The origin of geothermal waters in Morocco: Multiple isotope tracers for delineating sources of water-rock interactions

Abstract

The geochemical and isotopic (strontium, boron, radium, oxygen, hydrogen) variations of geothermal waters from five different regions in Morocco were investigated in order to evaluate the sources of solutes and the mechanisms of water-rock interaction. During 2008 to 2010, twenty-two geothermal water samples were collected from the southern, central, and northern parts of Morocco. The water samples were analyzed for major and trace elements, stable isotopes (δ18O, δ2H) naturally occurring radionuclides (226Ra, 228Ra, 224Ra), and isotopes of dissolved strontium (87Sr/86Sr) and boron (δ11B). The Moroccan geothermal waters exhibited a wide range of salinity, ranging from 590 to 25,000 mg/L with predominance of chloride, sodium, sulfate, calcium, and bicarbonate ions. Integration of the geochemical and isotope data suggests that the geothermal waters in Morocco originate from recharge of meteoric water and water-rock interactions. The predominance of chloride and sodium and the relatively low Br/Cl ratios (<1.5 × 10−3), combined with Ca/SO4 ratios ∼1 suggest that halite and sulfate (gypsum/anhydrite) minerals are the major rock sources for mineralization of the geothermal waters. The variations of 87Sr/86Sr ratios (0.7076–0.7122) and δ11B (5.3‰–29.3‰) were used to distinguish two rock sources (1) dissolution of marine sulfate/carbonate rocks with Sr isotope composition that correspond to the expected ratios in seawater during time of deposition; and (2) non-marine sources with higher 87Sr/86Sr and δ11B, presumably from interactions with clay minerals and shale rocks. The variations of radium isotopes indicate that the alpha-recoil of the parent nuclides on the host rocks caused mobilization of the short-lived 224Ra and 228Ra to the geochemical waters. The low 228Ra/226Ra ratios (activity ratios of 0.04–0.14) measured in the geothermal waters mimic the Th/U ratios in the source rocks, which indicate predominance of uranium over thorium. Our data show that some of the geothermal waters in Morocco are characterized by high salinity, and in some systems also by elevated radioactivity above international drinking water standards, which limits their utilization for agricultural and domestic applications. Adequate treatment is therefore required to use geothermal waters in Morocco as an alternative or additional water source. Other geothermal waters with low mineralization can be utilized as alternative resource for the agricultural and domestic sectors without any restrictions.