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 short-term 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 2022 (represented by measurements made during November 2021 through March 2022) (Ramage and Adams, 2023a), long-term water-level changes in the Chicot and Evangeline aquifers (undifferentiated) (1977–2022 and 1990–2022) and the Jasper aquifer (2000–22) (Ramage and Adams, 2023b), and short-term water-level changes for the Chicot and Evangeline (undifferentiated) and Jasper aquifers (2017–22 and 2021–22) (Ramage and Adams, 2023b) are featured. Except where noted, the text and figures in this report were modified from the previous report in this annual series of reports (Braun and Ramage, 2022).

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 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 percentage of clay and other fine-grained, clastic material generally increases downdip (Baker, 1979). The two uppermost of the three aquifers, the Chicot and Evangeline aquifers (undifferentiated), are contained in Holocene- and Pleistocene-age (Quaternary-age) sediments and 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 northwestern Grimes County southeastward through Waller, Montgomery, Harris, and Brazoria Counties before terminating at the Gulf of Mexico coast in Galveston County.

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 geologic stratigraphy and hydrogeologic unit thicknesses depicted 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 based on a 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 into upper, middle, and lower parts; in their revised hydrogeologic framework, the entirety of the upper part of the Lagarto Clay was included in the sediments that contain the Evangeline aquifer. The lower part of the Lagarto Clay was also included in the revised definition of the sediments that contain the Jasper aquifer, similar to Turcan and others (1966). Additionally, the Goliad Sand was subdivided into upper and lower parts following Young and others (2012).

Through time, geologic and hydrologic processes created accretionary sediment wedges (stacked sequences of sediments) more than 7,600 ft thick at the coast (fig. 2) (Chowdhury and Turco, 2006). These sediments, which compose the Gulf Coast aquifer system, were deposited by fluvial-deltaic processes and subsequently were 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). The Gulf Coast aquifer system consists of hydrogeologic 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 hydrogeologic units composing 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 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), 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 stratigraphic 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 updip sediments composing 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).

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 in 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 as part of a groundwater-level monitoring network of wells in the Houston area. Gabrysch (1967, 1972, 1980) published 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 springs 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, in 1979 (Gabrysch, 1979). In cooperation with the FBSD, which adopted its groundwater management plan in 1990 (Fort Bend Subsidence District, 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 of 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, revised 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).

Water-Level Measurements

Water-level measurements were made at 670 wells in the study area by USGS personnel during November 2021 through March 2022 (Ramage and Adams, 2023a). At each well, a measuring point was established (if one had not been previously established) to determine the depth to water. The height above 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 November 2021 through March 2022 time period to provide a “snapshot” of water-level altitudes in the aquifers associated with individual wells.

In 2022, 15 additional water-level measurements representing about 2 percent of the total number of water-level measurements were provided by municipal utility districts, industrial entities, and powerplants. Municipal utility districts supply water to the businesses and residents of the greater Houston area (Municipal District Services, 2022). 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 electric tape; therefore, all measurements provided by other entities were rounded to the nearest foot. Groundwater levels that were provided by municipal utility districts, industrial entities, or powerplants are identified as such in the “source” field in Ramage and Adams (2023a); additional information about these wells can be obtained from the USGS National Water Information System (NWIS) (U.S. Geological Survey, 2022b).

Most 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 and 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 or provided for this study were carefully reviewed by USGS personnel and were compiled in a companion data release (Ramage and Adams, 2023a). All collected water-level measurements were uploaded into NWIS (U.S. Geological Survey, 2022b); the provided water-level measurements were not uploaded into NWIS.

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 grid water-level-measurement data from 2022 and to grid long-term water-level changes for the Chicot and Evangeline (undifferentiated) and Jasper aquifers. 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 are spatially dependent. The spatial dependency is quantified by the spatial correlation of the measured locations and is assumed to be more similar in spatial proximity and 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), allowing for evaluation of the estimation reliability (Todini, 2001). This estimation uncertainty can be useful for determining 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 in the Chicot and Evangeline (undifferentiated) and Jasper aquifers are described in Ramage and others (2022).

Determination of Water-Level Altitudes

The 2022 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 November 2021 through March 2022 (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 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 2022, a corresponding land-surface datum was obtained for each well by using a USGS National Geospatial Program 1-meter DEM that provides three-dimensional elevation values referenced to NAVD 88 (Arundel and others, 2015; U.S. Geological Survey, 2022a). In 1977, land-surface-elevation data were extracted from USGS 1:24,000-scale 7.5-minute topographic quadrangle maps for the Gulf of Mexico 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 2022 provides an accuracy of 0.5 ft.

Color-shaded gridded representations (shaded grids) of the 2022 water-level altitudes in the Chicot and Evangeline (undifferentiated) and Jasper aquifers were constructed; each color shade represents increments or “bins” of water-level altitudes (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 shaded grids are published in a companion data release (Ramage and Adams, 2023b). The water-level-altitude grids are approximate, regional-scale depictions of the water-level altitudes in the Chicot and Evangeline (undifferentiated) and Jasper aquifers. The areal extents and locations of the shaded areas represented by these grids represent the combined effects of total groundwater withdrawals from all wells screened in the Gulf Coast aquifer system within the study area.

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 2022 for the Chicot and Evangeline aquifers (undifferentiated) and from 2000 to 2022 for the Jasper aquifer. A long-term change map for the period from 1990 to 2022 for the Chicot and Evangeline aquifers (undifferentiated) was also constructed to highlight changes in groundwater levels that may have occurred since 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. Long-term water-level-change maps were constructed from gridded water-level-measurement data from the current year (2022) and from each respective beginning year for the period of record (1977 and 1990, respectively, 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 (20- or 40-ft range).

A mask was applied to the gridded 2022 water-level altitudes to retain only those grid cells with variance values less than or equal to the mean variance of the 2022 grid (as a whole) as a method for either removing potential outliers or hiding grid cells with insufficient data density. Water-level-measurement data from wells measured in the historical year (1977, 1990, or 2000) were gridded using the same methods that were used to generate the 2022 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). The resulting grids derived from water-level-measurement data from 2022 and from 1977, 1990, or 2000 were then cropped to a common geospatial extent before subtracting the historical grid from the 2022 grid. (The grids were cropped to a common geospatial extent because the network of wells where groundwater levels were measured in each aquifer changed over time as wells were added in response to expansion of development throughout the greater Houston area or were taken out of service in certain parts of the study area as surface-water usage increased and groundwater usage decreased, among other reasons.) The area of water-level change shown on the maps is based primarily on the availability of groundwater-level data in both 2022 and the historical year. Another factor when determining the area of mapped water-level change is the computed variance associated with each estimated water-level altitude at each grid cell for both the current year water-level-altitude and historical year water-level-altitude gridded surfaces. If the computed variance at any given grid cell is less than or equal to the overall mean variance for each respective gridded surface for the current year and historical year, the data are deemed valid for inclusion in the mapped area depicting water-level change (Ramage and others, 2022). The datasets of water-level-change values (difference between 2022 and historical year water-level-altitude values) are available in Ramage and Adams (2023b); 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 Ramage and Adams (2023b).

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 (2017–22) and during the past year (2021–22). Because these change maps represent a much shorter time period than do the long-term change maps, changes at the local scale (individual wells or small groups of proximal wells) can be more pronounced in them than in long-term change maps, which are more representative of regional-scale variations in groundwater levels. As a result, using a shaded grid to represent these short-term changes tends to lead to pronounced concentric patterns (bullseyes) in the short-term change maps. Therefore, short-term water-level changes were instead depicted by shading a circular symbol at each of the wells measured during both years representing the short-term interval in question, where the shading represents water-level rises or declines at 10-ft intervals. Kriging was not used to depict the short-term water-level changes. Instead, water-level change determined by direct subtraction of water levels at each well measured at both the beginning and ending of the periods of interest (2017 and 2022 for the 5-year maps; 2021 and 2022 for the 1-year maps) is represented by the shaded symbol for each well in the short-term change maps.

Chicot and Evangeline Aquifers (Undifferentiated)

Water-level-measurement data from 470 wells (Ramage and Adams, 2023b) were used to depict the 2022 water-level altitudes for the Chicot and Evangeline aquifers (undifferentiated) (fig. 4). In 2022, observed water-level altitudes for the Chicot and Evangeline aquifers (undifferentiated) were lowest (250–270 ft below NAVD 88, which corresponds to the interval from −270 to −250 ft in fig. 4) at and near a few clusters of wells, which are obscured by the well symbols on the map, in relatively small areas in the south-central part of Montgomery County between The Woodlands and Spring and in an area of west-central Harris County between Interstate 10 and U.S. Highway 290. The highest observed water-level altitudes (150–195 ft above NAVD 88, which corresponds to the interval from greater than [>] 150 to 195 ft in fig. 4) for the Chicot and Evangeline aquifers (undifferentiated) in 2022 were in an area west of Todd Mission, near the western border of Grimes County.

The largest decline in water-level altitudes (320–344 ft, which corresponds to the interval from −344 to −320 ft in fig. 5) indicated by the 1977–2022 long-term water-level-change map for the Chicot and Evangeline aquifers (undifferentiated) was southeast of The Woodlands (fig. 5), whereas the 1990–2022 long-term water-level-change map for the Chicot and Evangeline aquifers (undifferentiated) depicts the largest declines in water-level altitudes (280–292 ft, which corresponds to the interval from −292 to −280 ft in fig. 6) in localized areas at or near certain wells, often obscured by the well symbols on the map, in parts of northwestern Harris County and southern Montgomery County (fig. 6). The largest rise in water-level altitudes in the Chicot and Evangeline aquifers (undifferentiated) (200–236 ft, which corresponds to the interval from >200 to 236 ft in fig. 5) was observed in a relatively large area in southeastern Harris County for 1977–2022, whereas the largest rise in water-level altitudes for 1990–2022 (200–208 ft, which corresponds to the interval from >200 to 208 ft in fig. 6) was observed in a relatively small area in central Harris County.

The 5-year (2017–22) short-term water-level-change map for the Chicot and Evangeline aquifers (undifferentiated) depicts the largest declines (greater than or equal to [≥] 50 ft) at three wells in northern Fort Bend County and the largest rises (>50 ft) at five wells in southwestern and central Harris County (fig. 7). The 1-year (2021–22) short-term water-level-change map for the Chicot and Evangeline aquifers (undifferentiated) depicts the largest decline (≥50 ft) at a well in north-central Harris County and the largest rises (>50 ft) at a well in north-central Fort Bend County and a well in southwestern Harris County (fig. 8).

Jasper Aquifer

Water-level-measurement data from 104 wells (Ramage and Adams, 2023b) were used to depict the approximate 2022 water-level altitudes for the Jasper aquifer (fig. 9). In 2022, water-level altitudes for the Jasper aquifer were lowest (200–213 ft below NAVD 88, which corresponds to the interval from −213 to −200 ft in fig. 9) in an area of south-central Montgomery County and north-central Harris County between The Woodlands and Spring, whereas water-level altitudes for the Jasper aquifer were highest (250–285 ft above NAVD 88, which corresponds to the interval from >250 to 285 ft in fig. 9) in northeastern Grimes, northwestern Montgomery, and southwestern Walker Counties (fig. 9).

Annual water-level-measurement data have been collected from wells completed in the Chicot and Evangeline aquifers (undifferentiated) since 1977, whereas annual water-level-measurement data have been collected from wells completed in the Jasper aquifer since only 2000. The 2000–22 long-term water-level-change map for the Jasper aquifer (fig. 10) depicts water-level declines throughout most of the study area where water-level-measurement data from the Jasper aquifer were collected, with the largest decline (≥200 ft, which corresponds to the interval −210 to −200 ft in fig. 10) in north-central Harris County and south-central Montgomery County south of The Woodlands. One small area of the map in northwestern Montgomery County and east-central Grimes County depicts slight rises of >0 ft (which corresponds to the interval >0 to 11 ft in fig. 10).

The 5-year (2017–22) short-term water-level-change map for the Jasper aquifer depicts the largest decline (≥60 ft) at a well in Conroe and the largest rise (>50 ft) at a well in south-central Montgomery County (fig. 11). The 1-year (2021–22) short-term water-level-change map for the Jasper aquifer depicts the largest decline (≥30 ft) at a well in south-central Montgomery County east of The Woodlands and the largest rise (>20 ft) at a well in Conroe (fig. 12).