Above: Major structural areas used to divide the Marathon Aquifer after King (1937). Blue and purple units tend to be aquifers, while brown units tend to be aquitards. Tan unit (alluvium) can be an aquifer where saturated.
Daniel B. Stephens and Associates has completed its model of the Marathon Aquifer for the Texas Water Development Board (TWDB), a project that began in the fall of 2020. Currently, the full draft of the final report is available on the TWDB website. This report details the complex science and computer modelling used to get a clearer picture of how water moves through the aquifer, how much water is recharged, and how much is being used. The following excerpts give a snapshot of the findings.
The Marathon Aquifer is the most structurally complex aquifer in Texas. The area of the aquifer is 576 square miles, or 636 square miles including a proposed expansion resulting from observations made during this study.
The Marathon Aquifer consists of two types of aquifer systems delineated based on the porosity and water yielding characteristics of the rocks and sediments. The first type of aquifer occurs within Quaternary alluvium that overlies consolidated rocks between prominent ridges and along the edges of the Marathon Aquifer extent. The alluvium, deposited relatively recently by rivers and erosion, is permeable to the flow of groundwater and, where there is a significant amount of saturation, it can yield useable quantities of water to wells. Wells completed in alluvium are generally shallow and highly susceptible to drought conditions. Porosity of the alluvial sediments is referred to as primary porosity, as it is a function of the type of sediment (or rock) as deposited.
The second type of aquifer consists of dipping Paleozoic formations that have been deformed and faulted by structural activity. Left unaltered, the permeability of these units to the flow of groundwater is slight, but the breaking of these rocks through structural movement has created fault and fracture zones through which groundwater can flow. In some units, most notably the Marathon Limestone, the permeability of the rocks has been enhanced by solutioning (when water dissolves the minerals in surrounding rock such as limestone) and enlargement of the void spaces and interconnected fractures—a process known as karstification. The creation of porosity and permeability through faulting, fracturing, and karstification is called “secondary” porosity. Both types of aquifers are addressed in the report, although the Marathon Limestone is the most significant aquifer unit based on its ability to provide relatively large quantities of water. The town of Marathon obtains its water supply from wells completed in the Marathon Limestone.
Aquifers and aquitards (areas where water can’t permeate the rock structures well) that occur in the Marathon Aquifer region are the result of interactions of rock composition and the cumulative effects of a complex geologic history that has produced numerous fractures, faults, and folds. Lithology influences the mechanical properties of the rock, which affect the susceptibility of the rock to fracturing and determine whether the rock exhibits a brittle or ductile response to stress. Prior tectonic and structural events have resulted in the development of localized, well-developed fracture and joint permeability systems. Minor karstification (cavities) within the Marathon Limestone has also occurred, as confirmed by local well drillers.
Grouping formations by age and water well productivity is a useful starting point for conceptualizing the aquifer system, although it must be kept in mind that the stratigraphic units vary significantly, and some units may include both aquifer and aquitard intervals. For example, TWDB well records may indicate if water is produced from limestones of the upper or the lower Marathon Formation. Bentonites at the top and the mega-conglomerate in the middle of the Marathon Limestone are unlikely to produce water, but the layers of limestone in between can.
As part of the conceptual model development, a hydrostratigraphic framework was created to form the geometry for a future flow model. To examine the project data and test the geometry in three-dimensional space, a three-dimensional geologic model was created using the software package Leapfrog Works® by Seequent. The three-dimensional geologic model is a digital representation of the hydrostratigraphic structure. Once the composition and number of layers were established, the Leapfrog model was constructed using a variety of data sources.
The lateral extent of the three-dimensional geologic model is the study area as defined by the Marathon Aquifer extent with an addition to the northeast of the town of Marathon, plus a 1‑mile buffer around the aquifer boundaries. The vertical extent is defined from the topography to a horizontal plane surface set at an elevation of 2,000 feet above mean sea level. This allows the computer model to predict what the layers of geology and water look like for approximately 2,000 feet below the surface.
A 30-meter digital elevation model grid obtained from the USGS National Map Seamless Server defines the land surface of the three-dimensional geologic model. Surface geology maps originally surveyed by King in 1937 served as general guides for the interpretation of surface geology. The historical maps were scanned, georeferenced, and imported into a three-dimensional workspace, and then draped on top of the digital elevation model.
Using a combination of the surface geology maps and the Geologic Atlas of Texas shapefiles, the model surface geology was interpreted by comparing the two data sources and blending them together through hand-drawing polylines using drawing tools in Leapfrog. During this process, care was taken to preserve the major features while generalizing some of the finer detail. This was done while keeping in mind an interpretation of the subsurface structure. Defining the hierarchy of the three-dimensional geologic model surfaces (i.e., how the software will direct the surfaces to interact in space that result in digital volumetric shapes) was also an important consideration in the construction of the model files.
To create the subsurface interpretations, 19 cross sections (A–A’ through S–S’) from King (1937) were scanned, georeferenced, and imported into the three-dimensional workspace.
The majority of the existing recorded wells are completed in the alluvium, Tesnus Formation, and Marathon Limestone, which are aquifer units. However, there are also a lesser number of wells completed in what are generally considered to be aquitard units, such as the Gaptank and Haymond formations. This is not surprising, as the units may produce small amounts of water from fracture zones even though they are not as productive as some of the other formations. In the Marathon Aquifer area, as with many rural areas, landowners will often drill to the depth where they first encounter a source of water sufficient for their needs, and for local livestock or domestic use, relatively small well yields will suffice.
Available information on changes in water levels through time is limited. DeCook (1961) states that water levels on the Marathon area fluctuate chiefly in response to changes in the rates of recharge and discharge. He measured water levels at four wells over the period September 1956 through November 1957 and observed “very little net change in water level during this period” (DeCook, 1961, p. 18).
Two well hydrographs were constructed from water levels in the TWDB groundwater database. Well 52-55-104 is the town of Marathon public supply well 1. Water levels at this well have fluctuated over a range of about 20 feet for the period 2007 to early 2022. The highest water levels recorded during this more recent period are about 10 feet lower than the earliest recorded water level of 3,974 feet above mean sea level in 1969, when the well was installed, indicating that over the long term, there has been some moderate water level decline at this well.
The second hydrograph is also for a Marathon public supply well installed in 2015 near supply well 1. Observed water levels at this location fluctuate over the period 2016 through early 2022, but there is no clear upward or downward trend. As would be expected, the observed water levels at this well are similar to those at well 1.
Some of the water levels measured during this study were obtained at wells that had prior water level information from either the TWDB groundwater database or the Texas Department of Licensing and Regulation submitted drillers reports database. A total of 14 of these wells were identified; reported water levels are summarized in Table 4-3. Of the 14 wells, 9 exhibited a water level decline from the prior measurement and 5 exhibited a water level increase. The first 2 wells showed almost no change in water levels over the 64-year span of time—one increased by 9 feet and the other declined by 2 feet. Water level at the third well, however, declined by 46 feet. Details regarding this well are not available, but it is likely in a low-permeability aquifer (or confining) unit with limited recharge potential.
Overall, because water use in the Marathon Aquifer is primarily for domestic and stock purposes, water levels will primarily fluctuate based on changes in groundwater recharge. The exception to this would be the town of Marathon, where there is a public water system and water use for other purposes common to a small community. Based on the water levels at Marathon supply well 1 and some of the other wells in the town, some moderate water level decline appears to have occurred over time.
The groundwater in the Marathon Aquifer is generally fresh. The vast majority of samples indicate fresh water, with total dissolved solids concentrations less than 1,000 milligrams per liter. These analyses are generally indicative of a good quality, fresh water aquifer.
The final estimated groundwater recharge is presented for mean annual conditions for the period 1981 through 2021, the lowest water year of recharge (water year 2011), and the highest water year of recharge (water year 2004).
Higher recharge generally occurs where soils are thin and soil hydraulic conductivity and bedrock hydraulic conductivity are highest, as would be expected. In addition, higher rates of recharge occur along drainages because (1) the drainages occur in alluvial sediments (bedrock at those locations), which has high permeability, and (2) storm flows are collected in the drainages and provide source water to be recharged.
The mean annual groundwater recharge over the entire Distributed Parameter Watershed Model domain for 1981 through 2021 is estimated at 48,864 acre-feet per year (5.2 percent of precipitation). For the portion of the Distributed Parameter Watershed Model domain overlying the Marathon Aquifer, recharge is estimated at 18,504 acre-feet per year (5.1percent of precipitation and run-on). The recharge for the entire model domain was 3,850 acre-feet (2.4 percent of precipitation) in water year 2011 (the driest year in the study period) and 184,183 acre-feet (10.6 percent of precipitation) in water year 2004 (the wettest year in that period).
Groundwater discharge occurs through groundwater pumping from wells. Very little pumping has occurred from the Marathon Aquifer to date. The study indicates that the total pumpage from the aquifer has increased from approximately 100 acre-feet per year in 1980 to approximately 250 acre-feet per year in 2019, with a maximum pumping of approximately 450 acre-feet per year in 2010. For most of the time period from 1980 to 2019, the majority of pumping from the Marathon Aquifer has been for municipal use. Municipal pumping was slightly lower from 1980 to the mid-1990s, but overall has remained relatively stable at approximately 100 acre-feet per year. Municipal pumping from the Marathon Aquifer is mostly by the town of Marathon, though there are also a number of domestic wells present in the study area, most of which are in the vicinity of the town of Marathon.
In summary, there has been very little historical pumping from the Marathon Aquifer. Groundwater conditions are likely unaffected by pumping, with the possible exception of the area around the town of Marathon, where the majority of historical pumpage from the aquifer has occurred. The lowest year of aquifer recharge, 2011, saw approximately 3,850 acre-feet added to the aquifer, while the highest recorded pumpage, in 2010, only removed 450 acre-feet of water from it.
For a detailed description of the geology of the area, examples of the 3-D model of the aquifer, and a complete description of the methods used in the study, download the full report at https://www.twdb.texas.gov/groundwater/models/gam/mrtn/mrtn.asp.
