Developing a new municipal water supply well.
Initially, an exploratory phase is undertaken to locate potential sites for a water supply well. A good place to start is with United States Geologic Service (USGS) Hydrologic Atlases, which are available for many river basins. (These atlases are sometimes referred to as "Groundwater Favorability Maps".) The atlases identify water-bearing soils and classify them with respect to the potential of the geologic formation to support a water supply well. The areas possessing the ability to yield sizeable quantifies of water are color coded based on the availability of water; i.e. greater depth and soils with a better ability to transmit water.
The USGS also publishes topographic maps for much of the country. These maps can be used to locate geologic features that typically contain the transmissive sand and gravel capable of yielding large quantities of water. A community should use an experienced hydrogeologist to make a detailed interpretation. A less experienced individual, however, can locate potential sites by identifying geologic "eskers" on the topographic maps. These distinctive land features are basically islands of sand and gravel and are higher in elevation than the surrounding ground surface. In the northeastern United States, glacial meltwater created eskers that are typically oblong in shape, with the longest axis being parallel to river valleys. Many times these eskers run in a north-south direction, parallel to the northward retreat of the glaciers, and may extend out into the valley itself as a finger of land.
Sand and gravel mining operations are also good indicators of transmissive material within a river valley. Since sand and gravel operations remove the esker, it may be difficult to locate the potential well site by topography alone. Fortunately, USGS topographic maps indicate locations of former sand and gravel removal operations fairly accurately.
Once all potential sites have been located on these maps, community officials or their consultant should determine the feasibility of sites with respect to property ownership, sources of potential contamination, and accessibility. Property ownership is a primary concern of the regulatory community. For example, Massachusetts requires the water supplier to own or control the land use on the area within a 400-ft radius of the well. This requirement translates into the purchase of 11.54 acres of property; a substantial financial commitment, given the increasing cost of real estate. Land ownership requirements vary from state to state; however, most are determined using a set radial distance from a well.
Site visits produce valuable information regarding the type of soil at the potential well location. The species of tree growing on the site can also help determine the type of soil. Tree species that prefer well-drained soils will typically grow on sites which contain soils of good water-transmitting qualifies. In Massachusetts, for example, white pine and white oak trees are good indicator species.
When assessing the site, the community should identify potential sources of groundwater contamination located within the capture zone of the well (the area containing water from which the well would draw). The capture zone extends from a point termed the downgradient stagnation point, located downstream of the well with respect to the direction of groundwater flow, to an upgradient location usually defined by a geologic boundary or by a surface water divide. The capture zone cannot be adequately delineated prior to performing a pumping test at the site, therefore, potential sources of contamination within a half mile radius from the site should be assessed.
Accessibility is usually self-evident from the topographical maps or by field reconnaissance. The location of the site with respect to the water distribution system is also a concern as the installation of extended lengths of water main will increase the cost significantly.
Exploratory Test Wells
After reviewing potential sites and eliminating the unfavorable locations, the community should prioritize sites with respect to their potential. A test well program is then initiated to further aid in determining the best site at which to perform extensive well testing and an ensuing pumping test. Typically the test well program will consist of installing one or two 2 1/2-in. diameter test wells at each site. Test well logs are used to record the characteristics of the soil material (type, color, gradation, case of penetration, etc.) encountered during installation, depth of well, and the well rating. The well rating refers to the rate at which the well was pumped (usually in gallons per minute) and the vacuum developed within the well during pumping. The vacuum (given in inches of mercury), is important as it indicates the ease at which the soil yields water and can be used to theoretically indicate the well drawdown (the distance the water level dropped during pumping).
Upon completion of this initial phase of the test well program, the well logs should be compared to determine the most favorable site for further testing. Further testing includes installation of additional 2 1/2-in. observation wells across the site in an effort to determine the best location at which to perform a prolonged pumping test (typically run for a minimum of five days). It is important to locate these additional wells at distances and directions such that they can be used as observation wells during the prolonged pumping test. An experienced professional will locate these wells based on additional criteria, including the type of withdrawal system best suited for the site (wellfield vs. single withdrawal point), increasing or decreasing depth of wells across the site, site geology, the distance to property lines, and the well ratings.
The final 2 1/2-in. well installed at the site during this phase of the project is usually located two ft from the most productive test well at the site. Typically, a four-hour pumping test is then performed. During this short test the water levels in the nearby wells are noted. This information allows the hydrogeologist to predict the pumping rate that can be obtained during the prolonged pumping test, based on the impact the pumping had on the local water table. At this point a water sample is taken for water quality testing. Typically, testing for volatile organic contaminants (VOC) using EPA method 524 and a general chemistry analysis are performed. The VOC analysis will indicate any petroleum or solvent contamination in the groundwater. The general chemistry analysis evaluates parameters which by themselves are not deemed to be harmful, but which may affect the esthetic and corrosive nature of the water.
Prolonged Pumping Test
State regulatory agencies typically establish the protocol for the prolonged pumping test. Usually, several additional observation wells will be required at locations further from the pumping well(s) in an attempt to determine the impact on the groundwater table toward sensitive entities such as private wells, surface water features, and potential sources of contamination. The logs of these observation wells can be used to determine the areal extent of the soil layers and, upon surveying the wells to the same datum, geologic cross-sections can be developed for use in the pumping test analysis.
Of increasing concern to regulators is the impact pumping will have on the local surface water features such as wetlands, streams, rivers, and ponds. The effect on surface water features is difficult to quantify; however, there are several methods that can be used to indicate the impact pumping has on these features. The first of these methods involves the installation of a shallow (usually less than ten ft in depth) and deep observation well "nest" located on the edge of the surface water feature. This nest is located between the pumping center and the water body. By comparing the water levels in the two observation wells during the pumping test, the consultant can determine from what depth the recharge is originating; i.e. from deeper in the aquifer or from the surface. Many times a blanket layer of finer sediments underlies these surface water features and retards the impact on the surface water feature. Well logs and the resulting water levels in the shallow and deep wells can be used to make this determination. A second method commonly used is to determine the temperature, color, odor, conductivity, and pH of the pumped water and the water in the surface water feature every other day during the pumping test. Any increasing or decreasing trends in pumped water quality toward that of the surface water might indicate recharge originating from the surface. Well recharge originating from a surface water feature is commonly referred to as induced infiltration.
Pumping Test Well Configuration
An experienced hydrogeologist should be able to determine the most appropriate pumping well configuration to optimize the yield from the site. This initial decision is based on whether a single or multiple point withdrawal system is appropriate. Factors influencing this decision include depth at which the well is to be screened, areal extent of the soil layer in which the well is screened, soil particle gradation of the soil layer in which the well is screened, and land ownership. A site of relatively shallow saturated thickness (where the depth between the water table and bottom of the screen is less than approximately 20 ft) would indicate that withdrawal should be distributed across the site in an attempt to keep the water level above the well screen during pumping. A typical wellfield includes 2 1/2- or 4-in. wells on a 20 to 30 ft grid. A more innovative approach at a site where the transmissive soil layers diminish away from the preglacial valley or where the saturated thickness is quite shallow is to construct multiple withdrawal points consisting of two or more pumping wells and one well in which the water level is monitored. This configuration helps to combat the inefficiency of a single small diameter well, thus allowing a greater withdrawal throughout the pumping test.
Single withdrawal point pumping tests commonly use a 4-ft diameter circle of five or six 2 1/2-in. pumping wells with one well in the middle of the circle to monitor water levels. This arrangement costs less than installing an 8- or 12-in. diameter test well at a location where the depth to transmissive deposits is not especially deep. The 4-ft diameter circle mimics a gravel-packed production well with an outside gravel pack diameter of 4 ft (a common gravel pack outside diameter used in New England).
At locations where depth to the screened interval is greater than approximately 35 ft and a higher pumping rate is possible, an 8- or 12-in. test well is installed and a vertical turbine pump is usually used. At locations where the maximum withdrawal during a pumping test is desired, a 12-in. well should be installed. The 12-in. well has an efficiency (measure of head loss as water passes through the screen) of approximately 50 to 60 percent, while an 8-in. well has an efficiency of approximately 25 to 30 percent. The use of a 12-in. well reduces the head loss (drawdown) due to high well screen entrance velocities compared to that of an 8-in. well. A 12-in. well also permits larger pump bowls to be used, translating to higher pumping rates. Another benefit is that in highly transmissive sand and gravel deposits, a properly designed 12-in. test well can be left in the ground to be used as the finished production well, thereby saving the additional cost of pulling the test well and installing the production well. A disadvantage of the 12-in. test well is the 20 to 25 percent higher cost compared to that of an 8-in. test well.
A final option is to construct the finished gravel-packed well as the test well. This option is rare and is used only when extensive hydrogeologic information is available or if a pumping test has been conducted at the site in the past. These gravel-packed wells consist of an inner casing and screen (usually between 12 and 24 in. in diameter) encased by a designed gravel pack usually varying in diameter between 18 and 48 in. The actual size depends on the native material, pumping rate, and the inner casing diameter.
Upon installation of the observation wells and pumping test wells, the prolonged pumping test can proceed. Throughout the pumping test, water levels in the surrounding observation wells are measured at scheduled intervals.
Typically the pumping test is run for a minimum of five days and is stopped when the water level in the pumping well or observation well has stabilized. In Massachusetts, stabilization is achieved when the water level in the well does not vary more than 0.5 in. over a time period of 24 hours. Upon shutting off the pump at the site, water levels are once again monitored during what is called the aquifer recovery period. These recovery water levels are monitored until the water level in the pumping well or observation well has returned to a certain percentage of its pre-pumping level. In Massachusetts, this level is 95 percent of the pre-pumping water level.
The observation well water levels recorded during the pumping test are initially reduced to drawdowns (difference between pre-pumping test and end of pumping test water levels) at each observation well. This transformation is completed for both the pumping and the recovery data. The drawdowns are then plotted using three methods: time-draw-down, distance-drawdown, and recovery (residual drawdown) vs. time since pumping stopped. Other methods to evaluate pumping test data are available but are somewhat more complex. The aquifer characteristics, transmissivity and storativity, are determined from the graphs. Transmissivity is a measure of an aquifer's ability to transmit water while storativity is the measure of the volume of water an aquifer can release or take into storage per unit surface area per unit change in head. These aquifer characteristics are then used to theoretically determine the area from which the pumping well(s) will obtain its water, previously referred to as the "capture zone".
Throughout the pumping test, samples of the pumped water are taken for water quality testing. The parameters tested for typically include volatile organic contaminants, general chemistry, inorganic contaminants, herbicides, pesticides, radionuclides (including radon), and coliform bacteria. The results of this testing are compared to standards set by the U.S. EPA to ensure the water produced from the site is of safe drinking water quality.
The Production Well
Following the analysis of the pumping test data and the corresponding capture zone delineation, the production well must be designed and constructed. Shallow wellfields typically consist of 4-in. diameter wells arranged in a fashion similar to that used during the pumping test. At sites where pumping tests used groups of 2 1/2-in. diameter wells, usually larger (8- or 12-in. diameter), more efficient production wells are installed. At sites where larger diameter test wells were pumped, either the previously described gravel packed well or a naturally-developed well is constructed. A naturally-developed well is constructed where the soil material in the screened interval is graded so that the appropriately sized screen will retain the larger soil particles and the head loss of the water passing through it is minimized. The gravel-packed well is the most common design of a deep well.
A common misconception in the development a new municipal water supply is that the process is relatively quick, simple, and inexpensive. In reality, the need for a new municipal water supply should be recognized upwards of five years in advance. This allows time for all potential sites to be investigated, proper review by the state regulatory agencies, and time to obtain the needed funds. The cost of developing a new municipal water supply well is high both in dollars and in time, therefore, best results are achieved by thorough planning. The new source approval process described above is generic in that the state regulations/requirements vary from state to state; however the key points outlined will serve as a roadmap for water departments to find their way through the maze of developing a new water supply.
ROBERT A. LEITCH Project Engineer, Dufresne-Henry, Inc., Westford, Massachusetts
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|Author:||Leitch, Robert A.|
|Date:||Jul 1, 1995|
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