CHAPTER 7: REGIONAL GROUNDWATER FLOW

EASC 304 Lecture Notes
Gwenn Flowers, Simon Fraser University
Courtesy of course creator, Diana Allen

Steady-State Assumptions: Made to analyze regional flow systems, implying minimal fluctuations in the water table.
- This leads to a stable condition in groundwater flow characteristics over time.
Geometric Orientation:
- The analyzed section is perpendicular to the strike of parallel ridges and valleys.
- Geological materials are assumed to be homogeneous and isotropic.
Base Conditions:
- An impermeable base is essential (e.g., crystalline basement rock).

Groundwater Divides: Symmetric characteristics result in the establishment of groundwater divides.
- These divides may coincide with surface water divides, indicating a relationship between surface and groundwater systems.
Water Table Modelling:
- The water table can coincide with the ground surface in valleys, behaving as a subdued replica of surface topography.
Hydraulic Head:
- Head on any equipotential line corresponds to water table elevation where the equipotential intersects, with the condition that Ψ = 0 at the water table, leading to the relationship h = z (hydraulic head equals elevation).
Flow Direction:
- Groundwater flows from higher elevations (recharge areas) to lower elevations (discharge areas), with an upward flow component observed in valleys.

Groundwater has a downward flow component; hydraulic head decreases with depth.
Typically located on topographic highs, with water table positioned at some depth.
Characteristics:
- Net saturated flow directed away from the water table.
- Low concentrations of Total Dissolved Solids (TDS); isotopically young water (dating using H-3, C-14).

Exhibits an upward flow component; hydraulic head increases with depth.
Typically located near topographic lows, with a shallow water table, sometimes above surface.
Characteristics:
- Flow directed toward the water table.
- Features include seepage (e.g., springs) and vegetation indicative of phreatophytes.

Separates the recharge area from the discharge area, crucial for understanding groundwater flow dynamics.

Mountain Block Recharge (MBR): Significant in estimating groundwater movement.
Baseflow (BF): Reflects the seepage of groundwater into surface water bodies.
Recharge to Water Table (R): Indicates the processes replenishing the water table in various geographic settings.
- Example: Lambly (Bear) Creek, located on the west side of Okanagan Lake, serves as a study reference (Welch, PhD 2012).
Mountain Front Recharge (MFR): Explains additional recharge mechanisms at the mountain front interface.

Initial Findings: Tóth (1963) was the pioneering researcher on the impact of hummocky topography on regional groundwater flow.
- Flow Systems Described:
1. Local Flow System: Typically associated with local relief highs.
2. Intermediate Flow System: Acknowledges the transition between local and regional systems.
3. Regional Flow System: Characterized by low relief features and large areas of influence.

Uniform Water Table Characteristics (Figure (a)):
- Displays a gentle incline; single flow system with all upland designated as recharge area.
- Hinge line located on the valley wall.
Undulating Water Table Characteristics (Figure (b)):
- Water table reflects the surface topography, especially visible in glaciated terrains.
- Represents numerous groundwater cells (local flow systems) with recharge/discharge dynamics:
- Proximal in shallow cells; distal in deeper regional flow systems.

Heterogeneity Impact: The presence of geological variability influences groundwater behavior significantly.
- Scenarios Presented:
- (a) Gently sloping water tables, with valley on left-hand side and a high K layer at depth (note tangent law).
- (b) Higher K contrast results in equipotential lines that are nearly horizontal in the upper unit.
- (c) The effect of hummocky terrain truncating local systems.
- (d) High K units can create discharge areas within the middle of the geographical domain.
- (e) Layered inclined heterogeneity fosters complexity in equipotential and flow lines.
Key Takeaway: The interaction of geology and surface topography plays a pivotal role in defining local and regional groundwater flow systems.

Subsystem Flow Contribution: Regional flow from subsystems A and B contributes to discharge into the major valley.
- Calculation of Flow Rates:
  - $Q_A = 2.8 imes 10^{-3} ext{ m}^3/ ext{s}$
  - $Q_B = 2.0 imes 10^{-4} ext{ m}^3/ ext{s}$
- These calculations require known gradients, hydraulic conductivity (K) values, and the dimensions of the domain involved.

Dynamic Equilibrium Considerations: Groundwater systems are often viewed as being in dynamic equilibrium.
- Perturbations: Any disturbances to this equilibrium result in adjustments in the water table position and flow directions.
- Restoration to New Equilibrium: This transition to a new equilibrium can occur over varying timelines ranging from days to centuries.

Definition and Types:
- Non-cyclical groundwater refers to water that is not part of the hydrologic cycle, encompassing:
- Fossil Water: Groundwater that has been isolated from the atmosphere for an extended duration.
- Connate Water: Pore water that has been trapped during geological burial processes.
- Magmatic Water: A unique category of water that may never have been in contact with the atmosphere, offering insight into geological processes.