Groundwater Aquifers, Pumping Effects, and Fluvial Dynamics

Groundwater Aquifers and Definitions

What is an Aquifer?

An aquifer is a geologic formation capable of storing and transmitting enough water to supply wells.

Essential Requirements for Aquifers

1. Must be below the water table.
2. Must have sufficient pore space (porosity) to hold water.
3. Must allow water flow (sufficient permeability).
4. Must receive a sufficient amount of recharge (typically via infiltration of precipitation).

Types of Aquifers

Unconfined Aquifer

• The water table often intersects stream channels.
• The unsaturated zone directly recharges the saturated zone (no impermeable layer capping it).

Perched Aquifer

A perched aquifer is localized and occurs due to variations in the porosity, permeability, and properties of soil, sediment, and rock.

• An aquiclude (an impermeable layer) traps water above the main water table.
• This may form a perched water table.
• This geological structure could result in a spring emerging from the side of a mountain.

Note: Beneath the surface, different volumes of material exist, possessing varying properties in terms of porosity and permeability.

Confined Aquifer

A confined aquifer is bounded above and below by impermeable layers (aquicludes or aquitards).

Groundwater Pressure and Well Dynamics

Piezometric Surface

• When a well is dug into a confined aquifer, water rises up to the level of the piezometric surface (potentiometric surface).
• Confined aquifers are under greater pressure, allowing water to rise higher than the top of the aquifer itself (potentially creating an artesian well).
• In contrast, water in an unconfined aquifer only reaches the level of the water table.

Cone of Depression

The cone of depression is a localized drop in the water table or piezometric surface caused by pumping.

• It occurs when water is pumped out of a well faster than it can flow back in.
• If pumping rates are excessive, the water table is significantly lowered, potentially causing the well to run dry.
• This phenomenon is common in areas where groundwater resources are heavily relied upon.

Fluvial Geomorphology: The Lane Balance

The Lane Balance describes the relationship between sediment load and stream power, determining whether a stream channel experiences net deposition or net erosion.

Conditions Leading to Deposition

Deposition occurs when there is a decrease in stream power (e.g., decreased slope or discharge) or an increase in resistance to transport (e.g., increased sediment size or supply volume).

Causes of Deposition

• Increase in Sediment Supply: Resulting from activities like mining, clear-cutting, or forest fires.
• Change to Drier Climate: Leading to reduced discharge.
• Dam Construction: Decreases the slope in the reservoir area. Sediment builds up upstream of the dam, creating a shallower slope profile.

Conditions Leading to Erosion

Erosion occurs when there is an increase in stream power (e.g., increased slope or discharge) or a decrease in resistance to transport (e.g., reduced sediment size or supply volume).

Causes of Erosion

• Decrease in Sediment Supply: Often resulting from dam retention (trapping sediment upstream).
• Change to Wetter or Stormier Climate: Leading to increased discharge.
• Increased Slope: Caused by accelerated mountain building, avulsion (sudden change in river course), or dam removal.

Impacts of Excessive Groundwater Pumping

Excessive groundwater withdrawal, often referred to as groundwater mining, poses significant environmental and resource management challenges.

Key Issues Associated with Over-Pumping

• Significant decline in the water level (water table drawdown).
• Potential for severe water quality issues (e.g., saltwater intrusion or contamination migration).
• Ability to reverse the natural flow direction of groundwater, altering contaminant pathways.