Depth to Water Table and Depth To Water Level in Wells

Author:  Steve Custer
Earth Sciences, Montana State University, Bozeman, MT 59717-3480
Compiled by Matt Trebesch, David Moody, Stewart Dixon and Steve Custer
from data in
Hackett, et al. (1960), Dunn (1977) and the Ground-Water Information Center
Last modified 7 June 2000; 02 January 2010

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Example Questions the Database Can Answer

  1. What is the depth to the water table in the area in 1977?
  2. What is the depth to the water table in the area in 1953?
  3. What is the expected depth to water in water wells in the area based on other wells drilled near by?
  4. Where does depth to water suggest flowing artesian wells?
Additional depth to water table can be obtained for analysis of soil data as well. This map contains four themes:

1.  1977 Water Table Depth
2.  1953 Water Table Depth
3.  Water Level in Wells on the Date Drilled
4.  Flowing Artesian Wells (Wells with water level above ground surface.)

The first two depict areas where Hackett et al. (1960)  and Dunn, (1977) report water table within 10 feet of ground surface. These maps do not cover the whole Local Water Quality District (LWQD).   No absolute water levels are provided.  The map of water level in wells on the date drilled simply reports the water level the driller found when the well was drilled.  As a result, the water levels reported may not be the shallowest that occurs.  Areas of high water level (close to the surface) may reflect high water table or high ground-water pressure.  Which is the case can only be determined by site-specific investigation.  The last map depicts areas where water levels are reported above ground surface (negative depth to water in the well). Soil data on depth to water table (Restrictive Feature called Wetness) may also be helpful given the ambiguity of the water-level data in wells which arises from the difference between water level and water table (see below)  the variable data of measurement of water wells, and the age of the depth-to-water maps.


Glossary

There is a difference between the concept of water table and water-level in wells.  The water table is the location beneath the ground where water saturated rock or sediment is first encountered AND which is linked to the atmosphere through pores as opposed to being isolated from the atmosphere by a confining unit such as a clay layer.  Depth to water in a well used for drinking water or stock water or irrigation is likely NOT the water table.  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 actually has 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.  As a result, the water level in a well not completed to the first saturated sediment but drilled to obtain sustainable water for drinking may not be representative of the water table.  The maps of depth to water by Hackett et al. (1960) and Dunn (1977) can be considered water table depths because many of the wells monitored were hand driven well points.  On the other hand, maps by Slagle et al. (1995) and presented in the ground water gradient grids may be representative of a mixture of water table and ground-water level.   In all cases where very shallow ground water is suspected and home or septic system installation is contemplated,  shallow wells should be drilled to the first saturated zone, and the water levels should be monitored during a period when the highest water levels are expected (commonly July-August in the irrigated valley).  Such monitoring will help determine what if any special drains or septic system installation strategies are required.  Indeed the water table may be sufficiently high that alternative construction sites should be considered.

About the Data

Wtdepth53.shp Metadata
Wtdepth77.shp Metadata
The 1953 and 1977 water table maps and depth to the ground-water table were digitized from Hackett et al., 1960 and from Dunn, 1977 at an original map scale of 1:126720 in polyconic projection.  The depth to water-table  maps are generalized in the original publication to show depth to water more than 10 feet or less than 10 feet).   The Hackett et al. (1960) and Dunn (1977) maps were digitized from maps in polyconic projection and the data was reprojected to UTM Zone 12 NAD83.  The settings for polyconic conversion in Arc/Info were:  Central Meridian: 111 degrees 00  minutes 00 seconds; Latitude of Projection Origin: 45 degrees, 22 minutes, 30 seconds;  False Easting: 0; False Northing: 0.  The Slagle (1995) map was digitized from UTM Zone 12 NAD 27 and reprojected to UTM Zone 12 NAD83.   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 data base.  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.  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 generalization that lots are typically 1 acre in size.

References Cited

Dunn, D.E., 1977, Ground-water levels and ground water chemistry, Gallatin Valley Montana, Blue Ribbons of the Big Sky Areawide Planning Organization report, #11, 62 p.

Hackett, O.M., Visher, F.N., McMurtrey, R.G., and Steinhulber, W.L., 1960, Geology and ground water resources of the Gallatin Valley, Gallatin County, Montana: U.S. Geological survey Water Supply Paper 1482, 282 p.

Slagle, S.E., 1995, Geohydrologic conditions and land use in the Gallatin Valley, South Western Montana: U.S. Geological Survey Water-Resources Investigations Report 95-4030.