Previous studies on one soil showed that silicone polymer (polydimethylsiloxane, or PDMS) degrades to dimethylsilanediol (DMSD). This study examines PDMS degradation on seven U.S. soils differing in pH, 070 organic matter, texture, mineralogy, and geographic origin. Moist soils were amended with 350-centistoke (cs) [I4C]PDMS at 100 mg kg-I, and soils were dried at 23°C for 0, 2, 4, 7, 10, or 14 d. Foam plugs were inserted in tube necks to trap volatiles. Samples were extracted with water to monitor silanol formation, or with THF (tetrahydrofuran) for analysis of molecular weight changes and identification of degradates. In all soils, PDMS degraded extensively to low-molecular-weight, water-soluble products. Gas chromatography-mass spectrometry (GC-MS) identified the main product in all soils as DMSD. Other small silanols and cyclic siloxanes were either not detected or were present in only trace amounts. No volatile I4C was captured by the plugs, and quantitative recovery of I4C showed no loss of unidentified volatiles. PDMS degradation was thus similar in a wide range of soils, and DMSD was the main degradate. A lower limit of 4,900 -I-1,250 L kg-' for the k, of this PDMS suggests that the polymer should be immobile in soil.

The saturation‐paste extract (SPE) chemistry of samples collected in the wet‐meadow and shallow‐marsh zones of seven North Dakota wetlands was related to SPE electrical conductivity to investigate the development of hydric‐soil salinity. Study wetlands represent a local, depression‐focused groundwater‐flow system. Recharge wetlands recharge the groundwater whereas discharge wetlands receive the majority of their water as groundwater discharge. Throughflow wetlands receive water from as well as yield water to the system. Development of soil salinity generally followed the Hardie and Eugster model of closed‐basin brine evolution, which considers the composition of solutions undergoing evaporation to be the result of chemical changes imposed by the successive formation of evaporite minerals. Hydric soils of recharge wetlands were nonsaline and free of calcite (CaCO3) and gypsum (CaSO4·2H2O). The chemistry of these soils results from evapotranspiration, recharge hydrology, ionic mobility, and exchange relationships. Increases in SPE Mg2+, Na+, and SO2‐4 dominance in more saline throughflow and discharge wetlands are caused by calcite and gypsum precipitation, with the former controlling alkalinity and the latter Ca2+ concentrations. At high salinities produced by concentration through freezing, mirabilite (Na2SO4·10H2O) crystallizes and controls Na+ levels, resulting in hypersaline solutions enriched in Mg2+ and SO2‐4. Additional variation in the patterns of salinity development can be explained by dominance of recharge over discharge, mixing with fresh or chemically discrete water, and valence dilution effects.

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The morphology and selected physical, mineralogic, and chemical characteristics of pedons collected in seven North Dakota wetlands were examined to investigate the development of hydric soils associated with wetlands of the Northern Plains. The wetlands were chosen using established field criteria to be representative of ground-water recharge, flowthrough, and discharge conditions. All profiles collected in recharge wetlands were nonsaline, flee of carbonate, and exhibited clay illuviation to some degree, the result of seasonal ponding, fluctuating water tables, and downward, saturated water flow characteristic of recharge conditions. Recharge profiles also exhibited continuous sediment aggradation due to erosion of the surrounding upland and deposition in the wetland. High organic matter production under such conditions resulted in overthickened A-horizons. Soil classes ranged from Typic'Argiaquolls if argillic horizons are present, to Cumulic Haplaquolls if illuviation was not as well expressed. Soil development in the wet-meadow and shallow-marsh zones of flowthrough wetlands was influenced by higher, more stable, more brackish water tables. Profffle morphology reflected a continuum from dominantly-recharge to dominantly-discharge hydrology. All profiles were calcareous. The more saline profiles also contained gypsum. Soil classes ranged from Cumulic HaplaquoUs of calcareous, mixed mineralogy to Typic Calciaquolls. The presence of high, saline water tables and mechanical sorting of soil surfaces by wave action retarded soil development in the discharge wetland periphery. Soils were uniformly saline, calcareous, and gypsiferons. Soil classes ranged from Fluvaquentic HaplaquoUs to Typic Fluvaquents.

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