UNDERSTANDING LOCAL HYDRAULIC CONNECTIVITY AND SUBSURFACE FLOW PATHS TO OPTIMIZE EXISTING PLUME CONTAINMENT METHODS

Tedros Tesfay (Ph. D.)

Conventionally, hydraulic connectivity among a network of wells is verified by aquifer tests, tracer tests, lithologic correlations, etc. These are expensive experiments and/or may not represent ambient groundwater conditions. The data collected are also fragmentary by nature and scaling up of test results must be presumed to represent a larger domain. Moreover, regional hydraulic gradients computed from the test results may not capture local preferential flow paths due to aquifer heterogeneity and anisotropy. The complexity of the subsurface and the difficulty of characterizing it are some of the major causes for failing to meet remediation goals (NRC, 1994). Containment remedial systems, such as hydraulic cage, pump and treat systems, are commonly evaluated and optimized through analytical and numerical solute transport modelling. However, the level of uncertainties associated with the simulated results makes it difficult to quantify a small but chemically impactive leakage of water through the systems. Hence, a complementary or alternative approach of understanding local hydraulic connectivity is deemed necessary.

Stiff (1951), Piper (1953) and Mazor (2004) have successfully shown, through various graphics, how regional groundwater chemistry evolves from source to downgradient zones. Source water chemistry, mixing of distinct waters, and water-rock interactions are some of the major underlying processes for a systematic change in chemical composition. However, due to aquifer heterogeneity and interrelated processes (e.g. dissolution and precipitation reactions, ion-exchange reactions, etc.), chemically distinctive flow paths may be concealed locally.

A Hydro-Geochemical Trend Analysis (HGTA) method was developed to understand local hydraulic connectivity and was applied successfully to multiple projects. HGTA decodes the relative proportional relationship of major ions through numerous bivariate and multivariate compositional and statistical analyses to find hidden patterns and trends. The method reveals unique chemical signatures that serve as natural tracers among hydraulically interconnected wells. Delineating geochemically and hydrogeologically distinct local flow path lines supports optimization of existing remedial methods. For example, a leakage from containment systems via preferential flow paths could be identified and substantiate appropriate corrective actions to meet clean up goals.

The HGTA method also characterizes mixing of distinct water sources (e.g. surface water-groundwater, tailing-groundwater, mining pit lake-groundwater, etc.). Geochemical modeling and statistical programs play vital roles in identifying governing geochemical processes that affect all dissolved analytes alike, including constituents of interest. This low-cost method is verified through coherence of various independent data sets collected from the same wells (e.g. plume configuration, hydraulic heads/gradients, field parameters, stable and radiogenic isotopes, etc.).

The limitations of this approach include the reliance on dissolved constituents and, thus, effects of contaminant transport by diffusion will not be known. As with other methods, errors associated with the collection and analysis of the data incur unknown level of uncertainty.

Mazor, E. 2004. Applied Chemical and Isotopic Groundwater Hydrology, (1997, third edition 2004): Marcel Dekker Publ.; New York, 453 pp.

NRC (National research Council). 1994. Alternatives for Groundwater Cleanup. Washington DC: National Press.

Piper A.M., 1953. A graphic procedure in the geochemical interpretation of water analyses [M]. U.S. Geol. Survey, Groundwater. No. 12.

Stiff, H.A., Jr., 1951. The interpretation of chemical water analysis by means of patterns: Journal of Petroleum Technology, v. 3. no. 10, p. 15-17.