Groundwater

Borehole Casing

A component used to protect the upper, unstable part of a borehole.

Slotted Screen

A component of borehole design used alongside solid casing and a sump to allow water ingress while filtering particles.

Gravel Pack

A layer placed around the borehole screen with specific dimensions ranging from $50-100\,mm$ to $75-100\,mm$.

Rotary Drilling

A method of borehole construction that utilizes a pump as part of the drilling process.

Percussion Drilling

A drilling method characterized by the use of a winch, casing, and a percussion device.

Pumping Test

A procedure performed on each new well to learn about aquifer and well behavior under pumping conditions, but not for learning about long-term pumping effects or recharge.

Transmissivity ($T$)

An aquifer property analyzed through pumping test data to predict well performance; calculated in Jacob's equation as $T = \frac{0.183Q}{\Delta s}$.

Storativity ($S$)

An aquifer parameter representing the volume of water released from storage; calculated in Jacob's equation as $S = \frac{2.25Tt_0}{r^2}$.

Groundwater Flow Equation

The principle stating that $\text{Rate of inflow} - \text{Rate of outflow} = \text{Rate of change of storage}$.

Drawdown ($s$)

The difference between the original head ($h_0$) and the pumping head ($h_1$), represented as $s = h_0 - h_1$.

Cone of Depression

The curve formed by drawdown around a well that expands over time until inflows match the well extraction.

Thiem Equation

An equation for steady radial flow to a well in a confined aquifer: $Q = \frac{2\pi Kb(h_2 - h_1)}{\ln(r_2/r_1)}$.

Theis (1935) Solution

A method to solve for drawdown ($s$) using pumping rate ($Q$), transmissivity ($T$), distance ($r$), storativity ($S$), time ($t$), and the well function $W(u)$.

Well Function ($W(u)$)

An infinite series of terms involving the dimensionless parameter $u$, defined as $W(u) = -0.5772 - \ln(u) + u - \frac{u^2}{2.2!} + \frac{u^3}{3.3!} \dots$

Cooper-Jacob Equation

A simplified version of the Theis equation valid only when the parameter $u$ is small (u < 0.01).

$$\Delta s$$

The measure of slope in a Cooper-Jacob plot, defined as the drawdown observed over one log cycle of time.

$$t_0$$

The intercept on the time axis at which the drawdown ($s$) is equal to zero in a Cooper-Jacob analysis.

Recovery Test

A test that monitors the levels of water returning to a well after pumping has stopped.

Step-Drawdown Test

A well test or well test involving varying discharge rates to distinguish between aquifer drawdown and well drawdown.

Biofouling

A biological cause of decreased well efficiency alongside factors like iron and manganese deposits or pump deterioration.

Electrical Resistivity Tomography

A geophysical method used to assist in borehole siting, particularly where detailed investigation is required.

Infiltration gallery

A method for withdrawing shallow groundwater ($5-8\,m$ depth) near a pond or river that requires sandy soil and can offer protection against contamination by storing water underground.

Safe Yield

The amount of naturally occurring groundwater that can be withdrawn from an aquifer on a sustained basis without creating undesirable effects, such as environmental damage or impaired water quality.

DNAPLs

Dense Non-Aqueous Phase Liquids, which are immiscible in water, denser than water, and often persist for decades because they sink to the full thickness of an aquifer and dissolve slowly.

Sand dams

Structures built on the riverbed that capture deposited sand to store saturated water, reducing evaporation loss and protecting against water-based vectors like mosquitoes and bilharzia.

TTC (Thermo Tolerant Coliforms)

A microbiological indicator used to assess water quality, specifically referring to $\text{E. coli}$, which is used to measure faecal contamination.

Dynamic equilibrium

The natural condition of an aquifer where inflows equal outflows, meaning storage and water levels remain unchanged over time.

Spring box

An engineered structure used for spring protection that includes features such as an inspection hatch, overflow, screening layers (sand, gravel, stones), and an outlet.

Caissons

Concrete rings used as well linings; below the water table, 'no fines' porous concrete versions allow water to enter the well.

Source Protection Zones (SPZs)

Designated areas around a water source characterized by the time it takes for a contaminant to reach the source, such as $50$ days or $100$ days.

Nitrate Vulnerable Zones (NVZs)

Designated regions (such as $55\%$ of England) where groundwater is protected from nitrate pollution, which primarily originates from agriculture, fertilizers, and on-site sanitation.

Fluoride

A chemical contaminant often found in granite, mica, and apatite, particularly in arid climates with long residence times; the WHO limit is $1.5\,mg/L$.

Arsenic

A groundwater contaminant found in delta sediments and magnetite, often released when irrigation pumping draws water into mineral-rich sediment; the WHO limit is $0.01\,mg/L$.

Saltwater intrusion

The movement of saline water into freshwater aquifers, often caused by a 'cone of depression' or 'cone of ascension' near coastal wells.

Cut-off drain

A component of spring protection designed to divert surface water and prevent faecal contamination from entering the spring area.

Recharge

An input into the water balance of an aquifer, often estimated from meteorological data, representing the proportion of precipitation that enters the aquifer.

Baseflow

The groundwater output to 'gaining streams' where the water table intersects with the riverbed.

Water table

The top of the saturated zone and the level below which the ground is completely saturated with water.

Unsaturated zone

The area between the ground surface and the water table where the soil and rocks contain both air and water.

Saturated zone

The area below the water table where all pore spaces in the soil or rock are filled with water.

Aquifer (USGS Definition)

A formation that contains sufficient saturated permeable material to yield significant quantities of water to wells and springs.

Aquifer (EA Definition)

Permeable strata that can transmit and store water in significant quantities.

Unconfined aquifer

An aquifer whose upper water surface (water table) is at atmospheric pressure and is able to rise and fall.

Confined aquifer

An aquifer trapped between two impermeable layers (confining layers), where the water is under pressure.

Potentiometric surface

An imaginary surface that represents the total head of groundwater in a confined aquifer, defined by the level to which water will rise in a well.

Gaining stream

A stream where the direction of groundwater flow is into the stream because the water table is higher than the stream level.

Losing stream

A stream that loses water to the underlying aquifer because the water table is lower than the stream level.

Primary porosity

Porosity that develops during the original deposition of the material, often referred to as intergranular porosity.

Secondary porosity

Porosity that develops after a rock is formed, typically through fractures or solutional openings.

Total head

The sum of the elevation head and the pressure head; expressed as $\text{Total head} = \text{pressure head} + \text{elevation head}$.

Darcy’s Law (1856)

The equation describing the flow of a fluid through a porous medium: $Q = K A \frac{\Delta h}{L}$.

Hydraulic conductivity (K)

A measure of the ease with which a fluid can move through pore spaces or fractures, typically measured in units like $\text{m/d}$ or $\text{m/s}$.

Transmissivity (T)

The rate at which water is transmitted through a unit width of an aquifer under a unit hydraulic gradient, calculated as $T = K \cdot b$, with usual units in $m^2/d$.

Storativity (S)

The volume of water released from or taken into storage per unit surface area per unit change in head; it is a dimensionless value.

Specific yield ($S_y$)

The storativity for an unconfined aquifer, representing the amount of water that will drain by gravity from a saturated rock.

Capillary fringe

The subsurface layer in which groundwater seeps up from a water table by capillary action to fill pores.

Hydraulic gradient ($i$)

The change in total head per unit distance of flow in a given direction, represented as $\frac{\Delta h}{L}$.