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Geophysical surveys, soil and geological mapping and hydrologic studies have been used to show an association between dolerite dykes and the occurrence of saline seeps on two catchments in southwestern Australia. The role of the dykes in the location of the saline seeps is explained and the potential contribution of routine geophysical surveys to salinity studies is highlighted. Magnetic surveys and soil and geological mapping identified dolerite dykes crossing the main drainage line of each catchment. Soil resistivity and conductivity surveys showed that these dykes are associated with saline soils. Seismic refraction surveys over these dykes indicate deeper weathering profiles. Groundwater pressures and hydraulic conductivities measured in bores across one of the weathered dykes showed that the clay saprolite formed above the dolerite is less permeable than the surrounding weathered granite. The lower permeability is probably due to the finer texture of the saprolite and/or a lower frequency of preferred flow pathways. The clay formed above the dolerite acts as a linear hydraulic barrier to lateral groundwater flow and results in the discharge of saline groundwaters into surface soils.

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Subsoil pans regarded as cemented are present in certain Andisols of the Olympic Peninsula region of western Washington. Soils formed in basalt‐derived alluvium or colluvium over Pleistocene‐aged compact alpine till. Theories of pan formation include pedogenic cementation by silica, allophane, or humic‐metal complexes; or geogenic till compaction with no subsequent cementation. We examined the properties of three pedons with compacted and cemented subsoil horizons to better understand the genesis of these soils and pan horizons. Pan horizons, designated as Cm, are very brittle, nearly impossible to dig with hand tools, and massive in structure. Soils are acidic, with pH(H2O) for all horizons ranging from 3.7 to 6.0. Crystalline phyllosilicates identified are gibbsite, kaolinite, chlorite, vermiculite, and hydroxy‐interlayered vermiculite. Allophane was detected in most mineral horizons, including Cm horizons, and allophane Al/Si molar ratios vary from 1.2 to 3.4. Low NaOH‐extractable Si, low Si/Al molar ratios from NaOH extracts, high rainfall, and the presence of gibbsite decrease the likelihood of pedogenic opal as a cementing agent in pans. Limited organo‐metallic deposition, low organic C, and low pyrophosphate‐extractable Fe and Al in pan horizons suggest that podzolization processes related to ortstein formation are not responsible for Cm horizon cementation. Micromorphologic examination of Cm horizons detected a light brown to nearly colorless substance that appears to be allophane around mineral grains and in pore channels. Soil chemistry data support allophane as the primary cementing agent in these pan horizons, with kaolinite, gibbsite, and Fe oxides acting as possible accessory agents. Glacial compaction, parent material mineralogy, and cycles of wetting and drying are important factors affecting cementation of these pans.

The application of irrigation water on farmland throughout the arid and semiarid areas poses severe engineering challenges for gypsiferous soils. Subsidence and corrosion also pose severe engineering problems when gypsiferous soils are used for urban development or home sites. Subsidence is attributed to the dissolution and removal of gypsum by water, and it is usually used to estimate potential subsidence of soils. However, a number of other water-soluble minerals may occur in association with gypsum in these soils. Thus, subsidence should be attributed to the dissolution and removal of both gypsum and other water-soluble minerals in soils. The objectives were to (i) develop a laboratory method (equivalent gypsum content, or EGC) to estimate the content of both gypsum and other water-soluble minerals in gypsiferous soils and (ii) apply the EGC method to estimate potential subsidence of these soils. We used the relationship between dissolved minerals and electrical conductivity in soil/water solutions under equilibrium to estimate the EGC for soils. The EGC is defined as the quantity of both gypsum and other water-soluble minerals and expressed as gypsum percentage (by weight) in soils. We measured the EGC for 92 gypsumrich soil samples collected from different arid and semiarid areas in the United States. A highly significant correlation (r = 0.97 ÃÃ ) was found between the EGC and gypsum determined by the standard acetone method. The EGC was greater than gypsum content for almost all soils investigated. The average of EGC was 26.3% compared with 20.2% for gypsum. Using gypsum percentage would underestimate subsidence of these soils. We suggest the application of EGC rather than the gypsum content to estimate potential subsidence. Further, the EGC could be applied to provide a reasonable estimation of gypsum percentage for soils. (Soil Science 2007;172:209-224)