Data for Wells with M-Number

Author:  Steve Custer
Data Compiled by: Stewart Dixon, Steve Custer,  Robert Snyder, and Julie LaBranche
Last modified 15 June 2000; 02 January 2010

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Example Questions the Database Can Answer

1.  What are the M-Numbers for wells nearby?
2.  Where are wells that may have drillers logs?
3.  What is the well yield reported by drillers for  wells drilled in the area?
4.  What is the specific capacity reported by drillers for wells drilled in the area?
5.  How deep are wells in the area?
6.  What is the water level reported by drillers on the date the well was drilled in this area?
7.  Are there wells that have been redrilled or deepened in the area?
8.  What are the nitrate levels reported for ground water in the area?
9.  What total water hardness is expected for ground water in the area?
10.  Are there wells that have data on water level through time in the area?

This coverage contains 9 themes.  These data are described further in the section about the data.

1.  Well Location  are only shown for wells are that have records in the Ground-Water Information Center and can be located on a Gallatin County road book through address or subdivision block and lot.  Each point has many attributes which can be determined by highlighting the well location coverage, clicking on the "i" tool and then clicking on the well point or by examination of the attribute table.  Wells which could not be located on the maps may exist but are not included in this theme.  These wells can be found in the Ground-Water Information Center database.
2.  Well Yield shows the yield reported by drillers at the time the wells were drilled in gallons per minute.
3. The Specific Capacity theme shows the specific capacity for wells located in the LWQD. See the  glossary for discussion of specific capacity.
4.  The Well Depth theme shows the total well depths reported by drillers to the Ground-Water Information Center database for wells located on the County Road Book (1995).
5.  Water Level in Wells on the Date Drilled provides some perspective regarding water levels but the level depends in part on which aquifer the well is completed in and the levels may change through the year.  This data does not necessarily reflect the water table (see discussion in the Depth to Water Table Page)
6. The  Redrilled Wells theme shows where wells that have been deepened or redrilled.
7.  Nitrate represents the nitrate-N values in mg/l reported to the Montana Bureau of Mines and Geology.
8.  Hardness represents the calculated hardness of ground water for which calcium and magnesium data are available. The data are reported as grains per gallon.  Grains per gallon can be converted to mg/l by multiplying by 0.0584.  Similarly, mg/l can be converted to grains per gallon of total hardness expressed as calcite by multiplying by 17.1.    The formula used to calculate total hardness as mg/l calcite is:

2.5 * Ca (mg/l) +4.1 Mg (mg/l) = Total hardness in mg/l calcite.

9.  The Hydrograph Location theme shows where the Ground Water Monitoring Program of the Montana Bureau of Mines and Geology has multiple measurements of water level in wells through time (a hydrograph of water levels).


Glossary

(Additional information on word meanings can be obtained from the Ground-Water Information Center by clicking on their glossary.)

Well Yield

Well yield is the rate of water production the driller reported at the time of drilling to the Montana Bureau of Mines and Geology Ground-Water Information Center.  The yield is reported in gallons per minute (gpm).  The reported yield depends on a variety of factors including the size of the well which influences pump size, method of water extraction used by the driller (bailer, air lift, or pump are commonly used), whether the well is screened, and the aquifer characteristics.

Well Depth

Well depth refers to the total depth to which the well was drilled as reported by the driller to the Montana Bureau of Mines and Geology Ground Water Information Center.  If the hole is left open and material falls into the hole from the walls or there are quick conditions (like quick sand), the well depth may actually be a different depth than that at the time of drilling.

Specific Capacity

Specific capacity refers to the well yield divided by drawdown during a pumping test.  The english units for this parameter are gallons per minute per foot of drawdown (gpm/ft).  Drawdown is the difference between the water level in the well before pumping began and the water level in the well after the pump has been running for some time.  The specific capacity varies somewhat with pumping rate and pumping time.  As a result the specific capacity is at best an approximate value.  Specific capacity  can be used to estimate transmissivity because a relationship between these two terms has been developed from  recovery tests in the Gallatin Valley (Dixon, In preparation).  The calculation has been made for many of the wells in the LWQD and is an attribute in the well coverage. Specific capacity and transmissivity data are presented in the attribute table in both english and metric units.  Specific capacity can be used to estimate the maximum expected well yield.  The difference between water level and the total well depth minus about 10 feet for the pump is the available drawdown (feet).  If one multiplies the specific capacity (gpm/ft) times the available drawdown (ft), the expected maximum yield can be estimated. Considerable caution should be used with this approach given the approximate nature of the specific capacity values.  A factor of safety should be used.  The result is an estimate and an on-site pumping test conducted by a ground-water professional is strongly urged.

Water level

Water level is the depth from ground surface to the water in the well.

Water Table

Water table is the location where pressure head is zero in an unconfined aquifer.  There may be considerable difference between water table and water level.    For ground-water wells, there is often a pressure component that raises the water level above the bottom of the casing.  This level may or may not reflect the water table.  An extreme example of this issue is the flowing artesian well.  In a flowing artesian well,  the ground water is under sufficient pressure to rise above the ground surface and flow onto the ground.  The water level in the well in this case cannot reflect the water table, for if it did the area would be under water.  Similarly, there may be pressure in the well which does not take the water level above ground surface, but raises the water level in the well above the water table none the less.  The reverse can also occur where the ground-water-flow system is recharging and the pressures may actually be lower and water levels may be below the water table. The water table is at the depth where the well driller first encounters water in an aquifer open to the atmosphere and not constrained by a confining unit such as a clay layer.

Redrilled Well

A redrilled well is one that has been deepened or has failed and a new well is drilled for a new supply of water.  There are many possible explanations for why such redrilling might occur.  These include casing damage, need for a higher capacity well for a new use, occlusion of well screen by rust or biofilms, or falling water level in the well. A redrilled well may not be reported if a new well is drilled and is perceived to simply be a new well by the reporting individual.

Nitrate

Nitrate represents the nitrate values in mg/l reported to the Montana Bureau of Mines and Geology.  The data is reported as mg/l nitrate-nitrogen (nitrate-N).  The Maximum Contaminant Level (MCL) for Nitrate recommended by the Environmental Protection Agency is 10 mg/l.  This MCL value is equivalent to a level of 45 mg/l expressed as nitrate rather than nitrate-N.  Value may represent nitrate + nitrite- N if preserved in the field.

Hardness

Hardness reflects the tendency of the ground water to scale in boilers and hot water heaters and the amount of soap needed to produce suds.  The harder the water the more scale (lost hot water heating efficiency) and the more soap to create suds. Hardness represents the calculated hardness of ground water for which calcium and magnesium data are available. The data are reported as grains per gallon.  Grains per gallon can be converted to mg/l by multiplying by 0.0584.  Similarly, mg/l can be converted to grains per gallon of total hardness expressed as calcite by multiplying by 17.1.    The formula used to calculate total hardness as mg/l calcite is:

2.5 * Ca (mg/l) +4.1 Mg (mg/l) = Total hardness in mg/l calcite.

Hydrograph

A hydrograph is a graph of water level through time.  Time is usually on the x-axis and water level on the y-axis.  A hydrograph provides insight into water level trends and whether the water levels are rising and falling in response to recharge and discharge as the seasons progress.  Hydrograph decline can be caused by a variety of factors such as droughts, or elimination of flood irrigation,  or ground-water withdrawal at rates greater than recharge rates.  Hydrograph rise can be caused  by a variety of factors such as wet years, cessation of water withdrawal, or increases in irrigation.

About the Data

Welfnl.shp Metadata
The water level maps were created from data in the Ground Water Information Center (GWIC). This data represents the water level in wells on the date drilled.  Not all wells were allowed into the database.  Wells were located based upon address and subdivision block and lot data.  The locations of the wells were marked on a Gallatin County road book published in 1995 which shows structures as dots.  If the address or subdivision block and lot matched the road book, the dot was entered and a  M-Number (GWIC identification number) was noted.  There is some potential for the owner address to be incorrect since owner address may reflect the location the person lived while the well was drilled and not the location of the well.  The owner address was used only if there was evidence that the address reflected the location of the well via septic permit records, subdivision name block and lot, or address of owner the same as the address in the phone book if the well was drilled prior to 1997.  Some potential for error still exists since rentals with an owner address and no subdivision might be incorrectly associated with the place of residence of the rental owner and not the well location.  The data was digitized from the road book by coordinating to the section corners and entering a code.  Later information from the GWIC system was added to the coverage by linking on the M-Number.  Where more than one well existed at a site, additional wells were "spawned" 10 m from the original well so each well was unique. The first well was spawed to the east, the second to the south, the third to the west, the fourth to the north, and the fifth (if present) to the northeast.   The map of water level in the well  and of flowing artesian wells uses depth to water information from GWIC available for wells that were located.  Well location is estimated to be within 100 m of the correct location based on the idea that lots are typically 1 acre in size.

1.  Well Location  Wells were located based upon address and subdivision block and lot data.  The locations of the wells were marked on a Gallatin County road book published in 1995 which shows structures as dots.  If the address or subdivision block and lot matched the road book, the dot was entered and a  M-Number (GWIC identification number) was noted.  This data was digitized from the road book by coordinating to the section corners and entering a code.  Later information from the GWIC system was added to the coverage by linking on the M-Number.  Where more than one well existed at a site, additional wells were "spawned" 5 m from the original well so each well was unique.  Well location is estimated to be within m of the correct location based on the fact  that lots are typically 1 acre in size (43000 square feet) and about 200 feet on a side.  Since locations are based on lot location the location is likely better than +-200 feet or 100 m. The wells shown are those that  have records in the Ground-Water Information Center and can be located on a Gallatin County road book through address or subdivision block and lot.  Note these wells have many attributes attached to them that can be obtained from the Ground-Water Information Center or from the ArcView table of attributes either through the "i" tool, or by selection of wells in an area and examination of the attribute table.

2.  Well Yield shows the yield reported by drillers at the time the wells were drilled.  The yields are reported in gallons per minute.  The yields are to some degree controlled by the method of testing (pump, bail, air lift) and by the intended use. Wells which could not be located on the maps may exist but are not included in this theme.  These wells can be found in the Ground-Water Information Center database.

3. The Specific Capacity theme shows the specific capacity for wells located in the LWQD. See the  glossary for discussion of specific capacity. The specific capacity was calculated by dividing the well yield by the difference between water level and pumping level for wells that had yields determined by pumping or air lift.  Bailed wells were excluded from the data set because bailed wells were found to have significantly lower specific capacities than wells that were either pumped or air lifted.  Specific Capacity for air lift and pumped wells could not be distinguished in single geologic units (Dixon, in preparation). Wells which could not be located on the maps may exist but are not included in this theme.  These wells can be found in the Ground-Water Information Center database.

4.  The Well Depth theme shows the total well depths reported by drillers to the Ground-Water Information Center database for wells located on the County Road Book (1995).  Wells which could not be located on the maps may exist but are not included in this theme.  These wells can be found in the Ground-Water Information Center database.

5.  Water Level in Wells on the date drilled provides some perspective regarding water levels but the level depends in part on which aquifer the well is completed in and the levels may change through the year and between years.  This data does not necessarily reflect the water table.  (Further discussion is presented in the Depth to Water Table Page).  This data should NOT be interpreted as depth to water table without site investigation and careful assessment of the data.  The depth to water data does provide a sense of what typical water levels are in the area, however.  The depth to water can be expected to vary with date, aquifer penetrated, and climatic patterns and the like.  The water level maps were created from data in the Ground Water Information Center (GWIC)

6. The  Redrilled Wells theme shows where wells that have been deepened or redrilled as reported by the Ground-Water Information Center.  There are many possible explanations for why such redrilling might occur.  These include casing damage, need for a higher capacity well for a new use, occlusion of well screen, or falling water level in the well. A redrilled well may not be reported if a new well is drilled and is perceived to simply be a well by the reporting individual.  Wells which could not be located on the maps may exist but are not included in this theme.  These wells can be found in the Ground-Water Information Center database.

7.  Nitrate represents the nitrate values in mg/l reported to the Montana Bureau of Mines and Geology.  The data is reported as mg/l nitrate-nitrogen (nitrate-N).  The small number of data points results from lack of coordinated reporting from laboratories throughout the state.  Wells which could not be located on the maps may exist but are not included in this theme.  These wells can be found in the Ground-Water Information Center database.

8.  Hardness represents the calculated hardness of ground water for which calcium and magnesium data are available. The data are reported as grains per gallon.  Grains per gallon can be converted to mg/l by multiplying by 0.0584.  Similarly, mg/l can be converted to grains per gallon of total hardness expressed as calcite by multiplying by 17.1.    The formula used to calculate total hardness as mg/l calcite is:

2.5 * Ca (mg/l) +4.1 Mg (mg/l) = Total hardness in mg/l calcite.

Wells which could not be located on the maps may exist but are not included in this theme.  These wells can be found in the Ground-Water Information Center database.

9.  The Hydrograph Location theme shows where the Ground Water Monitoring Program of the Montana Bureau of Mines and Geology has multiple measurements of water level in wells through time (a hydrograph of water levels).  This data can be obtained from the Ground-Water Information Center if you determine the Township, Range and Section from the LWQD sections theme or if you click on the point of interest and find the M-Number with the theme activated.  Wells which could not be located on the maps may exist but are not included in this theme.  These wells can be found in the Ground-Water Information Center database.

The attributes in the wells attribute table are defined in the attached file.