Impact of pit-toilet leachate on groundwater chemistry and role of vadose zone in removal of nitrate and E-coli pollutants in Kolar District, Karnataka, India

Assessment of chemistry of groundwater infiltrated by pit-toilet leachate and contaminant removal by vadose zone form the focus of this study. The study area is Mulbagal Town in Karnataka State, India. Groundwater level measurements and estimation of unsaturated permeability indicated that the leachate recharged the groundwater inside the town at the rate of 1 m/day. The average nitrate concentration of groundwater inside the town (148 mg/L) was three times larger than the permissible limit (45 mg/L), while the average nitrate concentration of groundwater outside the town (30 mg/L) was below the permissible limit. The groundwater inside the town exhibited E. coli contamination, while groundwater outside the town was free of pathogen contamination. Infiltration of alkalis (Na+, K+) and strong acids (Cl-, SO4 (2-)) caused the mixed Ca-Mg-Cl type (60 %) and Na-Cl type (28 %) facies to predominate groundwater inside the town, while, Ca-HCO3 (35 %), mixed Ca-Mg-Cl type (35 %) and mixed Ca-Na-HCO3 type (28 %) facies predominated groundwater outside/periphery of town. Reductions in E. coli and nitrate concentrations with vadose zone thickness indicated its participation in contaminant removal. A 4-m thickness of unsaturated sand + soft, disintegrated weathered rock deposit facilitates the removal of 1 log of E. coli pathogen. The anoxic conditions prevailing in the deeper layers of the vadose zone (> 19 m thickness) favor denitrification resulting in lower nitrate concentrations (28-96 mg/L) in deeper water tables (located at depths of -29 to -39 m).

CFD analysis of one-dimensional infiltration in vadose zone

This study presents a CFD analysis constructed around PHYSICA, an open framework for multi-physics computational continuum mechanics modelling, to investigate the water movement in unsaturated porous media. The modelling environment is based on a cell-centred finite-volume discretisation technique. A number of test cases are performed in order to validate the correct implementation of Richard's equation for compressible and incompressible fluids. The pressure head form of the equation is used together with the constitutive relationships between pressure, volumetric water content and hydraulic conductivity described by Haverkamp and Van Genuchten models. The flow problems presented are associated with infiltration into initially dry soils with homogeneous or layered geologic settings. Comparison of results with the problems selected from literature shows a good agreement and validates the approach and the implementation.

Travel times in the vadose zone: Variability in space and time

Peer reviewed

塔里木盆地奥陶系不整合面岩溶作用特征与油气储层研究

Based on the principle and methods of carbonate sedimentology and reservoir geology, and guided by the theories of carbonate reservoir geology, the palaeokarst of Ordovician carbonate rocks in Tarim Basin has been comprehensively studied with multiple methods from different branches of geology. It is indicated that the features and distribution of palaeokarstification have developed in Ordovician carbonates. The controlling of karstification to Ordovician carbonate reservoirs has been discussed. Regional distribution of carbonate reservoirs controlled by karstification has been predicted within this basin. The main consents and conclusions of the this dissertation is as follows: Nine key indicators to the recognition of palaeokarst are proposed in terms of careful observation upon the well cores, lithological and geochemical analyses, and drilling and logging responses to the karst caves and fractures. The time and environment of cave filling are documented from careful research of lithofacies, mineralogy, and geochemistry of the physical and chemical fillings within karst caves. The caves in Ordovician carbonates were filled in Early Carboniferous in Lunnan area. The muddy filling in upper caves was deposited under subaerial fresh-water setting, while the muddy filling in lower caves was formed in the mixed water body of fresh-water and dominated sea water. Although most chemical fillings are suggested being precipated in the burial diagenetic environment after karstification but mineralogic and geochemical characteristics of some chemical fillings indicates they formed in meteoric environment during the karstification. It is obvious that the palaeokarst has been zoned in vertical profile. It can be divided into four units from top to bottom: surface karst, vadose karst, phreatic and tranquil flow zones. Between two types of limestone karst and dolostone karst are firstly differentiated in Tarim Basin, based on the comparison of features of each karst zone in limestone and dolostone regions. In Tabei area, the lowest depth of karstification is approximately 300 m below the Upper Ordovician unconformity interface, while the bottom depth of karstification in Tazhong area ranges commonly from 300 to 400 m, in rare cases may be up to 750 m below the upper Ordovician unconformity interface. In Lunnan and Tazhong areas, the palaeokarst morphology and the surface hydrosystem are firstly reconstructed based on the top of carboniferous "Shuangfeng limestone bed (Double-Peaks limestone)" as basal. According to the palaeomorphologic feature, karst topography can be divided into three units: karst upland, karst slope, and karst valley. Vadose zone was well developed in karst upland, and it can be found in a quite depth. Both vadose and phreatic zones were well developed in karst slope and upstream valley. In downstream valley, the karstification is not strong, the vadose and phreatic zones are thin in thickness. In Tazhong and Yingmaili areas, karstification is also developed in relict carbonate palaeo-hills which existed as isolated blocks admits clastic strata.

Viscous-Flow Approach to In Situ Infiltration and In Vitro Saturated Hydraulic Conductivity Determination

Infiltration is dominantly gravity driven, and a viscous-flow approach was developed. Laminar film flow equilibrates gravity with the viscous force and a constant flow velocity evolves during a period lasting 3/2 times the duration of a constant input rate, qS. Film thickness F and the specific contact area L of the film per unit soil volume are the key parameters. Sprinkler irrigation produced in situ time series of volumetric water contents, θ(z,t), as determined with TDR probes. The wetting front velocity v and the time series of the mobile water content, w(z,t) were deduced from θ(z,t). In vitro steady flow in a core of saturated soil provided volume flux density, q(z,t), and flow velocity, v, as determined from a heat front velocity. The F and L parameters of the in situ and the in vitro experiments were compared. The macropore-flow restriction states that, for a particular permeable medium, the specific contact area L must be independent from qS i.e., dL/dqS = 0. If true, then the relationship of qS ∝ v3/2 could scale a wide range of input rates 0 ≤ qS ≤ saturated hydraulic conductivity, Ksat, into a permeable medium, and kinematic-wave theory would become a versatile tool to deal with non-equilibrium flow. The viscous-flow approach is based on hydromechanical principles similar to Darcy’s law, but currently it is not suited to deduce flow properties from specified individual spatial structures of permeable media.

Applying HYDRUS to flow in a sodic clay soil with solution composition–dependent hydraulic conductivity

Management of sodic soils under irrigation often requires application of chemical ameliorants to improve permeability combined with leaching of excess salts. Modeling irrigation, soil treatments, and leaching in these sodic soils requires a model that can adequately represent the physical and chemical changes in the soil associated with the amelioration process. While there are a number of models that simulate reactive solute transport, UNSATCHEM and HYDRUS-1D are currently the only models that also include an ability to simulate the impacts of soil chemistry on hydraulic conductivity. Previous researchers have successfully applied these models to simulate amelioration experiments on a sodic loam soil. To further gauge their applicability, we extended the previous work by comparing HYDRUS simulations of sodic soil amelioration with the results from recently published laboratory experiments on a more reactive, repacked sodic clay soil. The general trends observed in the laboratory experiments were able to be simulated using HYDRUS. Differences between measured and simulated results were attributed to the limited flexibility of the function that represents chemistry-dependent hydraulic conductivity in HYDRUS. While improvements in the function could be made, the present work indicates that HYDRUS-UNSATCHEM captures the key changes in soil hydraulic properties that occur during sodic clay soil amelioration and thus extends the findings of previous researchers studying sodic loams.

Water balance modelling in a tropical watershed under deciduous forest (Mule Hole, India): Regolith matric storage buffers the groundwater recharge process

Accurate estimations of water balance are needed in semi-arid and sub-humid tropical regions, where water resources are scarce compared to water demand. Evapotranspiration plays a major role in this context, and the difficulty to quantify it precisely leads to major uncertainties in the groundwater recharge assessment, especially in forested catchments. In this paper, we propose to assess the importance of deep unsaturated regolith and water uptake by deep tree roots on the groundwater recharge process by using a lumped conceptual model (COMFORT). The model is calibrated using a 5 year hydrological monitoring of an experimental watershed under dry deciduous forest in South India (Mule Hole watershed). The model was able to simulate the stream discharge as well as the contrasted behaviour of groundwater table along the hillslope. Water balance simulated for a 32 year climatic time series displayed a large year-to-year variability, with alternance of dry and wet phases with a time period of approximately 14 years. On an average, input by the rainfall was 1090 mm year(-1) and the evapotranspiration was about 900 mm year(-1) out of which 100 mm year(-1) was uptake from the deep saprolite horizons. The stream flow was 100 mm year(-1) while the groundwater underflow was 80 mm year(-1). The simulation results suggest that (i) deciduous trees can uptake a significant amount of water from the deep regolith, (ii) this uptake, combined with the spatial variability of regolith depth, can account for the variable lag time between drainage events and groundwater rise observed for the different piezometers and (iii) water table response to recharge is buffered due to the long vertical travel time through the deep vadose zone, which constitutes a major water reservoir. This study stresses the importance of long term observations for the understanding of hydrological processes in tropical forested ecosystems. (C) 2009 Elsevier B.V. All rights reserved.

Numerical studies on channel formation and growth during solidification: Effect of process parameters

In the present work, solidification of a hyper-eutectic ammonium chloride solution in a bottom-cooled cavity (i.e. with stable thermal gradient) is numerically studied. A Rayleigh number based criterion is developed, which determines the conditions favorable for freckles formation. This criterion, when expressed in terms of physical properties and process parameters, yields the condition for plume formation as a function of concentration, liquid fraction, permeability, growth rate of a mushy layer and thermophysical properties. Subsequently, numerical simulations are performed for cases with initial and boundary conditions favoring freckle formation. The effects of parameters, such as cooling rate and initial concentration, on the formation and growth of freckles are investigated. It was found that a high cooling rate produced larger and more defined channels which are retained for a longer durations. Similarly, a lower initial concentration of solute resulted in fewer but more pronounced channels. The number and size of channels are also found to be related to the mushy zone thickness. The trends predicted with regard to the variation of number of channels with time under different process conditions are in accordance with the experimental observations reported in the literature.

Universal flame propagation behavior in packed bed of biomass

This article aims at seeking the universal behavior of propagation rate variation with air superficial velocity (V-s) in a packed bed of a range of biomass particles in reverse downdraft mode while also resolving the differing and conflicting explanations in the literature. Toward this, measurements are made of exit gas composition, gas phase and condensed phase surface temperature (T-g and T-s), and reaction zone thickness for a number of biomass with a range of properties. Based on these data, two regimes are identified: gasificationvolatile oxidation accompanied by char reduction reactions up to 16 +/- 1cm/s of V-s and above this, and char oxidationsimultaneous char oxidation and gas phase combustion. In the gasification regime, the measured T-s is less than T-g; a surface heat balance incorporating a diffusion controlled model for flaming combustion gives and matches with the experimental results to within 5%. In the char oxidation regime, T-g and T-s are nearly equal and match with the equilibrium temperature at that equivalence ratio. Drawing from a recent study of the authors, the ash layer over the oxidizing char particle is shown to play a critical role in regulating the radiation heat transfer to fresh biomass in this regime and is shown to be crucial in explaining the observed propagation behavior. A simple model based on radiation-convection balance that tracks the temperature-time evolution of a fresh biomass particle is shown to support the universal behavior of the experimental data on reaction front propagation rate from earlier literature and the present work for biomass with ash content up to 10% and moisture fraction up to 10%. Upstream radiant heat transfer from the ash-laden hot char modulated by the air flow is shown to be the dominant feature of this model.

Role of degree of saturation in denitrification of unsaturated sand specimens

Nitrate contamination of groundwater arises from anthropogenic activities, such as, fertilizer and animal manure applications and infiltration of wastewater/leachates. During migration of wastewater and leachates, the vadose zone (zone residing above the groundwater table), is considered to facilitate microbial denitrification. Particle voids in vadose zone are deficient in dissolved oxygen as the voids are partially filled by water and the remainder by air. Discontinuities in liquid phase would also restrict oxygen diffusion and therefore facilitate denitrification in the vadose/unsaturated soil zone. The degree of saturation of soil specimen (S (r)) quantifies the relative volume of voids filled with air and water. Unsaturated specimens have S (r) values ranging between 0 and 100 %. Earlier studies from naturally occurring nitrate losses in groundwater aquifers in Mulbagal town, Kolar District, Karnataka, showed that the sub-surface soils composed of residually derived sandy soil; hence, natural sand was chosen in the laboratory denitrification experiments. With a view to understand the role of vadose zone in denitrification process, experiments are performed with unsaturated sand specimens (S (r) = 73-90 %) whose pore water was spiked with nitrate and ethanol solutions. Experimental results revealed 73 % S (r) specimen facilitates nitrate reduction to 45 mg/L in relatively short durations of 5.5-7.5 h using the available natural organic matter (0.41 % on mass basis of sand); consequently, ethanol addition did not impact rate of denitrification. However, at higher S (r) values of 81 and 90 %, extraneous ethanol addition (C/N = 0.5-3) was needed to accelerate the denitrification rates.