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Chapter 3 HYDROGEOLOGY

INTRODUCTION

The groundwater systems of interest in the Oklahoma County area can be classified into two distinct groups ­ the bedrock aquifer system (Garber-Wellington aquifer) and the Quaternary system (alluvial aquifers), which is generally redeposited bedrock found in close proximity to the river systems such as the North Canadian River. The Garber-Wellington formation is the major aquifer in Central Oklahoma. The GarberWellington Aquifer is Lower Permian, Leonardian in age (Woods and Burton, 1968, Simpson, 1973). The water-bearing portions of the Garber and Wellington formations cover an area roughly two thousand square miles and contain approximately 5 trillion gallons of water. Over 400 public water-supply wells and more than 20,000 domestic wells tap into this resource. Figure 3-2 shows the generalized area of the Garber-Wellington aquifer, which covers most of Oklahoma and Cleveland Counties. The saturated thickness of the aquifer is quite variable. In the center of the aquifer near Draper Lake, fresh water can be found at depths of 1,200 feet. Saltwater is encountered at more shallow depths as one approaches nearing the edges of the aquifer, usually at about 400 feet around the city of Guthrie in Logan County and south of Slaughterville in Cleveland County (Figure 3-3). The western edge of the aquifer can be generally described as terminating at the Oklahoma-Canadian County border. Well yields in this aquifer vary considerably; a municipal well generally produces in the range of 100-250 gallons per minute (gpm), depending on the thickness of the saturated section. Occasionally wells may reach up to 400 gpm, depending on location and style of well construction. The productive alluvial aquifers in Oklahoma County are generally confined to the area within one mile of either side of the North Canadian River. The thickness of this aquifer generally averages 50 feet, although in the river channel itself the thickness may be up to 150 feet. Agricultural activities requiring large amounts of water can drill irrigation wells that produce 400 gpm in these areas.

REGIONAL GROUNDWATER CHEMISTRY

Groundwater quality data spans about 25 years in this area. Much of the groundwater quality data represents discrete sampling events from the Hennessey Shale, Garber-Wellington sandstones, and the alluvial terrace deposits. The water can be generally characterized as bicarbonate-rich, hard water in the shallow portions of the aquifer with softer, sodium-rich water towards the base of fresh water. Figure 3-1 is a bar chart showing the ranges of the major ions commonly found in central Oklahoma groundwater. Groundwater chemistry in each of the geologic units is quite different. Alluvial terrace water is highly variable, depending on land uses such as agribusiness. The alluvial terrace deposits contain extremely variable quality water depending on source. During the rainy months of April and May, the water may reflect runoff chemistries. These waters can be high in nitrates, sulfates, and occasionally chlorides.

Association of Central Oklahoma Governments Garber-Wellington Association Assistance ID C6-400000-44 Grant Number: 604(b) FY: 2004 Task: 300 Page 16 of 80

Figure 3-1 Ranges of Selected Ions in Central Oklahoma Groundwater (ACOG, 2004)

Water qualities in the bedrock units generally reflect the geology of the unit. Hennessey Shale groundwater is generally low quality, while Garber-Wellington formation groundwater is high quality. The waters in the Hennessey Shale are quite hard, containing heavy metals and high levels of sulfates, sodium, chlorides and total dissolved solids (TDS). While groundwater chemistry changes occur in the Garber-Wellington Aquifer, they are not as dramatic as the alluvial terrace waters. The water quality of the aquifer is good, especially in the eastern portion of the aquifer, where the chemistry rarely varies. The water is slightly hard to hard and contains few other elements. In the western portion of the aquifer, the GarberWellington dips deeper into the subsurface. In some wells in the western portion of the aquifer, water quality deteriorates as TDS, sodium, chlorides and metals increase.

WELL CONSTRUCTION

Well construction is an important consideration in any groundwater analysis, since the construction directly affects the chemistry of the water sampled in many cases. Several styles of well construction are found in the central Oklahoma area. This discussion will be limited to the domestic well, which is the dominant well type in the project area.

Domestic Wells

Most of the operational domestic wells are screened and gravel pack construction. Eight and one-half to 12-inch boreholes are drilled. Casing installation is usually a matter of driller's preference and experience in the area ­ rarely are samples taken or a well log run. The domestic wells completed in the Garber-Wellington Aquifer can be quite varied in depth and construction. Most wells are 100-500 feet deep and cased with five- to seven- inch steel casing. The bottom 25-200 feet of the casing is slotted. The entire casing except the top ten feet is gravel packed with 15-20 to 30-40 "Colorado Frac Sand". The top ten feet of the casing must be cemented to reduce surface water pollution. These wells yield 10-100 gpm. A vast majority of these domestic wells only penetrate the upper portion of the aquifer.

Association of Central Oklahoma Governments Garber-Wellington Association Assistance ID C6-400000-44 Grant Number: 604(b) FY: 2004 Task: 300 Page 17 of 80

Figure 3-2 Garber-Wellington Aquifer Limits (ACOG, 2004)

Association of Central Oklahoma Governments Garber-Wellington Association Assistance ID C6-400000-44 Grant Number: 604(b) FY: 2004 Task: 300 Page 18 of 80

Figure 3-3 Approximate Depths to Saltwater (ACOG, 2004)

Association of Central Oklahoma Governments Garber-Wellington Association Assistance ID C6-400000-44 Grant Number: 604(b) FY: 2004 Task: 300 Page 19 of 80

Figure 3-4 Yield Statistics on Domestic Wells in the Thunderhead Hills Area (OWRB, 2007)

Association of Central Oklahoma Governments Garber-Wellington Association Assistance ID C6-400000-44 Grant Number: 604(b) FY: 2004 Task: 300 Page 20 of 80

Figure 3-4 illustrates some statistics concerning domestic wells in the Thunderhead Hills area. As one can see, domestic wells in the area range from approximately 100-250 feet in depth, although an occasional well can go as deep as 450 feet. Yields do not necessarily correspond with depth; a well 250 feet deep would have about 170 feet of saturated section, yet may yield anywhere from 5 to 50+ gallons per minute (gpm), depending on the local transmissivity of the sand. A histogram of the domestic yields is bimodal. Most wells have yields in the 5 gpm range for household purposes, but another group of domestic wells have yields peaking in the 25 gpm range, presumably for small irrigation uses. Figure 3-5 shows a startling increase in the drilling activity in 2006. This triple-fold activity in domestic well drilling is due to a confluence of a number of factors ­ the start of the latest drought cycle, the increased development in the east side of Edmond, better record keeping among the well drillers ­ all have contributed to the apparent surge of water well drilling in the area. The combination of the drought, plus the development activity, will ensure that domestic drilling levels will remain robust for the next few years.

Figure 3-5 Drilling Activity for Domestic Wells in the Thunderhead Hills Area (OWRB, 2007)

Association of Central Oklahoma Governments Garber-Wellington Association Assistance ID C6-400000-44 Grant Number: 604(b) FY: 2004 Task: 300 Page 21 of 80

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