Purpose and Scope

The purpose of this report, prepared by the USGS in cooperation with the HGSD, City of Houston, FBSD, LSGCD, and BCGCD, is to depict water-level altitudes and long-term and recent short-term (1- and 5-year) water-level changes in the Chicot and Evangeline (undifferentiated) and Jasper aquifers in the greater Houston area, Texas. An overview of the hydrogeology of the study area is provided, and regional-scale maps depicting approximate water-level altitudes for 2024 (represented by measurements made during December 2023 through March 2024) (Adams and Ramage, 2024a, b), long-term water-level changes in the Chicot and Evangeline aquifers (undifferentiated) (1977–2024 and 1990–2024) and the Jasper aquifer (2000–24) (Adams and Ramage, 2024a, b), and recent short-term water-level changes for the Chicot and Evangeline (undifferentiated) and Jasper aquifers (2019–24 and 2023–24) (Adams and Ramage, 2024a, b) are featured. Except where noted, the text and figures in this report were modified from the most recent edition in this annual series of reports (Ramage, 2024).

Hydrogeology of the Study Area

The following overview of the hydrogeology of the study area is summarized from Kasmarek and Ramage (2017). From land surface down, the hydrogeologic units in the study area include the Chicot and Evangeline aquifers (undifferentiated), Burkeville confining unit, Jasper aquifer, and Catahoula confining unit (figs. 2 and 3). The geologic units containing the three primary aquifers in the Gulf Coast aquifer system in the study area (the Chicot, Evangeline, and Jasper aquifers) are composed of laterally discontinuous deposits of gravel, sand, silt, and clay (Baker, 1979). The geologic units that contain the Chicot, Evangeline, and Jasper aquifers and Burkeville and Catahoula confining units are shown in figure 3. The percentage of clay and other fine-grained, clastic material generally increases downdip (Baker, 1979). The Chicot part of the Chicot and Evangeline aquifers (undifferentiated) is contained in Holocene- and Pleistocene-age (Quaternary-age) sediments, and the Evangeline part of the Chicot and Evangeline aquifers (undifferentiated) is contained in Pliocene- and Miocene-age (Tertiary-age) sediments (Baker, 1979, 1986; Young and others, 2012, 2014; Casarez, 2020; Young and Draper, 2020; Teeple and others, 2021) (figs. 2 and 3). The more deeply buried Jasper aquifer is contained in Miocene-age sediments (Baker, 1979, 1986; Young and others, 2012, 2014; Casarez, 2020; Young and Draper, 2020; Teeple and others, 2021) (figs. 2 and 3). The stratigraphic relations between these hydrogeologic units are shown on hydrogeologic cross section A–A´ (figs. 1 and 2) of the Gulf Coast aquifer system, which extends through the greater Houston area from western Grimes County southeastward through Waller, Montgomery, Harris, and Brazoria Counties before terminating at the Gulf of America coast in Galveston County.

Through time, geologic and hydrologic processes resulted in the accumulation of sediment wedges (stacked sequences of sediments) more than 7,600 ft thick at the coast (fig. 2) (Chowdhury and Turco, 2006). These sediments are the inland part of a “vast wedge of terrigenous sediments which composes the northern continental margin” of the Gulf of America (Garrison and Martin, 1973, p. 1). Within the stacked sequences of sediments in the greater Houston area are the interbedded layers of sediments that compose the geologic units that contain the Gulf Coast aquifer system. These layers of sediments were deposited by fluvial-deltaic processes and subsequently eroded and redeposited by worldwide episodic changes in sea level that occurred as a result of oscillations between glacial and interglacial climate conditions (Lambeck and others, 2002). During these climatic oscillations, clay and silt layers accumulated in interbedded layers with coarser grained sediments as part of the complex depositional history of the Gulf Coast aquifer system (Chowdhury and Turco, 2006). The Gulf Coast aquifer system is contained in geologic units that dip and thicken from northwest to southeast; the hydrogeologic units representing the aquifers and confining units thus crop out in bands inland from, and approximately parallel to, the coast and become progressively more deeply buried and confined toward the coast (figs. 1 and 2) (Kasmarek, 2012). The aquifers receive recharge where the geologic units containing the aquifers crop out (Kasmarek and Robinson, 2004) (figs. 1 and 2). The Burkeville confining unit is stratigraphically positioned between the Chicot and Evangeline aquifers (undifferentiated) and the Jasper aquifer (fig. 2), thereby restricting groundwater flow between the Chicot and Evangeline aquifers (undifferentiated) and the Jasper aquifer. Supporting evidence of the interaction of groundwater flow between the Chicot and Evangeline aquifers includes the absence of a confining unit and similarities between long-term water-level-change maps for each aquifer (for example, figs. 5 and 7 in Braun and others [2019] and figs. 5 and 7 in Braun and Ramage [2020]), which indicate that the areas where groundwater levels have risen or declined are approximately spatially coincident. Additional evidence of the hydraulic connection between the Chicot and Evangeline aquifers was provided by Borrok and Broussard (2016, p. 330); their geochemical evaluation of the Chicot aquifer system from 1993 to 2015 in Louisiana indicated that in some years (1998, 2008, and 2011) certain wells screened in the Chicot aquifer “appeared to be tapping water with a geochemistry (temperature, salinity, alkalinity, [and so forth]) matching the underlying Evangeline aquifer.”

Hydraulic properties of the Chicot aquifer do not differ appreciably from those of the hydrogeologically similar Evangeline aquifer (Carr and others, 1985, p. 10). From aquifer-test data, Meyer and Carr (1979) estimated that the transmissivity of the Chicot aquifer ranges from 3,000 to 25,000 square feet per day (ft²/d) and that the transmissivity of the Evangeline aquifer ranges from 3,000 to 15,000 ft²/d.

Proceeding updip and inland from the Quaternary- and Tertiary-age sediments containing the Chicot and Evangeline aquifers (undifferentiated) in the southeastern part of the study area, the older geologic units (containing the Burkeville confining unit, the Jasper aquifer, and the Catahoula confining unit) sequentially crop out (figs. 1 and 2). Additionally, in the downdip parts of the aquifer system, the Jasper aquifer can be differentiated from the overlying Chicot and Evangeline aquifers (undifferentiated) on the basis of hydrostratigraphic position relative to the altitude of the Burkeville confining unit (figs. 2 and 3).

Precipitation falling on the land surface overlying these aquifers returns to the atmosphere as evapotranspiration, becomes surface runoff until it reaches streams, or infiltrates as groundwater recharge to the unconfined sediments containing the aquifers (Williamson and others, 1990; Healy, 2010). The infiltrating water moves downgradient toward the coast, reaching the intermediate and deep zones of the aquifers southeastward of the outcrop areas, where it can be withdrawn and discharged by wells or is naturally discharged by diffuse upward leakage in topographically low areas near the coast (Kasmarek and Robinson, 2004). Water in the coastal, deep zones of the aquifers is denser, and this higher density water causes the fresher, lower density water that has not been captured and withdrawn by wells to be redirected as diffuse upward leakage to shallow zones from the confined downdip areas of the aquifer system (Kasmarek and Robinson, 2004).

Recent updates to the hydrogeologic framework are detailed in Ramage and others (2022), which summarized the work of Young and others (2012, 2014) and Young and Draper (2020). Young and Draper (2020) updated the Gulf Coast aquifer system stratigraphy and hydrogeologic unit thicknesses for the study area (figs. 2 and 3) by using a coupled chronostratigraphic and lithostratigraphic approach. The coupled chronostratigraphic and lithostratigraphic approach used by Young and Draper (2020) was applied to a prior representation of the hydrogeologic framework proposed by Young and others (2014), which in turn was based on earlier work by Young and others (2012). Young and Draper (2020) subdivided the Lagarto Clay (Miocene) into upper, middle, and lower parts; in their revised hydrogeologic framework, the upper part of the Lagarto Clay was included in the sediments that contain the Evangeline aquifer, which is undifferentiated from the Chicot aquifer for the purposes of this report (fig. 3). The lower part of the Lagarto Clay was included in the revised definition of the sediments that contain the Jasper aquifer (fig. 3), similar to Turcan and others (1966). Additionally, the Goliad Sand (Pliocene) was subdivided into upper and lower parts following Young and others (2012).

Previous Studies

From the early 1900s until about 1930, periodic measurements of groundwater levels were documented in the greater Houston area. Groundwater-level data became more widely available beginning in December 1930 when systematic USGS groundwater measurement surveys began in cooperation with the Texas State Board of Water Engineers (predecessor to the Texas Water Commission) and the City of Houston (White and others, 1939, 1944). From these data, the first water-level-altitude maps of the Chicot and Evangeline aquifers for 1928 and 1931 in the Houston area were published (figs. 1 and 2 in White and others [1939]). USGS groundwater measurements have continued since 1930 in a groundwater-level monitoring network of wells in the Houston area. Gabrysch (1967, 1972, 1980) produced water-level-altitude maps of the Chicot and Evangeline aquifers for 1957, 1966, 1970, and 1975. As described in the “Introduction” section of this report, the USGS established an extensive network of wells in 1977, and groundwater-level data collected during the spring seasons of 1977 and 1978 were used to construct water-level-altitude maps of the Chicot and Evangeline aquifers in the greater Houston area, the first to be published in an annual series (Gabrysch, 1979). In cooperation with the FBSD, which was established in 1989 and adopted its groundwater management plan in 1990 (FBSD, 2013), an increased number of wells were inventoried by the USGS in Fort Bend, Harris, Brazoria, and Waller Counties in 1989 and 1990. A more comprehensive water-level-altitude report for the Chicot and Evangeline aquifers depicting water-level altitudes for 1990 was published by the USGS in 1991 (Barbie and others, 1991) and was revised in 1997 when updated well data became available (Kasmarek, 1997). Similarly, after the establishment of the LSGCD in 2001, the USGS began publishing water-level-altitude maps for the Jasper aquifer in the greater Houston area (primarily Montgomery County) (Coplin, 2001). In 2004, 2006, and 2007, as additional wells with reliable water-level data were inventoried, updated water-level-altitude maps for the Jasper aquifer were published (Kasmarek and Lanning-Rush, 2004; Kasmarek and others, 2006; Kasmarek and Houston, 2007). Since 2007, comprehensive maps for the Jasper aquifer have been included in the annual series of reports that depict water-level altitudes and water-level changes in the greater Houston area (Kasmarek and Houston, 2007, 2008; Kasmarek, Houston, and Ramage, 2009; Kasmarek and others, 2010, 2012, 2013, 2014, 2015, 2016; Johnson and others, 2011; Kasmarek and Ramage, 2017; Shah and others, 2018; Braun and others, 2019; Braun and Ramage, 2020, 2022; Ramage and Braun, 2023; Ramage, 2024).

Water-Level Measurements

Water-level measurements were made at 573 wells in the study area by USGS personnel during December 2023 through March 2024 (Adams and Ramage, 2024a). At each well, a measuring point was established (if one had not been previously established) to determine the depth to water. The height above or below land surface of the measuring point at each well was measured with an engineering ruler. The depth to water in each well was then measured by using a calibrated steel tape, airline, or electric groundwater-level tape in accordance with methods described in Cunningham and Schalk (2011). Each well was measured during the December 2023 through March 2024 time period to provide a “snapshot” of water-level altitudes in the aquifers associated with individual wells.

In 2024, a total of 11 additional water-level measurements (representing about 2 percent of the overall total number of water-level measurements) were provided by municipal utility districts, industrial entities, and powerplants. Municipal utility districts supply water to some of the businesses and residents of the greater Houston area (Municipal District Services, 2023). Water is used by oil and gas industrial entities during hydrocarbon processing (Allison and Mandler, 2018) and by powerplants to generate electrical power (Torcellini and others, 2003). The provided water levels were typically measured by using air pressure to determine the saturated thickness above the pump intake, and multiple pressure measurements were usually made as a quality-control measure. Air pressure measurements tend to provide less precision than do measurements made with either a calibrated steel tape or an electric tape; therefore, all provided measurements were rounded to the nearest foot. Water levels that were provided by municipal utility districts, industrial entities, or powerplants are identified as such in the “source” field in Adams and Ramage (2024a); additional information about these wells can be obtained from the USGS National Water Information System (NWIS) (USGS, 2024b); however, the provided water-level data were not entered into NWIS.

Median water-level values, based on measurements made during the previous 10 years where at least six water levels were available, were computed for three wells that could not be measured in 2024. This step was undertaken to maintain the spatial resolution of the dataset relative to previous iterations of the dataset associated with prior years in the annual series of USGS reports. The decision to include median water levels for these three wells to generate the maps included in this report was deemed appropriate because the three wells were considered to have low variability (standard deviations of less than [<] 2 ft) in the previous 10 years of measured values. Median water levels are identified in the “remarks” field in Adams and Ramage (2024a).

Many of the measured wells were being pumped at least once daily (and some more frequently) during the study period. Therefore, well pumps were turned off for at least 1 hour when possible before the water-level measurements were made in order to obtain a water-level measurement that approximated the static conditions within the aquifer. Antecedent withdrawal rates and pumping status of nearby wells were not always known and, in such instances, could have affected the representativeness of the water-level data that were collected. To ensure that the recorded water-level measurement was accurate, at least two water-level measurements were made at each well while the well was not being pumped. After the water-level-measurement data were collected, they were thoroughly evaluated, reviewed, and entered into NWIS (USGS, 2024b), and these data were then incorporated into a geographic information system as point-data layers and subsequently used for the construction of water-level-altitude and water-level-change maps. The water-level measurements made, provided, or computed for this study were carefully reviewed by USGS personnel and were compiled in a companion data release (Adams and Ramage, 2024a).

Kriging

This section is modified from the “Use of Universal Kriging Geostatistical Interpolation Methods To Depict Water-Level Altitudes” section of Ramage and others (2022). Kriging was used to create a grid of estimated water-level-altitude and water-level-change values for the Chicot and Evangeline (undifferentiated) and Jasper aquifers both at measured well locations and between measured well locations using water-level-measurement data from 2024 (Adams and Ramage, 2024a, b). Kriging has long been recognized as an effective method for interpolating hydrologic data, particularly water-level data, as an alternative to contouring methods (Varouchakis and others, 2012) and is a well-established geostatistical method (Matheron, 1963; Paramasivam and Venkatramanan, 2019) that provides an optimized unbiased linear prediction with a minimum mean estimation error. Kriging, like other interpolation methods, uses a weighted average of all measured values to predict values at unmeasured locations. The weights assigned to measured values when predicting values at unmeasured locations are spatially dependent. The spatial dependency is quantified by the spatial correlation of the measured locations and is assumed to be more similar with spatial proximity and to diverge as spatial distance increases. Correlation between measured locations and their spatial distance relative to one another is defined by the experimental semivariogram and the best-fit model that attempts to minimize the estimation error (Olea, 1975, 2009). Kriging methods for depicting water-level altitudes and water-level changes provide several advantages compared to manual contouring methods, such as those described in Kasmarek and Ramage (2017). One of the advantages of kriging over traditional, manual contouring methods is that kriging provides estimated values of the variable of interest at unmeasured locations. Kriging also provides a means of quantifying the estimation uncertainty (by standard deviation, the square root of the computed variance), thereby allowing for evaluation of the estimation reliability (Todini, 2001). This estimation uncertainty can be useful for understanding and quantifying the spatial variability of the data and allowing for possible optimization of the network of wells. Additional details related to the kriging processes used to depict shaded, gridded representations of water-level altitudes and water-level changes in the Chicot and Evangeline (undifferentiated) and Jasper aquifers are described in Ramage and others (2022).

Determination of Water-Level Altitudes

The 2024 regional-scale depictions of water-level altitudes presented in this report were derived from water-level measurements made throughout the 11-county study area from December 2023 through March 2024 (water levels usually are higher because groundwater use is at its lowest during these months compared to the rest of the year). Water-level altitudes were calculated by subtracting the water-level measurement, in feet below land surface, made at each well from the land-surface elevation at that well referenced to NAVD 88 (National Ocean Service, 2021). Prior to 2016, water-level altitudes published in this annual series of USGS reports were referenced to either the National Geodetic Vertical Datum of 1929 or NAVD 88, whereas all water-level altitudes published in 2016 and after are referenced to NAVD 88.

The accuracy of land-surface-elevation data at wells used in the annual series of reports has gradually improved over time as digital elevation models (DEMs) progressively replaced traditional methods of determining land-surface elevations from topographic maps. The most accurate land-surface-elevation data available were used by the USGS for each historical annual depiction of water-level altitudes in the study area. To determine land-surface elevations in 2024, a corresponding land-surface datum was obtained for each well (if one had not already been established) by using a USGS National Geospatial Program 1-meter DEM that provides three-dimensional elevation values referenced to NAVD 88 (Arundel and others, 2015; USGS, 2024a). In 1977, land-surface-elevation data were extracted from USGS 1:24,000-scale, 7.5-minute topographic quadrangle maps for the Gulf of America coastal area, which have a 5-ft contour as described in Gabrysch (1979) and thereby provide an accuracy of approximately 2.5 ft. The DEM used to determine land-surface elevations in 2024 provides an accuracy of 0.5 ft.

Color-shaded representations of the 2024 water-level-altitude grids of the Chicot and Evangeline (undifferentiated) and Jasper aquifers produced by the kriging process were constructed such that each color shade represents increments or “bins” of water-level altitudes (50-ft range, except at the extreme ranges where estimates are less than the 50-ft range). To depict the upper and lower bounds of the data on each map, the water-level-altitude increment for the first bin and for the last bin for a given map extends to the lowest and highest estimated water-level-altitude value, respectively. The gridded water-level altitudes are published in a companion data release (Adams and Ramage, 2024b). The kriging-derived water-level-altitude grids are approximate, regional-scale depictions of the water-level altitudes in the Chicot and Evangeline (undifferentiated) and Jasper aquifers.

Depicting Long-Term Water-Level Changes

Maps depicting long-term water-level changes in the Chicot and Evangeline (undifferentiated) and Jasper aquifers were constructed for the period of record available for each of the aquifers; data are from 1977 to 2024 for the Chicot and Evangeline aquifers (undifferentiated) and from 2000 to 2024 for the Jasper aquifer. With the establishment of the FBSD in 1989 and the subsequent increase in the number of wells that were inventoried by the USGS in Fort Bend, Harris, Brazoria, and Waller Counties in 1989 and 1990, a long-term water-level-change map for the period from 1990 to 2024 for the Chicot and Evangeline aquifers (undifferentiated) was also constructed to highlight changes in water levels since their establishment in 1989. Long-term water-level-change maps were constructed from kriging-derived grids of estimated water-level data from the current year (2024) and from each respective benchmark year for the period of record (1977 and 1990 for the Chicot and Evangeline aquifers [undifferentiated] and 2000 for the Jasper aquifer). Each color shade on the maps represents increments or “bins” of water-level changes of either 20 or 40 ft. As with the water-level-altitude maps, to depict the upper and lower bounds of the data on each map, the water-level-altitude increment for the first bin and for the last bin for a given map extends to the lowest and highest estimated water-level-change value, respectively.

In previous iterations of this report (Braun and Ramage, 2022; Ramage and Braun, 2023; Ramage, 2024), long-term water-level-change maps starting in 1977 and 1990 for the Chicot and Evangeline aquifers (undifferentiated) and starting in 2000 for the Jasper aquifer were produced by subtracting the current year water-level-altitude surface from the benchmark year water-level-altitude surface as described in Ramage and others (2022). In this report, long-term water-level- change surfaces were produced by using the same kriging methods (Ramage and others, 2022) that were used to produce the current year (2024) gridded surfaces of estimated water-level altitudes across the study area for the Chicot and Evangeline (undifferentiated) and Jasper aquifers. To produce the long-term water-level-change maps, the kriging process uses the calculated difference in water-level altitude between the benchmark year (1977, 1990, or 2000) and the current year (2024) at all wells where measurements were collected in both the benchmark year and the current year. At wells where a water-level measurement was collected in 2024 but not in the benchmark year, the well point was intersected with the kriged surface for the benchmark year, and the change was calculated as the difference between the water-level-altitude value at that well from the intersected point on the kriged surface for the benchmark year and the water-level altitude from 2024.

A mask was applied to each kriging-derived gridded surface (water-level altitudes and long-term water-level changes) as a method for either removing potential outlier data or masking (not showing) grid cells where there was insufficient data density. The threshold for the applied mask was chosen to retain (show) only those data for which the variance value of each grid cell was less than or equal to the overall mean variance of the entire gridded surface of each respective gridded surface. Water-level-measurement data from wells measured in a given historical year (1977, 1990, or 2000) were gridded using the same methods that were used to generate the 2024 water-level-altitude grids, and a mask was applied to the gridded historical altitudes to retain only those grid cells with variance values less than or equal to the mean variance of the historical grid (as a whole) (Ramage and Adams, 2023). The area of water-level change shown on the maps is based primarily on the availability of water-level data in both 2024 and the historical year. Long-term water-level-change shaded grids are available in Adams and Ramage (2024b); the grids and their associated variance distribution maps that were produced from the water-level data collected from the network of wells (along with a description of the processes by which these datasets were produced) are included in Adams and Ramage (2024a, b).

Depicting Short-Term Water-Level Changes

Maps depicting short-term water-level changes in the Chicot and Evangeline (undifferentiated) and Jasper aquifers were constructed to represent changes during the past 5 years (2019–24) and during the past year (2023–24). Because these change maps represent a much shorter time period than do the long-term change maps, changes in excess of 30 ft at the local scale (individual wells or small groups of proximal wells) tend to stand out because the majority of wells in the mapped area exhibit <20 ft of rise or decline over the shorter time periods; these local-scale changes tend to reflect short-term variations in demand, pumping status, or precipitation patterns. Because the estimates provided by the kriging process are weighted using data from other wells, these short-term variations tend to be smoothed out of long-term change maps, which are more representative of regional-scale variations in water levels. In order to preserve the representation of these short-term variations in demand, pumping status, or precipitation patterns, kriging and a shaded grid were not used to depict the short-term changes, which are instead depicted by shading a triangle symbol at each of the wells measured during both years representing the water-level change for the short-term interval in question, where the orientation of the triangle symbol represents water-level rises (pointing upward) or declines (pointing downward) and the shading represents water-level rises or declines at 10-ft intervals. Water-level change was determined by direct subtraction of water levels at each well measured at both the beginning and ending of the periods of interest (2019 and 2024 for the 5-year maps; 2023 and 2024 for the 1-year maps).

Chicot and Evangeline Aquifers (Undifferentiated)

Water-level-measurement data from 443 wells (Adams and Ramage, 2024b) were used to depict the 2024 water-level altitudes for the Chicot and Evangeline aquifers (undifferentiated) (fig. 4). In 2024, depicted water-level altitudes for the Chicot and Evangeline aquifers (undifferentiated) were lowest (300–301 ft below NAVD 88, which corresponds to the interval from −301 to −300 ft in fig. 4) in two small areas (so small that they are obscured by well symbols) in and near The Woodlands in Montgomery County. The highest depicted water-level altitudes (>150–184 ft above NAVD 88, which corresponds to the interval from greater than [>] 150 to 184 ft in fig. 4) for the Chicot and Evangeline aquifers (undifferentiated) in 2024 were in a small area (so small that it is obscured by the well symbol on the map) at and near a well south of Todd Mission, near the southeastern border of Grimes County.

The largest decline in water-level altitudes (360–397 ft, which corresponds to the interval from −397 to −360 ft in fig. 5) depicted by the 1977–2024 long-term water-level-change map for the Chicot and Evangeline aquifers (undifferentiated) was in south-central Montgomery County (fig. 5), whereas the 1990–2024 long-term water-level-change map for the Chicot and Evangeline aquifers (undifferentiated) depicts the largest declines in water-level altitudes (240–252 ft, which corresponds to the interval from −252 to −240 ft in fig. 6) in an area northwest of The Woodlands and in an area of northern Waller County (fig. 6). The largest rise in water-level altitudes in the Chicot and Evangeline aquifers (undifferentiated) (>200–215 ft, which corresponds to the interval from >200 to 215 ft in fig. 5) is depicted in an area of east-central Harris County for 1977–2024, whereas the largest rise in water-level altitudes for 1990–2024 (>160–161 ft, which corresponds to the interval from >160 to 161 ft in fig. 6) is depicted in an area of central Harris County.

The 5-year (2019–24) short-term water-level-change map for the Chicot and Evangeline aquifers (undifferentiated) depicts the largest declines (>50 ft, which corresponds to the interval <−50 ft in fig. 7) in several parts of the study area, primarily concentrated in northern Fort Bend County, southwestern Harris County, and south-central Montgomery County. The largest rise for 2019–24 (>50 ft) is depicted at a well in northern Fort Bend County (fig. 7). The 1-year (2023–24) short-term water-level-change map for the Chicot and Evangeline aquifers (undifferentiated) depicts the largest declines (>50 ft, which corresponds to the interval <−50 ft in fig. 8) at a well in northern Fort Bend County and a well in west-central Harris County. The largest rise (about 50 ft, which corresponds to the interval from 41 to 50 ft in fig. 8) is depicted at a well in east-central Fort Bend County.

Jasper Aquifer

Water-level-measurement data from 84 wells (Adams and Ramage, 2024b) were used to depict the approximate 2024 water-level altitudes for the Jasper aquifer (fig. 9). In 2024, water-level altitudes for the Jasper aquifer were lowest (250–255 ft below NAVD 88, which corresponds to the interval from −255 to −250 ft in fig. 9) in an area of south-central Montgomery County and north-central Harris County in and near The Woodlands, whereas water-level altitudes for the Jasper aquifer were highest (>300–321 ft above NAVD 88, which corresponds to the interval from >300 to 321 ft in fig. 9) in an area in eastern Grimes and northwestern Montgomery Counties (fig. 9).

Whereas annual water-level-measurement data have been collected from wells completed in the Chicot and Evangeline aquifers since 1977, annual water-level-measurement data have been collected from wells completed in the Jasper aquifer since only 2000. The 2000–24 long-term water-level-change map for the Jasper aquifer (fig. 10) depicts the largest water-level decline (360–378 ft, which corresponds to the interval from –378 to –360 ft in fig. 10) in an area of central San Jacinto County (fig. 10). The largest rise in water-level change (>120–132 ft, which corresponds to the interval from >120 to 132 ft in fig. 10) is depicted in an area of central Grimes County for 2000–24 (fig. 10). Note, however, that these areas of largest declines and rises are also in areas that are distant from water-level measurements; because they are interpreted values, the uncertainty associated with these values is considered large but within the threshold established for the methods used to develop the gridded surface. The extent to which shaded regions are depicted in figure 10 is explained in detail in Ramage and others (2022). Users should be aware of the uncertainty in the estimates of water-level change where the changes are depicted beyond where there were measured data for both 2000 and 2024.

The 5-year (2019–24) short-term water-level-change map for the Jasper aquifer depicts declines throughout most of the study area, with the largest declines (>50 ft, which corresponds to the interval <−50 ft in fig. 11) across parts of central and southern Montgomery County and at one well in north-central Harris County. The largest rise (>50 ft) for 2019–24 is depicted at a well centered on the Montgomery-Grimes County line (fig. 11). The 1-year (2023–24) short-term water-level-change map for the Jasper aquifer depicts the largest declines (31 to 35 ft, which corresponds to the interval from −35 to −31 ft in fig. 12) at two wells in south-central Montgomery County and at one well in northwestern Montgomery County on the west side of Lake Conroe. The largest rises (31 to 36 ft) during 2023–24 are depicted at one well in northwestern Montgomery County and one well in south-central Montgomery County (fig. 12).