Overview
Hydrogeology is the science of the movement, occurrence and distribution of water in the subsurface. It draws on geology, hydrology, physics, chemistry, mathematics and biology. This first module builds the vocabulary and the basic quantities you will use for the rest of the course.
- Describe the processes of the hydrological cycle and write a hydrologic (water) balance for any control volume.
- Estimate infiltration and runoff with the Horton equation.
- Use hydrogeology terminology correctly: vadose zone, water table, aquifer, aquitard, confined and unconfined, potentiometric surface, and gaining and losing streams.
- Compute porosity, void ratio, water content and bulk density from laboratory measurements.
- Explain sorting, the uniformity coefficient, specific yield and specific retention.
- Open, run and interpret a simple groundwater model in OpenHydroQual.
How to work through it: each section has a short animation, a reading and an interactive tool. Videos tagged required are part of the module; optional ones go deeper. Finish with the practice problems, take the Brightspace quiz, then do the OpenHydroQual exercise.
1Why groundwater matters
As engineers we work with groundwater in three main ways:
- Water resources. How much water can be pumped sustainably? How many wells, where, and with what long-term effects?
- Environmental engineering. How do contaminants move underground, and how can a polluted aquifer be contained or cleaned up (pump-and-treat, soil vapor extraction, air sparging, in-situ bioremediation, reactive barriers, phytoremediation)?
- Civil engineering. How do we dewater an excavation, estimate seepage under or through a dam, or design drainage?
Common problems include groundwater depletion, seawater intrusion, dry wells, reduced streamflow, land subsidence, and contamination by metals, organic chemicals, nutrients, pesticides and radionuclides. In the United States, groundwater supplies roughly 40% of the water for public supply and almost all drinking water for rural households not served by a public system. Demand is expected to grow with population and climate change.
2The hydrological cycle and the water balance
Where the water is
About 96.5% of Earth's water is in the oceans. Only about 2.5% is fresh, and roughly two-thirds of that is frozen in ice caps and glaciers. Groundwater is about 30% of all fresh water, and about 99% of the fresh water that is liquid. Lakes, rivers, soil moisture and the atmosphere together hold only a tiny fraction (data: Gleick, 1996).
Processes
| Precipitation | Water falling as rain or snow. |
|---|---|
| Evaporation, transpiration | Return of water to the atmosphere from open water and soil (evaporation) and through plants (transpiration). Together: evapotranspiration (ET). |
| Infiltration | Entry of water through the ground surface. Percolation is its continued downward movement through the vadose zone. |
| Recharge | Water that reaches the water table and adds to groundwater. |
| Overland flow (runoff) | Generated when rainfall intensity exceeds the infiltration capacity. Water trapped in puddles is depression storage. |
| Interflow | Lateral flow in the soil above a less permeable layer, before reaching the water table. |
| Baseflow | Groundwater discharge to streams and lakes where the water table is above the water surface. Return flow is overland flow generated by discharging interflow or groundwater. |
| Drainage basin (catchment) | The land area that drains to a point on a stream. A groundwater basin is the subsurface counterpart; the two do not always coincide. |
Hydrologic balance
Water is neither created nor destroyed. For any region (a reservoir, a watershed, an aquifer) the rate of change of storage equals inflows minus outflows:
Example 3.1. A reservoir of surface area 2.6 km² has an inflowing stream (10,000 m³/day) and an outlet at a dam (16,000 m³/day). There has been no rain for weeks, evaporation is 1.2 mm/day, and the level is falling at 3 mm/day. The only other flux is net groundwater exchange \(G\). Find \(G\).
\(\dfrac{dS}{dt} = I + G - O - E\). Here \(\dfrac{dS}{dt} = -0.003 \times 2.6\times10^6 = -7{,}800\) m³/day and \(E = 0.0012 \times 2.6\times10^6 = 3{,}120\) m³/day.
\(G = -7{,}800 - 10{,}000 + 16{,}000 + 3{,}120 = +1{,}320\) m³/day, so groundwater flows into the reservoir.
Residence time
The residence time of a reservoir is the average time a water molecule spends in it, \(T_r = S/Q\) (storage divided by the flux through it). Typical values range from about 9 days in the atmosphere, through weeks to months in rivers and soil moisture, to centuries in shallow groundwater and up to 10,000 years or more in deep aquifers. Long residence times explain why contaminated aquifers take so long to flush and why depleted aquifers recover slowly.
Infiltration: the Horton equation
A dry soil can absorb water quickly because capillary suction pulls water in. As it wets up, its infiltration capacity \(f_p\) drops toward a steady value \(f_c\). Horton described this as
where \(f_0\) is the initial capacity, \(f_c\) the equilibrium capacity, and \(k\) the decay rate. If the rainfall intensity \(i\) is less than \(f_p\), all the rain infiltrates; otherwise the excess \(i - f_p\) becomes overland flow. Integrating gives the cumulative infiltration while rain exceeds capacity:
\[ F(t) = f_c\,t + \frac{f_0-f_c}{k}\left(1 - e^{-kt}\right) \]Rainfall, infiltration capacity, infiltration and runoff
Predict, then check: what happens if you set i below fc?
Predict, then check: a sandy soil vs. a clay soil
Simplification: capacity is assumed to decay with time from the start of the rain, starting from a dry soil. More rigorous methods track cumulative infiltration; you will see Richards' equation later in the course.
optional HydroMohsen: Infiltration, the Horton model (6 min).
3Below the surface: terminology
| Vadose (unsaturated) zone | From the ground surface to the water table; pores hold both air and water. |
|---|---|
| Water table | The surface where pore-water pressure equals atmospheric pressure. |
| Capillary fringe | Nearly saturated band above the water table, held up by capillary suction (thicker in fine-grained soils). |
| Aquifer | A geologic unit permeable enough to store and transmit useful quantities of water. |
| Aquitard (leaky confining layer) | A low-permeability layer that slows but does not stop flow. |
| Aquifuge / aquiclude | A layer that transmits essentially no water. |
| Unconfined aquifer | An aquifer whose upper boundary is the water table. |
| Confined aquifer | An aquifer bounded above and below by confining layers; water is under pressure greater than atmospheric. |
| Potentiometric (piezometric) surface | The level to which water rises in wells tapping a confined aquifer. Above ground: flowing artesian wells. |
| Perched water table | A local saturated zone held up by a low-permeability lens above the main water table. |
| Gaining / losing stream | A stream that receives groundwater (water table above the stream) or loses water to the ground (water table below the stream bed). |
Check your vocabulary
Match each definition with its term.
4Properties of porous media
optional Michigan Geological Survey: The fundamentals of porosity and permeability (6 min).
Porosity and void ratio
For a sample of total volume \(V_t\) made of solids \(V_s\) and voids \(V_v\) (\(V_t = V_s + V_v\)):
Water content and bulk density
\(\theta = n\) only for a saturated sample. \(\rho_s\) is the density of the mineral grains (about 2.65 g/cm³ for quartz). Dry bulk density and grain density therefore give the porosity directly: \(n = 1 - \rho_b/\rho_s\).
Example 3.2. A sandy-silt core, 8.0 cm in diameter and 30.0 cm long, weighs 3265 g moist and 2931 g oven-dry, with \(\rho_s = 2.65\) g/cm³. Then \(V_t = \pi(4.0)^2(30.0) = 1508\) cm³, \(\rho_b = 2931/1508 = 1.94\) g/cm³, \(n = 1 - 1.94/2.65 = 0.27\), and \(\theta = (3265-2931)/1508 = 0.22\). Since \(\theta < n\), the core was not saturated.
Virtual soil sample (1000 cm³)
Try: which preset has e > 1? Why can e exceed 1 while n cannot?
Sorting and grain-size distribution
A well-sorted sediment has grains of nearly uniform size; a poorly sorted one has a wide range, so small grains fill the spaces between large ones and porosity is lower. Grain-size distributions are plotted as percent finer (by weight) against log grain diameter. \(d_k\) is the diameter for which \(k\)% of the sample is finer; \(d_{10}\) is the effective grain size. The uniformity coefficient is
Grain-size distribution curve (USDA size classes shaded)
Specific yield and specific retention
When a saturated sediment drains under gravity, some water stays behind, held by surface tension around grains and in small pores. A layer of water molecules about 0.1–0.5 µm thick is held so strongly to mineral surfaces that it is essentially immobile (pendular water).
Fine-grained materials have small specific yield and large specific retention: clay may have a porosity above 0.4 but give up only a few percent of its volume as drainable water. Typical values (Johnson, 1967): clay \(n \approx 0.42,\ S_y \approx 0.03\); silt 0.46, 0.08; fine sand 0.43, 0.21; coarse sand 0.39, 0.27; gravel 0.32, 0.23. Specific yield, not porosity, controls how much water an unconfined aquifer releases when the water table drops.
5Practice (ungraded)
New numbers every time. Aim for two correct attempts in a row on each type before taking the quiz. Scientific notation can be typed as 2.5e-4.
A · Core sample: bulk density, porosity, water content
B · Porosity and void ratio
C · Drainage test: specific yield and retention
D · Reservoir water balance
E · Horton infiltration and runoff
F · Uniformity coefficient
Concept checks
1. A clay and a coarse sand have the same porosity, 0.40. Both are saturated and then drained under gravity. Which releases more water?
2. During a long dry spell a stream keeps flowing. Where does its water come from?
3. A well screened in a confined aquifer has a water level 4 m above the top of the aquifer, but below the ground surface. This well is:
6Graded quiz
When you can do every practice problem without opening the solution, go to Brightspace → Quizzes → Module 1: Introduction and Porous Media.
- Terminology, the water balance, and porous-media calculations; numerical questions use different numbers for every student and attempt.
- 2 attempts, highest score counts. Calculator and notes allowed.
7OpenHydroQual exercise: your first groundwater model
OpenHydroQual (OHQ) is a free, open-source modeling framework, developed at Catholic University, for water flow and water quality. You will use it throughout the course. This first exercise is short: open a ready-made model of an aquifer draining to a stream, run it, and check that its answers obey the water balance and the porous-media relationships from this module.
Open the exercise instructions →