Fate and Transport of High Explosives in a Sandy Soil: Adsorption and Desorption

Several areas of the Massachusetts Military Reservation (MMR) have soils with significant levels of high explosives (HE) contamination because of a long history of training and range activities (such as open burning, open detonation, disposal, and artillery and mortar firing). Site-specific transport and attenuation mechanisms were assessed in sandy soils for three contaminants of concern: the nitramine hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX), and the nitroaromatics 2,4-dinitrotolune (2,4-DNT) and 2,4,6-trinitrotoluene (TNT). For all three contaminants, linear distribution coefficients (K_d) were dependent on the fraction of organic carbon in soil. The nitroaromatics sorbed much more strongly than RDX in both soils. Over 120 hours, the desorption rate of RDX from field contaminated surface soil was much slower than its sorption rate, with the desorption K_d (1.5 L/kg) much higher than K_d for sorption (0.37 L/kg). Desorption of 2,4-DNT was negligible over 120 hours. Thus, applying sorption-derived K_d values for transport modeling may significantly overestimate the flux of explosives from MMR soils. Based on multiple component column transport tests, RDX will be the most mobile of these contaminants in MMR soils. In saturated columns packed with uncontaminated soil, RDX broke through rapidly, whereas the nitroaromatics were significantly attenuated by irreversible sorption or abiotic transformations.     

Effects of dissolved organic matter from sewage sludge on the atrazine sorption by soils

The effects of dissolved organic matter (DOM), water soluble organic matter derived from sewage sludge, on the sorption of atrazine (2-chloro-4-ethylamino-6-isopropylamino-1,3,5-trazine) by soils were studied using a batch equilibrium technique. Six paddy soils, chosen so as to have different organic carbon contents, were experimented in this investigation. Atrazine sorption isotherms on soils were described by the linear equation, and the distribution coefficients without DOM (K_d) or with DOM (K_d ^*) were obtained. Generally, the values of K_d ^*/K_d initially increased and decreased thereafter with increasing DOM concentrations of 0–60 mg DOC · L^?1 in soil-solution system form. Critical concentrations of DOM (DOM_np) were obtained where the value of K_d ^* was equal to K_d. The presence of DOM with concentrations lower than DOM_np promoted atrazine sorption on soils (K_d ^* > K_d), whereas the presence of DOM with concentrations higher than DOM_np tended to inhibit atrazine sorption (K_d ^* < K_d). Interestingly, DOM_np for tested soils was negatively correlated to the soil organic carbon content, and the maximum of K_d ^*/K_d (i.e.K _max) correlated positively with the maximum of DOM sorption on soil (X_max). Further investigations showed that the presence of hydrophobic fraction of DOM evidently promoted the atrazine sorption on soils, whereas the presence of hydrophilic DOM fraction obviously tended to inhibit the atrazine sorption. Interactions of soil surfaces with DOM and its fractions were suggested to be the major processes determining atrazine sorption on soils. The results of this work provide a reference to the agricultural use of organic amendment such as sewage sludge for improving the availability of atrazine in soils.     

Depletion of Selenium in Soil Solution due to its Enhanced Sorption in the Rhizosphere of Soybean

The effect of plant roots on selenium (Se) mobility in soil was studied by a large-scale pot experiment in order to understand the environmental behavior of Se in agricultural soils under plant growth conditions. Soybean plants (Glycine max (L.) Merrill) were grown in a greenhouse for 84 d. The concentrations of Se and major elements (K, Ca, Mg, Na, and Al) in the soil solutions and in the plants were measured at different growth periods. Concentrations of Se and major cations in soil solution decreased as the soybean plants grew, while the concentrations of Al increased. It was assumed that the soybean roots released H⁺ with the uptake of cations; consequently, due to the acidification of the rhizosphere, Al³⁺ was released starting from the soil solid phase. The decreased Se concentration in the soil solution should be due to the enhancement of Se sorption onto the soil solid phase. The increase of Se sorption level in the rhizosphere was examined in a small-scale pot experiment. The soil–soil solution distribution coefficient of Se (K _d-Se) was observed as an index of Se sorption level. K _d-Se clearly increased in the rhizosphere soil after cultivation. The effects of pH and Al³⁺ in the rhizosphere on Se sorption were assessed by K _d-Se measurements at different levels of HCl and AlCl₃. In this third experiment, a decrease in pH increased K _d-Se values, but no specific effect was observed on Se sorption due to increased Al³⁺. These results show that the Se mobility in agricultural soil could be decreased by plant roots under plant growth conditions due to enhanced Se sorption in the rhizosphere.     

Characteristics of Copper Sorption by Various Agricultural Soils in China and the Effect of Exogenic Dissolved Organic Matter on the Sorption

To explore the effect of exogenic dissolved organic matter (DOM) on Cu(II) sorption in agricultural soils, 26 agricultural soils were collected across China. Exogenic dissolved organic matter, extracting from wheat straw (DOM_W) and swine manure (DOM_S), respectively, were added to the soils to conduct a series of batch sorption and characterization experiments. The solid-liquid partition coefficient (K_d) ranged from 0.02 to 76.46 L g^?1, suggesting different Cu(II) sorption on various soils. PCA analysis indicated that pH, free Fe/Al oxides, carbon, and total Cu content had a significant positive relationship with the Cu(II) sorption, respectively. And the contribution rate of pH was the highest (38.15%). Moreover, DOM markedly inhibited the Cu(II) sorption in alkaline soils while promoted the Cu(II) sorption in acidic soils, which were interacted by the soil properties and DOM characteristics. The effect of DOM_S on Cu(II) sorption were more obvious than DOM_W, which were further confirmed by Fourier transform infrared (FTIR) spectroscopy. FTIR also showed Cu(II) was primarily adsorbed on the specific functional groups, such as CO, OH, and CO, providing direct evidences for the binding of Cu(II) with DOM. This study can guide the rational use of organic fertilizers, and also provide baseline knowledge for the prevention and control of soil pollution.     

Evaluation and application of aquatic toxicity tests: use of the Microtox test for the prediction of toxicity based upon concentrations of contaminants in soil

The quality control for the reuse of cleaned soil from a contaminated site consisted in the determination of the main contaminants by analytical chemical methods such as GC and HPLC. Since it is not possible to analyze for all contaminants a toxicity test should be used to detect large concentrations of not routinely analyzed chemicals. The aim of the study was to develop a system for toxicity testing, which should be able to predict the toxicity of soil samples based on the concentration of chemicals in the soil and to detect toxic chemicals not analyzed by the routinely conducted soil analysis.Based upon the relative sensitivity to various contaminants as well as practical aspects such as test duration and costs the Microtox® test was favoured over the bioassays with Daphnia magna and Scenedesmus subspicatus. The Microtox® test was used to measure the toxicity of various pesticides and their major metabolites. The toxicity data of the pure compounds were used to predict the toxicity (EC₅₀ and % inhibition of the bioluminescence reaction) of defined mixtures of chemicals in water by applying two different mathematical appriaches which are based on the additivity of the effects of the single chemicals. The predicted values were compared with the experimental data and showed good agreement.In order to be able to predict the toxicity of soil samples using the Microtox® test the soil/water partition coefficient (K _d) was measured for the main contaminants. The toxicity of soil samples was predicted by calculating the concentration of the contaminants in the leachate by using the corresponding concentration in the soil and applying the K _d values determined. From the calculated composition of the leachate the expected toxicity was estimated. This value was compared with the toxicity experimentally determined in the Microtox® test.     

Decomposition and sorption characterization of plant cuticles in soil

The sorption of organic compounds by plant cuticular matter has been extensively investigated; however, little has been studied regarding the effect of plant cuticle degradation on their role in the sorption of organic compounds in soils. The sorption of phenanthrene was studied in soil samples which had been incubated for up to 9 months with three different types of plant cuticle isolated from tomato fruits, pepper fruits and citrus leaves. The main change in the diffuse reflectance Fourier-transform infrared (DRIFT) spectra during incubation of the cuticles was related to cutin decomposition. The peaks assigned to methyl and ethyl vibration and C=O vibration in ester links decreased with decomposition. In general, with all samples, the phenanthrene sorption coefficients calculated for the whole incubated soils (K _d) decreased with incubation time. In contrast, the carbon-normalized K _d values (K _oc) did not exhibit a similar trend for the different cuticles during incubation. The origin of the cuticle also affected the linearity of the sorption isotherms. With the tomato and citrus cuticle samples, the Freundlich N values were close to unity and were stable throughout incubation. However with the green pepper cuticle, the N values exhibited a significant decrease (from 0.98 to 0.70). This study demonstrates that the structural composition of the plant cuticle affects its biodegradability and therefore its ability to sorb organic compounds in soils. Of the residues originating from plant cuticular matter in soils, the cutan biopolymer and lignin-derived structures appear to play a dominant role in sorption as decomposition progresses. Responsible Editor: Alfonso Escudero.     

A Study of Vadose Zone Transport Model VLEACH

Application of vadose zone transport models has been hampered by lack of model validation. Difficulties to validate vadose zone models using field data not only come from model assumptions that are uncertain to the subsurface transport processes but also from the uncertainties associated with soil contaminants’ release time and quantity, soil sampling, sample transport, and analytical procedures. This article first conducts a test of a popularly used vadose zone transport VLEACH by comparing model results with a set of laboratory soil column infiltration and volatilization study data. The comparison shows a close agreement between the VLEACH model results and the laboratory data. Second, the sorption coefficient K_d calculated in VLEACH is compared with field data. The comparison indicates that VLEACH may overestimate the mass leached from soil to groundwater. The article also discusses the selection of the model simulation timestep, the vertical dimension increment, the Courant criterion, and the lower boundary condition using the sensitivity analysis method based on a case study of soil remediation for trichloroethylene. The procedures presented in this paper are important to practical model application and modification. This level of work should be routinely conducted for any new or modified version of vadose zone models.     

Migration of fallout-radionuclides in the soil: effect of non-uniformity of the sorption properties on the activity–depth profiles

Experimentally observed activity–depth profiles of fallout radionuclides in the soil frequently exhibit a comparatively fast moving tail in soil layers below the peak concentration (tailing). Monte Carlo calculations on the basis of the convection-dispersion model show that this phenomenon can be explained by assuming that either the hydraulic properties of the soil (characterised by the diffusion/dispersion coefficient and pore water velocity) or the sorption properties of the soil (characterised by the distribution coefficient K _d ), or both, exhibit a horizontal variability according to a log-normal distribution. Modifications of the activity–depth profile due to a K _d value which decreases linearly with depth were examined by using a random walk approach, based also on the convection-dispersion model. In this case, however, a pronounced tailing effect of the activity–depth profile did not result. Interpretation and realistic modelling of an experimentally observed activity–depth profile which exhibits a tailing effect is thus not unambiguously possible without any additional information on the spatial variability of the hydraulic parameters and, independently, also for the sorption properties. Received: 5 January 2001 / Accepted: 1 May 2001     

The biological and physico-chemical uptake of radiocesium by particulate matter of natural origin (Baltic Sea)

The kinetics of radiocesium (¹³⁷Cs) uptake by natural suspended matter collected from coastal waters in the southern Baltic has been studied under laboratory conditions. The uptake of radiocesium from seawater by the suspended matter took place immediately after introduction of the isotope and attained equilibrium within a few hours. Summer and winter suspended matter displayed equal K_d values, indicating similar sorption characteristics of radiocesium. The amounts of radiocesium sorbed from sea water were proportional to the suspended matter concentration studied,i.e. up to 312 mg dry wt dm^–3. The relative uptake of radiocesium by live and dead plankton appeared to be the same. The desorption of radiocesium from dead plankton proceeded more rapidly and more intensively than sorption. There are no significant differences between the K_d values for plankton determined in laboratory experiments and those found for plankton populations under field conditions.     

Effects of biochar amendment on the sorption and degradation of atrazine in different soils

ABSTRACT

Sugarcane top-derived biochar was added to an alluvial soil, a moist soil and a paddy soil at the rate of 0.2% and 0.5% (w/w). After the addition of 0.2% and 0.5% biochar, the sorption coefficients (K_d) of atrazine (Ce = 10 mg L^?1) were increased by 26.97% and 79.58%, respectively, in the moist soil with a low level of total organic carbon (TOC), while it increased by 31.43% and 60.06%, respectively, in the paddy soil with a high TOC content. The half-time persistence values of atrazine in the alluvial soil, moist soil and paddy soil were 28.18, 23.74 and 39.84 d, respectively. In the 0.2% biochar amended soils, the corresponding half-times of atrazine for the alluvial soil, moist soil and paddy soil were extended by 10.33, 11.81 and 1.42 d, and they were prolonged by 16.83, 17.52 and 14.74 d, respectively, in the 0.5% biochar amended soils. Atrazine degradation products (deisopropylatrazine and desethylatrazine) decreased after they accumulated to 3.2 and 1 mg kg^?1, respectively. Generally, increasing sorption was accompanied by decreasing degradation of atrazine which is found in biochar-amended soils.     

Sorption Behavior of Cesium in Two Greek Soils: Effects of Cs Initial Concentration,Clay Mineralogy,and Particle-size Fraction

The characteristics of Cs sorption behavior in two soils (soil 1 and soil 2) with nearly the same clay content and exhangeable K concentration, but with different clay mineralogy, were studied by the quantification of the distribution coefficient (k_d). It was observed that as the initial Cs concentration increased from 4 to 50 mg L^?1, the k_d values decreased in both soils, suggesting a progressive saturation of Cs available sorption sites. However, the presence of expansible 2:1 phyllosilicates minerals in the clay fraction of soil 2 maintained a high Cs sorption ability for this soil, even at high Cs concentrations. The experimental data were also fitted to the Freundlich isotherm and the results showed that parameters of the Freundlich equation could be used to estimate the degree of Cs sorption and the nature of the available sorption sites. For the studied soils, the k_f and the k_d values followed a similar trend and the n Freundlich constant values provided a reliable indicator for the soils’ clay mineralogy. The removal of the sand fraction enhanced Cs sorption in both soils and the absence of sorbed Cs ions on the quartz minerals, as observed by the SEM analysis, additionally supported the effect of particle-size fraction on Cs sorption.     

The Distribution of Metals in Soils and Pore Water at Three U.S. Military Training Facilities

Small arms firing ranges at military training facilities can have enormous heavy metal burdens (percent level) in soils. Currently there are few published works that quantify the metal content of soils and waters at military installations or speculate on the potential for migration of these contaminants into groundwater. This article documents metals in soils and waters at nine small arms training ranges at three military installations in the U.S. Soil samples were collected from the surface and shallow subsurface. The results demonstrated that lead, antimony, copper, and zinc were the principal contaminants of interest and mapping a site's lead and copper surface distributions would adequately define the extent of impacted soil. Lower metal concentrations at three of the ranges reflected previous remediation by means of physical separation and mechanical removal of metallic fragments followed by fixation treatment with Maectite^TM. Except for the treated ranges where mixing had occurred, subsurface soil samples indicated limited vertical migration. Several of the ranges were also monitored for trace element migration in the vadose zone by means of suction-cup lysimeters. This pore-water sampling indicated ceramic suction-cup lysimeters are useful for assessing relative concentrations but require care in evaluation because of potential sorption losses. Monitoring of soil water at ranges should include antimony and zinc; the former because, in contrast to the other metals, it is typically soluble in an anionic form, and the latter because of its greater solubility and mobility.     

Impacts of Soil Organic Matter and Temperature on Sorption of Acetazolamide on Four Soils

Batch sorptions of acetazolamide (AZ) were conducted using four soils from China. Sorption of AZ was found to be impacted by OC, clay content, and soil pH, with higher k_d values for soils with higher clay content. The k_d values of SOM-removed soils are much lower than those of bulk soils. Sorption data were well fitted with a Freundlich model (r² > 0.99). Chelating with the metal ions on the surfaces of soil particles was probably involved. With pH increase, the electrostatic attraction between anionic AZ and positively charged soil surface may increase. The sorption capacity decreased when the temperature increased from 20 to 40°C, and the calculated thermodynamics parameters of ΔG₀, ΔH₀, and ΔS₀ indicated that the sorption was a non-spontaneous, physisorption, and exothermic process. Sorption coefficients (k_d) for the compound in soil were low (ranging from 0.42 to 1.19 L·kg^?1) and indicated that low level sorption of AZ with appreciable risk of ground water contamination.     

Hysteretic Behavior of Imidacloprid Sorption-Desorption in Soils of Croatian Coastal Regions

Sorption and desorption are important processes that influence the transport, transformation, and bioavailability of imidacloprid in the soils. Equilibrium batch experiments were carried out using six coastal Croatian soils. The equilibrium sorption and desorption experimental data showed the best fit to the Freundlich equation. Sorption parameters predicted with the Freundlich model, K_F ^sor and 1/n ranged from 2.92 to 5.74 (mg/kg)/(mg/L)^1/n, and 0.888 to 0.919, respectively. The sorption of imidacloprid was found to be sensitive to organic carbon (OC) content. The highest sorption was observed in Krk soil (OC 4.74%) and the lowest in Zadar soil (OC 1.06%). Fitted desorption parameter values, K_F ^des , were consistently higher than those associated with sorption. The opposite trend was observed for the exponential parameter 1/n. Results also suggested that imidacloprid sorption-desorption by soil is concentration-dependent, i.e. at lower imidacloprid concentrations a greater sorption percentage and lower desorption percentage occurred. Desorption studies revealed that there was a hysteresis effect in all the tested soils. Hysteresis coefficient values (H) varied from 0.656 to 0.859. The study results emphasize that the controlled application of imidacloprid is obligatory, especially in soils with a low organic carbon content, in order to minimize a risk of environmental and groundwater pollution.     

Evaluating the Adsorption Potential of Alachlor and Its Subsequent Removal from Soils via Activated Carbon

Alachlor, a globally used aniline herbicide, has great agronomic interest for controlling the development of broadleaved weeds and grasses. This research aspires to evaluate the sorption attributes of Alachlor through batch equilibrium method and its successive removal through biomass based activated carbon prepared from Sawdust (Cedrus deodara). Six soil samples were collected from selected regions of Pakistan to assess the adsorption and removal phenomena. Adsorption capacity for Alachlor varied in soils depending upon their physicochemical properties. Adsorption coefficient (K_d) values ranged from 12 to 31 µg ml^?1 with the highest K_d value observed in soil sample with highest organic content (1.4%) and least pH (5.62). The Gibbs free energy values ranged from ?17 to ?20 kJ mol^?1 proposing physio-sorption and exothermic interaction with soils. Values of R² (0.96–0.99) exhibited the best fit to linear adsorption model. Adsorption coefficient displayed a negative correlation (r = ?0.97) with soil pH and positive correlation with organic matter (r = 0.87). The effect of contact time and pesticide concentration on the removal efficiency by activated carbon was investigated. The highest removal percentages observed through activated carbon were 66% and 64% at concentrations of 5 and 7.5 ppm respectively. Activated carbon from sawdust (Cedrus deodara) was investigated as a suitable adsorbent for the removal of Alachor from selected soils. Biomass based activated carbon can prove to be an effective and a sustainable mean to remove pesticides from soil.     

Comparative Sorption of Phenanthrene and Benzo[α]pyrene to Soil Humic Acids

Sorption of phenanthrene (Phen) and benzo[α]pyrene (BaP) by humic acids (HAs) extracted from four typical soils in China, including cinnamon soil, fluvo-aquic soil, red soil, and mountain meadow soil were investigated. All sorption data were fitted well by the Freundlich model, but BaP exhibited stronger and more nonlinear sorption than Phen by a given HA. For Phen isotherms, there was a positive relation between K _oc values and aliphaticity of HAs, whereas a negative correlation was observed between n values and aliphaticity. This indicated the importance of aliphatic groups in Phen sorption capacity and nonlinearity. Compared to Phen, a similar trend was obtained between n values of BaP and aliphaticity or aromatic carbon. However, no correlation existed between K _oc values for BaP and aliphaticity or aromatic carbon.     

Influence of Dissolved Organic Matter on Sorption and Desorption of 1,2,4-trichlorobenzene and 1,2,4,5-tetrachlorobenzene onto Wood Char

Sorption and desorption of 1, 2, 3-trichlorobenzene (TCB) and 1,2,4,5-tetrachlorobenzene (TeCB) onto wood char prepared from maple wood shavings heated at 500°C were studied in the presence of dissolved organic matter (DOM), including humic acid (HA), L-malic acid (L-MA), and peptone. Compared to TCB, TeCB exhibited more nonlinear and stronger sorption onto wood char. Nonlinearity of the sorption isotherms increased in the presence of DOM. The presence of HA enhanced the sorption capacity and desorption hysteresis of TCB and TeCB on wood char mainly due to the strong sorption of HA on wood char surface. Moreover, there were positive relations between K_d values of TCB and TeCB and the HA concentration (p < 0.01). In contrast, peptone reduced the sorption capacity and increased the sorption reversibility because of the partition of TCB and TeCB in peptone solution. L-MA at 50-200 mg·L^?1 also leads to a decrease in sorption capacity and irreversibility attributed to solubilization, although the sorbed L-MA on the wood char surface can slightly increase TCB and TeCB sorption. At the same concentration, peptone leads to a higher decrease in TCB sorption than L-MA. Also, negative correlations were found between K_d values of TCB and TeCB and the L-MA and peptone concentration (p < 0.01). Our results may help to understand the different impacts of DOM on the transport and fate of halogenated aromatic hydrocarbons in aquatic environments polluted with chars.     

Natural Attenuation of Trichloroethylene in Rhizosphere Soils at the Savannah River Site

Extensive trichloroethylene (TCE) groundwater contamination has resulted from discharges to a former seepage basin in the A/M Area at the Department of Energy's Savannah River Site. The direction of groundwater flow has been determined and a seep line where the contaminated groundwater is estimated to emerge as surface water has been identified in a region of the Southern Sector of the A/M Area. This study was undertaken to estimate the potential of four rhizosphere soils along the seep line to naturally attenuate TCE. Microcosms were setup to evaluate both biotic and abiotic attenuation of TCE. Results demonstrated that sorption to soil was the dominant mechanism during the first week of incubation, with as much as 90% of the TCE removed from the aqueous phase. Linear partitioning coefficients (K_d) ranged from 0.83 to 7.4?mL/g, while organic carbon partition coefficients (K_oc) ranged from 72 to 180?mL/gC. Diffu-sional losses from the microcosms appeared to be a dominant fate mechanism during the remainder of the experiment, as indicated by results from the water controls. A limited amount of TCE biodegradation was observed, and attempts to stimulate TCE biodegradation by either methanotrophic or methanogenic activity through amendments with methane, oxygen, and methanol were unsuccessful. The appearance of cis-1,2-dichloroethylene (c-DCE), and trans-1,2-dichloroethylene (t-DCE) confirmed the potential for anaerobic reductive dechlorination. However, these daughter products represented less than 5% of the initial TCE added. The sorption results indicate that natural attenuation may represent a viable remediation option for the TCE plume as it passes through the rhizosphere.     

Activating Effect of NO on Ca2+-Dependent K+ Channels in Smooth Muscle Cells of the Main Pulmonary Artery of the Rabbit

Using a patch-clamp technique in the whole-cell configuration, we studied the effect of a nitric oxide (NO) donor, nitroglycerin (NG), on outward transmembrane ion current in isolated smooth muscle cells (SMC) of the main pulmonary artery of the rabbit. We also studied the characteristics of unitary high-conductance Ca²⁺-dependent K⁺ channels (K_Ca channels) in the SMC membrane in the cell-attached and outside-out configurations. Nitroglycerin in a 10 M concentration increased the amplitude and intensified oscillations of outward transmembrane current induced by step depolarization. In this case, the threshold of activation of the current (–40 mV) did not change. If the potential was +70 mV, the transmembrane current in the presence of NG increased, as compared with the control, by 32.6 ± 19.4% (n = 6), on average. Simultaneous addition of 10 M NG and 1 mM tetraethylammonium chloride (TEA), a blocker of K_Ca channels, to the external solution at the potential of +70 mV decreased the amplitude of outward transmembrane current with respect to the control by 25.2 ± 11% (n = 6) and suppressed oscillations of this current. In the series of experiments carried out in the outside-out configuration (concentration of K⁺ ions in the external solution was 5.9 mM), we calculated the conductance of a single K_Ca channel, which was approximately 150 pS. In the case where the potential was equal to +40 mV, 1 mM TEA suppressed completely the current through unitary K_Ca channels. In the series of experiments performed in the cell-attached configuration, 100 M NG to a considerable extent intensified the activity of unitary high-conductance K_Ca channels by increasing the probability of the channel open state (P ₀), on average, by 80 ± 1%, as compared with the control. In this case, NG did not influence the conductance of single K_Ca channels. We concluded that the NO donor NG increases the amplitude of outward transmembrane current in SMC of the rabbit main pulmonary artery by stimulation of the activity of TEA-sensitive high-conductance K_Ca channels. Our experiments carried out on single K_Ca channels demonstrated that the activating effect of NG on K_Ca channels is realized at the expense of an increase in the P ₀ of these channels, but not of a change in the conductance of single channels.