Mercury Dispersal to Arroyo and Coastal Sediments from Abandoned Copper Mine Operations, El Boleo,

Abstract Evidence for mercury dispersal in an aridcoastal region of central Baja California (Mexico) suggeststhat abandoned copper mining operations are a noticeablesource of mercury in the environment. There is a generallyelevated level of mercury in alluvium of arroyos through-out the mining district (0.14–0.18 mg kg -1 ). In the firstseveral dozen meters surrounding two of the biggest mines,mercury levels range from 0.26 to 3.16 mg kg -1 , forming ahalo of anomalously high concentrations. The coastalmarine sediments, particularly those close to the coppersmelter in the town of Santa Rosali´a, also display somemercury enrichment.Keywords Mercury halo Abandoned mines El Bole´o mining district Coastal marine sedimentsMercury has long been recognized as a potentially valuabletracer when prospecting for various types of mineraliza-tion, primarily because many of its forms naturally displayhigh vapor pressure. Saukov (1946) was the main earlyproponent, recognizing that this attribute could produceanomalies around certain types of mineral deposits. Sincethe 1960s, Hg has become an important geochemicalexploration tool for water, vegetation, soil, bog, streamsediment, and rock studies (Lentz 2005). Weathering ofabandoned metal ore mines may produce a noticeable Hghalo that can be traced by examination of the sediments inthe local drainage basin. While metal mine pollution hasbeen extensively studied in temperate and cold regions, theimpact of mining activities on terrestrial and marine eco-systems of arid and semi-arid regions has received muchless attention (Shumilin et al. 2000).The aim of the present study was to trace the Hg dis-sipation halo of copper mines under arid conditions,namely at the abandoned mines of the El Bole´o miningdistrict near Santa Rosali´a, east-central Baja California(Mexico). Intensive copper mining and smelting werecarried out there between 1885 and 1985. According toWilson and Rocha (1955) and Conly et al. (2005), thecopper deposits were formed within geologic formationsrelated to Miocene continental rifting (approx. 11 Ma)during the early stages of the opening of the Gulf of Cal-ifornia. The area is characterized by laterally extensivestrata of laminated claystone and claystone breccia thatcontain finely disseminated Cu–Co–Zn sulfides and oxi-dized sulfides (Conly et al. 2005).The marine sediments near Santa Rosali´a are affected bymaterial discharged by arroyos and may serve as a relevantindicator of mine metal dissipation. The alluvium in thearroyo below the town includes waste products from thesmelter.