18.3: Groundwater
- Page ID
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\(\newcommand{\avec}{\mathbf a}\) \(\newcommand{\bvec}{\mathbf b}\) \(\newcommand{\cvec}{\mathbf c}\) \(\newcommand{\dvec}{\mathbf d}\) \(\newcommand{\dtil}{\widetilde{\mathbf d}}\) \(\newcommand{\evec}{\mathbf e}\) \(\newcommand{\fvec}{\mathbf f}\) \(\newcommand{\nvec}{\mathbf n}\) \(\newcommand{\pvec}{\mathbf p}\) \(\newcommand{\qvec}{\mathbf q}\) \(\newcommand{\svec}{\mathbf s}\) \(\newcommand{\tvec}{\mathbf t}\) \(\newcommand{\uvec}{\mathbf u}\) \(\newcommand{\vvec}{\mathbf v}\) \(\newcommand{\wvec}{\mathbf w}\) \(\newcommand{\xvec}{\mathbf x}\) \(\newcommand{\yvec}{\mathbf y}\) \(\newcommand{\zvec}{\mathbf z}\) \(\newcommand{\rvec}{\mathbf r}\) \(\newcommand{\mvec}{\mathbf m}\) \(\newcommand{\zerovec}{\mathbf 0}\) \(\newcommand{\onevec}{\mathbf 1}\) \(\newcommand{\real}{\mathbb R}\) \(\newcommand{\twovec}[2]{\left[\begin{array}{r}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\ctwovec}[2]{\left[\begin{array}{c}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\threevec}[3]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\cthreevec}[3]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\fourvec}[4]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\cfourvec}[4]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\fivevec}[5]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\cfivevec}[5]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\mattwo}[4]{\left[\begin{array}{rr}#1 \amp #2 \\ #3 \amp #4 \\ \end{array}\right]}\) \(\newcommand{\laspan}[1]{\text{Span}\{#1\}}\) \(\newcommand{\bcal}{\cal B}\) \(\newcommand{\ccal}{\cal C}\) \(\newcommand{\scal}{\cal S}\) \(\newcommand{\wcal}{\cal W}\) \(\newcommand{\ecal}{\cal E}\) \(\newcommand{\coords}[2]{\left\{#1\right\}_{#2}}\) \(\newcommand{\gray}[1]{\color{gray}{#1}}\) \(\newcommand{\lgray}[1]{\color{lightgray}{#1}}\) \(\newcommand{\rank}{\operatorname{rank}}\) \(\newcommand{\row}{\text{Row}}\) \(\newcommand{\col}{\text{Col}}\) \(\renewcommand{\row}{\text{Row}}\) \(\newcommand{\nul}{\text{Nul}}\) \(\newcommand{\var}{\text{Var}}\) \(\newcommand{\corr}{\text{corr}}\) \(\newcommand{\len}[1]{\left|#1\right|}\) \(\newcommand{\bbar}{\overline{\bvec}}\) \(\newcommand{\bhat}{\widehat{\bvec}}\) \(\newcommand{\bperp}{\bvec^\perp}\) \(\newcommand{\xhat}{\widehat{\xvec}}\) \(\newcommand{\vhat}{\widehat{\vvec}}\) \(\newcommand{\uhat}{\widehat{\uvec}}\) \(\newcommand{\what}{\widehat{\wvec}}\) \(\newcommand{\Sighat}{\widehat{\Sigma}}\) \(\newcommand{\lt}{<}\) \(\newcommand{\gt}{>}\) \(\newcommand{\amp}{&}\) \(\definecolor{fillinmathshade}{gray}{0.9}\)California's hydrogeology, the study of how water interacts with geological formations, plays a vital role in sustaining its diverse ecosystems and human activities. Groundwater, the water stored beneath the Earth’s surface, is essential for agriculture, industry, and municipal water supplies, especially in a state where surface water resources are highly variable. This section explores the origins, movement, and significance of groundwater within California's complex geological framework.
Aquifers
Groundwater forms as precipitation infiltrates into the ground, filling the spaces within sediments and rocks—spaces known as porosity. While groundwater is sometimes found in large underground caverns, particularly in karst systems, it is far more commonly stored in small, connected pore spaces between grains of sediment. Groundwater must be extracted carefully based on an understanding of both the porosity and permeability of the materials it resides in. Video \(\PageIndex{1}\) demonstrates how porosity and permeability define the characteristics of subsurface water reservoirs, known as aquifers.
Video \(\PageIndex{1}\): "What is an Aquifer?" by Geoscience Videos licensed under CC BY-NC 4.0.
As water seeps into the ground, some of it is held in the unsaturated zone, near the surface, while the rest moves downward into the saturated zone, where all pore spaces are filled with water. The boundary between these two zones is known as the water table. The depth of the water table varies significantly across California, from a few feet below the surface in river valleys to hundreds of feet underground. Seasonal changes and droughts can cause substantial fluctuations in the water table, with droughts sometimes causing it to drop tens of feet.
Groundwater moves slowly, migrating from areas of high pressure to lower pressure, typically from higher to lower elevations. The rate of movement can vary significantly, ranging from more than 1 foot per day to as little as 1 foot per year. In California, much of the state's groundwater is stored in unconsolidated, young alluvial deposits beneath valleys and basins, particularly in regions like the Central Valley. Figure \(\PageIndex{2}\) illustrates the different types of aquifers found throughout the state, including confined and unconfined aquifers.
Modern groundwater extraction primarily relies on aquifers at relatively shallow depths, typically ranging from a few feet to just over a hundred feet. Confined aquifers, situated between layers of impermeable rock or sediment, store water under pressure. When tapped by a well, this pressure can cause water to rise to the surface without pumping, a phenomenon known as an artesian well (Figure \(\PageIndex{3}\)). In contrast, unconfined aquifers are open to the atmosphere through permeable materials, making them easier to recharge through surface water infiltration but also more susceptible to surface contamination. Figure \(\PageIndex{2}\) illustrates the characteristics of both confined and unconfined aquifers, emphasizing the importance of their permeability and pressure dynamics in groundwater management. Groundwater extraction involves drilling wells into the saturated zone of these aquifers.
However, continuous extraction of groundwater, especially during prolonged drought periods, causes the water table to drop. In confined aquifers, this reduction in pressure can eliminate artesian conditions, necessitating the use of mechanical pumps to bring water to the surface. The declining water table is a growing concern across California, particularly in agricultural regions where groundwater serves as a critical water supply.
As the water table decreases, the artesian conditions diminish, necessitating the use of mechanical pumps to lift water to the surface. With continuous declines in the water table and a lack of recharge due to drought, wells can eventually run dry.
California Aquifer Systems
California is home to a variety of aquifer systems, each shaped by the state’s diverse geology and playing a critical role in meeting the water needs of agriculture, municipalities, and industry. However, these aquifer systems face significant challenges, including over-extraction and prolonged drought. Table 18.3.1 provides an overview of California's primary aquifer systems, highlighting total water use and the sectors that consume the most water from each system, measured in billion gallons per day.
| AQUIFER SYSTEM | TOTAL WATER USE (bgd) | PRINCIPAL CONSUMPTIVE WATER USE |
|---|---|---|
| Great Valley Aquifer System (Sacramento Valley, Delta, San Joaquin Valley, and Tulare) | 9.81 | Irrigation (9.91 bgd) |
| Basin and Range Carbonate and Basin Fill Aquifers | 5.70 |
Irrigation (4.55 bgd); |
| Coastal Basin Aquifers (Santa Clara Valley and Southern CA Coastal) | 3.45 |
Irrigation (1.76 bgd); |
| Pacific Northwest Basaltic Rock and Basin Fill Aquifers | 1.34 | Irrigation (1.21 bgd) |
Figure \(\PageIndex{4}\) complements Table 18.3.1 as it shows the approximate locations and spatial distribution of California’s major aquifer systems.
California's aquifers play a crucial role in supplying water for agricultural, municipal, industrial, and environmental needs across the state. These underground reservoirs of water, stored within porous rock formations and sedimentary deposits, vary in size, depth, and geological characteristics, reflecting the diverse hydrogeological conditions present in different regions of the state. This section provides an overview of California's aquifers, including their locations, associated hydrologic regions, and geological attributes.
Basin and Range Basin-Fill Aquifers
The Basin and Range Basin-Fill Aquifers are found primarily in eastern California, stretching from the eastside of the Modoc Plateau, in Nevada, to the Mojave Desert in the southeast of California. These aquifers are composed of unconsolidated to semi-consolidated alluvial deposits, including gravel, sand, silt, and clay, which have accumulated over millions of years from weathering and erosion of adjacent mountains. The permeability and hydraulic conductivity vary widely due to the heterogeneous nature of the deposits, with coarse-grained materials forming high-permeability zones and finer-grained materials creating low-permeability layers. The aquifers are recharged primarily by precipitation and runoff from surrounding mountains and by infiltration from rivers and streams, with discharge occurring through springs, seeps, and pumping. Faulting and tectonic activity influence groundwater movement, while the arid to semi-arid climate results in limited natural recharge. Over-extraction in some areas has led to declining water levels and land subsidence, necessitating careful groundwater management t
Basin and Range Carbonate-Rock Aquifers
The Basin and Range carbonate aquifers of California, predominantly located in the eastern part of the state, north of the Mojave Desert. These aquifers consist of extensive limestone and dolomite formations, which have developed significant secondary porosity and permeability through the processes of dissolution and fracturing. The carbonate rocks form highly productive aquifers with zones of enhanced permeability along faults and fractures, facilitating substantial groundwater flow. Recharge primarily occurs from precipitation and runoff in the surrounding mountainous areas, with infiltration through sinkholes and karst features. Discharge happens through springs, seeps, and wells. The hydrogeology is heavily influenced by faulting and tectonic activity, which create conduits for groundwater movement and storage. The arid to semi-arid climate limits natural recharge, making sustainable management of these aquifers crucial to prevent over- extraction, declining water levels, and land subsidence. Effective groundwater management is essential to balance use with recharge rates, ensuring the long-term viability of these critical water resources.
California Coastal Basin Aquifers
The California Coastal Basin aquifers are located along the state’s coastal regions, including areas from the San Francisco Bay in the north to the Los Angeles and San Diego basins in the south. These aquifers are primarily composed of unconsolidated to semi-consolidated alluvial deposits, including sand, gravel, silt, and clay, which allow for varying degrees of permeability and water storage. In some areas, these aquifers are bounded by impermeable bedrock, creating confined aquifers with significant water storage potential.
Recharge primarily occurs through precipitation, stream infiltration, and artificial recharge methods, though the arid to semi-arid climate in many coastal regions limits natural recharge. Over-extraction of groundwater in these aquifers, driven by urban, industrial, and agricultural demands, has resulted in declining water levels and the associated threat of saltwater intrusion, where seawater encroaches into freshwater aquifers. This issue is particularly prevalent in densely populated areas like the Los Angeles Basin.
The hydrogeology of these aquifers is heavily influenced by tectonic activity, with faults and fractures providing pathways for both water flow and potential contamination. Groundwater management strategies in the Coastal Basin aquifers focus on mitigating over-extraction, addressing saltwater intrusion, and ensuring sustainable use. Techniques such as managed aquifer recharge, desalination, and the use of “guardian wells” to create freshwater barriers have become essential tools for maintaining the viability of these critical water resources in California’s coastal regions.
Central Valley Aquifer System
The Central Valley Aquifer System spans over 450 miles (724 km) from the Cascade Mountains in the north to the Tehachapi Mountains in the south, encompassing the Sacramento River Region, San Joaquin River Region, and Tulare Lake Region. This aquifer system, one of the most important in the United States, is composed of unconsolidated sedimentary deposits, including gravel, sand, silt, and clay, deposited by ancient rivers. These sediments create layers with varying permeability and hydraulic conductivity.
Groundwater extraction from the Central Valley Aquifer System exceeds natural recharge rates, leading to a steady decline in water levels, with some areas seeing drops of over 40 feet. Recharge is primarily dependent on precipitation and overland flow, which varies widely across the state. For example, the arid southern half of the Central Valley receives little recharge, while the northern region benefits from greater precipitation and snowmelt. Despite this, groundwater extraction continues to outpace recharge, raising concerns about sustainability.
Pacific Northwest Basaltic-Rock Aquifers
The Pacific Northwest basaltic-rock aquifers in California are primarily found in the northern part of the state, extending from the Klamath Mountains to the Modoc Plateau. These aquifers are composed of layered basalt flows, which have developed secondary porosity and permeability through fracturing and weathering. The basaltic-rock formations create aquifers with varying degrees of permeability, where joints, fractures, and interflow zones facilitate significant groundwater movement. Recharge occurs mainly from precipitation and runoff in the higher elevations, with infiltration enhanced by the porous and fractured nature of the basalt. Groundwater discharge takes place through springs, streams, and wells. The hydrogeology is influenced by the region's volcanic history and tectonic activity, which shape the distribution and flow of groundwater. Due to the region's climate, natural recharge can be substantial, but localized variations in permeability and groundwater flow paths necessitate careful management. Sustainable management practices are crucial to maintain water levels and prevent over-extraction, ensuring the long-term viability of these important groundwater resources.
Pacific Northwest Basin-Fill Aquifers
The Pacific Northwest basin-fill aquifers in California, located primarily in the northern part of the state, extend from the Klamath Mountains to the Modoc Plateau. These aquifers consist of unconsolidated to semi-consolidated alluvial deposits, including gravel, sand, silt, and clay, accumulated over millions of years from weathering and erosion of the surrounding mountains. The permeability and hydraulic conductivity of these aquifers vary widely, with coarse-grained materials forming high-permeability zones and finer-grained materials creating low-permeability layers. Recharge primarily occurs from precipitation and runoff in the surrounding highlands, with additional infiltration from rivers and streams. Groundwater discharge happens through springs, seeps, and wells. The hydrogeology is influenced by the region's complex depositional history and tectonic activity, which affect groundwater flow and storage. The relatively wetter climate of the Pacific Northwest allows for substantial natural recharge, but localized variations in aquifer properties and groundwater flow paths require careful management.
Groundwater Resources to Water Infrastructure
California's diverse aquifer systems are essential for meeting the state's water needs, but ensuring their sustainability requires an integrated approach to water management. As we have explored, the hydrogeological characteristics of these aquifers dictate their recharge rates, storage capacities, and vulnerabilities, especially in the context of over-extraction and changing climate conditions. To maintain a balanced water supply, California relies not only on its aquifers but also on an extensive water infrastructure system that manages surface water resources. In the next section, we will delve into the state's water infrastructure, including dams, reservoirs, aqueducts, and levees, which work in concert with groundwater management to transport and store water across the state. These systems are vital for supporting agriculture, urban areas, and ecosystems, and their management involves the collaboration of federal, state, and private entities.
References
- California Department of Public Health, 2009, https://www.epa.gov/wqs-tech/water-quality-standards-regulations-california
- California Department of Water Resources, 2003, California’s Groundwater, Bulletin 118, https://water.ca.gov/programs/groundwater-management/bulletin-118
- United States Office of Ground Water and Drinking Water, December 2023, https://www.epa.gov/ground-water-and-drinking-water/national-primary-drinking-water-regulations
- U.S. Geological Survey. (n.d.). General facts and concepts about ground water. In Sustainability of Ground-Water Resources: Circular 1186. Retrieved April 12, 2024 from https://pubs.usgs.gov/circ/circ1186/html/gen_facts.html