FROM THE TEXAS WATER DEVELOPMENT BOARD FiNAL REPORT , SUBMiTTED BY DANiEL B . STEPHENS AND ASSOCiATES
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