Modelling hydrogeology

Two assumptions about the surface have been made to determine the effect of natural attenuation on the contaminated groundwater. First, despite the fractured nature of the bedrock, it has been assumed that the subsurface is homogeneous so as to facilitate the evaluation. Second, the potential for reduction in TCE concentrations has been assessed using a hydrogeologic model in which the fact that the cap would reduce existing leachate production by 75% is taken into account. This model is assumed to predict that the concentration of TCE in the groundwater would be reduced to an excess cancer risk level of 28 pg/L in 60 yr and an excess cancer risk level of 5 pg/L, approximately equal to the MCL, in approximately 100 yr. 

In most instances, hydrodynamic dispersion is not great enough to require detailed consideration in hydrogeologic modeling for fate assessment of deep-well-injected wastes. Flowever, regional variations (such as the presence of an USDW in the same aquifer as the injection zone, as is the case in parts of Florida) should be evaluated before a decision is made to exclude it. 

At the time of the submission of this extended abstract the background research on arsenic mobility is complete and the acquisition of the hydrogeological and geochemical datasets as well as the development of the preliminary conceptual model and hydrogeological framework is... 

A pumping test may be performed to evaluate hydrogeologic characteristics of the aquifer. Derived data can be applied to computer models or at least included in the conceptual site model. 

Sequestration of CO2 in a Depleted Oil Reservoir. This project will investigate down-hole injection of CO2 into depleted oil reservoirs in New Mexico. It will conduct a comprehensive suite of computer simulations, laboratory tests, field measurements, and monitoring efforts to understand the geomechanical, geochemical, and hydrogeologic processes involved. It will also use the observations to calibrate, modify, and validate the modelling and simulation tools. 

The initial and final waters used in these simulations were selected from among the analyses listed in Table 7. Although a number of simulations were made for various different flowpaths deemed likely on the basis of the hydrogeological model, the presentation here is restricted solely to investigations of the evolution of the mature pumped waters at Whittle Drift Borehole (Fig. 6) from a range of possible precursor waters. The mature Whittle Drift pumped water is of Ca- Na SC)4 facies, with a pH around 7.5 and <45 mg/L dissolved Fe. The possible precursor waters were identified as follows ... 

Fig. 13. Evolution of the pe and pH of waters along the flow paths suggested from the hydrogeological analysis of the Cigar Lake site. Thermodynamic database for uranium from Grenthe et al. (1992) and Bruno Puigdomenech (1989). (a) Uranium modelling. Equilibrium assumption. Evolution from borehole 220. (b) Uranium modelling. Equilibrium assumption. System evolving from borehole 211.

Winberg, A. Stevenson, D. 1994. Hydrogeological modelling. In Cramer, J. Smellie, J. (eds) Final Report of the AECL/SKB Cigar Lake Analog Study. Svensk Kambranslehantering, AB, Stockholm, Sweden, SKB TR 94-04, 104-142. 

Norris, R. D. Falotico, R. J. (1994). Modeling of hydrogeological field data for design and optimization ofi situ bioremediation of contaminated aquifers. In Bioremediation Field Experience, ed. P. Flathman et al., pp. 287-307. Boca Raton, FL CRC Press. 

Jakubick, A.T., Jenk, U. and Kahnt, R. (2002) Modelling of mine flooding and consequences in the mine hydrogeological environment flooding of the Koenigstein mine, Germany. Environmental Geology, 42(2-3), 222-34. 

Pham, V.N., Boyer, D., Le Model, J.-L. and Kim Thoa Nguyen, T. (2002) Hydrogeological investigation in the Mekong delta around Ho-Chi-Minh City (South Vietnam) by electric tomography. Comptes Rendus Geoscience, 334(10), 733-40. 

Figure 7.26. Hydrogeologic cross-section of the Devonshire groundwater lens. The X s denote a calculated fit to the 50% isoline of relative salinity (e) using the Ghyben-Herzberg-Dupuit model. (After Vacher, 1974, 1978.)...

Another optimization approach was followed by Wagner [68 ]. Wagner developed a methodology for performing simultaneous model parameter estimation and source characterization, in which he used an inverse model as a non-linear maximum likelihood estimation problem. The hydrogeologic and source parameters were estimated based on hydraulic head and contaminant concentration measurements. In essence, this method is minimizing the following ... 

Historic aerial photographs provide the accuracy and documentation required to compile a precise record of site boundaries, points of access, and adjacent land use. Engineering borings for construction projects in the vicinity of suspected sites are integrated with geologic information to construct reasonable hydrogeologic models to evaluate potential leachate impact on water wells or nearby inhabitants. 

Hydrogeological characteristics of potential sites of the region, assumed depth of the storage placement, as well as possibilities of the PORFLOW program code [16], using which the study was accomplished, allowed us to make up a conceptual representation of the migration model. 

Fig. 8 presents vertical cross-section of the migration model. The imdergroimd water flow and radioactivity spread from the storage is considered in a three-layer model. The model layers have different coefficients of conductivity and porosity. For the layer of placement of the nuclear material storage in the hydrogeological task, condition of zero flow is used [10]. 

Huguet J. M. (2001) Testing and validation of numerical models of groundwater flow, solute transport and chemical reactions in fractured granites a quantitative study of the hydrogeological and hydrochemical impact produced. ENRESA Tech. Publ. 06/2001, 253pp. 

Plummer L. N. (1984) Geochemical modeling a comparison of forward and reverse methods. In First Canadian/American Conference on Hydrogeology. Practical Applications of Ground Water Geochemistry (eds. B. Hitchon and E. 1. Walick). National Water Well Association, Dublin, OH, pp. 149-177. 

As mentioned above, direct groundwater age estimation methods may not be well suited for all study areas or with all groundwater tracers. Often, groundwater age estimates are more easily and correctly achieved through incorporation data for natural tracers in analytical and/or numerical transport models. This is particularly true when multiple sources contribute to particular tracers, or when an accurate hydrogeological representation is needed for complex groundwater systems. 

Figure 14 The conceptual hydrogeological model of the Palmottu, Finland research site. The arrows indicate the measured flow directions, the distribution of groundwater types is shown, and some of the measured and inferred geochemical processes are also indicated (Blomqvist et al., 2000) (reproduced by permission of European Commission from The Palmottu Natural Analogue Project, Phase II Transport of Radionuclides in a Natural Flow...

The application of the above-mentioned modelling techniques allow the assessment of the relative importance of the possible pressure generating mechanisms and the different characteristics of the hydrogeological framework in relation to the observed present-day hydrodynamic conditions in the basin. 

---