Hydrogeological and geochemical controls on trace metal concentrations in lake sediment in the Holyrood granite

MacLeod, Robert (1992) Hydrogeological and geochemical controls on trace metal concentrations in lake sediment in the Holyrood granite. Masters thesis, Memorial University of Newfoundland.

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Abstract

A regional lake/pond sediment survey over the Avalon Peninsula in 1976, by the Nfld. Department of Mines, showed a number of lakes with anomalous concentrations of uranium and other metals in the area of the Holyrood Pluton. These anomalies are not reflected in the average host granite rock geochemistry. The hydrogeological framework, including detailed analysis of the fracture systems as major groundwater conduits, has been studied in an attempt to determine the role of groundwaters in transporting and localizing concentrations of trace metals in lake sediment in four lakes in the area. -- Groundwater discharge into the lakes was estimated to range from 20 to 35 percent of their water balances, based on characteristic chemical differences between groundwater and surface waters. Detailed sampling of lake sediment, on a grid pattern, showed a non-uniform areal distribution of metals in these lakes. This sampling also showed that peak concentrations were not restricted to the centres or deepest points in the lakes. The maximum concentration of uranium found in these lakes ranged from 59 to 309 ppm, which is higher than those recorded in the regional survey. In some cases, the elongated shape of the uranium-rich area of sediment is aligned with the orientation of one of the major fracture sets in the study area. Sampling of vertical sediment profiles showed a considerable variation of metals in the sediment column. However there appeared to be no direct relationship between metal concentration in surface sediment and maximum concentrations at depth. Although concentrations of most metals in the sediment are near levels expected for a detrital granite-source origin, anomalous concentrations of uranium at depth in the cores appear to be associated with a rapid change in organic content. A mechanism is proposed whereby mobile dissolved uranium in oxidizing groundwaters is reduced to its tetravalent state when it encounters organic rich mud as it discharges into a lake. The magnitude of seepage flux values and uranium concentrations found in deep groundwaters in the granite suggest that groundwaters are a possible mechanism for the transport concentration of metals in these post-glacial lake sediments.

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