ASSESSING INTER-WELL HYDRAULIC CONNECTIVITY AND SURFACE WATER-GROUNDWATER INTERACTION THROUGH MULTIPLE VERIFICATION METHODS

Tedros Tesfay (Ph. D.), Hydrogeochemical/Hydrogeological Consultant at Inherent Water Tracers Geochemistry, LLC., Water Resources and Water Quality.

A sound understanding of geochemically and hydrogeologically distinct flow path lines is one of the major challenges of subsurface characterization. The complexity of the subsurface lithologies and the financial liabilities associated with management and remediation of surface water-groundwater resources demand use of multiple methods to discern flow paths. Commonly, empirical hydraulic connectivity index based on geology and hydrogeology, pumping tests, natural and artificial tracer tests are employed to understand hydraulic connectivity. Three pioneering project works are selected to demonstrate the consistency of the various independent hydraulic connectivity verification methods (Smith and Hunt, 2009; Oyarzún et al. 2014; Richard et al. 2014). A low-cost hydrogeochemical trend analysis (HGTA) method was also applied to replicate the conclusions reached by the authors. HGTA, developed as an extension of Ph.D. work at UND, Grand forks, North Dakota, decodes the relative proportional relationship of major ions through numerous bivariate and multivariate compositional and statistical analyses. The method reveals unique chemical signatures that serve as inherent well-to-well or local surface water-groundwater tracers.

One of the project works is done by Oyarzún et al. (2014, 2016). They applied various methods to characterize surface water-groundwater interactions in the Grande River basin in Chile (the work was supported by Conicyt under grant Fondecyt 11100040). Surface water-groundwater connectivity was assessed via major ion hydrochemistry, isotopes of water, radiogenic isotope-radon (222Rn), multivariate statistical analysis and hydraulic connectivity index (Oyarzún et al. 2014). For major ions, differences between the samples of the two systems were estimated using the central rhombus of the piper diagram (<20% => high connectivity, 20% - 40% => moderate connectivity and > 40% low connectivity) (Piper, 1951). A similar approach was used for the isotopes of water. Multivariate statistical cluster analysis that groups samples based on the similarity of their hydrochemical data was also employed. The connectivity index (Cl), assigns weighted values, based on four factors: the water-table depth (dw), the river-channel sediments (rs), the dominant geology (ge), and the site geomorphology (gm) (Ransley et al. 2000; Braaten and Gates 2003). The different approaches categorically show, using the samples collected in autumn, summer and winter, the presence of a strong interaction between the surface water and groundwater (Oyarzún et al. 2014, 2016).

Fig.1 Stiff diagrams for surface waters (light grey polygons) and groundwater (black polygons), summer sampling campaigns (Taken from Oyarzún et al. 2016).

The results were replicated using the HGTA method and contribution of groundwater (10% - 13%) at a specific reach (G7) was estimated (Figure 1). Oyarzún et al. (2016) have used 222Rn activities to conclude about a 10% contribution from groundwater for the same reach. The authors believe it could change in time and location as the interaction between the two systems is dynamic (personal communication with Professor Ricardo Oyarzún).

Secondly, Richard et al. (2014) have conducted pumping tests, permeability tests, and geophysical logging along with hydrochemical analysis to assess hydraulic connection between a weathered bedrock and overlying granular aquifers in the Canadian Shield (funded by the Fondation de l’UQAC (FUQAC)). Rapid response of piezometers (PZ-35DD and PZ-104DD) screened in the granular aquifer was observed during pumping of the nearby wells screened in the underlying bedrock aquifer (PZ-35R and PZ-104R) at two out of the three sites (Figure 2). Falling-head permeability tests were conducted to differentiate natural hydraulic connectivity from the hydraulic short-circuit. No hydraulic connection was observed at the third study site. Hydrochemical data provided an independent line of evidence. In the absence of a natural or artificial (defective sealing) hydraulic connection, each aquifer unit is expected to display distinctive geochemical signatures characteristic of the hosting formation. Hydrochemical comparison via Schoeller (1962) diagram indicate that the chemistry of the two distinct samples are similar and represent a mixture of the water of the two formations (Richard et al. 2014).

Fig.2 Conceptual scheme of natural hydraulic connections occurred at larger scale (blue arrows) and artificially (defective sealing at the contact zone) created hydraulic connections (orange dashed arrows) (Modified from Richard et al. (2014).

The hydrochemical data was analyzed using the HGTA method and has replicated hydraulic connections. The chemical signatures indicate a mature interaction between the aquifer units in piezometer PZ-35 (PZ-35DD and PZ-35R). While the chemical signatures share some common patterns in piezometer PZ-104 (PZ-104DD and PZ-104R), the extent of interaction is limited, suggesting perhaps a leakage of water due to a defective sealing consistent with the finding of the authors. 

Finally, the assessment of hydraulic connection between two superposed fractured aquifers, the Edwards and the Trinity Aquifers, in central Texas, is presented here to demonstrate the benefit of applying multiple verification methods. Smith and Hunt (2009) used three Edwards/Trinity well pairs and a deep multiport well (Westbay?) screened at fourteen discrete zones, separated by permanent inflatable packers, to collect representative hydrochemical data and corresponding piezometric heads. Potentiometric heads (the head difference ranges from 50 ft to 160 ft) and hydrochemical facies indicate that the two major aquifers have a negligible hydraulic connection due to the existence of low permeability units in between them and any cross-fault flow is found to be very limited (Figure 3). 

Fig.3 Difference in potentiometric heads and distinction in hydrochemical facies rule out significant hydraulic connection between the Edward and the Trinity aquifers (Modified from Smith and Hunt 2009).

The HGTA method confirms the distinct nature of the chemical signatures of the Edwards and the Trinity Aquifers. They reflect characteristically the lithological composition of the hosting regimes and potentially the influence of recharge for the overlying aquifer.  

Overall, hydrogeochemistry is an underused method, which could provide an independent line of evidence for hydraulic connectivity investigations. Management of water resources and water quality issues are better understood by analyzing water's intrinsic components and more so in hydrogeologically complex conditions (e.g. fractured aquifers). Major ions plus other ions of interest (~90% - 95% of total dissolved solutes) are an integral part of the universal solvent, water. The ions evolve owing to source water chemistry, mixing of distinct waters, and water-rock interactions (Mazor, 2004). Most of the natural processes that release or retain ions are common to all and the effect of some ion-specific processes is limited due to the relatively high-level concentrations of major ions. However, the use of absolute major ion concentrations as tracers is still less attributable due to aquifer heterogeneity and interrelating processes (e.g. dissolution and precipitation reactions, ion-exchange, etc.). Alternatively, getting insight into the relative and proportional existence of the ions, via HGTA and other methods, reveals unique chemical signatures that serve as inherent tracers. As a result, strongly interconnected, seasonally/weakly connected and totally disconnected aquifer units or groundwater-surface water sampling points are identified. The method, as a low-cost pre-screening tool, has multiple practical applications, such as designation of GWUDI (groundwater under the direct influence of surface water contamination sources), understanding of mine water impact to groundwater, coal seam gas related inter-aquifer mixing, amended-water-groundwater interaction, aquifer storage and recovery studies, etc. Spatially and temporally representative end members and robust quality control and quality assurance processes are required, during field sample collection and analysis, for a successful assessment of hydraulic connectivity using dissolved ions.  

References:

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

Oyarzún, Ricardo, Sandro Zambra, Hugo Maturana, Jorge Oyarzún, Evelyn Aguirre & Nicole Kretschmer (2016) Chemical and isotopic assessment of surface water–shallow groundwater interaction in the arid Grande river basin, North-Central Chile, Hydrological Sciences Journal, 61:12, 2193-2204, DOI: 10.1080/02626667.2015.1093635

Oyarzún, R., Barrera, F., Salazar, P. et al.(2014b). Multi-method assessment of connectivity between surface water and shallow groundwater: the case of Limarí River basin, north-central Chile. Hydrogeology Journal, 22, 1857–1873. doi:10.1007/s10040-014-1170-9

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Ransley T, Tottenham R, Sundaram B, Brodie R (2007) Development of method to map potential stream aquifer connectivity: a case study in the Borders rivers catchment. Bureau of Rural Sciences, Dept. of Agriculture, Fisheries and Forestry, Australian Gov., Canberra, Australia

Richard, S.K., Chesnaux, R., Rouleau, A. et al. Field evidence of hydraulic connections between bedrock aquifers and overlying granular aquifers: examples from the Grenville Province of the Canadian Shield. Hydrogeol J (2014) 22: 1889. 

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Smith B. A., Hunt B. B., (2009) Potential hydraulic connections between the Edwards and Trinity aquifers in the Balcones fault zone of Central Texas. South Texas Geol Soc Bull L(2):1–34