Groundwater and surface-subsurface exchange

Simplified models of watersheds often show groundwater as a one-way path from deep percolation to surface water exports. However, research in recent decades has demonstrated the importance of bi-directional movement between surface and ground waters in determining both water quality and quantity in exports from watersheds, highlighting the importance in understanding the recharge of groundwater from both soil water and surface water sources at multiple scales of exchange. This module will work through the fundamentals of aquifers and water movement below the water table in the context of both baseflow generation and surface-subsurface exchanges (3:17 min).

Characterizing water in the subsurface first requires a review of the ratios of volumes we typically use to quantify the amount of pore space available for water and the amount of water relative the total amount of substrate. Under saturated conditions, water content and porosity are equal and below the water table we are thinking about water under positive pressure heads (12:26 min).

Based on these fundamentals, let's review the basic organization of subsurface waters with the water table being the demarcation between water under tension and water under pressure. In this module, we will be focusing on water storage and movement in aquifers which is by definition the water under positive pressure heads (8:37 min).

The context of unconfined vs. confined aquifers strongly influences the dominant mechanisms of water storage and the conceptualization of energy and water movement through the subsurface. Getting familiar with the basic context of storage in unconfined and confined aquifers is important toward thinking about their respective behavior (9:43 min).

Darcy's Law is the dominant theory for describing water movement through porous substrate. When considering water movement in an aquifer, Darcy's Law is simplified relative to its application to soil water because the hydraulic conductivity is a constant equal to saturated conductivity. As discussed in watershed delineation, we often make the assumption that the direction of water movement will be along the steepest hydraulic gradient. In aquifer flow, this introduces a three-dimensional perspective on the flow net. We can use the principles of Darcy's Law to establish a vertical perspective on subsurface flow nets in a simple example of an aquifer with uniform thickness and hydraulic gradient (11:13 min).

Detailed derivation of subsurface flow nets is a common topic in full classes on hydrogeology, and is outside the scope of this class. However, an appreciation for the mechanisms that drive curvature in flow nets is important to interpreting piezometric surfaces and flow net profiles you might encounter from hydrogeologic studies. Let's start with the potential for variation in subsurface topography to influence curvature in flow net profiles (6:14 min).

The surface topography created by watershed landform development also tends to create a nested distribution of groundwater flow paths, with local groundwater flow paths driven by local surface topography nested within intermediate and regional groundwater flow paths driven by larger-scale variation in surface topography (5:25 min).

Finally, variation in hydraulic conductivity in the subsurface also tends to bend flow nets, with flow lines converging on higher conductivity areas and flow lines diverging around lower conductivity areas (8:41 min).

The combination of surface water features and groundwater flow nets allows for interpretation of surface-subsurface exchange across the landscape. Let's get some practice visualizing how convergence and divergence in subsurface flow nets around streams and rivers suggests the nature of surface-subsurface exchange (6:31 min).

Map of piezometric surface of the high plains aquifer used in the above video.

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Subsurface flow nets in profile also demonstrate the distribution of surface-subsurface exchange along streams and rivers. Let's review a special kind of well called a piezometer and how they are used to characterize the potential for gaining and losing conditions in streams (5:44 min).

Stream depletion zones as defined by Montana water law are a rare case of water regulation recognizing the inherent connections between ground and surface waters. We can use stream depletion zones as a context to understand cones of depression in the piezometric surface created around pumped production wells and how the intersection of a cone of depression with a river might trigger stricter restrictions on water usage from exempt wells (6:42 min).

Complexity in stream and river channel structure leads to relatively small scales of surface subsurface exchange. Together with the larger scale drivers of exchange discussed so far, the collection of flow paths between surface and subsurface waters ultimately lead to a combination of surface water turnover and tailing in residence time distributions (6:24 min).

The simplified perspective of Darcy's Law in saturated conditions allow for relatively straightforward estimates of water flux and average water velocity in aquifer flows. Working through examples of the calculation of flux and velocity in aquifers helps solidify the concepts of flow in porous media (9:05min)

Study guide

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Study guides are designed to summarize the vocabulary, concepts, and mathematics learned in this module.

Readings from Dingman (3rd ed)

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A list of associated readings from Physical Hydrology by S. Lawrence Dingman (3rd edition)

Slides used in videos

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Laboratory preparation materials for this module

Link to online laboratory preparation materials

Video on exempt wells in Montana

This video outlines the contentious issue of exempt domestic wells in Montana, where conflict over limited water sources for both human and ecosystem use are likely to continue to expand.

US Geological Survey information on groundwater

The following is a useful summary reference covering the basic concepts of hydrogeology

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The following web page is a portal to USGS groundwater content

USGS groundwater basics web page

Montana Bureau of Mines and Geology groundwater programs

The following groundwater study in the Helena area includes some figures used as examples in lecture

Montana Bureau of Mines and Geology (MBMG) Scratchgravel Hills project report

The Groundwater Investigations Program (GWIP) takes on specific groundwater investigations across the state nominated by stakeholders needing support in understanding groundwater resources in their area

MBMG GWIP web page

The Groundwater Assessment Program (GWAP) runs a long term monitoring program for groundwater quantity and quality across the state

MBMG GWAP web page

The Groundwater Information Center (GWIC) manages the data collected from GWIP projects and GWAP monitoring, making it available to the public. Well logs that are required for all wells drilled in the state and are also available in the GWIC database. The well logs used in the laboratory activity were obtained from this database.

MBMG GWIC web page