The unique hydrologic connection between surface and groundwater in our area is why the City of Austin sought (and received) voter approval to purchase lands for water quality protection. It also explains why these lands are located where they are. To understand the connection, it is necessary to consider how water is moving through our area. While the movement of water across the topography of the area is logical and fairly straightforward, the characteristics of our local segment of the Edwards aquifer are more complicated, and at times contradict topography. Understanding groundwater flow relies on field studies and scientific data and some conceptualization to put it all together. So let's start with what is visible.

On the surface
Water that flows across the surface of the earth is referred to as "surface water" and includes creek, streams, rivers, and lakes. Its source can be rainfall or spring flow, the latter relating to groundwater that will be discussed later. Simply put, since land on the surface of the earth has changes in elevation (topography)-ranging from the obvious (mountains) to the subtle (plains)-the influence of gravity moves water from higher to lower points resulting in water flow. When a collection of high points or ridges direct water flow towards a common creek or river, it creates a drainage area or watershed. Watersheds vary in size and in the Austin area, small canyons or washes among rolling hills may only be referred to as "drainages" or "sub-watersheds" that are part of a larger system. Watersheds that drain to a specific creek or waterway are named in conjunction with the creek, i.e. Barton Creek Watershed.

The boundaries of a watershed, the high points and ridges, serve as dividing lines between different drainages. Rainfall that falls on one side of the boundary flows to one creek, while rainfall falling on the other side of the boundary flows to the other creek. This has resulted in a logical means of addressing pollutants or other environmental issues, referred to as the "watershed approach". (Read more on this approach at the U.S. EPA website. The U.S. Environmental Protection Agency has supported this approach throughout the country as a better way of addressing water quality issues in a streams, rivers, lakes, and their tributaries.

Everyone, not just in Austin but across the globe, lives in a watershed, though certainly some are larger than others. Activities in a watershed can have an impact, positive or negative, on the amount and quality of water that flows into creeks. Wildlands are areas where the land and ecosystem are not disturbed for purposes of agriculture, urban, or industrial use. In these areas, the land can function in its natural condition, as both a sponge and filter, absorbing rainfall, filtering it through plants communities and layers of soil and roots, and slowly releasing it into creeks. Alternately, some landscapes are changed by adding impervious cover such as roadways and rooftops, sculpting the terrain which changes water flow, or removing parts of the land in quarries and mines. These changes can affect the amount of water reaching a creek, as well as contribute contaminants that degrade water quality.

For these reasons, watersheds can be a way to organize and plan land management activities since those activities are believed to have a focused affect on the water quantity and quality of a particular creek. The City of Austin's Water Quality Protection Land program manages land in the Bull Creek, Barton Creek, Slaughter Creek, Bear Creek, and Onion Creek watersheds. All of these creeks contribute flow to the Colorado River, Bull Creek from the north side of the river and all others from the south side. Yet, surface water is only part of the story and doesn't solely explain the location of the lands. That answer requires a trip underground.

Underground
The rock underlying the Austin area that is often visible in road cuts and natural areas is predominantly limestone. It is formed from sedimentary processes resulting from the existence of shallow seas advancing and retreating over portions of what we know as Texas over hundreds of millions of years. A closer look at the layers of limestone will reveal that they have different characteristics and properties, among them their tendency to erode through natural processes. Faulting along what we now call the Balcones Fault Zone resulted in the eastern side of the zone dropping as much as 700 feet and disrupting continuous east/west trending layers of rock in our area. Rainfall, being naturally a bit acidic when it falls, chemically reacts with the alkaline layers of limestone to varying degrees and erodes away openings in the rocks. Water can also combine with organic elements in localized areas of soil to chemically react and erode rock inches or feet below the surface. Weathering effects such as tree roots can help create and widen cracks in the rock that facilitates the movement of rainfall deeper. The result of these ancient and ongoing processes are layers of limestone rock underground that are extremely porous and permeable forming a storage system of water, what we now call the Edwards aquifer. As a limestone aquifer, its surface expression is characterized by caves and sinkholes, a type of terrain known as karst.

The Edwards aquifer exists as an arc beginning in Bell County, trending southwest underneath Austin and San Antonio where it extends more westerly towards Kinney County. (Click here to view a map of the Edwards aquifer-Balcones Fault Zone presented by the Texas Water Development Board) Interestingly, while the aquifer is more or less a continuous layer of rock, a groundwater divide located in northern Hays County and the Colorado River located in Travis County serve to separate it into three hydrologically distinct segments-the northern segment, the Barton Springs segment, and the southern segment. This means that the pumping of, movement of, and recharge of groundwater in each segment in large part occurs independent of the other segments. Though it is not a large aquifer in comparison to others in the state or nation, the Edwards aquifer is one of the most productive in the country. One reason is that a tremendous amount of water is confined in the rock layer, filling all of the open spaces in the rock. Underneath the Edwards formation of limestone is a layer called the Walnut formation that, while also limestone rock, is not as permeable or porous, thus providing a barrier to water moving deeper. Dramatic shifts in area rock layers estimated at 10 to 20 million years ago left the Edwards faulted (the Balcones Fault Zone) and the underground layers tilted. Therefore, the sloped layers of rock underground create flow (allowing an apt comparison to an underground river), though sometimes in unpredictable directions due to the influence of the faulting combined with a network of eroded passageways.

Important to the quality and quantity of water in our aquifer is the very dynamic inflow and outflow of water. Input into the aquifer, known as recharge, depends on rainfall and subsequent creek flow. Cracks, fissures, caves, and sinkholes are openings that allow water on the surface to flow underground. The location of these openings in a very specific geographic area where Edwards limestone is exposed at the surface is called the aquifer's Recharge Zone. Filtration of any pollutants from the water must occur through vegetation and soil layers as no filtration occurs underground in a karst aquifer system. Natural outflow of groundwater through springs (distinct) and seeps (generalized) occurs when pressure underground causes water to rise and find discharge points in openings at the surface, thus becoming surface water again. Springs or seeps can occur in upland areas where rock is exposed on slopes or in ravines, canyons or in creek beds feeding seasonal or perennial creeks. Pollution carried into an aquifer can also make its way out through such seeps and springs, essentially transmitting contaminants from one creek (think watershed) to another depending on directional water flow underground. The other way water makes its way out of the aquifer is through wells where it is pumped up for human or livestock use.

Putting it together
The middle segment of the Edwards aquifer is known as the Barton Springs segment due to the name of its major discharge spring. Water flows out of this segment through a complex of springs in Zilker Park and at least one other known spring in the southern bank of the Colorado River. Unique to the Barton Springs complex is the endangered Barton Springs Salamander and the recently discovered Austin Blind Salamander. Recognizing the importance of protecting the water quality of aquifer recharge, Austinites voted to authorize funding and a directive for the City of Austin to purchase and manage land for that purpose. Owing to the importance of the geographic area where water enters the aquifer, the Recharge Zone, many of the land purchases were conducted in that area. Those properties contain numerous karst features such as caves and certainly transmit water through them as well as undiscovered caves and smaller openings. It is also known that a significant amount of recharge occurs through openings in the creek bottoms. Thus, simply managing portions of the creeks isn't enough since impacts to creek water quality happen throughout the watershed, even when that drainage area extends on the surface upstream of where the Edwards limestone is located. These upstream portions of the watersheds are in the Contributing Zone because flows from these areas contribute to the quantity and quality of water entering the aquifer downstream where the water crosses the Recharge Zone. In fact, approximately 90% of the water that enters the Recharge Zone originates in the Contributing Zone. For that reason, the City of Austin also manages land in its WQPL program in the Contributing Zone.

There is more to learn about the complex and fascinating relationship between the geology, topography, and hydrology of our area as it pertains to the Barton Spring segment of the Edwards Aquifer. Management of groundwater resources to try and prevent overpumping are conducted by the Barton Springs Edwards Aquifer Groundwater District an organization that also educates the public and posts research on its website. Studies about this segment of the aquifer have been conducted and are ongoing by the District and in partnership with the City of Austin's Watershed Protection Department periodically through data collection on WQPL properties. These and other geology and groundwater web resources can be found on the links .