Search

Water table data collected for hydric soil and wetland identification studies require supporting analysis of rainfall normality. Water table measurements made after periods when precipitation is within a normal range are believed to represent long-term trends, whereas data collected following periods of abnormally high precipitation represent rare events, potentially resulting in erroneous hydric soil determinations. The USDA-NRCS currently uses two different methods to assess normal precipitation ranges; both have been used to assess hydric soils. This study compared methodologies that identify normal precipitation periods by using: (i) the range defined by the 30th and 70th percentiles observed within a 30-yr period [i.e., the Climate Analysis for Wetlands Tables (WETS) method] and (ii) long-term monthly mean precipitation ± one SD (i.e., the U.S. Soil Taxonomy method). Comparisons were made for 30 geographically diverse locations and soil moisture regimes. The results demonstrated that the U.S. Soil Taxonomy method yielded normal precipitation ranges approximately twice as large as those from the WETS method. As a result, the U.S. Soil Taxonomy method precluded the occurrence of drier than normal conditions in many instances and displayed increased sensitivity to infrequent high rainfall events. Three case studies evaluated the implications of method selection on hydric soil identification, demonstrating that the U.S. Soil Taxonomy method identified normal conditions more frequently than the WETS method. As a result, the adoption of the WETS method, which accounts for the non-normal distribution of precipitation data, as the sole method to determine normal precipitation periods for hydric soil assessment is recommended. [ABSTRACT FROM AUTHOR]

Phosphorus (p) dissolution occurs commonly in wetland soils restored from agricultural land. Associated with p release are high concentrations of dissolved organic carbon (DOC) and Fe2+. This field study evaluated the effect of a fluctuating water table on the root dynamics of bald cypress (Taxodium distichum L. Rich.) to determine whether root death created soil reduction microsites, potentially contributing to p dissolution. The study site is a restored Carolina bay wetland with organic soils. Root growth and death were monitored on 16 6-yr-old bald cypress using minirhizotrons. Root dynamics, water table levels, and soil porewater chemistry and redox potential in the root zone were monitored for 2 yr. Soil solution samples were analyzed for Fe2+, pH, DOC, and p. High rates of root growth occurred during dry conditions, whereas root death occurred during sustained periods of saturation, particularly within 20 cm of the surface. Cyclic changes in concentrations of Fe2+, DOC, and dissolved total p (DTP) were related to water table position but not to changes in root numbers. After sustained periods of saturated condi-tions, redox potential decreased to 0 mV, Fe2+ increased to 1.75 mg Fe2+ L-1, and DOC increased to 350 mg L-1, resulting in peak DTP concentrations of 750 µg L-1, compared with 100 µg L-1 during dry periods. This study showed that in these high-C soils (~20% organic C) rooting dynamics had minimal impact on changes in p concentrations and that p dissolution was largely controlled by Fe reduction processes occurring within the C-rich soil matrix. [ABSTRACT FROM AUTHOR]

Lateral transport of waterborne contaminants is generally assumed to occur below the water table (WT), but recent laboratory studies have suggested that subsurface lateral solute transport could occur above the WT through the capillary fringe (CF). The objective of this study was to evaluate the horizontal transport of solutes in the CF and shallow groundwater (SGW). Two consecutive field experiments were conducted in a Leon sand (Aeric Alaquod) at a site with a shallow WT. In both experiments, a bromide (Br-) solution was applied to the bottom of an auger hole dug to within 10 cm above the projected CF. Movement of Br- in the subsurface was monitored by collecting CF and SGW samples using nests of tension samplers installed at radial distances of 60, 120, 220, and 320 cm from the application spot. Each nest of samplers contained a tension sampler at 45, 60, 75, 90, and 105 cm depth. Bromide transport was monitored for 58 d in the first experiment while the more detailed second experiment was conducted for 84 d. Peak Br- concentration generally occurred in the upper 60 cm of the soil where the CF was located for most of the experiment. The Br- plume that entered the CF moved horizontally in it until Br- was partially moved into the SGW by the fluctuating WT. In the second experiment, approximately 48% of the Br- detected in the CF at a distance of 320 cm from the application spot was still in the CF after 15 cm of rain through 59 d. [ABSTRACT FROM AUTHOR]

Abstract: It is commonly assumed that nitrate (NO3 −) and other anions entering the soil move downward through the vadose zone, and then move horizontally in the groundwater. Recent laboratory studies, however, indicate that water movement and transport of pollutants can also take place in the capillary fringe (CF) above the water table (WT). This field study evaluated the fate of NO3 − in the CF and shallow groundwater (SGW) for a sandy soil (Aeric Alaquod) with shallow water table. Ten L of a solution containing approximately 18 mmol L−1 nitrate [2.66 g L−1 Mg(NO3)2] and 77 mmol L−1 bromide (9.12 g L−1 KBr) were applied to the soil above the CF. The movement of both NO3 − and Br− was monitored for 84 days by using tension lysimeters installed at depths between 45 and 105 cm at radial distances of 20, 60, 120, 220 and 320 cm from the application point. Nitrate and Br− plumes that entered the CF from the unsaturated zone moved horizontally in the CF until both species were partially carried into the groundwater by the fluctuating WT following rain events. Normalized concentrations of NO3 −N and Br− remained comparable as they moved horizontally in the CF up to 320 cm from the tracer application spot. However, below the WT the detected normalized concentration of Br− was higher than that for NO3 − indicating nitrate loss, perhaps due to denitrification. When monitoring subsurface NO3 −, solely relying on collection of groundwater samples may lead to an underestimation of the extent of NO3 − contamination and transport in the subsurface. [Copyright &y& Elsevier]