SERPENTINITE-WATER INTERACTION AND CHROMIUM(VI) RELEASE IN SPRING WATERS: EXAMPLES FROM TUSCAN OPHIOLITES

Abstract

Since 2006 the Regional Environmental Agency (ARPA) of Tuscany has found Cr(VI) concentration in groundwater above the maximum acceptable con- centration (5 ppb) according to the Italian regulation (D.M. 25 October 1999, n. 471). Recently, hydrogeological investigations (Grassi, 2009) confirmed high total Cr and Cr(VI) contents in soils and waters related to ultramafic rocks from coastal Tuscany. To study the release of Cr(VI) to water from geogenic sources we examined the spring waters, soils and bedrocks compositions within two serpentinized bodies of the coastal Tuscany, namely Santa Luce and Querceto. These selected sites are characterized by Cr(VI)-free and Cr(VI)-bearing spring waters, respec- tively. The prevalence of Mg-HCO3 waters at these sites indicates that waters are primarily interacting with serpentinites. Samples from Querceto are enriched in Mg suggesting the leaching of brucite. Soil profiles and bedrocks were analysed for major and trace elements by XRF to assess the fate of Cr during weather- ing. Detailed petrographic and chemical analyses of both soil and rock samples revealed that Cr is mainly hosted within chromites. Chemical data and alteration features indicate a metasomatism coeval to the oceanic serpentinization leading to a Mn-enrichment of the alteration products of primary spinel. The co-occur- rence of Mn-bearing and Fe-rich oxides with Cr-rich spinels creates a chemical system potentially able to promote the Cr(III) to Cr(VI) oxidation. However, the occurrence of Mn-rich minerals seems to be not correlated to the oxidation state of chromium in the spring waters. In fact, they are ubiquitous in both the study localities, while Cr(VI) has been detected only at Querceto. Unlike Santa Luce, serpentinites from Querceto contain a significant amount of Fe-rich brucite and show evidence for ongoing brucite dissolution coupled with precipitation of hydromagnesite crusts and veinlets. Formation of complex hydroxy-carbonate min- erals could be an alternative way for producing the proper environment for oxidative leaching of chromium from serpentinites to spring waters.