Slug test
Encyclopedia
A slug test is a particular type of aquifer test
where water
is quickly added or removed from a groundwater
well
, and the change in hydraulic head
is monitored through time, to determine the near-well aquifer characteristics. It is a method used by hydrogeologists
and civil engineers
to determine the transmissivity
/hydraulic conductivity
and storativity of the material the well is completed in.
A slug can be added by either quickly adding a measured amount of water to the well or something which displaces a measured volume (e.g., a long heavy pipe
with the ends capped off). An alternative object is a solid polyvinyl chloride (PVC) rod, with sufficient weight to sink into the groundwater. The objective here is to displace water, not merely be "heavy". A slug of water can be removed using a bailer
or pump
, but this is more difficult to do since it must be done very quickly and the equipment for removing the water (pump or bailer) will likely be in the way of getting water level measurements. On occasion, a meat-slug may be used in place of a solid-pipe slug.
The size of the slug required is determined by the aquifer properties, the size of the well and the amount of time which is available for the test. For very permeable
aquifers, the pulse will dissipate very quickly. If the well has a large diameter, a large volume of water must be added to increase the level in the well a measurable amount.
Mathematically
, the Theis equation is the solution of the groundwater flow equation
for a step increase
in discharge rate at the pumping well; a slug test is instead an instantaneous pulse
at the pumping well. This means that a superposition
(or more precisely a convolution
) of an infinite number of sequential slug tests through time would effectively be a "standard" Theis aquifer test.
There are several known solutions to the slug test problem; a common engineering approximation
is the Hvorslev method, which approximates the more rigorous solution to transient aquifer flow with a simple decaying exponential function.
The aquifer parameters obtained from a slug test are typically less representative of the aquifer surrounding the well than an aquifer test
which involves pumping in one well and monitoring in another. Complications arise from near-well effects (i.e., well skin and wellbore storage), which may make it difficult to get accurate results from slug test interpretation.
Aquifer test
An aquifer test is conducted to evaluate an aquifer by "stimulating" the aquifer through constant pumping, and observing the aquifer's "response" in observation wells...
where water
Water
Water is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...
is quickly added or removed from a groundwater
Groundwater
Groundwater is water located beneath the ground surface in soil pore spaces and in the fractures of rock formations. A unit of rock or an unconsolidated deposit is called an aquifer when it can yield a usable quantity of water. The depth at which soil pore spaces or fractures and voids in rock...
well
Water well
A water well is an excavation or structure created in the ground by digging, driving, boring or drilling to access groundwater in underground aquifers. The well water is drawn by an electric submersible pump, a trash pump, a vertical turbine pump, a handpump or a mechanical pump...
, and the change in hydraulic head
Hydraulic head
Hydraulic head or piezometric head is a specific measurement of water pressure above a geodetic datum. It is usually measured as a water surface elevation, expressed in units of length, at the entrance of a piezometer...
is monitored through time, to determine the near-well aquifer characteristics. It is a method used by hydrogeologists
Hydrogeology
Hydrogeology is the area of geology that deals with the distribution and movement of groundwater in the soil and rocks of the Earth's crust, . The term geohydrology is often used interchangeably...
and civil engineers
Civil engineering
Civil engineering is a professional engineering discipline that deals with the design, construction, and maintenance of the physical and naturally built environment, including works like roads, bridges, canals, dams, and buildings...
to determine the transmissivity
Transmissivity
Transmissivity may refer to:* Transmissivity , the rate which groundwater flows horizontally through an aquifer* Transmittance, in optics...
/hydraulic conductivity
Hydraulic conductivity
Hydraulic conductivity, symbolically represented as K, is a property of vascular plants, soil or rock, that describes the ease with which water can move through pore spaces or fractures. It depends on the intrinsic permeability of the material and on the degree of saturation...
and storativity of the material the well is completed in.
Slug test method
The slug of water can either be added to or removed from the well — the only requirement is that it be done as quickly as possible (the interpretation typically assumes instantaneously), then the water level or pressure is monitored. Depending on the properties of the aquifer and the size of the slug, the water level may return to pre-test levels very quickly (thus complicating accurate collection of water level data).A slug can be added by either quickly adding a measured amount of water to the well or something which displaces a measured volume (e.g., a long heavy pipe
Pipe (material)
A pipe is a tubular section or hollow cylinder, usually but not necessarily of circular cross-section, used mainly to convey substances which can flow — liquids and gases , slurries, powders, masses of small solids...
with the ends capped off). An alternative object is a solid polyvinyl chloride (PVC) rod, with sufficient weight to sink into the groundwater. The objective here is to displace water, not merely be "heavy". A slug of water can be removed using a bailer
Bailer
A bailer in hydrogeology is a hollow tube used to retrieve groundwater samples from monitoring wells. Groundwater monitoring wells are drilled in areas where there are underground storage tanks or where there is environmental remediation occurring. The wells are typically built out of PVC casing...
or pump
Pump
A pump is a device used to move fluids, such as liquids, gases or slurries.A pump displaces a volume by physical or mechanical action. Pumps fall into three major groups: direct lift, displacement, and gravity pumps...
, but this is more difficult to do since it must be done very quickly and the equipment for removing the water (pump or bailer) will likely be in the way of getting water level measurements. On occasion, a meat-slug may be used in place of a solid-pipe slug.
Performing a slug test
A slug test is in contrast to standard aquifer tests, which typically involve pumping a well at a constant flowrate, and monitoring the response of the aquifer in nearby monitoring wells. Often slug tests are performed instead of a constant rate test, because:- time constraints (quick results, or results for a large number of wells, are needed),
- the well does not or cannot have a pumpPumpA pump is a device used to move fluids, such as liquids, gases or slurries.A pump displaces a volume by physical or mechanical action. Pumps fall into three major groups: direct lift, displacement, and gravity pumps...
installed on it (slug tests do not require pumping), - the transmissivity of the material the well is cased in is too low to realistically perform a proper pumping test (common for aquitards or some bedrock monitoring wells), or
- the general size (order of magnitudeOrder of magnitudeAn order of magnitude is the class of scale or magnitude of any amount, where each class contains values of a fixed ratio to the class preceding it. In its most common usage, the amount being scaled is 10 and the scale is the exponent being applied to this amount...
) of the aquifer parameters is all the accuracy that is required.
The size of the slug required is determined by the aquifer properties, the size of the well and the amount of time which is available for the test. For very permeable
Hydraulic conductivity
Hydraulic conductivity, symbolically represented as K, is a property of vascular plants, soil or rock, that describes the ease with which water can move through pore spaces or fractures. It depends on the intrinsic permeability of the material and on the degree of saturation...
aquifers, the pulse will dissipate very quickly. If the well has a large diameter, a large volume of water must be added to increase the level in the well a measurable amount.
Slug test interpretation
Because the flow rate into or out of the well is not constant, as is the case in a typical aquifer test, the standard Theis solution does not work.Mathematically
Mathematics
Mathematics is the study of quantity, space, structure, and change. Mathematicians seek out patterns and formulate new conjectures. Mathematicians resolve the truth or falsity of conjectures by mathematical proofs, which are arguments sufficient to convince other mathematicians of their validity...
, the Theis equation is the solution of the groundwater flow equation
Groundwater flow equation
Used in hydrogeology, the groundwater flow equation is the mathematical relationship which is used to describe the flow of groundwater through an aquifer. The transient flow of groundwater is described by a form of the diffusion equation, similar to that used in heat transfer to describe the flow...
for a step increase
Heaviside step function
The Heaviside step function, or the unit step function, usually denoted by H , is a discontinuous function whose value is zero for negative argument and one for positive argument....
in discharge rate at the pumping well; a slug test is instead an instantaneous pulse
Dirac delta function
The Dirac delta function, or δ function, is a generalized function depending on a real parameter such that it is zero for all values of the parameter except when the parameter is zero, and its integral over the parameter from −∞ to ∞ is equal to one. It was introduced by theoretical...
at the pumping well. This means that a superposition
Superposition principle
In physics and systems theory, the superposition principle , also known as superposition property, states that, for all linear systems, the net response at a given place and time caused by two or more stimuli is the sum of the responses which would have been caused by each stimulus individually...
(or more precisely a convolution
Convolution
In mathematics and, in particular, functional analysis, convolution is a mathematical operation on two functions f and g, producing a third function that is typically viewed as a modified version of one of the original functions. Convolution is similar to cross-correlation...
) of an infinite number of sequential slug tests through time would effectively be a "standard" Theis aquifer test.
There are several known solutions to the slug test problem; a common engineering approximation
Approximation
An approximation is a representation of something that is not exact, but still close enough to be useful. Although approximation is most often applied to numbers, it is also frequently applied to such things as mathematical functions, shapes, and physical laws.Approximations may be used because...
is the Hvorslev method, which approximates the more rigorous solution to transient aquifer flow with a simple decaying exponential function.
The aquifer parameters obtained from a slug test are typically less representative of the aquifer surrounding the well than an aquifer test
Aquifer test
An aquifer test is conducted to evaluate an aquifer by "stimulating" the aquifer through constant pumping, and observing the aquifer's "response" in observation wells...
which involves pumping in one well and monitoring in another. Complications arise from near-well effects (i.e., well skin and wellbore storage), which may make it difficult to get accurate results from slug test interpretation.