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Summary: The physically-based surface–subsurface HydroGeoSphere model is used to examine the hydrologic budget of the 286.6km2 Duffins Creek watershed, located 10km northeast of metropolitan Toronto. The primary objective of this study is to demonstrate the utility of the model to simulate the three-dimensional hydrologic response of the surface and subsurface flow systems in a large-scale watershed driven by multi-season precipitation events. The hydrologic model was calibrated to a consecutive sequence of hydrographs resulting from all precipitation events between the months of April and December 1986, while hydrographs from 1987 were used to test the model’s ability for prediction. Results demonstrate that a physically-based modeling approach which treats the surface and subsurface flow systems as an integrated continuum can capture the dynamic response of a large heterogeneous watershed as driven by transient precipitation events over a multi-season period. It was also revealed that full coupling of plant transpiration and surface–subsurface evaporation processes into the governing flow equations was critical for the successful calibration of the hydrographs. However, the ability of the model to predict the shape of individual hydrographs in the following year, during which the annual average precipitation rate decreased significantly, was diminished even though the annual average stream flows were closely replicated. We identify a number of data gaps that contribute to the decline in the predictive capacity of the model, including difficulty in establishing an appropriate initial condition, vertical mesh refinement issues within the root zone, and the reliance on a largely empirical relationship that distributes the net capacity for plant transpiration among various factors that may change from one year to the next. [Copyright &y& Elsevier]

Abstract: When considering natural attenuation as a remediation strategy at a site contaminated by a light non-aqueous phase liquid (LNAPL), it is important to consider the emission of contaminants from the source zone. A quantification of source-zone emissions is essential both for comparison with down-gradient mass fluxes to provide an estimate of fractional mass flux reduction, as well as for estimating the source lifetime. Because the spatial distribution of LNAPL at a field site is strongly dependent on both the spill circumstances and the heterogeneity of the geologic materials, which can be problematic for in-situ determination, alternative methods for estimating source-zone emissions are needed. In this work, a three-dimensional multiphase flow and transport modelling approach is used to investigate the relationship between the lateral extent of an LNAPL body and the emission of contaminants to groundwater at a contaminated site. For simulations involving an LNAPL release in an aquifer comprised of heterogeneous porosity and permeability distributions that were generated geostatistically, it is shown that a simple linear relationship exists between the lateral extent of the LNAPL body in the capillary fringe and the emission to the aqueous phase. The parameters describing the relationship are found to be linear functions of the groundwater flow velocity and the vertical infiltration rate. This site-specific relationship provides a simple method to estimate contaminant emissions to groundwater at LNAPL contaminated sites. [Copyright &y& Elsevier]

• Large scale GW-lake interactions are evaluated with a fully integrated GW-SW model. • Direct GW discharge ranges from 0.6 % to 1.3 % of positive basin supply for the 5 lakes. • Annual average direct GW discharge to the 5 lakes ranged from 11.6 to 38.6 m3/s. • GW discharge varies with lake level and terrestrial groundwater level fluctuations. • Direct GW discharge is highest in winter when lake levels are at their annual low. Modelling groundwater-surface water (GW-SW) interactions at scales of large river basins is a difficult challenge. In this study, a fully-integrated surface water-groundwater model accounting for hydrologic seasonality is developed for the 766,000 km2 Laurentian Great Lakes basin, and applied towards the characterization of groundwater-lake (GW-lake) interactions in the five Great Lakes under monthly normal climatology. The simulated annual average rates of direct groundwater discharge to Lakes Superior, Michigan, Huron, Erie and Ontario through the combined lakebed and 8 km wide band of shoreline surrounding each lake are 29.0, 38.6, 24.5, 11.9, and 11.6 m3/s, respectively. Thus, direct groundwater discharge accounts for a small component of positive basin supply; ranging from 0.6% for Lake Ontario to 1.3% for Lake Michigan, with an overall average of 0.8% for all lakes combined. Simulation results demonstrate that GW-lake interactions are strongest nearshore, and vary temporally in response to seasonal fluctuations in both lake levels and terrestrial groundwater levels in nearshore regions. In winter, direct groundwater discharge dominates the GW-lake interactions in both the distal and nearshore lakebed areas. In summer, the combined effects of rising lake levels and lowering terrestrial groundwater levels lead to notable reductions in direct groundwater discharge through nearshore areas. Direct groundwater discharge is also shown to vary spatially, with highest rates associated with areas containing thick Phanerozoic hydrostratigraphy, as opposed to Precambrian basement rock. The results from this study indicate that the Great Lakes primarily act as groundwater receivers, gaining considerable amounts of water directly from the basin's groundwater system. [ABSTRACT FROM AUTHOR]