SPATIAL DISTRIBUTION OF BORON
duced high boron concentrations in a shallow alluvial aquifer in Saudi Arabia (Rehman and Cheema 2017).
Food and drinking water contribute significantly to daily boron intake in humans (Anderson et al. 1994; FNB 2001; PDRIEW 2004). Long-term health advisories for boron in drinking water for children and adults are 2,000 ug/L and 5,000 ug/L, respectively (EPA 2008). Consuming higher than advisory levels of boron may be harmful; however, boron may also contribute to positive health outcomes. Boron consumed in food and water may reduce the risk of prostate cancer (Cui et al. 2004; Barranco and Eckhert 2004). Moreover, boron may improve the effectiveness of some che- mo-preventative agents. In laboratory studies, prior treatment of cells with boric acid sensitized them to ionizing radiation (Barranco et al. 2007).
Boron is also an essential nutrient for
plants, but potentially harmful in high concentrations. Sensitive, semitolerant, and tolerant crops can withstand up to approximately 1,250, 2,500, and 3,750 ug/L boron, respectively (USSL 1954). For some plants, the toxic concentra- tion of boron is as low as 1,000 ug/L (Hem 1985). The boron content of plants reflects concentrations in both soil and
irrigation water. An area of southwest Texas used for cotton farming, range- land, and oil and gas production, which may be vulnerable to boron contamina- tion from human activity and natural sources, was investigated in this study. The objectives were to compile, map, and evaluate boron levels in the Edwards- Trinity Plateau Aquifer.
Study Area The study area is located in the ru-
ral, semi-arid, rocky plains of southwest Texas (Figure 1). This region is used predominantly for raising livestock and producing oil and gas. Rangeland (shrub/scrub) covers most of the area (NLCD 2011), which contains thou- sands of active oil and gas production wells (TCEQ 2017). Irrigation wells are most prevalent in the northern part of the study area; however, less than one percent of land in the study area is irrigated (USDA 2012a). Similarly, fertilizer is applied in the northern part of the study area, but most of the land in the study area is not fertilized (USDA 2012b). The study area is used to raise cotton in the north and livestock throughout; common livestock include cattle, sheep, and goats. Beneath the land surface, the uncon- fined Edwards-Trinity Plateau Aquifer
supplies water for human and animal consumption, as well as irrigation and industrial needs (USGS 2002). The aquifer consists mainly of Cretaceous shallow-marine limestone, dolomite, and evaporites with secondary porosity (Edwards Group; upper part); and in- terbedded fluvial sand, sandstone, and clay (Trinity Group; lower part) (TWC 1989; Barker and Ardis 1992; Bush et al. 1994; USGS 2002). Precipitation is the main source of recharge to the aquifer. Annually, pre- cipitation averages approximately 34 cm, while lake evaporation is much higher, averaging approximately 71 cm (TWDB 2018a). With low rainfall and high evaporation rates, relatively little water reaches the water table, averag- ing approximately 1 cm/year (Hopkins 1995). Saturated thickness beneath the study area averages about 130 m (TWDB 2018b). Groundwater generally flows southeastward and discharges to wells, streams, and springs. Water wells in the study area yield an average of approximately 8 liters/s (USGS 2002).
Methods
Dissolved boron concentrations, well locations, and depth measurements were compiled, mapped, and analyzed for 198 water wells in a groundwater da-
Figure 1. Map of study area showing sampled water wells (+), Edwards-Trinity Plateau Aquifer (light green), and water well usage categories (D – domestic, I – irrigation, N – industrial, P – public supply, S – stock, U – unused); north toward top.
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