Kansas Geological Survey, Open-file Report 2007-28
by
P. Allen Macfarlane and Nicholas Schneider
KGS Open File Report 2007-28
Ground-water availability from the High Plains aquifer in Southwest Kansas Ground-water Management District 3 (GMD 3) is declining. Water-use density, defined as the annual amount of water pumped from the aquifer per unit of land area, is high over much of GMD 3 and greatly exceeds annual recharge. Coupled with the fact that the Ogallala is a classic example of a regionally unconfined heterogeneous aquifer, these factors support subdividing the aquifer into smaller subunits for tailored management of the remaining water resource. These factors argue for a tailored approach to manage the remaining supplies in the aquifer. A key element in determining the best method to subdivide the High Plains aquifer into smaller units for management purposes is the geospatial distribution of permeable, water-yielding zones. One approach to characterizing the distribution of permeable zones within the High Plains aquifer is to mine the information contained in the thousands of drillers' logs of test holes and production wells drilled for water supply in GMD 3. The sheer abundance of these logs makes them an attractive source of information even though many geologists and hydrogeologists downplay their value.
Managers and planners use saturated thickness (ST) as a measure of ground-water availability in unconfined aquifers. However, in using ST for this purpose, it is assumed that all water- saturated sediments contribute water to pumping wells equally. The practical saturated thickness (PST) concept considers only the net thickness of saturated sediments that significantly contribute to well yield. Thus, PST provides a more accurate picture of water availability and may also provide insight into future water-level trends at the scale of an individual well.
We conducted a project in GMD 3 to demonstrate the utility of the PST concept. The project objectives were to (1) delineate the 3- D distribution of permeable zones within the saturated and unsaturated Ogallala portion of the High Plains aquifer in GMD 3 derived from drillers' logs and (2) map the PST in order to assess the change in water availability from predevelopment through the year 2006.
In the order of their abundance, the data sources used in this project included (1) WWC-5 records of water wells completed since 1975 and maintained at the Kansas Geological Survey (KGS), (2) test-hole logs from water-well contractors, and (3) test-hole logs from the KGS county bulletins. A driller's log was selected preliminarily for inclusion in the project database if it appeared from the narrative description that the borehole penetrated the bedrock surface. There are 7,195 drillers' logs currently in the project database. Point-based well and test-hole locations were converted into a Geographic Information System (GIS) data layer based on geographic coordinates (public land survey or latitude-longitude) for each well location. The USGS National Elevation Dataset (NED) 30 m x 30 m grid was used to estimate a surface elevation for each well location.
Descriptions of the drill cuttings recorded on a driller's log are highly individualized and reflect the driller's experience and his observational and writing skills. For this project, permeable sediment fraction determination required formulation of rules to translate log-entry phrasing into the relative proportion of the permeable sediments within each interval noted on the driller's log. These rules were based on drillers' terminology, the phrasing of their descriptions, the authors' experience with interpretation of drill cuttings in the field, and interviews with drilling contractors. Permeable sediment thickness for each described interval was determined from interval permeable sediment fraction and its thickness.
Maps of permeable sediment thickness and fraction for the entire GMD were prepared from coverages developed in ARC-GIS software. To better interpret the ARC-GIS produced maps, two subregions and one local area of GMD3 were selected for detailed subsurface examination of the Cenozoic sequence using RockWorks 2004® software to produce fence diagrams derived from 3-D models. The two subregions of the aquifer included a (1) 1,296-mi2 area referred to as the central GMD 3 subregion, and a (2) 4,248-mi2 area referred to as the strip subregion. A 144- mi2 local study area centered on the recently drilled observation well and included in both subregions was also investigated to better understand local heterogeneity and its influence on larger-scale patterns of permeability revealed in the subregional models.
Across GMD 3 the permeable fraction varies from near 0 to 100% with mean and median values slightly higher than 50% and in the central part, the sequence is thicker and a higher percentage of it is permeable than elsewhere. The fence diagrams indicate that over most of both subregional areas 40% to 80% of the late Cenozoic sequence is in the permeable range. Intermediate permeable fraction values suggest that regionally the lower saturated part of the aquifer consists locally of thin, highly permeable zones interlayered with non-productive zones. Intervals of sediment greater than 80% permeable are considered highly permeable, whereas intervals less than 40% permeable are considered barriers to ground-water flow or confining layers in the saturated part of the sequence. In the subsurface, thick, subregionally extensive intervals of highly permeable sediment are present at various levels in this part of the GMD 3. The existence of these zones is a clear indication that recognizable, preferred pathways for lateral and vertical water transmission exist within the sequence. Likewise, sinuous and sheet-like zones of less permeable sediment can be traced laterally over appreciable distances in the subsurface covering large swaths of GMD 3 and could be considered subregional, leaky confining layers. The fence diagrams also reveal that the deepest parts of the sequence do not necessarily consist of highly permeable deposits.
The range and distribution of predevelopment PST thickness and PST/ST fraction values are similar to that of total permeable thickness and fraction and suggest that water availability was much less than previously assumed based on ST. Changes in the PST/ST fraction from predevelopment to 2006 were pronounced and not uniform and reflect variability in water-use density across GMD 3 and locally the permeability fraction distribution. Water-use density is highest in the central part of GMD 3 and these areas have seen the greatest decline in PST thickness from predevelopment to 2006. Significant decreases in the PST/ST fraction have not occurred across GMD 3. However, the PST/ST fraction has increased in some local areas and decreased in others. The arrangement of water-producing and non-producing zones implies that high annual decline rates are likely to continue and possibly accelerate depending on the local connectivity of these zones.
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Kansas Geological Survey, Geohydrology
Placed online Feb. 9, 2009
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