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Current conceptual runoff models hypothesize that stormflow generation on the Canadian Shield is a combination of subsurface stormflow and saturation overland flow. This concept was tested during spring runoff in a small (3.3 ha) headwater basin using: (1) isotopic and chemical hydrograph separation and (2) field mapping and direct tracing of saturated areas. Isotopic and chemical hydrograph separation indicated three runoff components: (1) pre-melt subsurface flow; (2) subsurface flow of new (event) water; and (3) direct precipitation on to saturated areas (DPS). During early thaw-freeze cycles, their relative contributions to total flow remained constant (65 per cent, 30 per cent, and 5 per cent respectively). It is hypothesized that lateral flow along the bedrock/mineral soil interface, possibly through macropores, supplied large volumes of subsurface flow (of both old and new water) rapidly to the stream channel. Much higher contributions of DPS were observed during an intensive rain-on-snow event (15 per cent of total flow). Mapping and direct tracing of saturated areas using lithium bromide, suggested that saturated area size was positively correlated to stream discharge but its response lagged behind that of discharge. These observations suggest that the runoff mechanisms, and hence the sources of stream flow, will vary depending on storm characteristics.

Abstract:The Mount Morgan Mine is a historic mine site located in Central Queensland, Australia. Between 1882 and 1981, a total of 7.6 million ounces of gold and 360,000 tonnes of copper were extracted using underground and later open mining methods. The mine closed in 1990 after the re-treatment of 28 Mt of tailings, which were placed into the open cut. Most of the mine waste is acidgenerating and acidic runoff and seepage has heavily impacted portions of the adjacent Dee River.A groundwater flow model was developed for the Mount Morgan mine site to evaluate current seepage conditions and assess closure options. The calibrated groundwater flow model indicates that the backfilled (and flooded) Open Cut/Sandstone Gully represents the largest single source of ARD seepage (8.0 L/s) on the site with tailings impoundments representing important secondary sources of seepage. An estimated 80% of all seepage is collected in a seepage interception system (SIS). The remaining 20% (or ~3 L/s) of ARD impacted seepage by-passes the SIS and enters the Dee River and underlying aquifer.The model predicts that seepage from the open cut would increase exponentially with a further increase in the water level in the open cut. The model suggests that a grout curtain or sealing the upstream side of Sandstone Gully Dam using a "blanket" of low permeability tailings would reduce seepage out of the open cut by about 40%. The amount of seepage reduction in response to placing a dry cover system onto mine waste (tailings and mine rock) is predicted to vary significantly across the site. The modeling results suggest that a combination of rehabilitation measures (including the placement of dry cover system and measures to control seepage out of the flooded Open Cut/Sandstone Gully) will be required to effectively control seepage at Mount Morgan. The calibrated groundwater flow model is currently being used to assess the effects of different closure scenarios (e.g. cover placement versus full relocation) on seepage rates and loading to the Dee River.

Stream water pH may be influenced by (1) the flow paths and (2) the residence time of water that contributes to streamflow, when these hydrologic factors interact with the biogeochemical processes that neutralize H + ions in the catchment. This paper presents measures of the volumes of groundwater contributing to streamflow, the groundwater residence times, and the sources of stream water acidity found during spring runoff in three basins on the Canadian Shield. Isotopic hydrograph separations were used to estimate the relative contributions of groundwater to spring runoff. The contributions of old (premelt) groundwater to spring runoff were greater (60%) in a well-buffered, third-order basin than in a more acidic first-order basin (49%). Using a simple mixing model, a larger groundwater reservoir (420 mm unit depth) and longer residence time (162 days) were estimated in the third-order basin. The lowest stream pH (4.8) was observed in a second-order basin with a wetland that collects drainage from about 79% of the basin. In this basin the principal source of H + ions was the conifer-sphagnum wetland. We conclude that the hypotheses that the pH of these streams was proportional to (1) a fraction of streamflow contributed by groundwater or (2) the residence time of water in a basin are rejected. More attention must be focused upon the source of acidity generated in wetlands, since these are ubiquitous in small basins.Weis, C., Streamflow generation in acidified headwater baSih s during spring runoff: An isotopic and geochemical approach, M.S. thesis, 264 pp., Trent Univ., Peterborough, Ont., Canada, 1989. Weis, C., and K. Devito, Tracing of flow paths in a conifer swamp, paper presented at

Experimental evidence is presented on the influence of specific surface area on the transport of the sorbing tracer strontium in fractures with uniform, but differing aperture. Specific surface area is defined as the ratio of the fracture surface area to the volume of mobile water in the fracture. Static sorption experiments on granite coupons suggest hysteresis in the sorption process, showing higher surface distribution coefficients for desorption than for sorption. Strontium was subject to significantly greater dispersion than the nonreactive tracer tritium. This enhanced dispersion is believed to be the result of chemical heterogeneity at the mineral grain scale, hysteresis in sorption, and limited transverse mixing across the fracture aperture. The influence of fracture aperture on retardation is much greater than predicted by the commonly used definition of the surface retardation factor. Strontium retardation was approximately an order of magnitude greater in a smaller‐aperture fracture (450 µm, Ra ∼ 45) than in a large‐aperture fracture (780 µm, Ra ∼ 3.5). We hypothesize that hysteresis in sorption, in conjunction with limited transverse mixing across the aperture, caused the apparent increase in sorption strength (Ka) with a decrease in fracture aperture.