One of the best methods for determining ground water aquifer characteristics is the pump test. A pump test not only provides information such as the potential yield and capacity for a given well; but it can also provide hydraulic information about the entire aquifer. This information can then be used to determine the effects of additional similarly-placed wells penetrating the same groundwater system. Hence, the environmental effects of certain water uses, depending on pumping rates, volumes, and recoveries can be ascertained.
Some of the predictive values of aquifer/pump testing include:
1. The effect of new withdrawals on existing wells.
2. The potential yield of future wells.
3. The future drawdown of these wells at varying discharges.
4. The radius of the cone of depression for individual wells or multiple well systems.
5. The environmental impacts of well drawdown and water usage.
Potential uses for ground water at the Kingbrook Site include snowmaking, irrigation, and a potable source of drinking water (even though potable supply is to be furnished by the town of Yorkshire). This section reviews some of the general principles of groundwater hydrology for those readers who have limited knowledge of the subject or who require background information to aid in the evaluation of subsequent sections of the report.
There are actually several groundwater systems operating at the Kingbrook Site. These include unconfined water table aquifers near the surface, a deeper confined very high yield aquifer in the Cattaraugus Creek Valley, and fractured bedrock aquifers beneath the two mountains at the site. A brief review of basic ground water hydraulics and terminology as well as a comparison between these various aquifer types will suffice in preparing the general reader to understand the results of the well testing and aquifer analyses of the Kingbrook Site that follow. A review of the Site Geology section of this report is also recommended since the surface and subsurface geology of the Kingbrook Site determine the ways and means by which water flows into, through, and out of these subterranean reservoirs.
Ground Water Hydraulics
Permeability is the natural inherent capability of a soil or rock unit to transmit fluids. The degree of permeability depends upon the size and shape of the pore spaces between grains of soil or rock and the degree of interconnectedness of these pores. It is measured by the rate at which a fluid of known viscosity (such as water) moves a given distance through a medium during a given interval of time. A standard unit of permeability is the DARCY, named for Henry Darcy, a French engineer, who first described these relationships mathematically via his systematic study of water through various porous media (Fetter, 1980).
Darcy’s Law is a derived formula for the flow of fluids (liquids or gasses) through a medium assuming that the flow is laminar (as opposed to turbulent) and that inertia (resistance to change in motion) is negligible. The numerical formulation of this law is used generally in studies of natural gas, oil, and water production from underground formations. Darcy’s Law states that the velocity of fluid flow is proportional to the measure of the pressure gradient multiplied by the ratio of permeability multiplied by the density, divided by the viscosity of the fluid. Darcy showed experimentally that specific discharge (v) is directly proportional to the difference in hydraulic heads (pressures) in the system and is inversely proportional to the flow-length (L) between those points (Fetter, 1980; Freeze and Cherry, 1980).
v hA – hb (1)
v 1/L (2)
Specific discharge (v) is defined as:
v = Q/A (3)
where flow-rate (Q) is expressed as volume per unit of time and A is the area of the discharging portion of the system. Specific discharge is, therefore, related to a gradient, h/l , times a constant of proportionality such that:
v = -K h/l (4)
where K is the hydraulic conductivity which has the same units as velocity (distance per unit of time) and is dependent upon BOTH the permeability of the porous medium and the viscosity of the fluid moving through it. (Specifically in the case of Kingbrook, the porous medium is buried glacial till or bedrock and the fluid is water.) By substituting Equation 3 into Equation 4, Darcy’s Law may also be expressed as:
v = -K h/l (5)
Sometimes h/l is expressed as i which represents the slope of the potentiometric surface (difference in head elevation per unit length for confined aquifers and the slope of the water table in unconfined aquifers) and is called the hydraulic gradient; hence, the classic expression of Darcy’s Law:
Q = K i A (6)
where:
Q=Flow rate (discharge)
K=Hydraulic conductivity
i =Hydraulic gradient
A=Surface area of discharging portion of the aquifer.
Figure 1 (below) depicts the classic experimental demonstration of the relationships expressed in Darcy’s Law for a cylinder of sand which for practical purposes may be considered as an isolated portion of a ground water aquifer. This hypothetical tube of sand resembles the deep confined aquifer at the Kingbrook Site in several ways. The values h a and h b represent static water levels (elevations) in idle (non-pumping) wells at the top and bottom of Blue Mountain respectively. h is the difference in elevation between these two elevations. l is the horizontal distance between the two wells.
The datum line (z=0) represents the Cattaraugus Creek a probable zone of natural discharge (Q) for the aquifer. The upper Q represents the entry or recharge of groundwater into the Kingbrook Aquifer. The value A represents the cross sectional area of the discharging portion of the aquifer and K is a natural inherent property of the aquifer materials… hydraulic conductivity which is a measure of the aquifer’s ability to transmit water.
Another intrinsic characteristic of an aquifer (and one that is determined by pump testing) is Transmissivity:
Transmissivity (T).
This is the measure of the amount of water that can be transmitted horizontally by the full saturated thickness of the aquifer under a hydraulic gradient (i) of 1. It is found by multiplying the hydraulic conductivity (K) by the saturated thickness of the aquifer (b) such that:
T = bK (7)
Units of transmissivity are volume/time/length of the full thickness of the aquifer hence transmissivity is a function of the properties of the water, the rock or soil medium, and the full thickness of the medium.
When the transmissivity of an aquifer is known, the following predictions can be made:
1. The drawdown of the water table or potentiometric surface at various distances from the pumping well.
2. The drawdown at various times during pumping.
3. The effects of multiple wells on the ground water system.
4. The efficiency of the well(s).
5. The drawdown at various pumping rates.
6. Hence, the environmental impacts of pumping wells at varying rates and volumes can be determined.
Key Ground Water Terminology and Definitions
Additional terminology of which the general reader should be aware for a basic understanding of ground water hydraulics and aquifer testing is listed below:
STORATIVITY
Storativity or storage coefficient (S) is another hydrologic parameter determined by pump testing. Storativity is the volume of water that an aquifer will absorb or expel from storage per unit surface area per unit change in head (pressure in linear units of water in a well). It is a dimensionless quantity. In an unconfined (water table) aquifer, storativity is the same as specific yield (see below). In confined aquifers, S is the result of decompression during pumping due to expansion of confined water as a result of pressure head reduction from the pumping process. The relief of this pressure, that accompanies well pumping, manifests itself in drawdown of the water level in the well. The value of S ranges from 0.01 to 0.3 for unconfined aquifers and from 10-5 to 10-3 for confined aquifers.
Static Water Level (also: Potentiometric Surface)
The “natural” level of water in a well prior to any disturbance such as pumping. In a confined (artesian) aquifer, it is the pressure head level reached by water in the well and is generally higher than the confining layer’s lower surface. In an unconfined aquifer, the water table is the static water level.
Artesian Spring
An artesian spring or well is one in which the static water level is above the ground surface… “a gusher”.)
Dynamic Water Level (Pumping Water Level)
The water level in the well reached during pumping or recovery at a given instant in time.
Drawdown
A measure of the difference in elevation between static and dynamic water levels in a well at a given instant in time during pumping. (Called RESIDUAL drawdown during recovery.)
Recovery
The process of allowing water to return to its static level following a well disturbance such as pumping or injection.
Head
A measure of pressure as evidenced by the distance between the water level in a well and some reference point such as the static water level, ground surface, etc. Recovery of a pumped well involves a rise in water level due to increased pressure from the recharging aquifer. Hence, the rise in water level is a reflection of the increasing pressure of the aquifer on the well (in feet of water) just as a rise in barometric pressure is reflected by a rise in column inches of mercury. (Incidentally, sudden changes in barometric pressure can affect the accuracy of well pump tests.)
Yield
The volume of water produced by a well during a given amount of time.
Yield is generally expressed in gallons per minute (gpm) or gallons per day (gpd).
Specific Capacity
The yield of a well per unit of drawdown at a given time after the start of pumping (varies with time). It is a measure of the productivity of a well. For example: if drawdown is 100 feet after 5 hours of pumping at 100 gallons per minute, the specific capacity is:
100 gpm / 100 ft. = 1 gpm / ft.
of drawdown after five hours.
Generally the specific capacity will decrease with length of time from start of pumping as drawdown increases.
Cone of Depression
The cone of depression represents the drawdown of the potentiometric surface of the aquifer in the vicinity of the well being pumped. The cone of depression is greatest at the well and extends radially outward from the well. The total distance between the well and the farthest extent of this cone is called the Radius of Influence.
Hydraulic Conductivity
Hydraulic Conductivity is a coefficient of proportionality for a specific porous material or permeable medium which describes the rate at which water can move through that medium expressed as a constant for that medium.
Confined (Artesian) Aquifer
A Confined Aquifer or Artesian Aquifer is an aquifer that is sandwiched between two layers of material that have significantly lower hydraulic conductivities than that of the aquifer material itself. When penetrated by a well, the pressure on the aquifer is reflected by a rise of water in the well to a level that is generally higher than the top of the aquifer. For example: a water balloon when penetrated will spout water upward as a reflection of the confining pressure within the balloon. As the pressure decreases due to the withdrawal of water, the height of the spout will decrease until internal and external pressures are equal.
Unconfined aquifer
An unconfined aquifer is one that is not capped off by a confining layer. Generally close to the surface. Unconfined Aquifers are often referred to as “Water Table Aquifers” since the static water level in a monitoring well is at the same level as the water table.
Fractured Bedrock Aquifer
Aquifer in which groundwater moves through fractures such as vertical joints, horizontal bedding planes, or other features which are more permeable than (higher values of K) and are secondary in nature to the original porosity and permeability of the rock. Water yield depends on the amount of fracturing in the rock, the interconnectedness of the fractures, and the degree to which the well penetrates those fractures.
Pumping Well
This is the well that is actually pumped during an aquifer (pump) test as opposed to the one or more observation / monitoring wells placed at some distance from the pumping well. Water levels are measured in all of these during a pump test. The more data points (wells) monitored, the more precise the results of the test.
Preliminary Drawdown Test
Several days prior to the start of the pump test, the pump should be installed in the well and all equipment should be field tested.
A preliminary drawdown test should be conducted to determine the following:
1. The maximum anticipated drawdown which usually occurs within the first three to four hours of the test.
2. The volume produced (pump rate) of the pump equipment at various engine speeds.
3. The best method to determine discharge (meter, weir, etc.).
4. The best method to remove discharge from site and to prevent inadvertent recharge of the aquifer.
5. Whether the observation wells are located so as to exhibit sufficient drawdown during the actual test.
6. Best methods for synchronously gathering and recording data.
NOTE: Do not begin a pump test until static water level has recovered from this preliminary drawdown test.
Parameters Measured During Pump Tests
The parameters that are measured during a pump test should be recorded on data sheets for the pumping well and for all available observation wells. All clocks and times of measurement should be synchronized and the following parameters should be measured:
1. The static water levels in all wells prior to the start-up of pumping test.
2. The time of the start of the test.
3. The pumping rate and the times of any variations therein.
4. The elapsed time since the start of the test.
5. The synchronous dynamic (pumping) water levels in all wells.
6. The total drawdown since start of the test at each interval.
7. The time of the end of the test.
8. Recovering water levels in each well and times of synchronous measurements until the pre-test static water level is reached.
Accuracy of the Pump Test Is Dependent Upon:
1. Maintaining a constant yield during the test.
2. Careful and accurate measurement of drawdown in the pumping and observation wells.
3. Taking synchronous drawdown readings at the appropriate times.
4. Determining how barometric pressure, stream levels, and tidal oscillations affect drawdown data.
5. Careful comparison between recovery test data and drawdown data from the pumping portion of the test.
6. Continuing the test for at least 24 hours at constant pump rate in a confined aquifer or until the rate of drawdown
