HYDROCHEMICAL CHARACTERIZATION OF BRACKISH GROUNDWATER IN LOWER RIO BRAVO AQUIFER, TAMAULIPAS, MEXICO

Groundwater contamination has been proposed in the last forty years as a paramount water supply problem, before then the main concern on water quality degradation was restricted to surface water. Such concern is highly justified given the large and increasing number of soluble chemicals injected to the ground; this is especially valid when we are dealing with the potential contamination effects of unconventional oil and gas development. Tamaulipas state has a prospective area of about 5,500 square miles. Some dams and reservoirs controlling water from the Rio Bravo support irrigation (about 1,000x10⁶ m³/year) for the Lower Rio Bravo (LRB) region (500,000 acres) this source is also the water supply for population along the northern border between USA-Mexico. This investigation presents a detailed geochemical characterization of groundwater in the LRB region, describing groundwater quality and its chemical evolution as it flows to discharge in wells and springs. One hundred and twenty water samples were collected from canals, and shallow and deep wells distributed between Marte R. Gomez Dam and Matamoros City, for chemical analysis of major ions, trace elements, isotopes of oxygen and hydrogen. The aquifer is highly heterogeneous contained in several geologic units of Tertiary and Quaternary age. In general, groundwater in the LRB aquifer is not chemically suitable for regulated uses such as public supply and irrigation. Dissolved solids concentration is high, with a median of about 3,200 mg/l, no correlation of spatial distribution with groundwater flow in the horizontal direction is evident; however, a trend with depth is defined. Other parameters with significant concentration above Mexico’s drinking water standard are total hardness, Cl, SO₄, Na, NO₃, Fe, Mn, Al, Cr, Cd, and Pb. Chemical and isotopic evidence indicate that salinization in the obtained water from the LRB aquifer is produced by: i) dissolution of minerals (carbonate and gypsum), ii) cation exchange, iii) irrigation return flow, and iv) mixing with deep saline water. Future unconventional shale gas development in the northern region of Tamaulipas suggests is not posing a water competition with agriculture and population, an additional available local source is brackish groundwater.