Remediation of Heterogeneous Aquifers Subject to Uncertainty

Optimal cost pump-and-treat ground water remediation designs for containment of a contaminated aquifer are often developed using deterministic ground water models to predict ground water flow. Uncertainty in hydraulic conductivity fields used in these models results in remediation designs that are unreliable. The degree to which uncertainty contributes to the reliability of remediation designs as measured by the characterization of the uncertainty is shown to differ depending upon the geologic environments of the models. This conclusion is drawn from the optimal design costs for multiple deterministic models generated to represent the uncertainty of four distinct models with different geologic environments. A multi scenario approach that includes uncertainty into the remediation design called the deterministic method for optimization subject to uncertainty (DMOU) is applied to these distinct models. It is found that the DMOU is a method for determining a remediation design subject to uncertainty that requires minimal postprocessing efforts. Preprocessing, however, is required for the application of the DMOU to unique problems. In the ground water remediation design problems, the orientation of geologic facies with respect to the orientation of flow patterns, pumping well locations, and constraint locations are shown to affect the preprocessing, the solutions to the DMOU problems, and the computational efficiency of the DMOU approach. The results of the DMOU are compared to the results of a statistical analysis of the effects of the uncertainty on remediation designs. This comparison validates the efficacy of the DMOU and illustrates the computational advantages to using the DMOU over statistical measures.     

Numerical Modeling of Emulsified Oil Distribution in Heterogeneous Aquifers

In situ anaerobic bioremediation using edible oil emulsions will be most effective if the oil droplets can be brought into close contact with the contaminant to be treated. However, uniformly distributing oil in heterogeneous aquifers can be difficult. The impact of injection conditions on emulsion distribution in a three-dimensional heterogeneous aquifer is examined using MODFLOW and RT3D. Emulsion retention is simulated using a rate-limited Langmuir isotherm. Volume and flow contact efficiency are shown to be functions of mass of oil injected, injection fluid volume, well spacing, and injection sequence. Regression equations are developed relating dimensionless scaling factors to expected contact efficiency for area treatment and barriers. Cleanup time for uncontacted zones is estimated using a mobile-immobile zone modeling approach.     

Transport and Variability of Fecal Bacteria in Carbonate Conglomerate Aquifers

Clastic sedimentary rocks are generally considered non‐karstifiable and thus less vulnerable to pathogen contamination than karst aquifers. However, dissolution phenomena have been observed in clastic carbonate conglomerates of the Subalpine Molasse zone of the northern Alps and other regions of Europe, indicating karstification and high vulnerability, which is currently not considered for source protection zoning. Therefore, a research program was established at the Hochgrat site (Austria/Germany), as a demonstration that karst‐like characteristics, flow behavior, and high vulnerability to microbial contamination are possible in this type of aquifer. The study included geomorphologic mapping, comparative multi‐tracer tests with fluorescent dyes and bacteria‐sized fluorescent microspheres, and analyses of fecal indicator bacteria (FIB) in spring waters during different seasons. Results demonstrate that (1) flow velocities in carbonate conglomerates are similar as in typical karst aquifers, often exceeding 100 m/h; (2) microbial contaminants are rapidly transported toward springs; and (3) the magnitude and seasonal pattern of FIB variability depends on the land use in the spring catchment and its altitude. Different groundwater protection strategies that currently applied are consequently required in regions formed by karstified carbonatic clastic rocks, taking into account their high degree of heterogeneity and vulnerability.     

A Waste Treatment/Disposal Site Evaluation Process for Areas Underlain by Carbonate Aquifers

Fractured carbonate or karst aquifers are highly susceptible to contamination. In Minnesota, the areas underlain by karst aquifers are often characterized by thin soil cover, poorly cased wells and/or collapse features. When coupled with improper land use decisions these aquifers have become extensively contaminated.
Evaluation of existing or potentially contaminated sites has required development of unique site evaluation techniques or combinations of techniques. Shallow geophysics have been used successfully to determine subsurface karst features, pinpoint locations for additional investigations and evaluate pump-test data. Soil texture, density and relative moisture content have been closely examined and criteria established to evaluate subsurface erosion susceptibility. Percent rock core recovery, mechanical response of drilling and drilling fluid loss have been used to characterize the secondary porosity and potential contamination susceptibility of the carbonate rock. Dye tracing via surface sinks and well injection have determined primary flow paths and have been useful in estimating flow times.
These techniques used solely or in conjunction have been very useful in evaluating contamination existing in the aquifers and as a predictive tool to evaluate the proposed land uses impacts.     

Review of Modeling Approaches to Groundwater Flow in Deformed Carbonate Aquifers

We discuss techniques to represent groundwater flow in carbonate aquifers using the three existing modeling approaches: equivalent porous medium, conduit network, and discrete fracture network. Fractures in faulted stratigraphic successions are characterized by dominant sets of sub-vertical joints. Grid rotation is recommended using the equivalent porous medium to match higher hydraulic conductivity with the dominant orientation of the joints. Modeling carbonate faults with throws greater than approximately 100 m is more challenging. Such faults are characterized by combined conduit-barrier behavior. The barrier behavior can be modeled using the Horizontal Flow Barrier Package with a low-permeability vertical barrier inserted to represent the impediment of horizontal flow in faults characterized by sharp drops of the piezometric surface. Cavities can occur parallel to the strike of normal faults generating channels for the groundwater. In this case, flow models need to account for turbulence using a conduit network approach. Channels need to be embedded in an equivalent porous medium due to cavities a few centimeters large, which are present in carbonate aquifers even in areas characterized by low hydraulic gradients. Discrete fracture network modeling enables representation of individual rock discontinuities in three dimensions. This approach is used in non-heavily karstified aquifers at industrial sites and was recently combined with the equivalent porous medium to simulate diffusivity in the matrix. Following this review, we recommend that the future research combines three practiced modeling approaches: equivalent porous medium, discrete fracture network, and conduit network, in order to capture structural and flow aspects in the modeling of groundwater in carbonate rocks.     

Interpretation of Pumping Tests in Heterogeneous Aquifers with Constant Head Boundary

This paper investigates the impact of heterogeneity of the transmissivity field on the interpretation of steady-state pumping test data from aquifer systems delimited by constant head boundaries such as aquifers adjacent to lakes or rivers. Spatially variable transmissivity fields are randomly generated and used to simulate the drawdown due to a pumping well located at different distances from a constant head boundary. The steady-state drawdown simulated at different observation wells are then interpreted using the Hantush method (Hantush 1959). The numerical simulations show that, in contrast to the case of infinite aquifer domains, the interpreted transmissivity varies depending on well locations and the separation distance between pumping well and boundary relative to the correlation length. The ensemble-averaged estimated transmissivity varies between the geometric mean and the arithmetic mean, and can even exceed the arithmetic mean in a narrow domain adjacent to the boundary. It approaches the geometric mean of the underlying transmissivity field only if the distance between the pumping well is more than 20 times the characteristic length of the transmissivity field.     

Estimating the Horizontal Gradient in Heterogeneous, Unconfined Aquifers: Comparison of Three-Point Schemes

At least two approaches may be used to estimate the horizontal components of the hydraulic gradient based on measured heads from three observation points. First, the gradient may be estimated by passing a plane through the measured heads (h-method). Second, if the elevation of the base of the aquifer is known to be spatially constant, an estimate of the gradient may be obtained using the squares of the measured heads (h²- method). In the present study, these methods are examined in application to a heterogeneous system. Using Monte Carlo analysis, we demonstrate that the magnitude of the gradient estimated via the h-method involved significant bias, which increased when the distance separating the wells increased. In contrast, bias in the estimated magnitude of the gradient based on the h²-method decreased with increasing separation among the wells. Estimation variances for both the magnitude and orientation of the gradient also decreased with separation distance. The variance in the orientation was observed to remain relatively high, however, even at relatively large separations among the wells (e.g., 10 integral scales). These results are Interpreted as implying that the best estimate of the gradient for steady flow in an unconfined aquifer is derived from the h²- method with the wells separated by significant distances. These results also demonstrate the uncertainty inherent in estimating the gradient based on limited field data.     

青海湖沉积碳酸盐矿物之间氧同位素分馏的评估及其环境意义

青海湖沉积岩芯的矿物学和稳定同位素研究结果显示自生碳酸盐矿物以单种或多种矿物形式保存在沉积物中;不同碳酸盐样品的δ18O值的变化范围为5.47‰至-1.04‰,差值最高达6.5‰;δ18O值与总碳酸盐含量呈正相关关系,与碳酸盐矿物种类的改变则无明显关联.结果表明,造成δ18O显著变化的主导因素是湖水氧同位素组成,自然条件下生成的不同碳酸盐之间的氧同位素分馏不明显.说明δ18O是指示闭流湖泊降水蒸发水量平衡变化的敏感指标之一.     

Attenuation of a Mixed Chromium and Chlorinated Etjeme Ground Wate Plume in Estuarine Influenced Glaciated Sediments

The natural attenuation behavior of a ground water contaminant plume containing chromium and chlorinated ethenes in glaciated sediments was assessed using traditional and nontraditional methods. The mixed waste is transported through and attenuated within an estuarine influenced ground water aquifer of spatially varying redox character and organic carbon content. Contaminant fate and speciation were assessed as a function of geochemical conditions. Total, speciation-based, and sequential chemical extraction analyses were performed to determine contaminant partitioning and the redox capacity of the aquifer. Chromium speciation and partitioning were correlated with the reductive capacity and redox conditions of the aquifer sediments spatially distributed within the aquifer. Reductive dechlorination and partitioning of chlorinated ethenes were correlated with the organic carbon content and redox conditions of the aquifer sediments. The data showed that sharp redox gradients existed within the aquifer. Active reduction and retardation of both chromium and chlorinated ethenes was exhibited. The aqueous hexavalent chromium concentrations decreased to near nondetect levels in the vicinity of the receptor, whereas degradation products of higher-order chlorinated ethenes increased as a fraction of the total chlorinated ethene concentrations along the length of the plume. The potential for competition for reducing power under specific cases within the aquifer was suggested by the data, highlighting the need to include contaminant interactions in natural attenuation assessments.     

云南中甸纳帕海古环境演化的有机碳同位素记录

中甸纳帕海位于云贵高原横断山脉腹地,其成因与碳酸岩类地层长期溶蚀有关,在丰水期和枯水期纳帕海的水位相差很大,枯水期常形成浅水池沼,因而造成枯水期和丰水期水生植被的发育状况有很大的差异,枯水期由于水生植物（飘浮和挺水植物）发育,造成总有机碳和氢指数的增加,有机碳同位素偏负,丰水期出现相反的演化趋势,进一步轮推表明在暖干的气候条件下纳帕海极易形成枯水位,在冷湿的气候下可维持相对高的水位,古环境重建表明纳帕海在大约32kaBP和15kaBP出现环境的重大于变迁,32-15kaBP出现高湖面。     

Mass and Momentum Balance Model of a Mixed Flowing/Powder Snow Avalanche

This work uses a one-dimensional, depth averaged model to compute the massbalance of a mixed flowing/powder snow avalanche. This model is comprisedof three basic components: the dense flowing avalanche, the powder cloud anda turbulent wake. The dynamics of a mixed avalanche is strongly dependent onthe interaction between the components and also on the snow cover and ambientair, in particular the exchange or entrainment of snow and air mass. Therefore, animportant first step for modelling mixed avalanches is a basic understanding of thenature of these mass exchanges. In this paper, the governing equations of mass,momentum and turbulent energy are briefly presented. Numerical simulations wererun for three avalanche tracks – Aulta, Galtür and Vallée de la Sionne – for whichdata from real snow avalanche events exist. Based on the results, conclusions weredrawn regarding the parameterisation of the mass exchanges. The mass balances forthese three contrasting avalanches are presented.