SOIL CONTAMINANT SAMPLING RESULTS FROM LOWER RIO GRANDE VALLEY NATIONAL WILDLIFE REFUGE'S BOCA CHICA TRACT, CAMERON COUNTY, TEXAS
The Boca Chica Tract of the Lower Rio Grande Valley National Wildlife Refuge is one of the last undeveloped, pristine coastal areas in Texas. About 4,452 ha (11,000 ac) in size, it encompasses wind-tidal flats, mangrove forests, oyster beds, bays, beaches, dunes, including more than 3,237 ha (8,000 ac) of highly productive wetlands near the mouth of the Rio Grande (Turner 1988). More than 175 plant species and 344 wildlife species have been documented on or near Boca Chica Tract including 184 species of birds, 14 species of mollusks, 23 species of crab and shrimp, 61 species of fish, 40 species of reptiles and amphibians, and 22 species of mammals (Chaney and Pons 1987). Wind-tidal flats of the Boca Chica Tract are important migration stopover sites for Peregrine Falcons, Falco peregrinus (cf. Maechtle 1987). The Boca Chica Tract also supports the highest concentrations of breeding Snowy Plovers, Charadrius alexandrinus, and Wilson's Plovers, Charadrius wilsonia, in the Lower Laguna Madre Region of Texas (Zdravkovic 2005). Boca Chica Beach is relatively narrow, from as little as 10–100 m wide, and extending for about 12 km from the mouth of the Rio Grande to the South Jetties of the Brazos Santiago Pass. Morton et al. (1983) noted this segment of beach is under threat of shoreline erosion due to lack of sand deposition from the Rio Grande due to construction of dams along its course. More inland from the beach and wind-tidal flats are brushy-covered clay “lomas” or hillocks containing dense growths of Tamaulipan thornscrub or “chaparral” (Clover 1937). Lomas are surrounded by wind-tidal flats and halophytic plant associations known locally as “salt prairie.” Dune hillocks or lomas themselves are quite rare (only found elsewhere in Russia, Australia, and Africa) and were formed from silt deposited by the Rio Grande shaped by prevailing southeasterly winds over time (Richard 2005).
This coastal area has remained largely undeveloped due to lack of infrastructure; however, in the last few years, there has been a sharp increase in urbanization and industrial development, particularly in eastern Cameron County. According to Leslie (2016), urbanized landscapes in Cameron County increased by nearly 53% from 1993 to 2003 and continue to increase to date. Within the study area alone, there are plans to convert part of the pristine coastal dunes and flats near Boca Chica Beach into a commercial launch facility for spacecraft. In addition, a 165-mile international natural gas pipeline extending to Mexico will soon be constructed through the area just north of the proposed launch facility near Brazos Santiago Pass. Several proposed Liquified Natural Gas (LNG) terminals are slated for construction adjacent to the northwestern part of the Boca Chica Tract along the Brownsville Ship Channel at the Port of Brownsville. Finally, there are active oil and gas facilities operating on and in close proximity to the Boca Chica Tract and offshore oil and gas facilities and oil tanker transport activities in the Gulf of Mexico. Therefore, there is a concern increased industrial development of this pristine area or oil spills may lead to a corresponding increase in petroleum-based contaminants on Boca Chica Tract. The objective of this study was to determine levels of petroleum byproducts that may presently occur in soils near the beach on the Boca Chica Tract.
Study Area & Methods
The study area was located about 32 km east of Brownsville, TX, on the Boca Chica Tract, centered near the end of State Highway 4 at Boca Chica Beach (Fig. 1). Soil samples were taken along a north–south route just to the west of Boca Chica Beach, at the interface between dune ridges and wind-tidal flats (Fig. 1). A total of 30 soil samples were collected consisting of 15 randomly sampled sites north of State Highway 4 and 15 similar samples south of the highway, over a total distance of about 10 km. Samples were collected during daylight hours from 28 June through 30 June 2017. At each site, about 170 g of soil was collected from a depth of about 15 cm with a small shovel. Sites with heavy sand (i.e., dunes) or with dense vegetation were avoided in order to sample more compact soils. Each sample was placed in a 118 ml (4 oz) sterile glass sampling jar and then packed in an ice container made for direct shipping to Pace Analytical Services LLC., in Allen, Texas, for analysis. Samples were tested for presence of TPH using the Texas Commission on Environmental Quality (TCEQ) analytical method 1005 (TNRCC 2001). Reporting levels of TPH were established for each sample for three general petroleum types (i.e., gasoline, diesel, and oil) and were derived using a dry-weight correction, as described in the TCEQ 1005 method. Soil moisture was also tested using the American Society of Testing and Materials (ASTM D2974-07) analytical method for organic soils. Soil samples were tested for pH at 25°C as per the Environmental Protection Agency (EPA) analytical method 9045.
Citation: Texas Journal of Science 70, 1; 10.32011/txjsci_70_1_Note2
Results & Discussion
Soil sampling results for TPH ranging from C6–C35 (number of carbons) covered a range of typical petroleum types from gasoline (C6–C12), diesel (C12–C28) to oil (C20–C35) products. All samples had non-detectable levels of TPH that were below the established reporting levels for the three petroleum types and well below ecological benchmark values established by the TCEQ for TPHs (TCEQ 2017) (Table 1). Percent moisture averaged 13.1% with a range from 3.5% to 33.8%, which is considered fairly low. Average pH value for the samples was 8.2, which was consistent with values found in seawater.
The results of this study confirm the general conclusion that the area of concern on the Boca Chica Tract did not show evidence of TPH contamination. Although significant levels of contaminants such as TPH were not suspected, there are concerns this area might be subject to such types of contamination in the future. Therefore, the primary purpose of this effort was to ascertain baseline TPH levels. The Boca Chica Tract contains rare habitats and remains highly productive for a wide variety of migratory and resident wildlife including finfish and shellfish nurseries, nesting grounds for endangered Northern Aplomado Falcons, Falco femoralis septentrionalis, and threatened and endangered sea turtles; breeding grounds for Snowy and Wilson's plovers, as well as critical wintering habitat for Piping Plovers, Charadrius melodus (cf. USFWS 1990; USFWS 2001; Zdravkovic 2005; NMFS et al. 2011; Leslie 2016). Therefore, despite the ongoing development of the area for industrial and commercial uses, it is recommended that future soil sampling or similar studies be undertaken to help ensure this sensitive area continues to provide essential, quality habitat for important fish and wildlife resources.
Location of soil sampling sites in the Boca Chica Tract, Lower Rio Grande Valley National Wildlife Refuge, Cameron County, Texas.
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