Available information on changes in water levels through time is limited . DeCook ( 1961 ) states that water levels on the Marathon

Extent of the Leapfrog model with a scanned map ( Plate 23 of King , 1937 ) draped on a U . S . Geological Survey 30-meter digital elevation model . The extent of the Marathon Aquifer is shown as a red line .

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