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Basics of groundwater hydrology
as it relates to brook trout ecology

“Truisms” for shallow groundwater systems:

• Water table roughly mimics the surface topography
• Groundwater temperature mean annual air temperature
• Groundwater discharge and recharge sites in lake and stream beds are spatially variable (topography, substrate composition, stratigraphy, fractures, etc.)

Hydrogeologic controls on sites of groundwater discharge: substrate type (1)

• in situations where soils mantle crystalline bedrock, groundwater may be restricted to relatively shallow flow systems
• actual flowpath followed in these shallow systems is complicated by the potential presence of conductive lenses of material (e.g. sands, gravels)
• in situations where soils mantle fractured bedrock, groundwater may follow much deeper flowpaths

Hydrogeologic controls on sites of groundwater discharge: substrate type (2)

• Large range in K_H for given type of unconsolidated material
• this range sometimes be as great as the range in K_H values between differing material types
• Can lead to great range in estimated groundwater fluxes from Darcy’s Law

Hydrogeologic controls on sites of groundwater discharge: substrate thickness

• the thickness of material overlying a confining layer (such as the bedrock surface) controls the maximum amount of water that can be held in the soil/regolith
• this in turn helps to dictate the duration of groundwater flow from slopes to receiving streams, wetlands and lakes

Topographic controls on sites of groundwater discharge

• at its simplest, groundwater moves from the subsurface to the surface (a process known as exfiltration) when the amount of water being supplied to a given location exceeds the ability of the subsurface material to conduct that amount further downslope
• two conditions that can lead to exfiltration are convergence in plan (the upslope cross sectional area of flow exceeds the downslope area) and convergence in profile (the hydraulic gradient, which in shallow flow systems is roughly parallel to the ground surface gradient, moving water past a particularly point is smaller than the upslope hydraulic gradient)

Hydrogeologic setting within a given landscape

• Lake position may dictate whether it receives inputs from local, intermediate and regional groundwater systems
• Increased water residence time is associated with water moving via regional flow systems
• This generally leads to greater biogeochemical alteration
• Differences in the occurrence and amount of water that a lake receives from local, intermediate and regional flow systems have implications for lake response to disturbance of terrestrial basin (e.g. Devito et al. 2000)
• Thus, forest harvest might be expected to have a greater impact on groundwater fluxes in a local flow system and the lake that receives such fluxes than in the case of a lake that receives most of its groundwater input via a regional flow system

Flow Reversals

• Can occur on a seasonal or event basis
• Result in transition from groundwater discharge to groundwater recharge
• Lakes may shift from being discharge to flow-through systems
• These reversals may help to explain the temporary nature of some groundwater discharge sites in streams and lakes

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