Applying Hydrogeochemical Trend Analysis (HGTA) Method to Environmental Impact Assessments of Water Resources

Tedros Tesfay (Ph. D.), Senior Project Geochemist, Aqueous Geochemistry, Water Resources, Metals and Mining.

In hydraulically interconnected aquifer units or connected aquifer-surface water systems, incremental changes in groundwater chemistry follow a predicted pattern (Fig. 1). Understanding this pattern provides insight into the source-pathways-receptor relationships. HGTA decodes the relative proportional relationship of major ions through numerous bivariate and multivariate compositional and statistical analyses to find the hidden patterns and trends. The method reveals unique chemical signatures that serve as inherent tracers among hydraulically interconnected sampling points.  

Figure 1. Monitoring wells screened to different lithologies show a small difference in regional groundwater levels but geochemically distinct flow path lines (adapted from Mazor, 2004).

In the mining industry significant progress has been made, over the last two-three decades, on characterizing point and nonpoint sources of contamination and establishment of risk-based levels for various receptors. Despite all this, out of ~ 200,000 inactive and abandoned mines nationwide, about 20 % of the mining facilities inspected by EPA and the States between August 1990 and August 1995 were violating one or more of the following environmental regulations: the Clean Water Act, the Clean Air Act, or the Resource Conservation and Recovery Act (EPA, 1997a).

The complexity (e.g. heterogeneity and anisotropy of aquifers) of the subsurface is one of the biggest challenges to simultaneously promote both mining-driven economic growth and environmental protection (NRC, 1994). Geochemical and physical properties (hydraulic connectivity) of the subsurface control the behavior and mobility of dissolved contaminants.

Application of HGTA method to an anonymous phosphate mine, with a focus on the fate and transport of selenium, is provided below as a case study. 

Figure 2. Illustration of the phosphate mine, pit lake, and monitoring Wells 1 and 2 screened in two geochemically and lithologically distinct subsurface groups (source: anonymous project)

HGTA method was used to evaluate the hydraulic connection of the pit lake with the underlying groundwater within the two distinct lithologies (Fig. 2). Well 1 has shown strong interaction with the pit lake, consistent with its shallow groundwater level. However, unlike to that of the pit lake, selenium’s concentration is below detection limit. Well 2 has no or insignificant hydraulic connection with both the pit lake and Well 1, as indicated by the deeper water level. Paradoxically, its selenium concentration is lower than that of the pit lake but still higher than the chronic aquatic screening level.

Geochemical evaluations of chemical data collected from the surface water, vadose zone and groundwater wells demonstrated that sulfate and selenium have a common source, most likely oxidation of selenium bearing sulfide minerals. Field measured redox potential, consistent with the lithologic composition, indicated that Well 1 is screened within a strong reducing environment. Geochemical modeling (PHREEQC) demonstrated that the fate and transport of selenium is controlled by the abundance of electron donors that converted oxidized forms of selenium into immobile reduced forms. On the other hand, Well 2 is screened within a fractured carbonate aquifer with an oxidizing environment and its groundwater level reflects regional water table. Water quality of Well 2 shows the occurrence of seepage from Waste Rock Dumb 2 but has shown negligible interaction with the pit lake and Well 1. The findings support waste rock dump management that limits exposure to oxygen and water.

Application of HGTA method to understand the geochemically and hydrogeologically distinct flow path lines of clean and mine waters is essential for all phases of a mine project. It substantiates development of a sound environmental impact assessments, economically feasible and environmentally safe mineral processing methods and waste rock and tailing management plans.

References:

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.

Parkhurst, D.L. and Appelo, C.A.J. 1999. User's guide to PHREEQC (Version 2) - A Computer Program for Speciation, Batch-Reaction, One-Dimensional Transport, and Inverse Geochemical Calculations. United States Geological Survey, Water-Resource Investigation Report 99-4259. 312 p.

USEPA. 1997a. EPA’s National Hardrock Mining Framework. Of?ce of Water: Washington, DC, EPA 833-B-97-003.