Simulating Nitrate Leaching Profiles in a Highly Permeable Vadose Zone

An approach is developed to simulate leaching of a dissolved chemical constituent in the vadose zone of an aquifer. Specifically, nitrate loading at the water table for different water table depths, for a range of aquifer permeability values, and for different cases of heterogeneity of the aquifer, are considered. Models from the literature are first used to derive soil–water characteristic curves (water retention and hydraulic conductivity) from a grain size distribution curve for unsaturated conditions. Given infiltration from the surface, the initial conditions for the chemical concentration, and the water content profile, leaching of the chemical in the vadose zone is simulated as a function of both time and depth. The methodology is illustrated for a permeable aquifer. Simulations are undertaken using a finite element code for saturated and unsaturated flow. Different scenarios are simulated depending on the heterogeneity of the aquifer and the depth of the water table. Modeling results show that in the example case studied, nitrate concentration loading at the water table does not depend strongly on the position of the water table, but rather on the material properties of the aquifer. The contribution of this endeavor resides in the methodology which allows a prediction of nitrate leaching using only the grain size property of the aquifer. It allows practitioners to obtain a first assessment of leaching with limited data.     

Numerical Modeling on the Effect of Dissolved Oxygen on Nitrogen Transformation and Transport in Unsaturated Porous System

Nitrogen pollution in groundwater resulting from wastewater application to land is a common problem, and it causes a major threat to groundwater-based drinking water supplies. In this study, a numerical model is developed to study the nitrogen species transport and transformation in unsaturated porous media. Further, a new mass transfer module for dissolved oxygen (DO) is incorporated in the one-dimensional numerical model for nitrogen species transport to describe the fate and transport of nitrogen species, dissolved oxygen, dissolved organic carbon (DOC), and biomass. The spatial and temporal variation of dissolved oxygen is incorporated in the model through the mass transfer from gaseous phase to water phase in an unsaturated porous system. The numerical results of the water flow model and single species and multispecies transport model in an unsaturated zone developed for this purpose have been validated with the available analytical/numerical solution. The developed model is applied in clay loam, silt, and sand soils to analyze the transport behavior of nitrogen species under unsaturated condition. The numerical results suggest that the high rate of oxygen mass transfer from the air phase to the water phase positively increases the dissolved oxygen in the applied wastewater and enhances the nitrification process. Because of this high oxygen mass transfer, the nitrate nitrogen concentration significantly increases in the unsaturated zone and the same is transported to a larger depth at higher simulation period. On the other hand, the low rate of oxygen mass transfer implicitly enhances the denitrification process and finally reduces the nitrate nitrogen concentration in the unsaturated zone. The numerical results also show that the nitrate nitrogen transport is rapid in sandy soil when compared with clay loam and silty soils under high oxygen mass transfer rate. In essence, the high oxygen mass transfer rate significantly increases the nitrate nitrogen in the unsaturated zone, especially at a greater depth at larger time levels and eventually affects the groundwater quality.     

Centrifugal study of zone of influence during air-sparging

Air sparging (AS) is one of the groundwater remediation techniques for remediating volatile organic compounds (VOCs) in saturated soil. However, in spite of the success of air sparging as a remediation technique for the cleanup of contaminated soils, to date, the fundamental mechanisms or the physics of air flow through porous media is not well understood. In this study, centrifugal modeling tests were performed to investigate air flow rates and the evolution of the zone of influence during the air sparging under various g-levels. The test results show that with the increase in sparging pressure the mass flow rate of the air sparging volume increases. The air mass flow rate increases linearly with the effective sparging pressure ratio, which is the difference between sparging pressure and hydrostatic pressure normalized with respect to the effective overburden pressure at the sparging point. Also the slope of mass flow rate with effective sparging pressure ratio increases with higher g-levels. This variation of the slope of mass flow rate of air sparging volume versus effective sparging pressure ratio, M, is linear with g-level confirming that the air flow through soil for a given effective sparging pressure ratio only depends on the g-level. The test results also show that with increasing sparging pressure, the zone of influence (ZOI), which consists of the width at the tip of the cone or lateral intrusion and the cone angle, will lead to an increase in both lateral intrusion and the cone angle. With a further increase in air injection pressure, the cone angle reaches a constant value while the lateral intrusion becomes the main contributor to the enlargement of the ZOI. However, beyond a certain value of effective sparging pressure ratio, there is no further enlargement of the ZOI.     

Two-dimensional finite elements model for selenium transport in saturated and unsaturated zones

A two-dimensional finite element model was developed to simulate species of selenium transport in two dimensions in both saturated and unsaturated soil zones. The model considers water, selenate, selenite, and selenomethionine uptake by plants. It also considers adsorption and desorption, oxidation and reduction, volatilization, and chemical and biological transformations of selenate, selenite, and selenomethionine. In addition to simulating water flow, selenate, selenite, and selenomethionine transport, the model also simulates organic and gaseous selenium transport. The developed model was applied to simulate two different observed field data. The simulation of the observed data was satisfactory, with mean absolute error of 48.5 μg/l and mean relative error of 8.9%.     

庞庄煤矿煤矸石淋溶液对周边水体影响分析

为研究庞庄煤矿张小楼煤矸石堆场淋溶液对其场地和周边地表水、地下水水质影响情况，围绕煤矸石堆场及周边采集13组水样做分析比对。结果表明，煤矸石堆场淋溶液对周边地表水体形成普遍污染，且污染物浓度随着污染距离增加逐步降低，一般化学污染指标主要包括溶解性总固体、硫酸盐、铁、锰、氨氮等，毒理学污染指标包括亚硝酸盐、氟化物、砷，超标倍率约为0.02~5.36。受距离及成井深度等因素影响，地下水体未遭受淋溶液明显的污染和影响。     

Phosphorus losses from arable fields in Sweden—effects of field-specific factors and long-term trends

Accelerated eutrophication of surface water is often caused by high phosphorus (P) losses from agricultural fields. Long-term measurements of P concentrations from arable fields are therefore important for understanding of processes and key factors behind losses. Unfortunately, long-time series are difficult to compare due to high variablity, non-normal distribution, precipitation variability and flow relatedness. The objective was to use locally weighted scatterplot smoothing (LOWESS) and Seasonal Kendall Test to account for discharge effects on concentrations of P forms and suspended soil material, and to identify trends for seven agricultural fields included in a Swedish water quality monitoring program. Soil P content and sorption capacity governed concentrations and losses of dissolved reactive phosphorus (DRP). Other factors, such as preferential flow may influence the importance of these factors. Suspended soil material and unreactive phosphorus (UP) concentrations in water discharge depended on soil texture, slope of the field and total soil P content. Statistically significant trends in concentrations of different P forms were found for two fields. These trends are probably a consequence of changes in P additions and soils P saturation levels.     

海南昌江石碌钴铜矿尾矿库重金属污染环境现状调查

为了解海南省昌江县石碌镇钴铜矿尾矿库环境特点,分别采集了矿砂、背景土壤、废水、纳污水体和植物进行分析。结果表明,钴铜矿尾矿库的土壤受As和Cu污染严重,周边土壤受到不同程度的重金属污染;废水经过尾矿库沉淀处理后,SS、As、Cu、Co、Fe等监测指标均有所降低,尾矿库起到沉淀吸附处理作用;纳污河流石碌河水质良好;尾矿库内自然生长的菖蒲和水竹均受到重金属不同程度污染,尾矿库内的甘蔗中Cr、Pb含量均超过食品中污染物限量标准,表明尾矿库内不宜种植可食用作物。     

Linear alkylbenzene sulfonate (LAS) in water of Lake Dianchi—spatial and seasonal variation, and kinetics of biodegradation

Agroforesty systems, which are recommended as a management option to lower the shallow groundwater level and to reuse saline subsurface drainage waters from the tile-drained croplands in the drainage-impacted areas of Jan Joaquin Valley of California, have resulted in excessive boron buildup in the soil root zone. To assess the efficacy of the long-term impacts of soil boron buildup in agroforesty systems, a mathematical model was developed to simulate non-conservative boron transport. The developed dynamic two-dimensional finite element model simulates water flow and boron transport in saturated–unsaturated soil system, including boron sorption and boron uptake by root-water extraction processes. The simulation of two different observed field data sets by the developed model is satisfactory, with mean absolute error of 1.5 mg/L and relative error of 6.5%. Application of the model to three different soils shows that boron adsorption is higher in silt loam soil than that in sandy loam and clay loam soils. This result agrees with the laboratory experimental observations. The results of the sensitivity analysis indicate that boron uptake by root-water extraction process influences the boron concentration distribution along the root zone. Also, absorption coefficient and maximum adsorptive capacity of a soil for boron are found to be sensitive parameters.     

Agricultural Chemicals in the Alluvial Aquifer of a Typical County of the Arkansas Delta

Statistical methods and a Geographic Information System (GIS) were used to investigate potential indicators of ground water vulnerability to agricultural chemical contamination in a representative area of the Mississippi River alluvial aquifer. A total of 47 wells were sampled for analysis of nitrate, phosphorus, potassium, and 13 pesticides commonly-used in the area. Ten soil and hydrogeologic variables and five ground water vulnerability indices were examined to explain the variations of chemical concentrations. The results showed that no individual soil or hydrogeologic variables or their linear combinations could explain more than 25% of the variation of the chemical concentrations. A quadratic response surface model with the values of confining unit thickness, slope, soil permeability, depth to ground water, and recharge rate accounted for 62% of the variation of nitrate, 43% of P, and 83% of K, suggesting that the interactions among soil and hydrogeologic variables were significant. Observed trends of decreasing nitrate and P concentrations with increasing well depth and/or depth to ground water seemed to correlate with carbonate equilibrium in the aquifer and more reduced environment with depth. In view of uncertainties involved, it was recognized that the limitations associated with input data resolution used in GIS and the formulation of leaching indices limited their use for predicting ground water vulnerability. Misuse of pesticides could be another factor that would complicate the relationships between pesticide concentrations and the vulnerability indices.     

The leachate composition of an old waste dump connected to groundwater: Influence of the reclamation works

This study has been carried out in an uncontrolled dump in the suburbs of Buenos Aires, Argentina. This dump still contains solid wastes of different types in direct contact with the human consumption aquifer. After 18 years of overturning the dump was closed in 1992, and the reclamation works started.Three leachate samplings were carried out in three different zones of the dump, selected according to the topography, the age of residues and the degree of water contact. These samplings were done before the reclamation works, at the end, and four months after they were finished. Several chemical parameters, including heavy metals in solid and dissolved fractions, were analyzed.Multivariate analysis (PCA) was performed to help to understand the effects of the labors on leachate composition. The works could be responsible for a temporal increase of dissolved heavy metals and several chemical variables in the leachate. At the sites of the dump where the modifications were carried out, the levels of some elements analyzed recovered their former values a few months after the reclamations works. At the farthest site, the impact and recovery effects of the works on leachate composition, were delayed.