Good well design prioritizes important information
- Intended use
- Incorporates exploratory borehole data with background information
- Economics – construction and pumping costs
The water well design phase is where all the data generated during the preliminary phases of the project are drawn together. There are many projects where key elements are unavailable, it is necessary call on experience gained from completing similar water wells. Accurate records and uniform field procedures can greatly increase a water wells chances of success.
“Falling Water” Destructive to well performance
- Air becomes entrained when water falls in the well from perforation is above the pumping water level
- Results in pump bowl cavitations
- Excessive wear pump impellers
- Significant drop in pump efficiencies
- Not a good thing
Water coming into a well above that pumping water level “falls” into the water above the pump bowl unit. As a result air bubbles become entrained in the water and are carried down in the pump bowl, or turbine. The air bubbles are “vaporpockets”. At some point the liquid will reach an area of higher pressure, causing the vapor pockets to collapse at such a rapid rate that a rumbling noise can be heard. The collapse of these pockets is so violent that it causes pitting on the impeller and bowl surface.
The only solution is construct the well in such a way that water does not enter into the well in an area open to the atmosphere.
Solutions to “Falling Water”
- Don’t perforate well casing in areas of shallow water bearing formations
- Outer Casing Design
- Air tight gasket seal below pump head
- Inflatable “boot”
The best solution is to avoid screening water bearing areas that may become exposed to the atmosphere. This is a condition that may only occur in the later part of the irrigation season.
When shallow areas are relied on to provide enough water bearing formation to meet the desired production, the use of an outer casing allows the water to enter the well below the pump bowl, and not be exposed to the atmosphere.
Gaskets below the pump head, and pump boots require maintenance, and rely on the casing being air tight.
Water Sampling Well Design
The top portion of the figure shows how a multiple zone sampling well is constructed. Note the seal intervals deep in the bore hole, these seals are designed to prevent the exchange of water between the different intervals of water bearing formation and differing water quality.
Large Diameter Pumping Well Design
When a water sampling well is completed, water quality analysis can be used to design a final water supply pumping well. Note in the final pumping well design it was decided not to include the water bearing intervals above 530ft. It was determined that the water quality was unsuitable in these zones for public water supply. The new well was constructed with the sampling well data, and the water quality from the new is suitable for public use with no required treatment.
Definition of Terms Relating To Well Performance
- Static Water Level (SWL)
- Pumping Water Level (PWL)
- Drawdown Residual Drawdown
Specific CapacityThe static water level at which water stands in a well or unconfined aquifer when no water is being removed from the aquifer either by pumping or free flow.
The pumping water level is where water stands in a well when pumping is in progress. In the case of an artesian well, it is the above ground level at which water flows from the well.
Drawdown is the difference between the static water level and the pumping water level. This difference represents the head of water (force) that causes water to flow through an aquifer toward a well at the rate that water is being withdrawn from the well.
Residual Drawdown- After pumping is stopped, the water level rises and approaches the static water level before pumping began. During the water-level recovery, the distance between the water level and the initial static water level is called residual drawdown (Quick Come back)
Yield is the volume of water per unit of time, discharged from a well, either by pumping or free flow. It is measured commonly as a pumping rate in gallons per minute or cubic meters per day.
Specific capacity of a well is its yield per unit of drawdown.
Key elements to consider for good well design
- Elevation of proposed site
- Anticipated static water level
- Estimated yield
- Anticipated production rate
- Best guess of pumping water level
- Adjust for seasonal fluctuations
There are a number of important factors that are reviewed prior to finalizing a well design. Developing a successful well in alluvial deposits include these key elements. A preliminary review of any and all relevant data: Static water levels from any existing wells; elevation of a proposed well site; any water quality parameters that must be met by the new well; placement of the new well in relation to existing or proposed water conveyance systems.
Elevation of proposed site
When working on a multiple well project it is important to consider elevation changes and the effects on the pumping water levels below the ground surface. In designing a new water well it is very important not to use any production intervals in the well thatare located above the pumping water level. If the pumping water level is drawn below the top production interval, pump bowl cavitations will result from the entrained air.
Accurate maps with GPS locations are very useful in correlating well information. The map indicates the distance between test holes drilled on a multiple well project and indicated the changes in elevation.
Water well locations and logs can now be kept in a data base, and accurately mapped with GPS navigation. This data base of information is very use full in replacing and improving wells in developed areas. This a really good way to organize unused test hole information for multiple wells on a single parcel. Large projects were many wells are drilled over many years are much easier with the new technology.
Water Well Construction
If a water well were a puzzle, this is were the pieces go together
Well Construction Materials
By methodically completing all the steps we have presented, the new water well construction phase of a project can be completed with few surprises. All well construction materials should be placed at the drilling site prior to the start of the final borehole, minimizing the possibility of avoidable delays. The faster the well can be completed the better the outcome of the project.
Reaming the Borehole
Large 28-inch bore holes are commonly drilled by the reverse circulation method in a single pass. In cemented deposits of alluvial transported formations it is necessary to expand the diameter of the bore hole through repeated passes, reaming the hole with increasingly larger sized bits.
- Construction Materials
- Well Casing
- Stainless Steel
- PVC Plastic