Abstract
Abstract. 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.
Highlights
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)
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)
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
Summary
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). 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.). – the relatively little nitrate contamination in aquifers under vertisols (Sect. 5)
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