ESCI 121 Lecture 19: Comprehensive Study Guide on Groundwater and Subsurface Hydrology

The Importance and Scope of Groundwater

• Availability and Abundance     * Groundwater is the most abundant and readily available source of liquid freshwater on Earth.     * In the United States, the use totals approximately $576 \times 10^6\,\text{gallons/day}$.

• Primary Uses     * Public and domestic water supply.     * Agricultural needs, including irrigation and livestock (representing approximately $78\%$ of usage).     * Industry and mining (representing approximately $12\%$ and $6\%$ respectively).

• Ecological and Geological Importance     * Feeds rivers during periods of no precipitation to maintain base flow.     * Facilitates the creation of caves and unique landscape features through dissolution.     * Serves as a primary source of fresh water via wells for drinking and irrigation.

• Associated Risks     * Excessive removal leading to depletion.     * Contamination from various pollutants.

Storage and Transmission: Porosity and Permeability

• Definition of Groundwater Placement     * Water is stored in and transported through open spaces (voids) underground.     * These spaces include pore spaces between clastic grains in unconsolidated sediment and sedimentary rocks.     * Water also resides in cracks, fractures, and solution cavities (caves) in limestone.

• Porosity: The Storage Capacity     * Porosity is defined as the volume of pore space divided by the total volume of the rock, soil, or sediment.     * $\text{Porosity} = \frac{\text{Volume of Pore Space}}{\text{Total Volume}}$     * It typically ranges from less than $1\%$ to over $30\%$ of the total volume.     * Factors affecting porosity:         * Sorting: Well-sorted sediment (equally sized grains) has higher porosity than poorly sorted sediment (where smaller grains fill the gaps between larger ones).         * Cementation: In rocks like sandstone, cement fills pore spaces, significantly reducing porosity.

• Permeability: The Ease of Flow     * Permeability is a measure of a material's ability (soil, sand, rock) to allow a fluid to pass through it.     * It depends on the connectivity of the pores.     * Distinction: Porosity determines the amount of storage, while permeability controls the movement of groundwater.

Underground Water Distribution and Movement

• Infiltration and Recharge     * Rainfall follows three paths: runoff, evapotranspiration, or soaking into the ground (infiltration).     * Groundwater Recharge: Any process that adds water to the groundwater system.

• Zonation     * Unsaturated Zone (Zone of Aeration): The upper region where pore spaces contain both air and water.     * Saturated Zone: The region where all pore spaces are completely filled with water.     * Water Table: The upper limit of the zone of saturation.

• The Water Table Surface     * The water table is not flat; it is usually a subdued replica of the surface topography.     * Its depth is highly variable depending on the location, season, and annual precipitation.     * The water level in a well corresponds directly to the level of the water table.

• Interaction with Surface Water     * Where the water table intersects the ground surface, bodies of water (streams, lakes, springs) appear.     * Gaining Streams: Streams that receive water from the groundwater system.     * Losing Streams: Streams that provide water to the groundwater system.

Mechanics of Groundwater Flow

• Driving Forces     * Groundwater movement is governed by gravity and pressure.     * Water moves from areas where the water table is high to areas where it is low (high pressure to low pressure).     * Flow paths typically curve; water moves toward lower pressure zones near streams, sometimes moving upward into the stream bed.

• Hydraulic Metrics     * Hydraulic Head ($h$): The difference in elevation between two parts of a water table ($h_1 - h_2$).     * Hydraulic Gradient: The slope of the water table, calculated as:     $\text{Hydraulic Gradient} = \frac{h_1 - h_2}{d}$     where $d$ is the horizontal distance.

• Darcy’s Law     * Established by Henri Darcy, this law determines the flow rate ($Q$).     * $Q = K \times A \times \frac{h_1 - h_2}{d}$     * In this formula:         * $Q$ is the flow rate.         * $K$ is hydraulic conductivity (permeability).         * $A$ is the cross-sectional area of flow.         * $\frac{h_1 - h_2}{d}$ is the hydraulic gradient.

• Velocity of Flow     * Groundwater movement in granular material is exceedingly slow, typically a few centimeters per day.     * Velocity increases as the slope (hydraulic gradient) of the water table increases, provided permeability is uniform.

Geologic Controls: Aquifers and Aquitards

• Classification of Materials     * Aquifer: A rock or sediment layer that can hold and transmit water effectively (e.g., gravel, sand, sandstone, conglomerate).     * Aquitard (or Aquiclude): A material through which water cannot flow easily (e.g., clay, shale, certain igneous or metamorphic rocks).

• Types of Aquifers     * Unconfined Aquifer: Has direct access to the surface; water in a well rises to the water table level.     * Perched Aquifer: Occurs locally above an aquitard within the unsaturated (vadose) zone.     * Confined Aquifer: Surrounded by aquitards; often recharged in highland areas where rainfall is higher.

• Artesian Systems     * Water in a confined aquifer is often under pressure.     * Artesian Well: A well where water rises above the top of the aquifer.     * Flowing Artesian Well: Water flows above the land surface if the potentiometric surface (pressure surface) is higher than the ground.     * City water systems are engineered as artificial artesian systems using water towers.

Environmental Consequences of Excessive Pumping

• Drawdown and Cones of Depression     * Pumping creates a "cone of depression" in the water table around a well.     * The shape and size depend on the rate of withdrawal versus the rate of recharge.

• Ground Subsidence     * Excessive removal of water causes sediment to compact, leading to the sinking of the ground surface.     * San Joaquin Valley, CA: Subsided over $8\,\text{meters}$ ($28\,\text{feet}$) between the 1920s and 1970 due to agricultural irrigation, with drops up to $1\,\text{foot/year}$.     * Notable subsidence has also occurred in Las Vegas.

• Aquifer Depletion     * High Plains (Ogallala) Aquifer: A major case of overuse over $100\,\text{years}$. Water levels have declined by more than $50\,\text{meters}$ in some areas. It feeds approximately $170,000$ wells in the Texas Panhandle alone.

• Saltwater Intrusion     * In coastal areas, excessive freshwater pumping causes a "cone of ascension" where underlying saltwater rises into the freshwater well.

Groundwater Quality and Contamination

• Dissolved Substances     * Water is a universal solvent.     * Total Dissolved Solids (TDS): Freshwater typically contains ~$1000\,\text{ppm}$. Seawater contains ~$35,000\,\text{ppm}$. Water becomes unpotable to taste at ~$2000\text{--}3000\,\text{ppm}$.

• Pollution Sources     * Point Sources: Identified, single locations (e.g., sewer outlets, factory discharge, leaky fuel tanks, abandoned mines).     * Non-Point Sources: Diffuse, unidentifiable locations (e.g., agricultural runoff with pesticides/fertilizers, lawn insecticides, detergent from car washing).

• Types of Pollutants     * Inorganic Chemicals: Heavy metals like Arsenic, Cadmium, Copper, Chromium, Fluoride, Lead, and Mercury.     * Organic Chemicals: PCBs, dioxins, halogenated solvents (dry cleaning), and pesticides like DDT.     * Radioactive Waste: From weapons manufacturing and power plants.     * Acid Drainage: From coal and sulfide ore mine waste.

• EPA Standards for Inorganics (in ppm or mg/L)     * Arsenic: $0.01$ (causes paralysis).     * Cadmium: $0.005$ (kidney damage).     * Lead: $0.015$ (affects nervous system/kidneys).     * Mercury: $0.002$ ($2\,\text{ppb}$).     * Fluoride: $4.0$ (mottles tooth enamel).

• Natural Pollution and Nuisances     * Leaching: Bedrock can naturally contain arsenic.     * Hard Water: High concentrations of dissolved Calcium and Magnesium; prevents soap suds and creates scales in pipes.     * Iron: Causes reddish stains on fixtures and clothing, especially in acidic water.

Geological Phenomena: Karst and Thermal Features

• Limestone Dissolution     * Rainwater is slightly acidic due to dissolved $CO_2$.     * $H_2O + CO_2 \rightarrow H_2CO_3$ (Carbonic Acid)     * $H_2CO_3 + CaCO_3 \rightarrow 2Ca^{2+} + HCO_3^-$

• Karst Topography     * A landscape characterized by sinkholes, sinking streams, and caves.     * Sinkholes: Formed by the collapse of cave roofs or gradual overburden collapse.     * Stages: Early phases involve sinkhole formation; later phases leave isolated rock towers of less-soluble material.

• Significant Cave Systems     * Mammoth Cave (KY): World's longest cave, over $425\,\text{miles}$ ($685\,\text{km}$).     * Jewel Cave (SD): $220\,\text{miles}$.     * Wind Cave (SD): $168\,\text{miles}$.     * Lechuguilla Cave (NM): $152\,\text{miles}$, discovered in 1986.     * Carlsbad Caverns (NM): Features the "Big Room" (>8 acres) and a cafeteria seating $750$ people.

• Speleothems (Dripstone)     * Stalactites (hang from ceiling), Stalagmites (grow from floor), and Columns (when they meet).

• Thermal Features     * Hot Springs: Grand Prismatic Spring (Yellowstone) features multicolored rings caused by thermophilic (heat-loving) bacteria.     * Geysers: Intermittent, forceful ejection of superheated groundwater and steam.

• Oases     * Formed in deserts like the Sahara when a confined aquifer, recharged in distant mountains (e.g., Atlas Mountains), reaches the surface.