Geology topics

Stephen P. Opsahl

Publications and source records attributed to Stephen P. Opsahl.

21 records · Page 2Linked to original sources

USGS2012

Quality of surface-water runoff in selected streams in the San Antonio segment of the Edwards aquifer recharge zone, Bexar County, Texas, 1997-2012

During 1997–2012, the U.S. Geological Survey, in cooperation with the San Antonio Water System, collected and analyzed water-quality constituents in surface-water runoff from five ephemeral stream sites near San Antonio in northern Bexar County, Texas. The data were collected to assess the quality of surface water that recharges the Edwards aquifer. Samples were collected from four stream basins that had small amounts of developed land at the onset of the study but were predicted to undergo substantial development over a period of several decades. Water-quality samples also were collected from a fifth stream basin located on land protected from development to provide reference data by representing undeveloped land cover. Water-quality data included pH, specific conductance, chemical oxygen demand, dissolved solids (filtered residue on evaporation in milligrams per liter, dried at 180 degrees Celsius), suspended solids, major ions, nutrients, trace metals, and pesticides. Trace metal concentration data were compared to the Texas Commission on Environmental Quality established surface water quality standards for human health protection (water and fish). Among all constituents in all samples for which criteria were available for comparison, only one sample had one constituent which exceeded the surface water criteria on one occasion. A single lead concentration (2.76 micrograms per liter) measured in a filtered water sample exceeded the surface water criteria of 1.15 micrograms per liter. The average number of pesticide detections per sample in stream basins undergoing development ranged from 1.8 to 6.0. In contrast, the average number of pesticide detections per sample in the reference stream basin was 0.6. Among all constituents examined in this study, pesticides, dissolved orthophosphate phosphorus, and dissolved total phosphorus demonstrated the largest differences between the four stream basins undergoing development and the reference stream basin with undeveloped land cover.

Texas↗

USGS2012

Science to support the understanding of south Texas surface-water and groundwater resources in a changing landscape

Against a backdrop of constant cycles of extreme hydrologic conditions ranging from oppressive droughts to life-threatening floods, the water-resource landscape of south Texas is undergoing constant change. Demands on water resources are increasing because of changes related to population growth, energy demands, agricultural practices, and other human-related activities. In south Texas, the Nueces, San Antonio, and Guadalupe River Basins cover approximately 50,000 square miles and include all or part of 45 counties. These stream systems transect the faulted and fractured carbonate rocks of the Edwards aquifer recharge zone and provide the largest sources of recharge to the aquifer. As the streams make their way to the Gulf of Mexico, they provide water for communities and ecosystems in south Texas and deliver water, sediment, and nutrients to the south Texas bays and estuaries. The U.S. Geological Survey (USGS) works in cooperation with other local, State, and Federal agencies to provide timely access to water data, publications, and information to foster a better understanding of the water resources of south Texas. The USGS and our cooperators are involved in a wide variety of programs for collecting hydrologic data and scientific information in the changing landscape of south Texas to help our cooperators effectively address water-resource issues in this part of the State. This fact sheet provides an overview of our collaborative scientific endeavors in the basins of the Nueces, San Antonio, and Guadalupe Rivers and lower Rio Grande. An overview of USGS capabilities pertaining to water resource issues in Texas, including recently completed and ongoing studies in south Texas, is available at http://tx.usgs.gov/Capabilities/index.html.

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USGS2011

Suspended material availability and filtration-biodeposition processes performed by a native and invasive bivalve species in streams

Unionid mussels are among the most threatened group of freshwater organisms globally. They are known for their ability to filter food particles from flowing and standing waters. However, invasive bivalve species, such as the Asian clam ( Corbicula fluminea ) in North America, have the potential to overlap in feeding and potentially out-compete the native species. Yet, the feeding preferences of unionid mussels and C. fluminea are incompletely understood. We hypothesized that Elliptio crassidens (native) and C. fluminea (invasive) would select for specific organic components present within seston. We examined changes in seston (dry mass and ash-free dry mass) resulting from bivalve feeding activity for three size classes of material that were isolated using gravimetric filtration. The treatments were also sub-sampled for flow cytometry (FC) which separated the suspended materials in the stream water into five categories: detritus, heterotrophic bacteria, picoautotrophs, nanoautotrophs, and heterotrophic nanoeukaryotes. Our results indicated that both species of bivalve showed preferences for organic and living materials. E. crassidens preferentially filtered nanoeukaryotes, whose decreases were associated with an increase in bacteria. In contrast, C. fluminea preferred smaller materials through selective filtration of picoautotrophs. In addition, both species increased the concentration of large materials toward the end of the experiment because of the suspension of their pseudofeces biodeposits. To our knowledge, this study is the first to examine grazing by bivalve species on natural stream particulate matter using FC. Our results suggest that native and non-native mussels have different functional roles, which has important implications for organic matter processing and food webs in streams.

Georgia↗