Field evidence for density-driven free convection, a potentially important groundwater transport process, has been examined at Padre Island National Seashore to determine if this phenomenon can develop under natural environmental conditions. Hitherto, this process had not been conclusively detected or measured in field scale hydrogeology. Field methods, including nested monitoring wells and time-lapse 3-D resistivity surveys, reveal evidence of variable-density groundwater flow in the wind-tidal flats. Evaporative concentration of groundwater near the water table resulted in unstable inverted density gradients, reduced groundwater levels, and reduced hydraulic gradients. These factors allowed plumes of dense fluid to migrate downward into less dense fluid which were observed in monitoring wells and 3-D resistivity surveys. This shows that the development and flow of variable-density fluids in groundwater can be detected and monitored through field techniques. It demonstrates that the development of density inversions may overcome the dissipating forces of dispersion and diffusion to create a sufficiently large unstable gradient to induce free convection. / text

Oblique dunes have orientations that are intermediate between those of transverse and longitudinal dunes. The oblique dunes studied are reversing dunes which undergo no net annual migration when associated with normal meteorological patterns. From April 1980 through September 1980, the dunes migrated up to 65 feet (19.8 m) northwestward under the influence of prevailing onshore winds. High velocity northerly winds (November 1980 through February 1981), associated with the passage of winter frontal systems, caused the dunes to rapidly migrate 65 feet (19.8 m) southward. Volumes of sand transported by these strong winds were commonly reduced by accompanying rainfall. In October 1980 and March 1981 neither wind direction was dominant, and frequent changes in wind direction caused many of the dunes to become flattened. Hurricanes, which strike the area in late summer, have had no lasting effects on the dunes. Three major stratification types were observed in trenches and on etched surfaces. Translatent strata were deposited by wind ripples; grainfall deposits accumulated when saltating grains settled on leeward slopes of the dunes, and grainflow cross-strata were developed by avalanching on leeward slopes. Preservation of these stratification types occurred in zones of net deposition, predominantly leeward of the dune crests. Strata deposited during the summer wind regime dip northeast, whereas the winter strata dip in a southerly direction. The winter deposits are best preserved in the central cores of the dunes. This suggests that either the high velocity winds of the initial winter frontal systems destroy large volumes of the summer deposits, or that the dunes migrate southward, under the influence of dry northerly winds, during droughts. Oblique dune deposits should be difficult to discern in the rock record, because they may contain aspects of either transverse or longitudinal dunes. It is likely that some ancient oblique dunes have been mistakenly described as other dune types in the literature. / text

Padre Island National Seashore provides important habitat for wintering grassland birds, which are declining throughout their breeding range, yet oil and gas development is widespread in the park. My objective was to evaluate the effects of resource extraction on the park's grassland birds and make management recommendations based on my findings. In January-March 2007 and 2008, I surveyed 5 active, 4 abandoned, and 4 road sites to investigate the relationship between distance from disturbance (well pads, access roads) and bird abundance. I also compared abundance among the 3 site types. At each site I recorded bird numbers and species in 10-m distance bands along all transects (4 transects/well, 2 transects/road), each extending 300 m from the road or pad. At road sites bird abundance was positively correlated with increased distance from road edge, but I found no linear relationship at active or abandoned well sites. However, mean bird abundance in the first (0-30-m) distance interval of active well transects was less than half that at the second interval, and was the lowest value for all active intervals except the ninth. First-interval abundance at active wells was lower than abundance at any abandoned well interval. Road transects likewise showed low abundance in the initial interval, although unlike at active wells abundance increased steadily with distance from the center of disturbance. This trend of lower overall numbers at the first interval of active well transects was driven largely by 1 species, the meadowlark. A combination of high noise levels near active well pads (up to 80 dB) and lack of tall vegetation (on average 30% lower than the 60-90-m interval) from which to sing may have contributed to low numbers of meadowlarks, which were the only birds to sing regularly during my study period. While most birds appear to be minimally affected by resource extraction at Padre Island, to ensure minimal impacts on sensitive species I recommend: 1) reducing noise at active sites, 2) limiting disturbance to vegetation near pads and roads, 3) maintaining existing perch sites, 4) restoring all vegetation to its pre-extraction condition, 5) limiting road construction.