Katie Thomson, Edwards Aquifer Research Data Center

The vital connection between all life on Earth and water necessitates the preservation of this invaluable natural resource on both a global scale and closer to home. Locally, residents and visitors of San Marcos, as well as bountiful wildlife, rely heavily upon the Edwards Aquifer and its subsequent water systems, such as the San Marcos River. The Edwards Aquifer Research and Data Center (EARDC) utilizes water quality monitoring to ensure that bodies of water in San Marcos remain a healthy and viable resource within the local ecosystem.

During the 2019 Summer semester, I completed an internship with the Edwards Aquifer Research and Data Center. In this report, I will first describe the characteristics of the Edwards Aquifer and illustrate its relationship to the waters of San Marcos. Then, I will share the function of the EARDC and recount some of my usual tasks interning with the facility. Next, I will elaborate on my duties and activities practicing first-hand some of the standard methods of water quality monitoring. Finally, I will conclude this report with a reflection upon my experiences interning at the EARDC and a projection of my future beyond Texas State University. 

Edwards Aquifer and the Headwaters of San Marcos

The Edwards Aquifer is an extensive groundwater reservoir that follows the Balcones Fault Zone throughout south central Texas. It is a karst aquifer, a type of aquifer featuring extremely high porosity and permeability, a result of its highly soluble limestone walls reacting with mildly acidic waters. The aquifer’s vast expanses of nooks, caves, sinkholes, springs and water wells make it possible for the groundwater levels to be quickly recharged with rainwater (www.edwardsaquifer.org/science-and-maps/about-the-edwards-aquifer).  

These factors produce an aquifer that has historically supported nearly two million users in Texas. A massive demand is placed on the freshwater pumped from wells and expelled from the artesian zone into springs, providing water for a diversity of needs ranging from agricultural to domestic usage. Drilled wells tapping into the 180-mile long aquifer span ten counties: Kinney, Uvalde, Zavala, Medina, Frio, Atascosa, Bexar, Comal, Guadalupe and Hays (www.edwardsaquifer.org/science-and-maps/about-the-edwards-aquifer). In areas where hydraulic pressure is powerful enough to force water to the surface through wells and fissures, the crystal-clear waters are discharged into springs. In San Marcos, hundreds of these small artesian springs combine to produce Spring Lake.

 Figure 1. Water bubbling up from the Edwards Aquifer into Spring Lake

Spring Lake, the headwaters for the bodies of water that I worked with throughout my internship, has supported human life for as long as this region has been populated. Located at the Meadows Center for Water and the Environment in San Marcos, Spring Lake is where the pressurized groundwaters of the Edwards Aquifer bubble up to the surface through artesian wells to transcend beyond the lake into the creeks, streams and rivers of our community. Underneath where the lake currently stands are the remnants of an ancient river; buried beneath the sediment are reminders of the early humans who long ago were sustained by the river’s resources. Archaeological evidence indicates that humans settled here 11,500 years ago, and have relied upon these springs during every known period of human habitation in Central Texas (www.edwardsaquifer.net/index.html). Thousands of prehistoric artifacts have been recovered and are on display in the Meadows Center’s Discovery Hall. Just as we heavily upon the waters of the Edwards Aquifer and Spring Lake today, so too did the first people to call this region home rely upon its bounty for survival. The history of the City of San Marcos has been and will be shaped by the history of this natural, limited, resource; these waters will eventually meet the capacity of life that they can sustain without careful measure on our part.

Interning at the EARDC

The EARDC (www.eardc.txstate.edu/about/Mission.html), located within the Freeman Aquatic Building at Texas State University, was established in 1979 by the university to make research related to the Edwards Aquifer accessible to the public. It is organized into four departments, each serving a distinct function relating to water quality: The Education Center, the Data Center, the Research Center, and the Technical Services Center. In this section, I will briefly describe the work of these departments, and then introduce some of my activities working in the Technical Services and Research Centers.

The Education Center hosts field days and a summer aquatic science camp for young students to learn the importance of protecting and preserving our planet’s waterways, for the sake of human life and nature, alike. The Education Center also improves environmental awareness in the community through public school curriculum, training programs for science teachers and materials intended for use by local government.

The Data Center provides different water resources available to the public concerning the Edwards Aquifer. This department catalogues and maintains records obtained from organizations such as the U.S. Geological Survey and National Weather Service, as well as information about local threatened and endangered species, like the Texas Blind Salamander.

The Research Center’s laboratory conducts environmental research concerning the aquifer’s aquatic biology, geochemistry and hydrogeology. Research associates of this department perform tests such as wet chemistry analysis, analyzing water samples sourced from various locations throughout Texas for their chemical properties. 

The Technical Services Center involves all field sampling, such as collection and distribution of local threatened and endangered species, water quality samples, and industrial pretreatment sampling. This department also includes the EARDC’s Analytical Lab, which is certified and accredited to perform water quality testing through both the National Environmental Laboratory Accreditation Conference (NELAC) and by the Texas Commission on Environmental Quality (TCEQ). Here, drinking water samples from the wells of San Marcos residents and non-potable samples from City officials and environmental organizations are tested for soft metal traces and coliform bacteria.

 For my internship at the EARDC, I predominantly worked in Technical Services and briefly assisted in the Research Center’s laboratory. The day-to-day work of Technical Services is highly varied and often active—at times we were out in the field, collecting water samples and organism specimens, and at others I was in the laboratory, performing microscope analysis on those very collections. In the Analytical Lab, I shadowed a research associate performing quality testing of water samples from the San Marcos River, which were sent in by the City, for the presence of E. Coli bacteria. An important service to public health, these tests allow citizens and government entities to make informed decisions regarding the integrity of their water sources.

I assisted the Research Center with acid-base titrations on samples of water that had been sourced from different cave locations throughout Texas. Titrations are a method of wet chemical analysis that involve using the known pH of a solution to determine the unknown pH of another. This procedure involved adding measured increments of sulfuric acid until the water sample reached the desired neutral pH. The measurements recorded during titrations on a water sample provide useful quantitative data to researchers, such as the comparative pH levels across the cave water samples in our analysis. 

In the following section of this report, I will describe in greater detail my work with the  EARDC’s Technical Services Center. 

Water Quality Monitoring in Action

Figure 2. Left, Drift nets in Sessom Creek; Right, Microscope used for sample sorting

During my internship, I learned several methods and procedures employed in the field and laboratory to monitor water quality, including drift net sample collection, sample sorting, and how to perform a Rapid Bioassessment. After briefly discussing these procedures and my associated duties, the remainder of this section will describe my central project, a two-week comparative field study of water quality parameters.

One of my primary duties for the duration of my time at the EARDC was collecting samples taken from drift nets in Sessom Creek. First, I assisted in the construction of these drift nets, which are designed to trap aquatic macroinvertebrates, organisms living in or on water that are tiny but visible to the naked eye. Then, we set up these traps, which allow water carrying organisms to flow through a fine mesh netting, and into a one-way collection tube, in Sessom Creek. Daily, we would empty these tubes into a bucket with the hopes that small aquatic specimens were captured inside.

After collection, I was responsible for sorting the macroinvertebrate specimens that accumulated in our drift net traps. Back at the lab, I would sieve through the water sample and use a microscope to sort out the specimens into visually similar groupings, and then transfer the organisms into vials of ethanol for preservation. Although many of the organisms are indistinguishable to the untrained eye, I am now able to identify the visual differences between the numerous organisms I commonly worked with. For example, both macroinvertebrate crustaceans Isopoda and Amphipoda are white-bodied, but the first looks like a termite and the latter is more akin to a shrimp.

Although water quality monitoring typically involves multiple samples taken over a period of time, another method utilizes one-time sampling to get a snapshot of a water’s ecological health based on counts of aquatic organisms. I performed a simplified Rapid Bioassessment, or RBA, of Sessom Creek. This is a biological survey technique that measures aquatic life richness and diversity of a waterway. My site supervisor and I waded out into the creek, and set up a D-frame net, which is made of a fine mesh, allowing water to flow through, and macroinvertebrates to be caught. My site supervisor held the net in place while I kicked and disturbed the sediment one square meter in front of it. I also brushed organisms off randomly selected rocks. We did this at two points in the creek: once in a shallow spot where water is turbulent and rocky, which is called a riffle, and again in a deeper, more calm section, called a pool. We dumped everything that could be collected from the net and rocks into a bucket, and in the laboratory, I separated the different specimens for counts. My species diversity consisted of five groups—Aquatic worms, gilled snails, other snails, beetles, and water pennies. Each group exhibited strong richness—for instance, my sample included about twenty-five writhing aquatic worms! I compared my findings to a chart that shows which species are indicative of good, bad, or neutral water quality, and overall, my RBA determined the water quality of that section of Sessom Creek in that particular instance to be good.

Figure 3. Map of site locations for field study

As the focal project of my internship, I conducted a two-week comparative field study of the water quality of four bodies of water found in San Marcos, which provided me with hands-on practice in actual water quality monitoring procedures. Using a multiprobe sonde device, I measured the water temperature, pH, conductivity, and dissolved oxygen levels of San Marcos waterways, in order to identify and explain trends and differences in these major water quality parameters.

The sites chosen for my study included an artesian well, a creek, a manmade pond, and a river. All four sites share the same water source, the Edwards Aquifer. The artesian well behind the Freeman Aquatic Biology building is in a catchment area for water rising from the underground Edwards Aquifer through a 400-foot well. Sessom Creek, running beneath East Sessom Drive, receives water from Spring Lake and collects runoff from rain events. The large, decorative holding pond near the J. C. Kellam Building at Texas State is manually filled and aerated by an ornamental fountain. The San Marcos River, located in Sewell Park at Texas State, is often heavily populated by swimmers and other recreational users, and has a constant flowing current.

My procedure for data collection was replicated at each site location for each of the eight days of the study. The equipment I used was a YSI 556 Multiprobe System. This sonde device measured the following parameters: temperature of the body of water in Celsius; pH value; conductivity in microsiemens per centimeter; and dissolved oxygen in milligrams per liter. Upon reaching each site, I recorded in my field notes the time of day and weather, as well as any noticeable changes in the area from the previous visit. I submerged the probe into the water, gently bobbing the probe in the middle of the water column. When fluctuations in the parameters were steadied, typically after 30 seconds in the water, I logged the sample on the sonde device. At the study’s conclusion, I summarized my recorded measurements into tables for each parameter, including daily readings and site means. 

I then identified trends and variations both within and between site locations.  The quantitative data indicated a general consistency in temperature and pH within each site, and the greatest within-site fluctuation was seen in conductivity. In cross-comparison, the three natural bodies of water yielded similar measurements, and the manmade body of water the relative outlier.

The pond was distinct in all measured areas from the artesian well, creek and river. The shaded locations of the well, creek, and river resulted in lower daily and average water temperatures, and the open and exposed location of the pond resulted in higher temperatures. The pond’s fountain for circulation and heavy algal growth from slow drainage creates an environment unique from the well, creek and river, thereby producing uniquely higher dissolved oxygen levels and pH. The pond, the body of water studied showing the least clear water, also had the lowest conductivity. Conductivity, charged particles in the water column, and turbidity, particles in the water column, have an inverse relationship. Waters that are clear and see-through, like those of the well, creek and river, have less particle resistance. The pond water, however, was heavily shrouded by algae and debris; in other words, the more turbid pond showed the lowest conductivity. From the results of my study, I deduced that the nature of the water body’s system, including its location, water exchange system, and the presence vs. lack of human design, influence major water quality parameters.


My work in water quality monitoring with the EARDC navigated the journey of the waters of San Marcos from the original source to the outputs that are the scenic highlight of the city. Each day presented a new opportunity, not simply to learn, but to get my hands dirty—and my feet wet— working in water quality monitoring and environmental health. Going forward, beyond my years at Texas State University, I plan to harness my passion for the environment, my interest for the curiosity that is our species, and my education gained from these experiences to pursue a career that strikes a balance between all three. If we expect the environment to protect and sustain us, then we must give back and do the same for the environment. 

            I am thankful for this incredibly enlightening opportunity I was given to learn, to ask questions, and practice what I was taught. As a student of anthropology, I am naturally fascinated by the complex relationship between humans and the environment that is perfectly calibrated to support our existence, all by pure luck of the universe. This perfection, however, balances precariously upon a capricious scale, and we must not rest on the belief that its current state of equilibrium is indefinite. When basking in the bright summer sun or floating lazily down the river, it is easy to forget that many of our activities both rely upon and consume the limited natural resource of the Edwards Aquifer. As Central Texas is projected to be one of the fastest growing regions in the United States, it becomes paramount to bring attention to the fact that our waterways can, and will be, mistreated, polluted, and eventually depleted irreparably if we do not stand to prevent this from occurring. However, through conservation science and sustainability efforts, such as those of the EARDC, the water that has allowed life to flourish in San Marcos for thousands of years will continue to do so for many future generations to come.