly defined as soils with 30 % or more clay to a
depth of 50 cm that have shrinking/swelling properties (Brady and Weil,
2002). More detailed definitions require the existence of a subsurface
vertic horizon in which slickenside features are formed by the shrink/swell
dynamics (FAO Corporate Document Repository, 2015; IUSS Working Group WRB,
2014). Other names used for these types of soils are vertosols (common in
Australian studies, e.g., Radford et al., 2009; Silburn et al., 2009;
Gunawardena et al., 2011; Ringrose-Voase and Nadelko, 2013), and the more
general cracking clays (e.g., Bronswijk, 1991; Liu et al., 2010). This
latter generic term emphasizes both the hydrological complexity of these
soils due to the inherent discontinuities (cracks) and their relevance for
agriculture, being heavy, relatively fertile soils in many semiarid regions
(good water-holding capacity, relatively higher organic content, etc.).
Vertisols usually form in lowlands (Yaalon, 1997) where, typically,
groundwater pools into alluvial aquifers. Hence, the interface between
agricultural activity on these soils and the underlying groundwater
resources is both complex and relevant. This review focuses on vertisol
studies that have implications for the underlying groundwater resources; it
does not cover the substantial body of literature concerning
shrinking/swelling dynamics and its modeling (e.g., Bronswijk, 1988; Chertkov
et al., 2004; te Brake et al., 2013), the purely agricultural and
mineralogical aspects of vertisols (e.g., Bhattacharyya et al., 1993; Ahmad
and Mermut, 1996; Hati et al., 2007) or environmental topics like the
capacity of vertisols to sequester carbon (Hua et al., 2014).
Vertisols cover 335 million hectares out of a total earth land area of 14.8
billion hectares (2.3 %). The largest areas covered with vertisols are in
eastern Australia, India, Sudan–Ethiopia and Argentina–Uruguay (FAO
Corporate Document Repository, 2015). Smaller areas of vertisols are found
in various countries (e.g., China, Israel, Mexico, Spain, Tunisia, USA and
many more). Although vertisols are very hard when dry, and very sticky when
wet (making them difficult to till), in semiarid regions, irrigated crops
such as cotton, corn, wheat, soybeans and others are grown on this soil. We
acknowledge the dominant contribution of Australia to the literature on
agro-hydrological aspects of vertisols. Conclusions from those studies have
strengthened and generalized some of the findings obtained by the authors of
this review in vertisol–groundwater studies in Israel, and have motivated
this review (e.g., Arnon et al., 2008; Kurtzman and Scanlon, 2011; Baram et
al., 2012a, b, 2013; Kurtzman et al., 2013).
The review is divided into four sections, which are partially connected and
together deal with major issues concerning aquifers under agricultural land:
recharge, salinization and nitrate contamination (other contaminants are
mentioned as well). The following four sections cover the most general and
relevant issues concerning soil–aquifers phenomena under vertisols:
soil cracks as preferential pathways for water and contaminants (Sect. 2)
soil cracks as deep evaporators and unsaturated-zone salinity (Sect. 3)
impact of cultivation on flushing of the unsaturated zone and aquifer
salinization (Sect. 4)
the relatively little nitrate contamination in aquifers under vertisols
(Sect. 5).
Soil cracks as preferential pathways for water and contaminants
There are probably hundreds of studies that acknowledge preferential flow
and transport through cracks in clays – too many to be mentioned here. This
section aims to review works from the soil-column and lysimeter scale to the
basin and aquifer scales that show the relations between preferential flow
via cracks, deep drainage, and aquifer recharge and contamination. It also
provides a short description of the development of models of preferential
flow and transport through soil cracks.
Preferential flow of water in vertisols – evidence from the lysimeter to aquifer scale
On a small scale, Kosmas et al. (1991) observed bypass flow through cracks
in clayey soils from Greece using undisturbed soil columns (authors'
terminology) with a diameter of 23 cm. Ringrose-Voase and Nadelko (2013)
measured flow in preferential paths directly using a field lysimeter that
was installed 2 m below the surface of a furrow-irrigated cotton field
without disturbing the overlying soil. Significant drainage was collected in
this study when the hydraulic gradient in the matrix was in the upward
direction, advocating drainage through preferential pathways that bypasses
the matrix. In a paragraph on tension-lysimeters measurements, Silburn et al. (2013) acknowledge that “deep drainage measured at 1m depth was
dominated by matrix flow, with only 10 % of drainage attributed to
preferential flow (note that the soil was never dry enough to crack)”,
pointing out that under well-irrigated vertisols matrix deep-drainage and
recharge may be of importance despite the low saturated hydraulic
conductivity of the clay. A weighing-lysimeter experiment in irrigated
vertisols in eastern Australia revealed a complex drainage mechanism
following spray irrigation, where only deep parts of the cracks act as
preferential pathways for the drainage when the top soil is moist and
uncracked (Greve et al., 2010).
On the field scale (∼ 100–1000 m2), a similar
phenomenon – i.e., open cracks at depth when surface cracks are mostly
sealed – was reported by Baram et al. (2012b) throughout the rainy season in
Israel. These authors compared transient deep (up to 12 m) water-content
data collected by vadose-zone-monitoring systems (VMSs; Dahan et al., 2009)
at various sites, including very sandy soils; the comparison showed that by
far, the fastest propagation of wetting fronts in deep vadose zones is
observed in cracking clays.
Ben-Hur and Assouline (2002) conducted measurements of runoff in a vertisol
cotton field in Israel that was irrigated with a moving sprinkler irrigation
system. They observed that the high infiltration of runoff through soil
cracks limited the overall surface runoff from the field. Other field-scale
vadose-zone studies reported that preferential flow through cracked clay
enhances infiltration from rice paddies (Liu et al., 2004). Losses of up to
83 % of the water to deep drainage (including preferential and/or matrix flows) during furrow irrigation of cotton and sugar cane in
vertisols were reported (Raine and Bakker, 1996; Dalton et al., 2001; Moss
et al., 2001; Smith et al., 2005). Losses to deep drainage averaged 42.5 mm
per irrigation (Smith et al., 2005), ranging from 50 to 300 mm yr-1 (Silburn
and Montgomery, 2004). Chen et al. (2002) and Bandyopadhyay et al. (2010)
showed that the transition from flood to micro-sprinkler irrigation and
careful scheduling of water-application rates can dramatically reduce water
losses and contaminant transport due to deep drainage. Observation from
groundwater supported this phenomenon: Acworth and Timms (2009) used nested
piezometers and automated logging of groundwater levels and electrical
conductivity to show evidence of shallow-aquifer (16 m depth) freshening
(decrease in salinity) due to fast deep drainage of irrigation water during
the irrigation season.
At the small-watershed scale (∼ 10 000 m2), Pathak et al. (2013) indicated that runoff from vertisols is smaller than runoff from
sandier soils (alfisols) in an agricultural watershed near Hyderabad, India.
The smaller runoff from the vertisols was attributed to preferential
infiltration of local runoff into the soil cracks. Similar observations of
minimal drainage and rapid recharge of shallow groundwater (∼ 3 m) below a vertisol–shale watershed in Texas following rainstorms were
reported by Allen et al. (2005) and Arnold et al. (2005). This process was
most dominant during the first rainstorms when the cracks were fully
developed (at the end of the dry season).
On the aquifer scale (100+ km2), Kurtzman and Scanlon (2011)
concluded that parts of the Israeli coastal aquifer overlaid by vertisols
were fresh (before the influence of modern intensive cultivation) only due
to recharge flow through preferential paths that bypassed the saline
vadose-zone matrix. Dafny and Silburn (2014) reported that following the
growing evidence of the feasibility of percolation through cracking clays,
several recent studies have included a component of diffuse recharge in
their assumptions or models of the Condamine River alluvial aquifer in
Australia. This diffused recharge originates in deep drainage flowing
through clay matrix and/or preferential paths.
Preferential transport in vertisols
In the last 2 decades, many transport studies with dyes and/or other
conservative tracers (e.g., bromide, Br-) have indicated the
pervasiveness of deep preferential transport through cracks in vertisols.
Bronswijk et al. (1995) sprayed a bromide solution on cracking clays in the
Netherlands that overlay a shallow water table (∼ 1 m from
ground surface). The authors reported rapid (on the order of days after rain
event) preferential transport of Br- into the groundwater, and
relatively fast (weeks to months) propagation within the vadose zone.
Bronswijk et al. (1995) concluded that large cracks control the rapid
transport of Br- to the groundwater, and preferential paths made up
of tortuous mesopores control transport in the unsaturated zone
(suggesting that transport through vertisols could be described as a triple
domain medium–macropores, mesopores and matrix). Van Dam (2000) used the
Crack module in SWAP to model the aforementioned experiment. This effort
improved fits to the observations (relative to a single-domain model), but
the variability of Br- in the unsaturated zone still could not be well
reproduced. Lin and McInnes (1995) used dye to study and model flow in
vertisols. They showed that infiltrating water passes first through the soil
cracks and then into the soil matrix; they concluded that uniform flow
through the soil cannot be used to describe the dye transport. A dye
experiment in a soil column consisting of a sandy A horizon and a vertic
clay B horizon showed preferential downward flow through the cracks in
horizon B, bypassing more than 94 % of the matrix (Hardie et al., 2011).
Kelly and Pomes (1998) estimated equivalent hydraulic conductivities from
arrival times of Br- and 15N-labeled nitrate in gravity lysimeters
installed above and under a clay pan in Missouri (USA). They reported
equivalent conductivities that were 4 orders of magnitude higher than the
saturated hydraulic conductivity of the clay matrix.
Unlike tracers used in experiments, fast transport of herbicides and
pesticides is of concern in aquifers and drainage systems down gradient from
cultivated fields. Graham et al. (1992) reported that in cultivated
vertisols in California (USA), herbicides were only found deep below the
root zone in samples taken from the cracks' walls and not within the matrix,
suggesting rapid transport of herbicides through the cracks, either as
solutes or on colloids. Transport of pesticides in preferential-flow paths
absorbed on colloids was also suggested for cotton fields on vertisols in
Australia (Weaver et al., 2012). Early and deep drainage of herbicides from
a lysimeter in cracking clays in the UK (early meaning well before reaching
field capacity in the matrix) was reported by Harris et al. (1994).
Similarly, fast arrival of herbicides to drains in cultivated clays was
observed by Tediosi et al. (2013) on a larger scale (small catchment).
Due to the fact that in semiarid regions vertisols are arable,
agriculture-oriented settlements have developed on these soils. In many
cases, these settlements include concentrated animal feeding operations
(CAFOs), such as dairy farms. Arnon et al. (2008) reported deep transport
(> 40 m) of estrogen and testosterone hormones into the
unsaturated zone under an unlined dairy-waste lagoon constructed in a 6 m
thick vertisol in Israel. They concluded that deep transport of such highly
sorptive contaminants can only occur by preferential transport. Baram et al. (2012a, b) reported that preferential infiltration of dairy effluents
through the cracks at the same site can transport water and solutes into the
deep unsaturated zone. Locally, groundwater under dairy farm areas also
shows relatively high concentrations of nitrate (Baram et al., 2014).
Figure 1 provides a visual summary of Sect. 2.1 and 2.2. It shows the
potential for matrix-bypassing groundwater recharge and pollution under
vertisols. Passing the biogeo-active matrix enables both freshwater recharge
and transport of reactive substances.
Illustration of potential fluxes of water and pollutants
that bypass the matrix, which is typical of vertisols.
Development of flow and transport models in cracking clays
The field evidence described above motivated the development of quantitative
methods to enable better predictions of flow and transport from ground
surface to water table under vertisols. Nevertheless, modeling of
unsaturated flow and transport as a dual (or multiple) domain in their
different variants (e.g., mobile–immobile, dual porosity, dual permeability)
did not develop exclusively to deal with cracking clays. Macropores such as
voids between aggregates, or worm holes, are the preferential-flow paths of
interest in many agricultural problems. Computer codes for modeling
unsaturated preferential flow include among others: MACRO (Jarvis et al.,
1994) and nonequilibrium flow and transport in HYDRUS (Šimůnek and
van Genuchten, 2008). For further information on the kinematic wave approach
used in MACRO, the reader is referred to German and Beven (1985); for
comparative reviews of the different models and codes see Šimůnek et al. (2003), Gerke (2006), Köhne et al. (2009) and Beven and Germann (2013). The latter is critical of the common use of the Richards (1931)
formulation in single and multiple-domain unsaturated-flow simulators.
One of the earlier crack-specific unsaturated-flow models was developed by
Hoogmoed and Bouma (1980), who coupled vertical (crack) and horizontal (into
the matrix) 1-D models using morphological data for parameterization of the
linkage between the two flows. Novák et al. (2000) attached a FRACTURE
module to HYDRUS in which a source term was added to the Richards equation
accounting for infiltration from the bottom of the fractures, bypassing
matrix bulks. Van Dam (2000), added a crack sub-model to SWAP (van Dam et al.,
2008) and Hendriks et al. (1999) used a code called FLOCR/AMINO,
to study flow and transport phenomenon in shallow and cracked clayey
unsaturated-zones in the Netherlands. A model of herbicide transport through
the preferential paths was fitted successfully with the improved MACRO
version 5.1 (Larsbo et al., 2005).
A more comprehensive dual-permeability module for 2-D and 3-D variably
saturated models was introduced into HYDRUS much later (Šimůnek et
al., 2012) following the formulations of Gerke and van Genuchten (1993).
Coppola et al. (2012) took another step forward in modeling flow and
transport in cracking clays by also introducing cracking dynamics (adopting
formulation of Chertkov, 2005) into a dual-permeability flow and transport
model.
Soil cracks as deep evaporators and unsaturated-zone salinity
Whereas during rain events or under irrigation, cracks are a concern in
terms of loss of water and fertilizers and/or contamination of groundwater
(Sect. 2), under dry conditions, deep soil cracks are relevant for their
evaporation capacity from deep parts of the soil column. Kurtzman and
Scanlon (2011), Baram et al. (2013) and others have reported the low water
content and high salinity typical of the sediment matrix under uncultivated
vertisols. Deep chloride profiles under native-land vertisols often show an
increase in salinity down to 1–3 m and a relatively constant concentration
in deeper parts of the vadose zone (e.g., Radford et al., 2009; Kurtzman and
Scanlon, 2011; Silburn et al., 2011). In the reported cases from Israel,
water uptake by roots was limited to the upper 1 m of the soil profile and
to the rainy season, and therefore could not fully explain the increase in
salinization in the deeper layers. Deep cracks form an additional mechanism
of deep evaporation that supports the chloride profiles and low water
content in the matrix under vertisols.
Sun and Cornish (2005) used SWAT to model runoff and groundwater recharge at
the catchment scale (∼ 500 km2) in a vertisolic catchment
in eastern Australia. Considering water balances at this scale, they
concluded that recharge models need to have a component that enables taking
moisture out of the lower soil profile or groundwater during dry periods.
Trees with roots in groundwater and deep soil cracks can maintain deep
evaporation in long dry periods. Another, indirect observation that supports
evaporation through cracks in vertisol was reported by Liu et al. (2010). In
this work discrepancies between satellite and model estimates of soil water
content in dry seasons in vertisols are assumed to be related to the extra
evaporation through the cracks. Both local- and higher-scale observations
and analyses point to a possible significant role of soil cracks as deep
evaporators in dry periods.
Desiccation-crack-induced salinization (DCIS), Baram et al. (2013). Convective instability of air in soil cracks, occurring mainly at
night, leads to drying and salinization of the unsaturated zone.
Baram et al. (2013) suggested a conceptual model termed desiccation-crack-induced salinization (DCIS) based on previous work on subsurface
evaporation and salinization in rock fractures (e.g., Weisbrod and Dragila,
2006; Nachshon et al., 2008; Kamai et al., 2009; Weisbrod et al., 2009). In
DCIS, vertical convective flow of air in the cracks is driven by instability
due to cold (and dense) air in the crack near the surface and warmer air
down in deeper parts of the crack at night or other surface-cooling periods.
The difference in the relative humidity between the invading surface air
(low humidity) and the escaping air (high humidity) leads to subsurface
evaporation and salt buildup (Fig. 2). Earlier studies that support the
significance of evaporation via cracks in vertisols through field and
laboratory observations include: Selim and Kirkham (1970), Chan and Hodgson (1981) and Adams and Hanks (1964). The latter showed enhanced evaporation
from crack walls due to increase in surface wind velocity, this is another
mechanism (in addition to surface cooling described before) causing
instability in the crack's air, hence convection, evaporation and salt build up.
Leaching of salts from horizontal flow through the crack network evident in
salinity rise in tail water of furrow-irrigated fields in cracking clays in
California was reported by Rhoades et al. (1997). This Californian study
acknowledge that this phenomenon was not observed in similar fields (crop
and irrigation technique) in lighter soils with no cracks.
In many semiarid regions, deep matrix percolation under non-cultivated
vertisols is very small due to the clay's high retention capacity and low
hydraulic conductivity, root uptake of the natural vegetation in the rainy
season and further evaporation through cracks in dry periods. Low water
content in the deeper unsaturated zone results in low hydraulic
conductivities and makes aquifer recharge through matrix flow very small
year-round. Matrix fluxes on the order of 1 mm yr-1 under the root/crack zone
were reported in a number of studies (e.g., Silburn et al., 2009; Kurtzman and
Scanlon, 2011; Timms et al., 2012). These very low water fluxes contain the
conservative ions (e.g., chloride) originating from 200–600 mm yr-1 of
precipitation (with salts from wet and dry fallout) that enter the matrix at
soil surface. Therefore, a dry (relatively immobile) and salty deep
unsaturated matrix, developed for centuries–millennia under these
non-cultivated vertisols. Nevertheless, some fresh recharge to the
underlying aquifer through preferential paths related to cracks during heavy
rain events creates an anomaly whereby relatively fresh water in the aquifer
(e.g., ∼ 250 mg L-1 chloride; Kurtzman and Scanlon, 2011) lies
beneath a salty and immobile unsaturated zone with porewater chloride
concentration of a few thousands of milligrams per liter (O'Leary, 1996;
Kurtzman and Scanlon, 2011; Tolmie et al., 2011; Baram et al., 2013). River,
mountain-front, paleo- or other types of recharge may contribute, as well, to
a situation where a relatively fresh aquifer exists under a saline vadose
zone.
Impact of cultivation on flushing of the unsaturated zone and
aquifer salinization
The anomalous situation of fresh groundwater under a saline unsaturated zone
found in some native-land vertisols in semiarid regions exists due to the
efficient evapotranspiration by natural vegetation and cracks (making deep
unsaturated matrix immobile and saline) and fresh groundwater recharge
through preferential flow in cracks or other types of recharge. However,
what happens when natural conditions are changed to less favorable for
native-vegetation and soil cracks (e.g., cultivated land and more ever
irrigated intensive cropping)? The answer is obvious: higher fluxes may
develop in the unsaturated matrix, which will flush salts and ultimately
cause salinization of the underlying aquifer.
Flushing of chloride down through the unsaturated zone
under cultivated vertisols: (a) Silburn et al. (2009) – 19 years of
flushing; (b) Radford et al. (2009) – full diamond, native vegetation;
empty, annual cropping; flushing from the top 3 m (c) Kurtzman and Scanlon (2011) – red, natural land; blue, irrigated cropping; flushing from 2–10 m
depth; (d) Tolmie et al. (2011) – flushing from the top 1.5 m. (e) Timms et al. (2012) – black, cropping; empty – grass; flushing from the top 2 m.
A large bulk of literature from eastern Australia has reported increased
deep-drainage and leaching of salts, and in some cases, salinization of
aquifers under cultivated vertisols. A typical increase in deep drainage
from < 1 mm yr-1 under native conditions to 10–20 mm yr-1 under
rain-fed cropping were reported by Silburn et al. (2009); Timms et al. (2012)
and Young et al. (2014); whereas variable deep fluxes often in the 100's mm yr-1 range were reported for irrigated fields (mostly furrow-irrigated
cotton; Gunwardena et al., 2011; Silburn et al., 2013; Weaver et al.,
2013). These deep fluxes desolate salts that accumulated in the vadose zone
in the native-vegetation period, moving them down towards the water table
(Fig. 3). Earlier studies reporting leaching of salts from the vadose zone
after clearing of natural eucalyptus trees for cropping include Allison and
Hughes (1983) and Jolly et al. (1989), who worked in semiarid zones in
southern Australia. In those studies, neither vertisols nor the role of soil
cracks was mentioned; however, deep eucalyptus roots act similar to cracks
to form a very saline and immobile deep-unsaturated-zone matrix, which
becomes more mobile and less saline after the land-use change. Timms et al. (2012) inferred, from combined soil and groundwater data, deep drainage and
salt leaching after conversion to cropping under gray vertosols
in the
Murray–Darling Basin. Fresh groundwater was found in that study under
shallower saline waters, strengthening the source of groundwater salinity
from the vadose zone.
Scanlon et al. (2009) compared mobilization of solutes in the vadose zone
after a change in the natural landscape to cultivated fields in three
semiarid regions: Amargosa Desert (southwestern USA), southern High Plains
(central USA) and Murray Basin (southeast Australia). Flushing of chloride
from the top 6–10 m of the vadose zone after cultivation was very clear
(e.g., Fig. 3 in Scanlon et al., 2009). Flushing has been observed in many
arid and semiarid regions, and not exclusively related to vertisols (e.g.,
Oren et al., 2004, in the arid Arava Valley, southern Israel). Nevertheless,
salinization of an aquifer due to cultivation and salt mobilization may be
more pronounced under vertisols due to preferential-flow paths related to
soil cracks (enabling the native aquifer to be relatively fresh) and the
cracks evaporative capabilities (making the native deep vadose zone more
saline).
Plan views of the Israeli coastal aquifer. (a) Soil type
(black polygons and red ellipses for spatial comparisons with panels d and
e, respectively). (b) Location map. (c) Cultivated land in the year 2000.
(d) Difference in chloride concentrations between 2007 and 1935 (modified
from Livshitz and Zentner, 2009). Black polygons are characteristic
cultivated areas that were severely salinized (southern polygons) and barely
salinized (northern polygon) relative to soil type (panel a). (e) Nitrate
concentration in groundwater wells in 2007 (modified from Hydrological
Service, 2008). Red ellipses – areas with many wells contaminated with
nitrate relative to soil type (panel a; modified from Kurtzman et al.,
2013).
A good example of an aquifer in which vertisols made a difference is the
Mediterranean coastal aquifer in Israel (Fig. 4). Although known as a
coastal aquifer, the phenomena discussed here are all a few kilometers inland and are
not related to seawater intrusions. The parts of this aquifer overlain by
vertisols were salinized a few decades after intensive-cultivation, whereas
the water in those parts of the aquifer overlain by cultivated loamy sand is
still potable (Kurtzman, 2011; Kurtzman and Scanlon, 2011; Fig. 4). Similar
to the Murray–Darling Basin (Timms et al., 2012), the upper groundwater
under vertisols in this aquifer were more saline than the deep groundwater
(e.g., Fig. 1 in Baram et al., 2014). Identification of the source of the
salt and the cause of the salinization in the deep unsaturated zone under
vertisols and land-use change, respectively, contradicted previous works,
which attributed the salinization of these parts of the Israeli coastal
aquifer to the intrusion of deep brines and intensive pumping (e.g., Vengosh
and Ben-Zvi, 1994; Avisar et al., 2004). A different and shorter temporal
trend that might also be interpreted in light of soils covering the recharge
area is the response of groundwater salinity of the Israeli coastal aquifer
to extreme precipitation (e.g., winter of 1991/1992): under vertisols,
freshening of the aquifer (decrease in salinity) was generally observed due
to recharge of freshwater through preferential paths, mostly under
uncultivated parts; under loamy-sand soils, salinization of the aquifer was
observed due to piston-flow recharge pushing relatively saline vadose-zone
porewater down to the water table (Goldenberg et al., 1996; interpreted by
Kurtzman and Scanlon, 2011).
Relatively little nitrate contamination in aquifers under
vertisols
Whereas the literature concerning salinization of aquifers and draining of
salts from the vadose zone under cultivated vertisols is abundant, much less
has been written about the contamination of groundwater by nitrate under
these soils. Nitrate is the most problematic groundwater contaminant
associated with agriculture worldwide (Jalali, 2005; Erisman et al., 2008;
Burow et al., 2010; Vitousek et al., 2010; Kourakos et al., 2012). Both
mineral nitrogen fertilizers (e.g., Kurtzman et al., 2013) and organic forms
of nitrogen (e.g., Dahan et al., 2014) are often applied in excess with
respect to the plants' ability to take up the nitrogen, leaving significant
quantities of nitrate as a potential groundwater contaminant.
While in the previous sections aquifers under vertisols were shown to be
vulnerable to salinization, due to the agricultural practice above, there is
an increasing number of observations that indicate lesser nitrate
contamination in groundwater under cultivated vertisols relative to
groundwater of the same aquifer located under cultivated land of lighter
soils. Kurtzman et al. (2013), dealing with nitrate contamination problems
of the Israeli coastal aquifer, showed that at the groundwater basin scale
(∼ 2000 km2) the contamination plumes of nitrate are
present in the aquifer only under cultivated sandy loams, whereas under
cultivated vertisols sporadic wells seldom produce water with nitrate
concentration above the drinking-water standard (Fig. 4). Dafny (personal communication, 2014)
revealed, by chi-square analysis, that groundwater under cultivated vertisols
and a thick clayey-alluvial unsaturated zone is less likely than groundwater,
under coarser sediments, to get contaminated by nitrate in the Condamine floodplain aquifer in Australia.
In contrast to the relatively high capability of vertisols to reduce nitrate
leaching from cultivated land, both Baram et al. (2014; Israel, Coastal
Aquifer) and Dafny (personal communication, 2014; Condamine floodplain, eastern Australia)
acknowledge that CAFOs can be
significant point sources of nitrate in vertisols as well. This might be due
to incidental percolation of CAFO wastewater through the crack systems.
Silburn et al. (2013) indicated that modern deep drainage and solutes are
still migrating down through the unsaturated zone in
vertisol–alluvial
systems in Australia and the nitrate is accumulating in the unsaturated
zone. Nevertheless in vertisols areas overlaying the Israeli coastal aquifer,
the rise in salinity and unsaturated-flow and transport models, indicate
that the cultivation effects reached the water table, yet nitrate
contamination is not severe, suggesting other mechanism are responsible for
the low levels of nitrate contamination.
Denitrification in clayey soils is thought to be the major reason for the
reduced deep leaching of nitrate in semiarid climates; this reduction of
nitrate to gaseous nitrogen is less likely to be significant in lighter
soils (Sigunga et al., 2002; Baram et al., 2012b; Boy-Roura et al., 2013; He
et al., 2013).
Jahangir et al. (2012) found that adding carbon to deeper soil horizons
significantly enhances denitrification in those layers. Profiles of
dissolved organic carbon (DOC) in deep vadose zones (down to 9 m below
ground) under citrus orchards on thick vertisols versus sandy-loam in Israel
were compared. Whereas DOC in the lighter soils was higher than 15 mg kgdry soil-1 only in the top 1 m, in the vertisols it was
above 30 mg kgdry oil-1 in the entire 9 m profile
(Shapira, 2012). These latter two studies support the notion that
denitrification in the root zone, and perhaps beyond, results in less
nitrate problems in aquifers under cultivated vertisols than under lighter
soils. Thayalakumaran et al. (2014) reported high DOC in shallow groundwater
overlain by irrigated sugarcane corresponds with the absence of nitrate in this
aquifer in northeast Australia.
Denitrification in the root zone and deeper in the soil profile explains the
small amount of nitrate leached to the groundwater under rice fields in
clayey soils in California, USA (Liang et al., 2014). Shallow groundwater
(< 1.5 m) under cultivated vertisols (e.g., the Netherlands) showed large
variability (spatial and temporal) in nitrate concentration, probably due to
the highly variable oxygen concentrations and therefore variability in
nitrogen transformations in these systems (Hendriks et al., 1999).
A more speculative mechanism that might explain the relatively lower
occurrence of groundwater nitrate contamination involves the anion-exchange
capacity of the clay. Harmand et al. (2010) observed very significant
adsorption of nitrate to kaolinite and oxyhydroxides under a fertilized
coffee plantation growing on an acrisol in Costa Rica. In vertisols,
montmorillonite is usually the dominant clay mineral; nevertheless, some
kaolinite is found in most vertisols (e.g., Singh and Heffernan, 2002; Krull
and Skjemstad, 2003; Baram et al., 2012b). Another drawback of this
mechanism as dominant in vertisols is the adsorption of anions to a positively
charged surface is more efficient at low pH, while vertisols in semiarid
regions are usually neutral to alkaline. Retardation of nitrate in the
vadose zone due to adsorption to positively charged sites within the clay
might slow down groundwater contamination under cultivated vertisols.
Nevertheless, if significant, this mechanism would only retard groundwater
contamination, whereas denitrification removes the nitrogen from the
soil – unsaturated zone – aquifer system. The idea of nitrate adsorption has
been tested as an engineered solution for reducing deep nitrate percolation.
Artificially synthesized materials that have nitrate-sorption capacity (e.g.,
[Mg0.822+ Al0.183+
(OH)2]0.18+[(Cl-)0.18 0.5(H2O)]0.18-)
are being tested as soil additives to buffer nitrate leaching
(Torres-Dorante et al., 2009).
Conclusions
Vertisols are considered arable soils in semiarid climates, and are
intensively cultivated. Located in lowlands, vertisols often overlie
aquifers. Flow and transport through the cracking clays is complex and
results in unique land–aquifer phenomena. Observations from the lysimeter
to basin scale have shown (directly and indirectly) the significance of
cracks as preferential-flow paths in vertisols that bypass matrix blocks in
the unsaturated zone. These preferential paths support recharge with
relatively fresh water in uncultivated vertisols, and groundwater
contamination from point sources such as CAFOs and under some conditions,
from crop fields. Deep soil samples under uncultivated vertisols in semiarid
regions reveal a dry (immobile), saline matrix, partly due to enhanced
evaporation through the soil cracks. This evaporation is related to
convective instability due to colder air at ground surface and warmer air
deep in the crack during the night. In some aquifers lying beneath vertisols
in these regions, relatively fresh groundwater exists under the saline
unsaturated zone. Land-use change to cropland promotes greater fluxes
through the saline matrix, which flush salts into the aquifer and eventually
cause groundwater salinization. In contrast to the vulnerability of
groundwater under vertisols to salinization, observations show that this
soil–aquifer setting has some resistance to groundwater contamination by
nitrate (the major agriculturally related groundwater contamination).
Denitrification is probably the main mechanism supporting this resistance,
whereas anion-exchange capacity may have a retarding effect as well.
Acknowledgements
The study was supported by the Agricultural Research Organization (ARO),
Israel.
Edited by: M. Vanclooster
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