Polycyclic Aromatic Hydrocarbons and Suspended Materials in a Semi-urbanized Tidal Creek after an Historic Flood Event and Implications for Water Quality Monitoring

Tidal creeks transport both dissolved and particulate natural organic carbon materials and contaminants, connecting land-based activities with estuarine surface waters. It is important to characterize these materials in tidal creeks because it provides insights as to their origins and potential for ecosystem impacts. Surface water samples were collected from Bull Creek, Charleston, SC, a semi-urbanized tidal creek wetland, on five sampling dates from fall 2015 to spring 2016 to measure total suspended solids (TSS), turbidity, dissolved organic carbon (DOC), SUVA254 (specific absorbance as an indicator of aromaticity of DOC), and total water concentrations of polycyclic aromatic hydrocarbons (PAHs), a ubiquitous class of hydrophobic organic contaminants of concern. Stream discharge was also measured to allow an estimation of material flux. One of the sampling dates captured these parameters following a historic rainfall related to Hurricane Joaquin in October 2015, and therefore the aim of the present study is to characterize the sources and to quantify the transport of carbonaceous materials and PAHs in Bull Creek, with a focus on the response to this storm event. The quality of suspended solids and DOC were different following the October storm event in comparison to the other sampling dates, and they were more terrestrially derived as shown by shifts in SUVA254 and correlations between TSS and turbidity. Elevated levels of PAHs were detected in Bull Creek after the storm, and diagnostic ratios indicated that additional mixed sources were mobilized by the event. Combining the measures of both carbonaceous material quality and PAH profile contributed to a better understanding of the sources to the tidal creek. Shifts in PAH sources and suspended materials have implications for PAH toxicity to aquatic life, as well as for the appropriate approach to water quality monitoring. Future work should aim to develop relationships between discharge, suspended materials, and PAHs to facilitate more continuous monitoring of material transport in tidal creeks, especially during storm events, which have a strong influence on water quality.

Geomorphic and sedimentary responses of the Bull Creek Valley (Southern High Plains, USA) to Pleistocene and Holocene environmental change

Fluvial geomorphology and stratigraphy often reflect past environmental and climate conditions. This study examines the response of Bull Creek, a small ephemeral creek in the Oklahoma panhandle, to environmental conditions through the late Pleistocene and Holocene. Fluvial terraces were mapped and their stratigraphy and sedimentology documented throughout the course of the main valley. Based on their elevations, terraces were broadly grouped into a late-Pleistocene fill terrace (T3) and two Holocene fill-cut terrace sets (T2 and T1). Terrace systems are marked by similar stratigraphies recording the general environmental conditions of the time. Sedimentary sequences preserved in terrace fills record the transition from a perennial fluvial system during the late glacial period and the Younger Dryas to a semiarid environment dominated by loess accumulation and punctuated by flood events during the middle to late Holocene. The highest rates of aeolian accumulation within the valley occurred during the early to middle Holocene. Our data provide significant new information regarding the late-Pleistocene and Holocene environmental history for this region, located between the well-studied Southern and Central High Plains of North America.

Ultrashallow depth imaging of a channel stratigraphy with first-arrival traveltime inversion and prestack depth migration: A case history from Bull Creek, Oklahoma

Depth imaging in ultrashallow ([Formula: see text]) environments presents twofold challenge: (1) coda available for depth migration is very limited; and (2) conventional time processing with limited coda generally fails to estimate reliable velocity models for depth migration. We studied the combining of first-arrival traveltime inversion and prestack depth migration (PSDM) for depth imaging of ultrashallow paleochannel stratigraphy associated with the Bull Creek drainage system, Oklahoma. Restricted by a limited number of geophones (24) we acquired data for inversion and migration through two coincident profiles. The first profile for inversion has a wider survey-aperture (115-m maximum shot-receiver spacing) and consequently sparse CMP spacing (2.5 m), whereas the second profile for PSDM has denser CMP spacing (1 m) and consequently a narrower survey aperture (46-m maximum shot-receiver spacing). We also found that the velocity model from traveltime inversion of the wider-aperture data set is more preferable for depth-migration than the velocity model from time processing of the denser data set. The preferred depth image showed three episodes of incision whose chronological order is resolved through radio-carbon dating of terrace sediments. Results suggested that even with limited geophones, depth imaging of ultrashallow targets can be achieved by combining first-arrival traveltime inversion and PSDM through coincident wide- and narrow-aperture acquisitions.