The Hydro-dynamics of the Woodbine Aquifer, Collin County, Texas

These materials were developed by Lance Christian, Department of Geography, University of Texas at Austin, 1998. These materials may be used for study, research, and education in not-for-profit applications. If you link to or cite these materials, please credit the author, Lance Christian, The Geographer's Craft Project, Department of Geography, The University of Colorado at Boulder. These materials may not be copied to or issued from another Web server without the author's express permission. Copyright © 1998. All commercial rights are reserved. If you have comments or suggestions, please contact the author or Kenneth E. Foote at k.foote@colorado.edu.

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The Research Question

The water of the Woodbine aquifer supplies water for agriculture and for urban and suburban development in Collin County, Texas.  How has this use altered the aquifer over the past twenty years?

Background

What is an Aquifer?

    We have all heard the term "aquifer" while growing up. It is used in the media, and it has been the subject of numerous political debates in Austin, but what exactly, is an aquifer? Translated from its Latin roots: aqua meaning "water" and ferre  "to bear", aquifer means water-bearing (Long 1974, p.466), but a water bearing what?  The term is applied geologically to rocks which store water within the earth. Hamblin (1989) defines an aquifer as "a permeable stratum or zone below the Earth's surface through which groundwater moves." Although this definition is from a geologic perspective, many hydro-geologists may add an additional parameter to the criteria. The volume of water and rates of flow are also important factors in specifically defining an aquifer. "Other kinds of material that are not good conductors of fluid can be induced to yield their water under special circumstances." (Long 1974, p.466) Therefore, because water can be extracted from a rock stratum does not necessarily mean that it is an aquifer. Drever (1988) addresses this problem with his definition: "an aquifer is a water-saturated rock with sufficient porosity and permeability to be a usable source of water for wells." This introduces a humanistic aspect to an often conceptualized physical phenomenon.

    Groundwater was recognized thousands of years ago and was utilized by the ancients through springs and wells. The source of groundwater, however, puzzled early scientists for centuries. Early theories by the philosphers proposed groundwater replenishment from the seas through "underground rivers" flowing inland, while percolation through the soils filtered the salts from the seawater. During the seventeenth century, two French scientists, P. Perrault and E. Mariotte, established the plausibility of continental precipitation as the source of recharge for groundwater reservoirs. Through experimental measurements of the Seine River they demonstrated that the annual discharge from the river was less than 1/6th of the annual precipitation in its catchment (Long, 1974, p.463). Where was the rest of the water going? Infiltration into the earth was theorized to account for the remaining precipitation, as well as, provide an alternative source of groundwater recharge. In 1856, modern studies of groundwater began with a French engineer named Henri Darcy "who was commissioned to develop a water-purification system for the city of Dijon, France". (Long, 1974, p.466) He constructed the first experimental apparatus to study the flow characteristics of water through the earth. From his experiments, he derived an equation known as Darcy's Law. This law describes the flow of water in nature and has become one of the fundamental building blocks in understanding groundwater systems.
 

Aquifers and the Hydrologic Cycle

    A natural theologian named John Ray presented a simplistic model of the hydrologic cycle in the seventeenth century . His model was generally accepted and though over-simplistic by todays standards, has become the standard model that appears in textbooks today (Hamblin, 1989 p.22) (Figure 1).

    Many different rock types may comprise an aquifer. In the Hawaiian islands, volcanic rocks provide a source of water while in the Texas Panhandle, the Ogallala sandstone provides millions of gallons a year for farming irrigation. Conglomerates may also comprise aquifers, but many of the most prolific aquifers occur in limestones. Large reserves of groundwater are stored in limestone aquifers in Florida and the Edwards Aquifer of Central Texas.
 

Geology of the Study Area

    Geologically speaking, north Texas is a relatively undisturbed straight stratigraphic column. Figure 2. is a cross-sectional diagram showing the underlying stratigraphy of the study area. The stratigraphy, or the division of the rocks into distinct units, is a time constant column without any major 'interruptions' or erosional events. It contains all of the representative rock units (or their age equivalents) known in Texas from the Cretaceous Period which is a unit of time from approximately 65 to 140 million years ago. Beneath the Cretaceous rocks lies a basement of older rocks from the Pennsylvanian and Permian Periods of the Paleozoic Era.

The Woodbine Aquifer

    The Woodbine Aquifer is one of several aquifers that underlies north Texas. It is particularly interesting because it underlies both prime agricultural land as well as land that is experiencing rapid suburban development. There was once a great demand put upon the aquifers of the region although the last few decades the area has begun relying on surface water to supply its needs. The North Texas Municipal Water District is the region's governing body on water supply affairs, and it oversees the numerous reservoirs built in the area during the last 40 or 50 years. North Texas receives enough precipitation to make surface water supply feasible, as opposed to some parts of the drier west which must rely heavily upon groundwater supplies. Today the Woodbine Aquifer is mainly used by small municipalities, irrigation, and industrial purposes. Heavy pumping for municipal and industrial water supplies in the Sherman-Denison area have led to water-level declines in excess of 100 feet. (Hopkins 1996) The undisturbed (unpumped) potentiometric surface typically parallels the dip direction of the sandstone unit.

    The Woodbine Sandstone is composed of fine-grained, cross-stratified fluvial sandstone interbedded with overbank deposits of clay and shale. (Hopkins, 1996) The unit dips eastward reaching a depth of 2,500 feet below the land surface while the regional dip is southeastward averaging approximately 35 feet per mile. Near the boundary of fresh to slightly saline water in the eastern portion of the aquifer, the dip of the rock unit increases to 75 feet per mile. The sandstone thickens both downdip and to the northeast achieving thicknesses of approximately 230 feet in the southern extent of the outcrop to 700 feet in the northeast. (Hopkins, 1996) Figure 2.

    The aquifer is divided into three water-bearing zones in the northern segment of the outcrop. Each zone varies significantly in productivity and quality, and only the lower two zones are being utilized for domestic and municipal water supplies. The upper zone often yields water with concentrations of iron that are too high to be a usable supply. Further to the east (and some to the south) the Woodbine produces both oil and gas. (Hopkins, 1996)