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Source-linked reports with geographic coverage including Texas.

At least 181 records · Page 10Linked to original sources
USGS2019

Emerging contaminants in groundwater, karst, and the Edwards Aquifer

Karst aquifers have hydrogeologic characteristics that render them uniquely vulnerable to contamination from emerging contaminants (ECs). ECs comprise numerous chemical groups, including pharmaceuticals, personal-care products, flame retardants, perfluorinated and polyfluorinated compounds, nanoparticles and microplastics. Many ECs have sources, transport pathways, and chemical characteristics that facilitate their infiltration into groundwater, either indirectly from surface water or directly from sources such as landfill leachate and septic systems. What little is known about the occurrence, fate, and transport of ECs in the Edwards aquifer indicates that the aquifer might be increasingly vulnerable to this type of contamination. The natural physical characteristics of this karst aquifer and anthropogenic sources of ECs associated with increased urbanization in central Texas contribute to this vulnerability. In this chapter, we review groups of ECs and their sources, occurrence of ECs in groundwater and karst, and what is known about occurrence of ECs in the Edwards aquifer. We conclude by discussing specific factors, such as rapid flow and contaminant sources, that contribute to the vulnerability of the Edwards aquifer to contamination by ECs.

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USGS2019

Nest survival of Black-necked Stilts (Himantopus mexicanus) on the upper Texas coast, USA

The Black-necked Stilt ( Himantopus mexicanus ) is a migratory shorebird of temperate and tropical America. Declining wetland quality and associated declines in hydrological integrity may contribute to widespread habitat loss for stilts nesting on the upper Texas Gulf of Mexico coast of the USA, as both fresh and brackish marshes are converting to open water and saline marsh. Nests ( n = 356) were monitored in three wetland types on the upper Texas coast from 21 April-30 June 2011-2012. Of these 356 nests, 151 were located in managed freshwater wetlands (16 in 2011 and 135 in 2012), 128 were located in managed intermediate wetlands (75 in 2011 and 53 in 2012), and 77 were located in rice fields (all in 2012). Collectively, nest success was 0.2% (0 in rice fields and as high as 4.3% in freshwater wetlands in 2012), among the lowest ever reported for the species. The most frequent cause of nest failure was predation by mammalian and avian predators (∼50%). Daily nest survival rate was positively related to mudflat nesting substrates and negatively related to colony size, rice field, and brackish coastal wetland habitats. Future efforts to minimize edge effects in managed wetlands may prove valuable to improve nest success of stilts and other species that nest in similar wetland types.

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USGS2019

Using pointing dogs and hierarchical models to evaluate American woodcock winter occupancy and densities

Use of dogs has increased for multiple wildlife research purposes ranging from carnivore scat detection to estimation of reptile abundance. Use of dogs is not particularly novel for upland gamebird biologists, and pointing dogs have been long considered an important research tool. However, recent advances in Global Positioning System (GPS) technology and the development of hierarchical modeling approaches that account for imperfect detection may improve estimates of occupancy and density of cryptic species such as the American woodcock (Scolopax minor; hereafter, woodcock). We conducted surveys for woodcock using a trained pointing dog wearing a GPS collar during the winters of 2010–2011 and 2011–2012 in East Texas, USA. We surveyed 0.5-km-radius circular plots ( n = 24; survey sites) randomly placed along secondary roads in Davy Crockett National Forest and on private timber property. Surveys lasted 1.5 hrs and were repeated 3–5 times each winter. We estimated woodcock occupancy and density using multiple modeling approaches at the survey site and forest stand scales within survey sites. Woodcock occupied 88% (21/24) of survey sites and 48% (39/82) of forest stands (i.e., unique cover types) within sites. Using a modified distance sampling technique, we estimated an average density of 0.16 birds/ha (SE = 0.13) throughout both study areas. We describe the first attempt to blend use of pointing dogs with hierarchical modeling approaches to derive estimates of regional diurnal woodcock occupancy and density, and describe relationships between these estimates of abundance and habitat covariates. Although forest stand occupancy estimates had the lowest coefficients of variation, our estimates of density provided the most useful inference of habitat use. Surveys using pointing dogs paired with hierarchical models of occupancy and density may provide a cost-efficient and effective approach to estimate habitat abundance at broad spatial scales.

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USGS2019

Nest-defense behavior of Mississippi Kites in urban and exurban areas

Mississippi kites ( Ictinia mississippiensis ) have become an abundant raptor in many urban and exurban areas throughout the Southern Great Plains of the United States. Unfortunately, human–wildlife conflicts have resulted from this juxtaposition of suitable breeding areas for kites and areas that humans frequent, with some kites responding aggressively to humans near nests. To date, there are no data describing the prevalence of aggressive nest defense in the species, making informed management of human and kite conflicts difficult. We assessed and compared the prevalence of aggressive nest-defense by Mississippi kites in an urban area and an exurban area by simulating nest disturbance with a trial pedestrian. Additionally, we examine the relationships between physical features of the nest tree where aggressive behaviors were and were not recorded. Individual kites breeding in the exurban area responded to the trial pedestrian by taking flight from the nesting area, circling overhead, swooping at the pedestrian, or remaining on the nest. In the urban area, kites displayed a more limited suit of responses and either remained on the nest or swooped at the pedestrian. Additionally, kites breeding in the exurban area appeared to respond to experimental disturbance at a greater distance than did urban breeding kites, but not with more attacks on pedestrians. Physical characteristics of the nest tree did not explain aggressive behaviors, thereby suggesting that aggression in Mississippi kites is caused by factors other than nesting location features.

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USGS2019

Guadalupe Bass flow-ecology relationships; with emphasis on the impact of flow on recruitment

Guadalupe Bass Micropterus treculii is an economically and ecologically important black bass species endemic to the Edwards Plateau ecoregion and the lower portions of the Colorado River in central Texas. It is considered a fluvial specialist and as such, there are concerns that the increasing demands being placed upon the water resources of central Texas by growing human populations have the potential to negatively impact Guadalupe Bass populations. Therefore, this study assessed the relationship between Guadalupe Bass growth, feeding ecology, and streamflow. Sagittal otoliths were removed from Guadalupe Bass collected from throughout their range during 2015-2017 and used to estimate the age and back-calculate the growth trajectory of each individual. Additionally, young-of-year (YOY) Guadalupe Bass were collected every 10-14 days from two second-order streams, the North Llano River and South Llano River, in the Colorado Basin on the Edwards Plateau. Stomach contents of these individuals were identified and the effect of streamflow on the occurrence of the taxa comprising the stomach contents assessed. Guadalupe Bass growth was greater in the Colorado and Guadalupe River basins, independent of stream order, and tended to increase with increasing stream order within a basin. Growth was higher in higher stream orders and during years with stable and lower spring and summer monthly median flows, lower minimum and maximum flows, slower rise and fall rates, and higher baseflows. Growth was not influenced by years with higher monthly median flows in winter. These results would seem to contradict previous research, but more likely represent a fuller picture of how Guadalupe Bass respond to flow conditions. The disagreement between the current study and past studies seem to be attributable, at least in part, to the fact that previous studies were conducted during a period of extensive drought, while the current study was conducted during relatively wet conditions. Taken together with previous studies, the current study suggests that Guadalupe Bass growth is sensitive to flow conditions and is lower in years with flow conditions that fall outside a basin- and stream order-specific optimal range for the species. A total of 21 unique taxonomic groups were recovered from the stomachs of YOY Guadalupe Bass collected from the North Llano River and South Llano River. Aquatic insects, especially larval mayflies (Ephemeroptera), damselflies (Odanata: Zygoptera), and caddisflies (Trichoptera), were the most frequently encountered taxa. While there was no difference between the two rivers in stomach content composition, there was a strong longitudinal gradient in both systems with aquatic insects predominating at upstream sample sites and fishes being more common at downstream sites. Stream discharge during the 24 hours prior to collection did not have any influence on the probability of a taxa being found in Guadalupe Bass stomachs. The results of this study support efforts to manage Guadalupe Bass populations at a sub-watershed scale and suggests that populations occupying the same stream order within a basin are likely to have similar responses to annual flow conditions. In addition, these results indicate that the lower Colorado River population may inhabit a unique set of conditions that has supported the development of a trophy Guadalupe Bass fishery. Further, this study highlights the need to incorporate a sufficient range of annual flow conditions to ensure that the influence of stream flow on fish growth is adequately assessed. While interannual variation in growth rates seem to be capable of serving as a proxy for recruitment and year-class strength, long-term monitoring of recruitment paired with assessment of growth is necessary to further clarify the relationship between population density, flow regime, recruitment and growth and allow the construction of predictive models.

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USGS2018

Raptor selection of captive reared and released Galliform birds

Captive rearing and release of birds in the order Galliformes remains a popular management tactic despite low survival rates. We investigated avian predator selection of captive‐reared northern bobwhites ( Colinus virginianus ) as a potential driver of their high mortality. We simulated avian predator hunts on a flushing pair of bobwhites during trials conducted from 6 February 2015 to 14 February 2015 in Texas, USA. When presented a choice of a captive‐reared bobwhite or wild bobwhite, a Harris's hawk ( Parabuteo unicinctus ) pursued a captive‐reared bobwhite in 8 of 10 flight trials. This selective rate, combined with observations from auxiliary research indicate avian predation may be a greater risk for captive‐reared birds than wild birds. The success of captive‐rearing and release efforts may benefit from a better understanding of how to more adequately prepare captive‐reared birds for release.

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USGS2018

The spatial scale of biotic change in Chihuahuan Desert fish assemblages

1. We examined riverine desert fish assemblages in the Chihuahuan Desert, USA at multiple spatial scales of similarity to assess long-term changes to assemblage distinctiveness, identify individual species responsible for changes, and determine the importance of geographic context and species resolution in interpreting patterns of change. 2. We used a well-documented historical data set on fish distribution and abundance, and recent collections of fishes that provided a paired analytical design across 36 localities spanning nearly three decades of time. Patterns of faunal homogenization and differentiation were assessed at basin-wide, sub-basin and river-reach scales with species occurrence and relative abundance data. Individual species responses were examined to identify the drivers of assemblage change across time. 3. Patterns of similarity varied across spatial scales and produced seemingly incongruous trends in assemblage similarity across time. Patterns of assemblage distinctiveness depended on the spatial extent of the analyses, the geographical structuring of the fish assemblages, and whether occurrence or relative abundance data were used. These dependencies led to interesting and conflicting patterns of homogenization and differentiation. The Rio Grande sub-basin showed strong homogenization with convergence between upstream and downstream reaches that corresponded to declining water quality and quantity from the Rio Conchos in Mexico. In contrast, the Pecos River sub-basin showed strong differentiation between upstream and downstream reaches that corresponded to the successful colonization and spread of the non-native gulf killifish (Fundulus grandis) in the highly degraded upper reach. Spatial variability in fish assemblages and their degree of change from historical conditions were largely dependent on anthropogenic modifications to the flow regime and variability in the success of invasive gulf killifish in the basin. 4. The use of species occurrence or abundance data, and the spatial scale of analysis are crucial choices in studies of faunal homogenization and differentiation, and we have demonstrated how these choices lead to variable results for our study system. Our multi-scale approach and examination of individual species responses identified the ultimate drivers of these differences and illustrated the importance of scale-dependent effects and geographical context on patterns of assemblage distinctiveness, especially with regard to species invasion, species loss and abundance shifts.

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USGS2018

Land-cover changes associated with oil and natural-gas production and concentrations of selected constituents in surface-water and streambed-sediment samples collected upstream from and within an area of oil and natural-gas production, south Texas, 2008–17

The extensive development of oil and natural-gas resources in south Texas during the past 10 years has led to questions regarding possible environmental effects of processes associated with oil and natural-gas production, in particular the process of hydraulic fracturing, on water and other natural resources. Part of the lower San Antonio River watershed intersects an area of oil and natural-gas production from the sedimentary rocks that compose the Eagle Ford Group. The rapid expansion of infrastructure associated with oil and natural-gas production increases potential pathways for inorganic and organic contaminants to enter surface-water systems. The U.S. Geological Survey, in cooperation with the San Antonio River Authority, analyzed geospatial data from different years (2008 and 2015) to evaluate changes in land cover associated with oil and natural-gas production activities in the lower San Antonio River watershed. Impervious surface in this study is defined as land cover consisting of well pads, oil- and gas-related features, or roads. The areal coverage associated with impervious surface increased from 201 acres to 5,390 acres (net increase of 5,189 acres) between 2008 and 2015. The total percentage of the study area accounted for by impervious surface resulting from oil and natural-gas production activities increased from 0.034 percent to 0.912 percent, which is an increase of approximately 27-fold. Collectively, 0.878 percent of the study area was converted to new impervious surface between 2008 and 2015. If the area associated with new storage ponds (0.066 percent) is added to the estimate of total land-cover changes as a result of oil and natural-gas production, then 0.944 percent of the study area was altered. During 2015–17, surface-water samples collected from 5 sites and streambed-sediment samples collected from 17 sites in the lower San Antonio River watershed were analyzed for a broad range of constituents that might be associated with oil and natural-gas production. All major elements, trace elements, semivolatile organic compounds (SVOCs), and volatile organic compounds (VOCs) measured in surface-water samples were detected at concentrations less than any of the U.S. Environmental Protection Agency’s water-quality standards. In general, the greatest SVOC and VOC concentrations were observed in samples collected from sites upstream from the area of active oil and natural-gas production and just downstream from urban areas. The lack of benzene, toluene, ethylbenzene, and all isomers of xylene (hereinafter referred to as BTEX) for most sites within the area of active oil and natural-gas production indicates that little, if any, local runoff associated with the area of active oil and natural-gas production has contaminated the surface water with BTEX compounds. Glycols, which are commonly used in hydraulic fracturing fluids as scale inhibitors, were detected in one surface-water sample from Ecleto Creek within the area of oil and natural-gas production; however, the presence of glycols does not necessarily indicate contamination from hydraulic fracturing fluid. The glycols detected also have other potential sources including the use of diethylene and ethylene glycols in antifreeze used in vehicles and the use of triethylene glycol in antibacterial air sanitizers. The concentrations of select constituents in the streambed-sediment samples were compared to sediment quality guidelines (SQGs). The SQGs evaluate the potential toxicity of bed sediments to sediment-dwelling organisms. Two SQG concentration levels are used: (1) a lower level, called the threshold effect concentration (TEC), below which harmful effects to benthic biota are not expected, and (2) a higher level, the probable effect concentration (PEC), above which harmful effects are expected to occur frequently. The PEC for arsenic was exceeded in a sample collected from one site on Ecleto Creek. The origin of the elevated arsenic concentration is unknown; the contamination likely is not related to oil and natural-gas production because the site of the sample collection is located upstream from the area of active oil and natural-gas production. Streambed-sediment samples were analyzed for selected polycyclic aromatic hydrocarbons (PAHs) because PAHs can be used as indicators of petroleum hydrocarbons associated with produced waters. Each streambed-sediment sample was analyzed for two size fractions of PAHs: less than (<) 63 micrometers (μm) and < 2 millimeters (mm). Total PAH concentrations in all samples, regardless of size fraction, were less than the TEC for total PAHs of 1,610 micrograms per kilogram. Total PAH concentrations generally were greater in the <63-μm size-fraction samples than in the <2-mm size-fraction samples, indicating that PAHs could potentially sorb more readily to the exclusively silt- and clay-sized particles that compose <63-μm size-fraction samples than to the mixture of silt and clay and larger sized particles that compose the <2-mm size-fraction samples. Total PAH concentrations typically were greater in the samples collected from the sites upstream from the area of active oil and natural-gas production compared to those collected from sites within the area in both the <2-mm and <63-μm size-fraction samples. The smaller PAH concentrations measured in samples collected from within the area of active oil and natural-gas production in comparison to the upstream urbanized areas indicate relatively minor additional local contributions of PAHs of uncertain origin to the watershed.

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USGS2018

Geologic framework and hydrostratigraphy of the Edwards and Trinity aquifers within Hays County, Texas

The Edwards and Trinity aquifers are classified as major aquifers by the Texas Water Development Board and are major sources of water in south-central Texas, where Hays County is located. Detailed maps and descriptions of the geologic framework and hydrostratigraphic units (HSUs) of these karstic aquifers in Hays County are needed for water managers to effectively manage groundwater resources in the area. During 2016–18, the U.S. Geological Survey, in cooperation with the Edwards Aquifer Authority, documented the geologic framework and hydrostratigraphy of the Edwards and Trinity aquifers for a large part of Hays County, characterizing approximately 560 square miles of the county. The report includes a 1:24,000-scale hydrostratigraphic map and descriptions of the geology and HSUs in the study area. In addition, parts of the adjacent upper confining unit to the Edwards aquifer are described. The rocks exposed within the study area are within outcrops of the Trinity and Edwards Groups and the overlying Washita, Eagle Ford, Austin, and Taylor Groups. The rocks are sedimentary and formed during the Cretaceous age. The principal structural feature in Hays County is the Balcones fault zone, which is the result of late Oligocene and early Miocene age high-angle normal faulting and fracturing. Hydrostratigraphically, the exposed rocks represent a section of the upper confining unit to the Edwards aquifer, the Edwards aquifer, the upper zone of the Trinity aquifer, the middle zone of the Trinity aquifer, and the upper part of the lower zone of the Trinity aquifer. Complexity in the aquifer system results from a combination of the original depositional history, bioturbation, primary and secondary porosity, diagenesis, fracturing, and faulting.

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USGS2018

Annual and approximately quarterly series peak streamflow derived from interpretations of indirect measurements for a crest-stage gage network in Texas through water year 2015

In 2006, the U.S. Geological Survey (USGS), in cooperation with the Texas Department of Transportation, began collecting annual and approximately quarterly series peak-streamflow data at streamflow-gaging stations in smallto medium-sized watersheds in central and western Texas as part of a crest-stage gage (CSG) network, along with selected flood-hydrograph data at a subset of these stations. CSGs record the peak stage during storm events, which is the maximum gage height (elevation of water surface above a local vertical datum), at each CSG station. Established and widely used indirect methods of peak streamflow estimation and interpretation, such as culvert-flow, slope-area, and flow-over-road methods, are used in conjunction with peak gage height data to create the database of peak streamflow described herein. The CSG network is focused on hydrology of small- to medium-sized watersheds in central and western Texas because additional streamflow data for this semiarid to arid study area will eventually provide for more statistical information and presumably reduced uncertainty in regional regression equations or other regionalized statistical methods for peak-streamflow frequency estimation at ungaged locations. The database of annual and approximately quarterly peak streamflow is published through USGS ScienceBase and described in this report.

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USGS2018

New insights into surface-water/groundwater exchanges in the Guadalupe River, Texas, from floating geophysical methods

In south-central Texas, the amount of streamflow in the Guadalupe River is a primary concern for local and downstream communities because of municipal, agricultural, wildlife, and recreational uses. Understanding the flow paths and rates of exchange between the surface water in the river and the groundwater in the underlying Carrizo-Wilcox aquifer is vital for understanding the water budget and streamflow variations. In areas where the Guadalupe River crosses the Carrizo-Wilcox aquifer outcrop, the surface-water and groundwater exchanges are not well characterized. Traditional methods to measure these interactions, such as measuring differences in surface-water flows at different locations to infer gains and losses between the locations, are not feasible along this stretch of the Guadalupe River because of upstream dams that cause large daily fluctuations in streamflow. Consequently, the U.S. Geological Survey, in cooperation with the Guadalupe-Blanco River Authority, applied geophysical methods in an exploratory study to identify reaches of the river where streamflow gains and losses (surface-water/groundwater exchanges) might be occurring.

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USGS2018

Status of groundwater-level altitudes and long-term groundwater-level changes in the Chicot, Evangeline, and Jasper aquifers, Houston-Galveston region, Texas, 2018

Since the early 1900s, most of the groundwater withdrawals in the Houston-Galveston region, Texas, have been from the three primary aquifers that compose the Gulf Coast aquifer system—the Chicot, Evangeline, and Jasper aquifers. Withdrawals from these aquifers are used for municipal supply, industrial, and irrigation purposes. This report, prepared by the U.S. Geological Survey in cooperation with the Harris-Galveston Subsidence District, City of Houston, Fort Bend Subsidence District, Lone Star Groundwater Conservation District, and Brazoria County Groundwater Conservation District, is one in an annual series of reports depicting the status of groundwater-level altitudes and long-term groundwater-level changes in the Chicot, Evangeline, and Jasper aquifers in the Houston-Galveston region. This report contains regional-scale maps depicting approximate 2018 groundwater-level altitudes (represented by measurements made during December 2017 through March 2018) and long-term groundwater-level changes for the Chicot, Evangeline, and Jasper aquifers. In 2018, groundwater-level-altitude contours for the Chicot aquifer ranged from 200 feet (ft) below the North American Vertical Datum of 1988 (hereinafter referred to as “datum”) to 200 ft above datum. The 1977–2018 groundwater-level-change contours for the Chicot aquifer depict a large area of decline in groundwater-level altitudes (120 ft) in northwestern Harris County. The largest rise in groundwater-level altitudes in the Chicot aquifer from 1977 to 2018 (180 ft) was in southeastern Harris County. Groundwater-level-altitude contours for the Evangeline aquifer ranged from 250 ft below datum to 200 ft above datum in 2018. The 1977–2018 groundwater-level-change contours for the Evangeline aquifer depict broad areas where groundwater-level altitudes either declined or rose. The largest decline in groundwater-level altitudes (320 ft) was in southern Montgomery County. The largest rise in groundwater-level altitudes in the Evangeline aquifer from 1977 to 2018 (240 ft) was in southeastern Harris County. In 2018, groundwater-level-altitude contours for the Jasper aquifer ranged from 200 ft below datum to 200 ft above datum. The 2000–18 groundwater-level-change contours for the Jasper aquifer depict groundwater-level declines throughout most of the study area where groundwater-level-altitude data from the Jasper aquifer were collected, with the largest decline (200 ft) in southern Montgomery County.

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USGS2018

Changing environmental gradients over forty years alter ecomorphological variation in Guadalupe Bass Micropterus treculii throughout a river basin

Understanding the degree of intraspecific variation within and among populations is a key aspect of predicting the capacity of a species to respond to anthropogenic disturbances. However, intraspecific variation is usually assessed at either limited temporal, but broad spatial scales or vice versa, which can make assessing changes in response to long-term disturbances challenging. We evaluated the relationship between the longitudinal gradient of changing flow regimes and land use/land cover patterns since 1980 and morphological variation of Guadalupe Bass Micropterus treculii throughout the Colorado River Basin of central Texas. The Colorado River Basin in Texas has experienced major alterations to the hydrologic regime due to changing land- and water-use patterns. Historical collections of Guadalupe Bass prior to rapid human-induced change present the unique opportunity to study the response of populations to varying environmental conditions through space and time. Morphological differentiation of Guadalupe Bass associated with temporal changes in flow regimes and land use/land cover patterns suggests that they are exhibiting intraspecific trait variability, with contemporary individuals showing increased body depth, in response to environmental alteration through time (specifically related to an increase in herbaceous land cover, maximum flows, and the number of low pulses and high pulses). Additionally, individuals from tributaries with increased hydrologic alteration associated with urbanization or agricultural withdrawals tended to have a greater distance between the anal and caudal fin. These results reveal trait variation that may help to buffer populations under conditions of increased urbanization and sprawl, human population growth, and climate risk, all of which impose novel selective pressures, especially on endemic species like Guadalupe Bass. Our results contribute an understanding of the adaptability and capacity of an endemic population to respond to expected future changes based on demographic or climatic projection.

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USGS2018

Water-quality observations of the San Antonio segment of the Edwards aquifer, Texas, with an emphasis on processes influencing nutrient and pesticide geochemistry and factors affecting aquifer vulnerability, 2010–16

As questions regarding the influence of increasing urbanization on water quality in the Edwards aquifer are raised, a better understanding of the sources, fate, and transport of compounds of concern in the aquifer—in particular, nutrients and pesticides—is needed to improve water management decision-making capabilities. The U.S. Geological Survey, in cooperation with the San Antonio Water System, performed a study from 2010 to 2016 to better understand how water quality changes under a range of hydrologic conditions and in contrasting land-cover settings (rural and urban) in the Edwards aquifer. The study design included continuous hydrologic monitoring, continuous water-quality monitoring, and discrete sample collection for a detailed characterization of water quality at a network of sites throughout the aquifer system. The sites were selected to encompass a “source-to-sink” (that is, from aquifer recharge to aquifer discharge) approach. Network sites were selected to characterize rainfall, recharging surface water, and groundwater; groundwater sites included wells in the unconfined part of the aquifer (unconfined wells) and in the confined part of the aquifer (confined wells) and a major discharging spring. Storm-related samples—including rainfall samples, stormwater-runoff (surface-water) samples, and groundwater samples—were collected to characterize the aquifer response to recharge. Elevated nitrate concentrations relative to national background values and the widespread detection of pesticides indicate that the Edwards aquifer is vulnerable to contamination and that vulnerability is affected by factors such as land cover, aquifer hydrogeology, and changes in hydrologic conditions. Greater vulnerability of groundwater in urban areas relative to rural areas was evident from results for urban groundwater sites, which generally had higher nitrate concentrations, elevated δ 15 N-nitrate values, a greater diversity of pesticides, and higher pesticide concentrations. The continuum of water quality from unconfined rural groundwater sites (least affected by anthropogenic contamination) to unconfined urban groundwater sites (most affected by anthropogenic contamination) demonstrates enhanced vulnerability of urban versus rural land cover. Differences in contaminant occurrences and concentration among unconfined urban wells indicate that the urban parts of the aquifer are not uniformly vulnerable, but rather are affected by spatial differences in the sources of nutrients and pesticides. In urban areas, the shallow, unconfined groundwater sites showed greater temporal variability in both nutrient and pesticide concentrations, as well as a greater degree of contamination, than did deeper, confined groundwater sites. In comparison to that of the shallow, unconfined groundwater sites, the water quality of the deeper, confined groundwater sites was relatively invariant during this multiyear study. Although aquifer hydrogeology is an important factor related to aquifer vulnerability, land cover likely has a greater influence on pesticide contamination of groundwater. Temporal variability in hydrologic conditions for the Edwards aquifer is apparent in data for surface water as a source of groundwater recharge, water-level altitude in wells, spring discharge, and groundwater quality. This temporal variability affects recharge sources, recharge amounts, groundwater traveltimes, flow routing, water-rock interaction processes, dilution, mixing, and, in turn, water quality. Relations of land cover, aquifer hydrogeology, and changing hydrologic conditions to water quality are complex but provide insight into the vulnerability of Edwards aquifer groundwater—a vital drinking-water resource.

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USGS2018

Characterizing the subsurface geology in and around the U.S. Army Camp Stanley Storage Activity, south-central Texas

Several U.S. Geological Survey projects, supported by the National Cooperative Geologic Mapping Program, have used multi-disciplinary approaches over a 14-year period to reveal the surface and subsurface geologic frameworks of the Edwards and Trinity aquifers of central Texas and the Arbuckle-Simpson aquifer of south-central Oklahoma. Some of the project achievements include advancements in hydrostratigraphic mapping, three-dimensional subsurface framework modeling, and airborne geophysical surveys as well as new methodologies that link geologic and groundwater flow models. One area where some of these milestones were achieved was in and around the U.S. Army Camp Stanley Storage Activity, located in north­western Bexar County, Texas, about 19 miles north­west of downtown San Antonio.

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USGS2018

New insights on scale-dependent surface-groundwater exchange from a floating self-potential Dipole

In south-central Texas the lower Guadalupe River has incised into the outcrop of the Carrizo-Wilcox aquifer. The river and the aquifer are hydraulically connected across the outcrop, although the connectivity is obscured at the surface by alluvium and surface-water and groundwater exchange dynamics are currently poorly understood. To investigate surface-water and groundwater exchange dynamics between the lower Guadalupe River and the Carrizo-Wilcox aquifer, a geophysical study was completed along a 14.86 km reach of the river by using water-borne gradient self-potential (SP) profiling and two-dimensional direct-current electric resistivity tomography. This paper explores the applicability of these water-borne geoelectric methods in delineating gaining and losing channel reaches, and demonstrates that geoelectric signals in the form of total electric field strength can be logged with an electric dipole and decomposed into component SP signals depicting regional and local groundwater flow patterns attributable to regional and localized hydraulic gradients. Localized SP anomalies of several tens of millivolts, indicative of hyporheic exchange flows, are observed and superimposed upon a 124 mV regional SP anomaly indicative of ambient groundwater exchange flows between the river and the aquifer. The observed SP signals are interpreted through two-dimensional finite-element modeling of streaming potentials attributable to ambient groundwater exchange and hyporheic exchange flow patterns. Variables of the channel environment such as temperature and concentration gradients, depth, and velocity are considered and subsequently eliminated as alternative sources of the SP signals that are presented.

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USGS2018

Green tree frog (Hyla cinerea) and ground squirrel (Xerospermophilus spilosoma) mortality attributed to inland brevetoxin transportation at Padre Island National Seashore, Texas, 2015

On 16 September 2015, a red tide ( Karenia brevis ) bloom impacted coastal areas of Padre Island National Seashore Park. Two days later and about 0.9 km inland, 30–40 adult green tree frogs ( Hyla cinerea ) were found dead after displaying tremors, weakness, labored breathing, and other signs of neurologic impairment. A rainstorm, accompanied by high winds, rough surf, and high tides, which could have aerosolized brevetoxin, occurred on the morning of the mortality event. Frog carcasses were healthy but contained significant brevetoxin in tissues. Tissue brevetoxin was also found in two dead or dying spotted ground squirrels ( Xerospermophilus spilosoma ) and a coyote ( Canis latrans ). Rainwater collected from the location of the mortality event contained brevetoxin. Mortality of green tree frog and ground squirrel mortality has not been previously attributed to brevetoxin exposure and such mortality suggested that inland toxin transport, possibly through aerosols, rainfall, or insects, may have important implications for coastal species.

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