Hydrogeochemistry and physical hydrogeology of the Newfoundland Zinc Mine, Daniel's Harbour, Newfoundland

Diaz, Wendy Doreen (1990) Hydrogeochemistry and physical hydrogeology of the Newfoundland Zinc Mine, Daniel's Harbour, Newfoundland. Masters thesis, Memorial University of Newfoundland.

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Abstract

The Newfoundland Zinc Mine, located in a karstified carbonate platform, has large groundwater inflows along joint, fault and bedding planes. Sinkholes are prominent along the hingeline of the dominant anticlinal structure in the area. The karst nature of this aquifer is exhibited by sinkholes and other surface features, as well as by the close correlation of groundwater geochemistry and flow rates with surface recharge fluctuations. Drill logs and previous hydrogeological work suggest that aquifer permeability decreases with depth and has both diffuse and conduit elements. -- Measured fracture orientations indicate t o dominant fracture plane strike and dip orientations of 039/89 and 310/88. The near horizontal bedding planes form an important third set due to their ability to conduct large volumes of water. The mine drawdown cone shape established by mine dewatering is a manifestation of underground workings and dewatering operations. Irregularities in its shape and anisotropy observed in pump tests may be caused by the orientation of these two fracture sets and faults with similar orientations. The highly variable tritium values (1-41 TU) and atypical mid-depth groundwater chemistry may be the result of localized fracture flow dominated by these sets. Local flow lines are a function of the mine drawdown and the topography. -- Groundwaters are Ca-HCO₃ and Na-Cl type waters and most are of a meteoric origin. The δ¹⁸O vs δ²H data indicate an evaporated surface water contribution to some groundwater samples and suggest a nearby lake as a source of shallow mine inflows. Apparent tritium ages indicate portions of these groundwaters have been recharged in the last 33 years. Inflows collected from different mine levels (L-Zone and T-Zone) are of two types 1) oxidized shallow inflows with low total dissolved solids and chloride (<40 ppm C1) that may carry up to 4457 ppb zinc and 2) reducing, deeper saline groundwaters having high chloride (114-990 ppm) and sulphate (170-370 ppm) concentrations. Dissolution of gypsum or other soluble minerals locally may explain the salt content of some shallow groundwaters. The δ¹³C vs DIC data suggest different evolutionary paths for the shallow and deep groundwaters and imply mixing of relict seawater with deep groundwater during the evolution of T-Zone groundwater. -- Geochemical modelling indicates shallow groundwater evolution requires the dissolution of dolomite first under open or closed pCO₂ system conditions. This suggests recharge areas for this local flow system must be underlain by dolostone. Groundwater undersaturated with respect to carbonate minerals may reflect greater mixing with rainwater and/or saline groundwater, or slow kinetics of mineral dissolution. The decrease in δ¹⁸O with increasing Cl in deeper waters suggests they are a mixture of seawater and regional groundwater recharged at higher elevations. The δ¹⁸O values inferred for the meteoric component of the deep saline water implies that recharge occurred during the last postglacial epoch.

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