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Irrigation with blended drainage water can damage soil structure, impair infiltration, and increase runoff and erosion. This loss of permeability is often attributed to the slaking of aggregrates and clay dispersion, which leads to pore plugging. Soil swelling is usually considered an important factor only when exchangeable sodium percentages (ESP) exceed 15%. We hypothesized that swelling is more important than generally recognized in reducing soil hydraulic conductivity (HC), and swelling can occur at low ESP. In this study we attempted to identify the mechanisms reducing HC and the reversibility of the processes during gypsum and sulfuric acid application. Synthetic drainage waters with sodium adsorption ratios (SAR) of 1, 3, 5, and 8 and electrolyte concentrations (C) of 0, 2.5, 5, 10, 25, 50, and 100 mmol c L were leached through machine-packed soil columns to evaluate the effects of clay dispersion and swelling on HC. Following the initial leachings, the soils were amended with surface applied gypsum and H 2 SO 4 , and leached with deionized water. Clay concentration in the leachate was used as a measure of clay dispersion, and internal soil swelling was assumed to be proportional to changes in the water holding capacity at -22 kPa tension. Internal soil swelling at low electrolyte concentrations (

Elevated concentrations of U are found in agricultural drainage waters from the San Joaquin Valley, CA, which are often disposed of in evaporation basins that are frequented by waterfowl. To determine the factors that affect aqueous U concentrations in the basins, sorption experiments with U(VI) were performed at various CO 2 partial pressures, dissolved Ca, Mg, and P concentrations, and carbonate alkalinities. Synthetic waters, comparable in inorganic constituents to irrigation and drainage waters, were prepared, spiked with 0.1 (soil) and 2 mg U(VI) L -1 (synthetic goethite), and analyzed for U, P (when applicable), and major ions. Total chemical analyses were input into the computer program FITEQL to determine U(VI) speciation and generate U(VI) adsorption constants with the diffuse layer model (also referred to as the two-layer model). Maximum adsorption occurred in solutions with low carbonate alkalinities (≤3 mmol L -1 ), ionic strengths (≤ 0.03 M), Ca concentrations (≤ 4 mmol L -1 ), and P concentrations (

Hexavalent chromium [Cr(VI)] is a common environmental pollutant that is mobile in soils and is a known mutagen The trivalent form [Cr(III)] has no known mutagenic properties and is highly insoluble and immobile above pH 5 5 Reduction to the trivalent state thus represents an effective mechanism for detoxification and immobilization of Cr(VI) in soil/water systems In this study, we conducted experiments to examine various parameters involving aerobic reduction of Cr(VI) in a field soil We found that organic matter content, bioactivity, and oxygen status were important factors Under optimum conditions, the soil reduced 96% of added Cr(VI) under aerobic, field moist conditions The pH of the system was shown to have little effect Both biological and nonbiological processes were influential, and, of 20 chromate resistant bacterial cultures isolated from soils, 9 were found to actively reduce Cr(VI) to Cr(III) in liquid media Our study suggests that organic amended soils can readily reduce Cr(VI) and could promote excellent removal efficiency either as a primary treatment technique, or in conjunction with bioreactor type wastewater treatment systems

Kesterson Reservoir and other impoundments in the San Joaquin Valley, California, have received large volumes of saline irrigation drainage water that is enriched with trace elements, including As, B, Mo, Se, U, and V. When these ponds are allowed to dry and revert to terrestrial ecosystemms, careful soil water and vegetation management may be needed to prevent toxicological hazards to wildlife and/or livestock. We conducted a 248-d column study in the greenhouse using a soil from Kesterson to assess the growth of salt-and B-tolerant genotypes, and to determine the uptake of As, B, Mo, Se, U and V by these genotypes (...)

The water infiltration rate of saline sodic soils often limits the rate of reclamation. Column leaching studies were conducted to determine if water-soluble, synthetic polymers would be beneficial in improving the hydraulic conductivity and aid reclamation of a heavy-textured, saltaffected soil. Soil samples from a swelling soil (fine, montmorillonitic [calcareous] Thermic Vertic Haplaquoll) were collected from a field site that bad exchangeable sodium percentages (ESP) of 8, 12, 20, 25, 32, and 35. The air-dried soil samples were treated with polyacrylamide polymers (one nonionic and two anionic) and one cationic guar-derivative polymer at a rate of 50 mg kg. Polymer treatments had a highly significant effect on increasing the saturated hydraulic conductivity of the soil samples with ESP values 15. The addition of gypsum increased the hydraulic conductivity from 0.0 to 0.063 mm h 1 in the soil with an ESP of 32. When polymers were used in conjunction with gypsum, the hydraulic conductivity increased to 0.28 mm h. We attributed the improvement in hydraulic conductivity with polymer treatment at low ESP values and in the gypsum-treated soil to a reduction in soil slaking and dispersion. At ESP values > 15, an additional mechanism that may have been controlling the hydraulic conductivity was swelling, and none of the polymers reduced soil swelling. View complete article To view this complete article, insert Disc 5 then click button8

Reclamation of salt-affected high shrink-swell soils using chemical amendments combined with deep loosening and tile drainage is very expensive. As an alternative, the utilization of cracks as pathways for water and salts is possible if these cracks could be stabilized by the application of a polyacrylamide (PAM) polymer and their complete closure prevented upon rewetting. A laboratory column study was conducted on two soil samples with exchangeable sodium percentages (ESP) of 8 and 25 from a heavy-textured, swelling soil. The soil samples were packed into columns, ponded with irrigation water for 24 h, then drained and dried to create cracks. After drying, the soil samples were ponded for 24 h with irrigation water containing 0, 25, 75, and 200 mg L⁻¹ of an anionic polyacrylamide polymer, allowed to drain, and dried. The treated soils were then ponded without drainage for 1, 6, 12, and 24 h to allow crack closure and the hydraulic conductivity (HC) and effluent salinity were measured. The experiment was repeated except that the polymer solution was added to the soil without initially creating the cracks. An additional study was carried out to test the durability of the polymer when subjected to wetting and drying cycles. Increasing the amount of polymer significantly increased the HC and salt removal in the cracked soils. When the polymer solution was applied directly onto soils without initially creating cracks, there was no increase in HC on the ESP 8 soil and a small increase on the ESP 25 soil. The polymer had a stabilizing effect on soil cracks, but the increase in HC did not persist through several wetting and drying cycles.

In order to accurately model soil reclamation with concentrated electrolytes such as CaCl2 or sea water, it must be determined if there is an effect of salinity on the Na-Ca exchange selectivity. A new procedure for determining Na-Ca selectivity in calcareous and gypsiferous soils was used to study the effects of ionic strength and mineralogy on Na-Ca exchange. Four specimen clay minerals and three whole soils were equilibrated with solutions ranging in concentration from 10 to 1000 mmolc L' and at Na adsorption ratios from 1 to SO (mmol L~'). Exchangeable-cation values were corrected for calcite and gypsum dissolution as well as anion exclusion. The selectivity data were compared with other reported values for Na-Ca exchange in order to determine if there were any trends in selectivity with mineralogy or surface charge density. Generally, NaCa selectivity was independent of ionic strength. Vermiculite exhibited near-ideal exchange when compared with the nonpreference isotherm. There was no effect of mineralogy on the Na-Ca selectivity, even though the soils and minerals had various surface charge densities. The Gapon selectivity coefficients (KG) for the calcareous Many Farms soil (a mesic Torrifluvent) and the gypsiferous Shiprock soil (a mesic Torriorthent) averaged 0.011 and 0.013 (mmol L~')~i respectively. Failure to correct for anion exclusion and minC. Amrhein, Dep. of Soil and Environmental Sciences, Univ. of California, Riverside, CA 92521; and D.L. Suarez, U.S. Salinity Lab., USDA-ARS, 4500 Glenwood Dr., Riverside, CA 92501. Submitted by the U.S. Salinity Lab. Received 26 Jan. 1990. *Corresponding author. Published in Soil Sci. Soc. Am. J. 55:698-706 (1991). eral weathering lowered these average A"G values to 0.0085 and 0.0064, respectively. The data were compared with the Vanselow (Kv = 4.3), nonpreference (Ay = 1.0), Gapon (JiTG = 0.015), and the diffuse double-layer (DDL) models. After the review of the problems associated with the various methods of determining exchangeable cations, we recommend that the alcohol rinse method to remove soluble salts not be used. M SALT-AFFECTED SOILS require the addition of chemical amendments prior to leaching to remove soluble salts and replace adsorbed Na. In order to calculate the amount of amendment needed to reclaim a sodic soil, knowledge is required of the equilibrium relationship between the solution and exchanger phase composition of the soil. Traditionally, this relationship has been described using the Gapon convention and conveniently expressed by the linear equation ESR = (Ac SAR) + x [I] where ESR (exchangeable sodium ratio) is equal to ^Na/E(ca+Mg)> where Ef is the equivalent fraction of cation i on the exchanger phase, SAR is the sodium adsorption ratio of the solution phase denned as Na*/ V^(Ca + Mg) with concentrations in mmol L". Since x is a small intercept value, often assumed to AMRHEIN & SUAREZ: SODIUM-CALCIUM EXCHANGE 699 be zero (U.S. Salinity Laboratory Staff, 1954), the value of KG can be equated to ESR/SAR and is generally assumed to be a constant over a wide range of soil types, salinity, and exchangeable Na. The U.S. Salinity Laboratory Staff (1954) reported an average value for Ka of 0.015 (mmol L-')-' on 59 soil samples from the western USA. Many workers have reported variations in KG, and it is felt by many to be site specific (Levy and Hillel, 1968; Doering and Willis, 1980; Nadler and Magaritz, 1981). Two factors that are known to affect the NaCa selectivity relationship are organic-matter content (organic matter tends to increase Ca preference [Pratt and Grover, 1964; Fletcher et al., 1984b]) and pH (particularly in soils containing large amounts of variablecharge materials [Pratt et al., 1962]). In addition, the DDL theory for cation adsorption predicts an increasing Ca preference with increasing ionic strength and decreasing exchangeable Na (Babcock, 1963). The reported changes in Na-Ca selectivity as a function of salinity and exchanger phase composition have been quite variable. Bower (1959) found that the Kc of a montmorillonitic clay soil was not significantly affected by solution concentration (50-200 mmolc L~') but increased with increasing exchangeable Na; that is, Na preference increased with increasing ENa. Similar results were found by Pratt et al. (1962) on a soil high in amorphous clays and kaolinite and equilibrated with solutions that ranged in concentration from 50 to 330 mmolc L". They also found that the selectivity coefficient of this soil was strongly pH dependent. On the other hand, Doering and Willis (1980) reported that the KG of a strip-mine spoil material (montmorillonitic) was constant with variations in £Na but showed a strong dependence on ionic strength, decreasing from 0.0163 to 0.0085 (mmol L-')~ at ionic strengths from 0.04 to 1.06 molc L", respectively. Jurinak et al. (1984) reported that KG was a constant for a montmorillonitic soil from the Central Valley of California and for a surface overburden material from Montana at all concentrations and SAR values studied (0.01-0.5 molc Land 5-80 (mmol L-'), respectively). On a sample of kaolinitic deep overburden material, KG varied from 0.0138 to 0.0047 (mmol L-i)-i/2 across the concentration range 0.01 to 0.5 molc L-, respectively. They suggested that variations in clay mineralogy were a prime factor determining the ESRSAR relationship. They also noted that correction for anion exclusion was necessary at concentrations > 100 mmolc Lfor accurate exchangeable-Na values. Another convention for describing cation exchange was proposed by Vanselow (1932), in which the activity of the exchangeable cations are equated to their mole fraction (N,) on the exchanger and activity is used for ions in solution. This is in contrast to the Gapon convention, in which solution concentration is used. The mass-action expression for Na-Ca exchange and the Vanselow selectivity coefficient (Kv) for the reaction are as follows

A procedure to determine the exchanger-phase composition (and thus the Na/Ca selectivity) of calcareous and gypsiferous soils was developed and tested on three soils and three phyllosilicate minerals. The procedure corrects for anion exclusion and mineral weathering by measuring the Cl, SO,,, and HCO3 in the soil solution and in the extracting solution. Anion exclusion was found to be significant in the montmorillonite system, but generally less than 10% of the cation-exchange capacity in the soil, illite, and vermiculite systems. Mineral weathering contributed significantly to the total extracted Ca. Thus, exchangeable-Ca values were overestimated by 30 to 500% in the gypsiferous soils and 3 to 20% in the calcareous soils when weathering was neglected. The proposed method allows the determination of cation-exchange capacity, exchangeable-cation composition, and anion exclusion at any solution composition, concentra

The kinetics of dissolution of soil minerals as a function of the ionic composition of the exchange phase were studied using a calcareous soil and a noncalcareous soil. It was concluded that mineral dissolution occurred as a two-step reaction when an exchanger phase is present. The initial fast reaction was completed in < 1 h and was linearly related to the amount of exchangeable sodium. The second slower reaction was independent of the exchangeable cation but dependent on the total mineral surface (soil-to-water ratio) available for weathering. The kinetics of dissolution for the second reaction was diffusion controlled.

Solutions equilibrated with soils under laboratory conditions often exhibit Ca and CO^~ activities that exceed calcite saturation. This supersaturation has been attributed to various possibilities, none of which have been fully able to explain the phenomenon. In this study we report on the attainment of calcite supersaturation in calcite + water + organic matter systems under controlled CO2 conditions. The decomposition of alfalfa shoots (Medicago sativa L.) and soybean roots (Glycine max L. merr.) in batch reactors under CO2 pressures of 1.0,10.0, and 99.5 kPa was found to result in HCO, as an end product of aerobic decomposition. The rate of HCO, production from mineralizing organic matter exceeded calcite precipitation rates and, thus, calcite supersaturation resulted. The decomposition of pure organic compounds (dextrose, inositol, glucosamine, glucuronic acid, citric acid, and sodium citrate) was studied to elucidate the mechanism of HCO, production. It was determined that the decomposition of organic anions associated with inorganic cations always results in HCO, as an end product. The rates of calcite dissolution and precipitation were greatly inhibited in the presence of decomposing organic matter. Additional Index Words: carbon dioxide, alkalinity, organic matter decomposition, mineral weathering, ion activity products, kinetAmrhein, C., and D.L. Suarez. 1987. Calcite supersaturation in soils as a result of organic matter mineralization. Soil Sci. Soc. Am. J. 51:932-937. S (1977) found calcite supersaturation (ion activity products three times that of calcite) in waters sampled from below the root zone in both field and lysimeter studies. Inskeep and Bloom (1986b) found that Calciaquolls in western Minnesota were up to 40-fold oversaturated with respect to pure calcite. Both studies found that these high levels of Ca and carbonate activities in solution could not be explained by Mg substitution in the calcite, calcite particle size, or the presence of a more soluble soil CaCO3 phase. Suarez (1977) attributed these large supersaturation values to the concentrating effects of evapotranspiration and the kinetic inhibitions of calcite precipitation, and further hypothesized that organic excretions from microorganisms blocked the crystal growth sites on calcite surfaces. Reynolds (1978) and Inskeep and Bloom (1986a) have shown that some organic matter constituents may inhibit calcite precipitation. Apparent calcite supersaturation in soil/water laboratory equilibrations have been noted by several investigators. In contrast to the field studies, calcite supersaturation in the laboratory equilibrations cannot be attributed to the concentrating effects of evapdtranspiration. Cole (1957), Olsen and Watanabe (1959) and Levy (1981) observed that Ca concentrations and calcite supersaturation increased as the soil-to-water ratio increased. Suarez and Rhoades (1982) found that calcareous soils which were reacted with water under controlled CO2 conditions resulted in calcite supersaturation when equilibrated from initially undersaturated conditions. However, calcite nodules isolated from soil gave CaCO3 ion activity product (IAP) values of 10~ to 10~. Organic complexes with Ca 1 Contribution from the U.S. Salinity Laboratory, USDA, ARS, 4500 Glenwood Drive, Riverside, CA 92501. Received 20 October 1986. 2 Soil Scientist and Geochemist, respectively. AMRHEIN & SUAREZ: CALCITE SUPERSATURATION IN SOILS 933 Table 1. Composition of each reaction bottle. Exp. Solution Organic material Calcite, g pCOj, kPa