Advanced Hydrogeology Course Notes

Surface Water Hydrology and Advanced Hydrogeology Course Notes

This course note covers advanced hydrogeology topics.
The instructor is Sileshi Mamo (PhD).
The year of the course note is 2018.

Types of Water in Soils/Rocks

Water in soils and rocks exists in different forms.
Bounded/Retained Water: Water attached to soil or rock particles by electromagnetic forces. Two types include:
- Hygroscopic water: Fills the micropores and irregularities on particle surfaces through adsorption.
- Pellicular water: Forms a thin film around soil particles and flows from denser to thinner films.
Capillary Water: Partially or completely fills pores or sub-capillary channels of soils and rocks due to surface tension. Two types include:
- Isolated/Perched Capillary: Located above the water table and capillary fringe. It occupies partially capillary voids and can't be removed by gravity.
- Continuous/Sustained Capillary: Located in the capillary fringe. It fills all capillary voids just above the water table and is influenced by gravity. Classified as free water.
Free/Gravitational Water: Not subject to attraction toward soil/rock surfaces. Moves by gravity and is subjected to hydrostatic pressure. This is the most active part of groundwater.

Distribution of Subsurface Water

Unsaturated Zone (Vadose or Aeration Zone): Located between the land surface and the water table. Subdivisions include the soil water zone, intermediate zone, and capillary fringe.
Saturated Zone (Phreatic or Groundwater Zone): All pore spaces are filled with water.
Zone of Isolated Pores and Chemically Combined Water: Water in this zone may not flow to a well because pores are not interconnected; water is chemically combined with rock minerals.

Water Content

Volumetric water content (θ) is the fraction of space occupied by water in a given volume of material.
θ = Vw/Vt (Vw = volume of water, Vt = total volume)
If pores are saturated with water, θ = n (n= porosity).
Field capacity is the maximum volume of water a soil can retain against gravity.

Unsaturated Zone Subdivisions

Soil water zone: Located between the land surface and the root zones of plants. Reaches up to 10 meters. Affected by climatic changes and evaporation/transpiration; important for plant life. Water content is less than field capacity during dry periods, and greater than field capacity during rainy periods. Can reach wilting point (negative fluid pressure head) during dry periods.
Intermediate zone: Extends from the lower edge of the soil water zone to the upper limit of the capillary zone. Can vary in thickness from zero at the water table surface to more than 100 m in the case of water tables far below the surface. Water moves downward under the influence of gravity.
Capillary fringe: Extends from the water table to about 3 m below the ground surface. Primarily influenced by capillary forces due to surface tension. Water content increases from the top to the bottom. The soil is saturated within this zone; however, pressure is less than atmospheric pressure because of surface tension. Absent from coarse soils. This is inversely proportional to the grain size of sediments.

Zone of Saturation/Groundwater

All interstices in soils and rocks are filled with water.
Part of this water moves due to gravity; the rest is retained by molecular and surface tension.
Water Table: A level at which water stands in a well. It has pressure equal to atmospheric pressure.
Below the water table, fluid pressure is greater than atmospheric pressure.
The depth can vary from shallow to very deep in sedimentary rocks.

Zone of Isolated Pores and Chemically Combined Water

Zones of saturated and unsaturated zones gradually grade into this zone.
Water may not flow to wells readily due to unconnected pores.
Chemically combined waters are stored in rock-forming minerals and are released only by melting the rocks.

Hydraulic Properties of Soil and Rock

Porosity: A measure of the volume not occupied by solid matter in soil, rocks. Can be primary (same geologic processes that give the rock its overall structure and pores) or secondary (developed after rock formation like joints, faults, solution openings). Total porosity = (Vv/Vt)*100 where Vv= volume of voids and Vt = total volume.
Specific Retention Coefficient (SR): Ratio of the volume of water the rock retains against gravity to the total volume.
Effective Porosity (ne): Quantifies the water in the rock that can move by gravity and that can be exploited.
Specific Yield (Sy): The volume of water a rock releases by gravity drainage to the total volume.

Coefficient of Permeability (k) and Hydraulic Conductivity (K)

Permeability is a function of effective porosity, structure of soil particles, and geological history.
Hydraulic conductivity is the capacity of a fluid to move through interconnected void spaces in sediments or rocks.
Values of K vary widely based on material type and are often reported relative to other materials. Units are m/day.

Transmissivity (T)

The discharge rate at which water transmits through unit width and full saturated thickness of a confined or unconfined aquifer under unit hydraulic gradient.

Specific Storage/Elastic Storage Coefficient (Ss)

Amount of water stored and expelled from storage per unit change in head due to compressibility of the mineral skeleton and pore water.

Storage Coefficient/Storativity (s)

Volume of water released from storage per unit storage area per unit change in head
In unconfined aquifers, the storage coefficient equals the specific yield; in confined aquifers it is usually smaller (510^-5 to 510^-3).

Homogeneity and Isotropy

Homogeneous aquifer: K is the same everywhere.
Heterogeneous aquifer: K varies spatially.
Isotropic aquifer: K is the same in all directions.
Anisotropic aquifer: K varies in different directions.

Average Hydraulic Properties

Average horizontal and vertical hydraulic conductivities (Kxa and Kza) can be calculated for layered homogeneous anisotropic systems based on layer thickness and conductivity.

Aquifer Concepts

Aquifer: A layer containing saturated porous and permeable media that stores and transmits groundwater to wells. Types of rocks include sedimentary rocks, sand, gravels.
Aquitard: Saturated but poorly pervious layer; may transmit substantial water to or from other aquifers if sufficiently thick.
Aquiclude: Saturated but cannot transmit groundwater.
Aquifuge: Neither stores nor transmits groundwater.

Types of Aquifers

Unconfined (phreatic) aquifer: Underlain by impervious layer; characterized by a water table. Well levels fluctuate with precipitation and recharge.
Perched aquifer: A special type of unconfined aquifer where groundwater is formed in the unsaturated zone due to clay layers impeding downward flow.
Confined (pressure) aquifer: Underlain and overlain by confining impermeable layers. The water level rises above bottom of upper confining bed in wells.
Leaky aquifer (semi-confined aquifer): Formed by an aquitard, which has lower conductivity, overlaying and/or underlying the aquifer.

Groundwater Occurences/Evaluation of Rocks and Soils as Aquifers

Unconsolidated Sediments: Forms important aquifers readily tapped at shallow depths; include glacial terrains, alluvial deposits of rivers, and tectonic valleys
Consolidated Sedimentary Rocks: Includes clastic (e.g, conglomerate, sandstone) and carbonate (e.g., limestone) rocks; primary and secondary permeability; groundwater occurrence depends on grain size and degree of weathering and cementation
Volcanic Rocks: Composed of multiple volcanic layers and characterized by varied hydraulic properties and groundwater potential based on composition, eruption mode, etc
Crystalline Rocks: Hard, crystalline, or recrystalline, mostly igneous or metamorphic origin. Groundwater occurs in weathered zones, joints, fractures; characteristics affected by regolith, fractures, and weathering.

Groundwater Flow

Water moves from higher to lower hydraulic head in response to energy distribution.
Hydraulic head combines fluid pressure head (P/pg), elevation head (z), and velocity head (v^2/2g).
Velocity head is typically negligible in groundwater systems; therefore, groundwater head is approximately P/pg+ z

Darcy's Law

The rate of groundwater flow through porous medium is proportional to -K(dh/dl) where K is hydraulic conductivity and (dh/dl) is the hydraulic gradient.
Darcy's velocity (v) = -K (dh/dl)
Q = rate of flow, A = cross-sectional area, K = hydraulic conductivity and i = hydraulic gradient
Va = actual velocity = Q/ne*A. Actual velocity is slower due to water moving through interconnected pores only.

Applicability of Darcy's Law

Darcy's law is applicable when flow is laminar; groundwater velocity is low.
Flow may be turbulent when velocities are high, and the medium is irregular/porous.
The Reynolds number (Re) = (pVD)/µ; if Re <1 flow is laminar and Darcy's law applies.

Groundwater Contour Lines (Equipotentials) and Flow Direction

Equipotential lines connect points with equal groundwater head.
Groundwater flow direction is perpendicular to equipotential lines.

Groundwater Recharge Area

Area where net flow of saturated groundwater is away from the water table.
Usually near topographic highs, deeper depths, and lower chemical concentration.

Groundwater Discharge Area

Area where net flow of saturated groundwater is toward the water table.
Usually near topographic lows, shallow depths and high chemical concentration.

Groundwater Flow (One-Dimensional)

Flow rate (Q) = Av, A = cross sectional area, v = Darcy's velocity and b= aquifer thickness
In confined aquifers, q = -T(dh/dl) where T = transmissivity.
In confined aquifers, Q=-T(∆h/L), where T is the transmissivity and ∆h is the head loss between 2 points in an aquifer with a distance L apart.

Groundwater Flow Net

Flow net is a set of streamlines and equipotential lines which are perpendicular to each other.

Estimation of Groundwater Flow Rate in Heterogeneous Medium

Qi = TWaii.

Remarks

Flow nets are not appropriate for karst and highly fractured aquifers where flow is discontinuous or takes place along preferential flow paths.

Groundwater-Surface Water Interaction

Groundwater flows from higher head to lower groundwater head. If a stream is losing water, it's because groundwater level is above the stream level. If a stream is gaining water, it's because groundwater level is below the stream level.

Recharge

Recharge is the addition of water to a groundwater reservoir. Natural and artificial recharge methods are used. Natural examples include precipitation/irrigation, surface water, and lateral inter-aquifer flows.
Artificial recharge examples include artificial groundwater injection.

Factors Affecting Recharge

Topography and Geology, Precipitation, Runoff/Pounding of Water, Irrigation, Rivers, Soil-zone, Unsaturated Zone, and Aquifer Condition variation with time.

Time Scale of Recharge

Present day recharge (days/months), short-term recharge (months/years), and long-term recharge (tens/thousands of years)

Recharge Estimation Methods

Direct methods: Measurement of infiltration.
Indirect methods: Water budget method; calculating difference between input and output, hydrograph separation/baseflow analysis, water table fluctuation method, chloride mass balance, basin recharge.