Movement and degradation of aldicarb residues in the saturated zone under citrus groves on the Florida ridge

A 1.7-ha section of citrus grove near Lake Hamilton was the site of a three-year field study designed to monitor the movement and degradation of the nematicide and insecticide aldicarb in the central ridge area of Florida. Soil cores were used to monitor the fate of aldicarb residues in the unsaturated zone and over 2,000 groundwater samples were collected from 174 monitoring wells to measure horizontal and vertical transport of aldicarb residues in the saturated zone. A simple saturated zone model was used to estimate the degradation rate of aldicarb residues and extrapolate findings to other ridge areas.The results of the study suggest that in the saturated zone aldicarb residues degrade at a rate corresponding to a half-life of approximately eight months. The predominantly horizontal movement of groundwater at this site limits aldicarb residues to the upper three to five meters of the saturated zone. Field data from this site together with unsaturated and saturated zone simulations suggest that in this area of Florida current restrictions on aldicarb used near potable wells are adequate to protect drinking water supplies.     

A sensitivity analysis of adsorption and degradation parameters in the modeling of pesticide transport in soils

The increasing use of deterministic models in predicting the movement of pesticides in soils, has focused attention on the evaluation of major parameters which represent attenuation factors of organics in the subsurface. These parameters are the degradation rate constant and the adsorption constant for the pesticide. In view of the large in situ variability of these parameters and of the difficulty in obtaining accurate field data, there is a high degree of uncertainty associated with the results obtained from deterministic models. A sensitivity analysis is performed here to quantify the impact of such variation in each of these input parameters on the output results of an unsaturated zone transport model (PRZM). Results show that variations in these parameters about their respective mean values greatly affect the predicted concentration distributions, obtained after three years, of the pesticide aldicarb in all the soil profile. A 15–22% variation in the degradation constant, or a 24% variation in the adsorption constant, lead to a 100% uncertainty in the various simulation results defined as the cumulative quantity of aldicarb or the dissolved aldicarb concentration leached below the root zone (or the unsaturated zone) of the soil. Such a deterministic model presents a high degree of sensitivity to these input parameters. Accurate field data are then needed to obtain reliable model results in predicting pesticide movement inthe unsaturated zone.     

Environmental fate studies of aldicarb and aldoxycarb applications to Wisconsin potatoes

Aldicarb and aldoxycarb pesticides were applied to potato fields in central Wisconsin to study the degradation and movement of their carbamate residues within the soil profile. Aldicarb and aldoxycarb residues degraded at similar rates with half-lives ranging from 0.9 to 1.4 months. Although unsaturated-zone residue measurements indicated relatively minor differences among the three plots, residues were detected in shallow groundwater beneath plots treated with aldicarb or aldoxycarb at planting. Residues beneath the plot treated with aldicarb at emergence were much lower and the timing of these residues suggests they might have resulted from migration of residues from the plot treated with aldicarb at planting. This study confirms the results of previous and concurrent research programs which suggest that emergence applications of aldicarb reduce the potential for residues to enter shallow groundwater without reducing insect control or potato yields.     

非正规垃圾填埋场包气带介质的污染物阻滞能力研究

非正规垃圾填埋场已成为地下水的潜在污染源,给城市供水安全带来了极大风险。以北京某非正规垃圾填埋场的3种典型包气带介质为研究对象,以Br-为示踪剂,采取土柱实验和数值模拟相结合的方法,通过对场区3种典型包气带介质饱和与非饱和状态下的Br-迁移速率的比较,探讨包气带介质对污染物的阻滞能力。结果表明,Br-在不同包气带介质饱和状态下的迁移速率为中砂>细砂>砂质粉土,这可能与不同介质中粉粒和粘粒的含量有关,同时介质的有机质含量也会影响Br-的迁移速率,但前者因素占了主导作用;在Br-迁移过程中,不同包气带介质的含水量是影响其迁移速率的主要因素,若能合理控制场区的地下水位,使得包气带介质长期处于非饱和状态,将会大大延缓垃圾渗滤液中污染物向地下水迁移;针对特定的非正规垃圾填埋场,可采取抽水等措施,控制场区的细砂和砂质粉土层处于非饱和状态,充分利用包气带介质在非饱和状态时较强的污染物阻滞能力,阻隔或延缓垃圾渗滤液中的污染物进入地下水中。     

Linking ground-water age and chemistry data along flow paths: implications for trends and transformations of nitrate and pesticides

Tracer-based ground-water ages, along with the concentrations of pesticides, nitrogen species, and other redox-active constituents, were used to evaluate the trends and transformations of agricultural chemicals along flow paths in diverse hydrogeologic settings. A range of conditions affecting the transformation of nitrate and pesticides (e.g., thickness of unsaturated zone, redox conditions) was examined at study sites in Georgia, North Carolina, Wisconsin, and California. Deethylatrazine (DEA), a transformation product of atrazine, was typically present at concentrations higher than those of atrazine at study sites with thick unsaturated zones but not at sites with thin unsaturated zones. Furthermore, the fraction of atrazine plus DEA that was present as DEA did not increase as a function of ground-water age. These findings suggest that atrazine degradation occurs primarily in the unsaturated zone with little or no degradation in the saturated zone. Similar observations were also made for metolachlor and alachlor. The fraction of the initial nitrate concentration found as excess N2 (N2 derived from denitrification) increased with ground-water age only at the North Carolina site, where oxic conditions were generally limited to the top 5 m of saturated thickness. Historical trends in fluxes to ground water were evaluated by relating the times of recharge of ground-water samples, estimated using chlorofluorocarbon concentrations, with concentrations of the parent compound at the time of recharge, estimated by summing the molar concentrations of the parent compound and its transformation products in the age-dated sample. Using this approach, nitrate concentrations were estimated to have increased markedly from 1960 to the present at all study sites. Trends in concentrations of atrazine, metolachlor, alachlor, and their degradates were related to the timing of introduction and use of these compounds. Degradates, and to a lesser extent parent compounds, were detected in ground water dating back to the time these compounds were introduced.     

Metal ion and other key water quality constituent attenuation in soil columns

A set of soil columns was constructed to simulate discharge of disinfected tertiary treated wastewater to a river via nearby land application or indirect discharge. The system was primarily designed to observe the fate of metal ions and nutrients. The following three experiments were conducted: (1) flow through saturated soils only, which simulates indirect discharge where water is directly applied to groundwater; (2) flow through unsaturated soil followed by saturated flow, which simulates vadose then saturated zone transport; and (3) saturated flow only using ethylene diamine tetraacetic acid-metal chelates, which determined effects of metal organic complexes on metal mobility through the soil. Metal ion attenuation was substantial but not complete in experiments 1 and 2 (removal: 68% Cu2+, 43% Ni2+, 98% Pb2+, and 96% Hg2+), which was somewhat contrary to modeling results. Cyanide attenuation was also monitored (92% removal). In experiment 3, lead attenuation was somewhat reduced (92% removal) and delayed (requiring additional residence time); copper attenuation was significantly reduced (38% removal) and delayed; and nickel concentrations were higher in the 28-day sample (> 80 microg/L) than in the column feed water (58 microg/L). Near-complete denitrification and total phosphorus attenuation were observed. For the water quality constituents studied, unsaturated (vadose zone) transport did not appear to add additional benefit.     

Migration of contaminants through the unsaturated zone overlying the Hesbaye chalky aquifer in Belgium: a field investigation

This paper presents the results of a detailed field investigation that was performed for studying groundwater recharge processes and solute downward migration mechanisms prevailing in the unsaturated zone overlying a chalk aquifer in Belgium. Various laboratory measurements were performed on core samples collected during the drilling of boreholes in the experimental site. In the field, experiments consisted of well logging, infiltration tests in the unsaturated zone, pumping tests in the saturated zone and tracer tests in both the saturated and unsaturated zones. Results show that gravitational flows govern groundwater recharge and solute migration mechanisms in the unsaturated zone. In the variably saturated chalk, the migration and retardation of solutes is strongly influenced by recharge conditions. Under intense injection conditions, solutes migrate at high speed along the partially saturated fissures, downward to the saturated zone. At the same time, they are temporarily retarded in the almost immobile water located in the chalk matrix. Under normal recharge conditions, fissures are inactive and solutes migrate slowly through the chalk matrix. Results also show that concentration dynamics in the saturated zone are related to fluctuations of groundwater levels in the aquifer. A conceptual model is proposed to explain the hydrodispersive behaviour of the variably saturated chalk. Finally, the vulnerability of the chalk to contamination issues occurring at the land surface is discussed.     

Degradation rates of CFC-11, CFC-12 and CFC-113 in anoxic shallow aquifers of Araihazar, Bangladesh

Chlorofluorocarbons CFC-11 (CCl(3)F), CFC-12 (CCl(2)F(2)), and CFC-113 (CCl(2)F-CClF(2)) are used in hydrology as transient tracers under the assumption of conservative behavior in the unsaturated and saturated soil zones. However, laboratory and field studies have shown that these compounds are not stable under anaerobic conditions. To determine the degradation rates of CFCs in a tropical environment, atmospheric air, unsaturated zone soil gas, and anoxic groundwater samples were collected in Araihazar upazila, Bangladesh. Observed CFC concentrations in both soil gas and groundwater were significantly below those expected from atmospheric levels. The CFC deficits in the unsaturated zone can be explained by gas exchange with groundwater undersaturated in CFCs. The CFC deficits observed in (3)H/(3)He dated groundwater were used to estimate degradation rates in the saturated zone. The results show that CFCs are degraded to the point where practically no (<5%) CFC-11, CFC-12, or CFC-113 remains in groundwater with (3)H/(3)He ages above 10 yr. In groundwater sampled at our site CFC-11 and CFC-12 appear to degrade at similar rates with estimated degradation rates ranging from approximately 0.25 yr(-1) to approximately 6 yr(-1). Degradation rates increased as a function of reducing conditions. This indicates that CFC dating of groundwater in regions of humid tropical climate has to be carried out with great caution.     

A simulation procedure for groundwater quality assessments of pesticides

A procedure is described for making regional assessments of pesticide residue loadings and movement in groundwater underneath and downgradient from treated fields. A Monte-Carlo numerical simulation technique is used to generate model parameters for both the unsaturated and saturated zones. Simulations are performed using the Pesticide Root Zone Model linked to a simple groundwater solute transport model.The procedure is useful for evaluating the potential for producing pesticide residues in drinking water wells before actual field applications are made. Appropriate land management options, including restrictions on pesticide application, also can be developed using this procedure.The procedure was used to assess aldicarb levels in northeastern North Carolina groundwater resulting from application of the pesticide to peanuts. Probability density functions for selected soil characteristics were developed using a direct-access soils information data base. Probability density functions for selected groundwater characteristics were developed from available data for the study area. Simulation results indicated that mass fluxes to groundwater exceeded 0.01 and 0.1 kg ha⁻¹ approximately 6.9 and 1.0 percent of the time, respectively. No fluxes exceeded 0.1 kg ha⁻¹ at a distance of 60 m downgradient in any of the cases evaluated.     

In situ fluidization for peat bed rupture,and preliminary economic analysis

This study concerns in situ fluidization (ISF), a new remediation method with potential application to the remediation of NAPL and heavy metal contaminants, by their release from the fluidized zone generated by a water jet. The present study examines the effect of ISF on layers of peat, of significance owing to its role as an important NAPL and metal contaminant trap. Once trapped, such contaminants are not readily accessible by most remedial methods, due to the low permeability and diffusivity of the peat. A simple tank experiment is used to demonstrate rupture of a peat layer by ISF, with removal of the peat as elutriated fines and segregated peat chunks. The application of ISF in the field is then examined by three field trials in uncontaminated sands, in both saturated and unsaturated conditions. Fluidized depths of up to 1.9 m in the saturated zone (with refusal on a peat layer) and 2.5 m in the unsaturated zone (no refusal) were attained, using a 1.9-m-long, 50 mm diameter jet operated at 5-13 1 s(-1). Pulses of dark turbidity and shell fragments in the effluent indicated the rupture of peat and shelly layers. The experiments demonstrate the hydraulic viability of ISF in the field, and its ability to remove peat-based contaminants. The issues of appropriate jet design and water generation during ISF are discussed, followed by a preliminary economic analysis of ISF relative to existing remediation methods.     

Numerical simulation of a fine-grained denitrification layer for removing septic system nitrate from shallow groundwater

One of the most common methods to dispose of domestic wastewater involves the release of septic effluent from drains located in the unsaturated zone. Nitrogen from such systems is currently of concern because of nitrate contamination of drinking water supplies and eutrophication of coastal waters. It has been proposed that adding labile carbon sources to septic distribution fields could enhance heterotrophic denitrification and thus reduce nitrate concentrations in shallow groundwater. In this study, a numerical model which solves for variably saturated flow and reactive transport of multiple species is employed to investigate the performance of a drain field design that incorporates a fine-grained denitrification layer. The hydrogeological scenario simulated is an unconfined sand aquifer. The model results suggest that the denitrification layer, supplemented with labile organic carbon, may be an effective means to eliminate nitrogen loading to shallow groundwater. It is also shown that in noncalcareous aquifers, the denitrification reaction may provide sufficient buffering capacity to maintain near neutral pH conditions beneath and down gradient of the drain field. Leaching of excess dissolved organic carbon (DOC) from the denitrification layer is problematic, and causes an anaerobic plume to develop in simulations where the water table is less than 5-6 m below ground surface; this anaerobic plume may lead to other down gradient changes in groundwater quality. A drain field and denitrification layer of smaller dimensions is shown to be just as effective for reducing nitrate, but has the benefit of reducing the excess DOC leached from the layer. This configuration will minimize the impact of wastewater disposal in areas where the water table is as shallow as 3.5 m.     

Comparison of the solution behaviour of a pyrite-calcite mixture in batch and unsaturated sand column

A successful application of reaction transport algorithms to calculate the chemical evolution of natural systems requires accurate methods to compute the rates of mineral/fluid surface reactions. Regarding the transport of radio-nuclides in mining dumps the dissolution of minerals is of special importance. Using a kinetic rate law of the mineral dissolution verified for unsaturated conditions will allow a realistic modelling of the mineral weathering in the environment. Dissolution rates of minerals in an aqueous solution are determined by several characteristics. These are surface reaction rates, morphology of the mineral's surface and, in case it is the unsaturated zone, the degree of the water saturation. For this process, the quantity of the particle surfaces which are in contact with percolating water is most decisive. In order to study the differences of mineral dissolution under saturated and unsaturated conditions batch and column experiments were carried out with a pyrite-calcite mixture. The experimental results were verified by calculations. Comparing the dissolution in batch with those in the column experiment, which was performed with a water flow velocity of 0.64 cm/day and was analyzed in the region of a water saturation of 0.11, one can conclude that only a small portion of about 5% of the grain surface is chemically reactive in this unsaturated flow.     

Effects of water content on reactive transport of 85Sr in Chernobyl sand columns

It is known that under unsaturated conditions, the transport of solutes can deviate from ideal advective-dispersive behaviour even for macroscopically homogeneous porous materials. Causes may include physical non-equilibrium, sorption kinetics, non-linear sorption, and the irregular distribution of sorption sites. We have performed laboratory experiments designed to identify the processes responsible for the non-ideality of radioactive Sr transport observed under unsaturated flow conditions in an Aeolian sandy deposit from the Chernobyl exclusion zone. Miscible displacement experiments were carried out at various water contents and corresponding flow rates in a laboratory model system. Results of our experiments have shown that breakthrough curves of a conservative tracer exhibit a higher degree of asymmetry when the water content decreases than at saturated water content and same Darcy velocity. It is possible that velocity variations caused by heterogeneities at the macroscopic scale are responsible for this situation. Another explanation is that molecular diffusion drives the solute mass transfer between mobile and immobile water regions, but the surface of contact between these water regions is small. At very low concentrations, representative of a radioactive Sr contamination of the pore water, sorption and physical disequilibrium dominate the radioactive Sr transport under unsaturated flow conditions. A sorption reaction is described by a cation exchange mechanism calibrated under fully saturated conditions. The sorption capacity, as well as the exchange coefficients are not affected by desaturation. The number of accessible exchange sites was calculated on the basis that the solid remained in contact with water and that the fraction of solid phase in contact with mobile water is numerically equal to the proportion of mobile water to total water content. That means that for this type of sandy soil, the nature of mineral phases is the same in advective and non-advective domains. So sorption reaction parameters can be estimated from more easily conducted saturated experiments, but hydrodynamic behaviour must be characterized by conservative tracer experiments under unsaturated flow conditions.     

Effects of degree of water saturation on dispersivity and immobile water in sandy soil columns

Three natural nonaggregated soil samples, with similar grain-size distributions, have been used to determine the dispersive behavior of porous media under steady, saturated and unsaturated flow conditions. Tritium was used as a tracer and was found to have no sorption on the solid matrix. Generated breakthrough curves (BTCs) for the unsaturated experiments were symmetrical with no evidence of tailing. The unsaturated experiments for two of the soils were adequately described by considering all the water in the pore volume as mobile. However, about 10% of the pore water, independent of the degree of saturation, was found to be immobile in the case of the third soil during unsaturated flow. For this soil, there was no mass transfer between the two water regions, indicating that the immobile water is essentially isolated from the flowing water fraction. For all three soils, dispersivity under unsaturated conditions was found to be higher, independent of the degree of water saturation, than the value determined for the saturated experiments. This is inconsistent with what would be expected from the simple bundle-of-capillary-tubes model and does not agree well with a more sophisticated conceptualization of the porous medium. The data, however, clearly indicate a wider range in pore-water velocities when these soils are desaturated.     

A high-resolution non-invasive approach to quantify oxygen transport across the capillary fringe and within the underlying groundwater

Oxygen transport across the capillary fringe is relevant for many biogeochemical processes. We present a non-invasive technique, based on optode technology, to measure high-resolution concentration profiles of oxygen across the unsaturated/saturated interface. By conducting a series of quasi two-dimensional flow-through laboratory experiments, we show that vertical hydrodynamic dispersion in the water-saturated part of the capillary fringe is the process limiting the mass transfer of oxygen. A number of experimental conditions were tested in order to investigate the influence of grain size and horizontal flow velocity on transverse vertical dispersion in the capillary fringe. In the same setup, analogous experiments were simultaneously carried out in the fully water-saturated zone, therefore allowing a direct comparison with oxygen transfer across the capillary fringe. The outcomes of the experiments under various conditions show that oxygen transport in the two zones of interest (i.e., the unsaturated/saturated interface and the saturated zone) is characterized by very similar transverse dispersion coefficients. An influence of the capillary fringe morphology on oxygen transport has not been observed. These results may be explained by the narrow grain size distribution used in the experiments, leading to a steep decline in water saturation at the unsaturated/saturated interface and to the absence of trapped gas in this transition zone. We also modeled flow (applying the van Genuchten and the Brooks-Corey relationships) and two-dimensional transport across the capillary fringe, obtaining simulated profiles of equivalent aqueous oxygen concentration that were in good agreement with the observations.     

Behavior of nine selected emerging trace organic contaminants in an artificial recharge system supplemented with a reactive barrier

Artificial recharge improves several water quality parameters, but has only minor effects on recalcitrant pollutants. To improve the removal of these pollutants, we added a reactive barrier at the bottom of an infiltration basin. This barrier contained aquifer sand, vegetable compost, and clay and was covered with iron oxide dust. The goal of the compost was to sorb neutral compounds and release dissolved organic carbon. The release of dissolved organic carbon should generate a broad range of redox conditions to promote the transformation of emerging trace organic contaminants (EOCs). Iron oxides and clay increase the range of sorption site types. In the present study, we examined the effectiveness of this barrier by analyzing the fate of nine EOCs. Water quality was monitored before and after constructing the reactive barrier. Installation of the reactive barrier led to nitrate-, iron-, and manganese-reducing conditions in the unsaturated zone below the basin and within the first few meters of the saturated zone. Thus, the behavior of most EOCs changed after installing the reactive barrier. The reactive barrier enhanced the removal of some EOCs, either markedly (sulfamethoxazole, caffeine, benzoylecgonine) or slightly (trimethoprim) and decreased the removal rates of compounds that are easily degradable under aerobic conditions (ibuprofen, paracetamol). The barrier had no remarkable effect on 1H-benzotriazole and tolyltriazole.     

Aldicarb and carbofuran transport in a Hapludalf influenced by differential antecedent soil water content and irrigation delay

Pesticide use in agroecosystems can adversely impact groundwater quality via chemical leaching through soils. Few studies have investigated the effects of antecedent soil water content (SWC) and timing of initial irrigation (TII) after chemical application on pesticide transport and degradation. The objectives of this study were to investigate the effects of antecedent soil water content (wet vs dry) and timing of initial irrigation (0h Delay vs 24h Delay) on aldicarb [(EZ)-2-methyl-2-(methylthio)propionaldehyde O-methylcarbamoyloxime] and carbofuran [2,3-dihydro-2,2-dimethylbenzofuran-7-yl methylcarbamate] transport and degradation parameters at a field site with Menfro silt loam (fine-silty, mixed, superactive, mesic Typic Hapludalf) soils. Aldicarb and carbofuran were applied to plots near field capacity (wet) or near permanent wilting point (dry). Half of the dry and wet plots received irrigation water immediately after chemical application and the remaining plots were irrigated after a 24h Delay. The transport and degradation parameters were estimated using the method of moments. Statistical significance determined for SWC included averages across TII levels, and significance determined for TII included averages across SWC levels. For the dry treatment, aldicarb was detected 0.10 m deeper (P<0.01) on two of the four sampling dates and carbofuran was detected at least 0.10 m deeper (P<0.05) on all of the sampling dates compared to the wet treatment. Pore water velocity was found to be higher (P<0.10) in the dry vs wet treatments on three of four dates for aldicarb and two of four dates for carbofuran. Retardation coefficients for both pesticides showed similar evidence of reduced values for the dry vs wet treatments. These results indicate deeper pesticide movement in the initially dry treatment. For aldicarb and carbofuran, estimated values of the degradation rate were approximately 40-49% lower in the initially dry plots compared to the initially wet plots, respectively. When the initial irrigation was delayed for 24h, irrespective of antecedent moisture conditions, a 30% reduction in aldicarb degradation occurred. This study illustrates the deeper transport of pesticides and their increased persistence when applied to initially dry soils.     

A large-scale experiment on mass transfer of trichloroethylene from the unsaturated zone of a sandy aquifer to its interfaces

A large-scale experiment was conducted to investigate the transport of trichloroethylene (TCE) vapors in the unsaturated zone and to determine the mass transfer to the groundwater and the atmosphere. The experiment involved injection of 5 1 of TCE in the unsaturated zone under controlled conditions, with multidepth sampling of gas and water through the unsaturated zone and across the capillary zone into underlying groundwater. The mass transfer of TCE vapors from the vadose zone to the atmosphere was quantified using a vertical flux chamber. A special soil water sampler was used to monitor transport across the capillary fringe. Experimental data indicated that TCE in the unsaturated zone was mainly transported to the atmosphere and this exchange reduced significantly the potential for groundwater pollution. The maximum measured TCE flux to the atmosphere was about 3 g/m(2)/day. Observed and calculated fluxes based on vertical TCE vapor concentration gradients and Fick's law were in good agreement. This confirms that TCE vapor transport under the experimental conditions was governed essentially by molecular diffusion. TCE vapors also caused a lower, but significant contamination of the underlying groundwater by dispersion across the capillary fringe with a corresponding maximum flux of about 0.1 g/m(2)/day. This mass transfer to groundwater is partly uncertain due to an inadvertent entry of some nonaqueous phase liquid (NAPL) from the source area into the saturated zone. Application of an analytical solution to estimate the TCE flux from the unsaturated zone to the groundwater indicated that this phenomenon is not only influenced by molecular diffusion but also by vertical dispersion. The mass balance indicates that, under the given experimental conditions (e.g. proximity of the source emplacement relative to the soil surface, relatively high permeable porous medium), nearly 95% of the initial TCE mass was transferred to the atmosphere.     

Determination of sorption coefficients of pharmaceutically active substances carbamazepine, diclofenac, and ibuprofen, in sandy sediments

Laboratory batch studies were conducted to characterize the sorption behavior of three pharmaceutically active substances (carbamazepine, diclofenac, and ibuprofen) in different sediment types. The sediments were natural sandy sediments from the water saturated zone and the unsaturated zone in the Berlin (Germany) area. They are characterized as medium and fine-grained sands, both with low organic carbon content. The results of the experiments show that sorption coefficients were generally quite low. Distribution coefficients (K(d) values) determined by the batch experiments varied from 0.21 to 5.32 for carbamazepine, 0.55 to 4.66 for diclofenac, and 0.18 to 1.69 for ibuprofen. Comparison of the organic carbon normalized sorption coefficient K(OC) values based on correlation equations with actual experimental data revealed that the calculated data for carbamazepine is of the same order as the experimental data. For diclofenac and ibuprofen the calculated values are much higher than the experimental data, showing a much higher mobility of diclofenac and ibuprofen in natural aquifer sediments than indicated by correlation equations based on octanol water distribution coefficients.     

The hydrogeology of a tailings impoundment formed by central discharge of thickened tailings: implications for tailings management

The Kidd Creek Cu–Zn sulfide mine is located near Timmins, Ontario. Mill tailings are thickened and deposited as a slurry in a circular impoundment with an area of approximately 1200 ha. Deposition of tailings as a thickened slurry from a central discharge ramp results in a conical-shaped tailings deposit with low perimeter dykes, a uniform grain-size distribution, uniform and low hydraulic conductivity, and a tension-saturated zone above the water table up to 5 to 6 m thick. These characteristics provide benefits over conventionally disposed tailings with respect to tailings management. The thick tension-saturated zone within the tailings limits the thickness of unsaturated tailings that are susceptible to rapid sulfide oxidation. The conical shape of the deposit results in the formation of a recharge area near the centre of the impoundment and discharge in the peripheral areas. In contrast, the elevated nature of many conventional, unthickened tailings impoundments results in recharge over most of the surface of the impoundment, with discharge occurring outside the impoundment through large containment dykes. Three-dimensional pore water flow modelling suggests that approximately 90% of the total discharge from the thickened tailings occurs within the tailings impoundment. When discharge is confined within the impoundment, there is improved control over low-quality effluent, and an opportunity to design passive control measures to reduce treatment costs and minimize environmental impacts.