Peer-Review Article Spatial Distribution of Boron
Concentrations in the Edwards-Trinity Plateau Aquifer, Southwest Texas, USA
Paul F. Hudak, University of North Texas, Department of Geography and the Environment
Abstract
Recent dissolved boron concentrations in 198 water wells in the Edwards-Trinity Plateau Aquifer of southwest Texas were compiled and analyzed. The aquifer consists predominantly of limestone and sandstone deposited in fluvial, deltaic, and shal- low marine environments. Boron concentrations ranged from less than 50 ug/L to 3,390 ug/L, with a median concentration of 102 ug/L. Only two boron measurements exceeded the 2,000 ug/L drinking water advisory level for children. However, 11 observations exceeded the 1,250 advisory ug/L level for sensitive crops. Wells open to sand and sandstone in the lower part of the aquifer had relatively high boron levels, with a median concentration of 527 ug/L. Natural interactions between ground- water and the aquifer matrix largely control observed boron concentrations in the study area.
Keywords: Boron, Groundwater, Edwards-Trinity Plateau Aquifer, Texas
Introduction Human actions have led to count-
less cases of groundwater contamination worldwide. However, unsafe groundwa- ter also results from natural interactions between rock, water, and gases that affect concentrations of many solutes. Some natural contaminants, such as arsenic, are well established. High arsenic con- centrations in groundwater beneath the Bengal Delta Plains and Bangladesh have contaminated approximately one million water wells (Nickson et al. 1998). More than 200,000 people in those regions have experienced toxic levels of arsenic caused by natural, reductive dissolu- tion of arsenic-rich iron and manganese oxyhydroxides (Bagla and Kaiser 1996; Nickson et al. 1998). A similar process enriched arsenic in groundwater at sever- al locations in China (Rodriguez-Lado et al. 2013). Elsewhere in China, desorption from mineral oxides in aerobic, alkaline environments contributed to high arsenic concentrations in groundwater (Smedley and Kinniburgh 2002).
Many naturally-occurring contami-
nants such as arsenic tend to be harm- ful at very low concentrations, with no significant health benefits for humans. However, some solutes, including boron, are beneficial at low concentrations and potentially harmful at high concentra- tions. Boron in groundwater originates
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mainly from natural sources. It is a com- mon constituent of brines and evaporite deposits in sedimentary rocks (Weast 1992). Seawater has an average boron concentration of 4.6 mg/L (Hem 1985). Boron is also found in other rocks and in geothermal settings.
High boron concentrations in sand-
stone and shale aquifers in Michigan, and in alluvial aquifers in Bangladesh, were attributed to desorption from min- eral surfaces during influx of freshwa- ter (Ravenscroft and McArthur 2004). Geothermal activity and seawater intru- sion led to high boron concentrations in groundwater of Greece (Dotsika et al. 2006; Voutsa et al. 2009). In Estonia, long-term dissolution of carbonate rock produced high boron concentrations in groundwater (Uppin and Karro 2013). Boron concentrations increased with content of terrigenous material in the rock. Upwelling of connate groundwa- ter accounted for high boron concentra- tions in groundwater of east-central Italy (Palmucci and Sergio 2014). Rock com- position also controlled boron concentra- tions measured at several groundwater sources in northern Poland (Wons et al. 2014).
While often natural, boron in ground-
water sometimes originates from human activity. For example, Gemici et al. (2008) attributed high boron concentra-
tions in groundwater to leaching of host rocks in borate mines of western Turkey. Paliewicz et al. (2015) documented haz- ardous concentrations of boron in gold mine tailings at a site in Ontario, Canada. Sodium borate was used in the refining process. Boron is also found in oilfield brine; it occurs naturally in produced water, but is also used to control viscosity in proppants and drilling mud (Floquet et al. 2017; Craddock 2018).
In addition to mining and oil and gas production, boron has been associated with urban and agricultural activity. Afaj et al. (2015) determined that irrigation water, aquifer composition, and recharge cycles controlled the spatial variability of boron concentrations in groundwater beneath southern Iraq. Especially in dry regions, evaporation and transpiration of irrigation water builds up salts con- taining boron and other elements in the vadose zone. Rain or excess irrigation water may eventually dissolve these salts and carry their constituents to the satu- rated zone. For example, Hudak (2004) documented high boron and selenium concentrations in groundwater beneath irrigated fields of south Texas.
Reed et al. (2017) showed that urban
activity, primarily wastewater discharg- es and fertilizer applications, affected boron levels in Volusia Blue Spring, Florida. Similarly, sewage effluent pro-
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