ARSENIC CONTAMINATION IN THE COPIAPÓ RIVER BASIN, ATACAMA REGION, CHILE

Copiapó, capital of the administrative Region of Atacama in the arid northern Chile, is located on the banks of the Copiapó River. Between late 1997 and March 2015 the river had dried mostly due to the lack of precipitation and partly due to overexploitation for irrigation and mining processes. Heavy rains in the catchment area of the river on March 25, 2015 not only caused reappearance of the river, but also led to heavy flood that affected the entire region with contaminated water and mud from the mineral rich higher regions, causing several casualties and substantial economic damage. Once dried, this mud converted into the dust blown over the entire basin. Likely presence of arsenic in the dust was a major concern for the local population inhabiting mining towns of the region. To address this concern, mud and airborne dust samples were analyzed using X-ray fluorescence technique, which gave the values very well below 100 ppm. It was corroborated further by atomic absorption analysis of the samples, which gave even lower values of arsenic concentration (5 ppm). The differences in the results can be attributed to low solubility of the arsenic bearing minerals. In May 2017, a fresh flood affected the region and caused the river to re-appear for a prolonged period. Water quality was investigated throughout the course of the Copiapó River, which included the measurement of arsenic concentration using ICP-MS. It is important to mention that the first two data points belong to different tributaries of the Copiapó River, as water from two different tributaries (Manflas and Jorquera) were sampled in two field campaigns. For the most part of the river (between Km. 20 and 100 from its headwaters, corresponding to Lautaro Dam and Piedra Colgada sectors respectively), it is below the Chilean norm (NCh409/1.Of2005) for potable water of 0,01 mg/L. In the catchment area of the river, located in the Andean Cordillera, some of the tributaries have relatively higher arsenic concentration, of both geogenic and anthropogenic origin, affirmed by the presence of arsenic bearing mineralization and mining activities in the catchment area. A peak observed close to the city of Copiapó (Km. 95 from the headwaters) can be attributed to a local contamination source that not only contributes arsenic, but a wide spectrum of different metals. Downstream, Piedra Colgada onward, the arsenic concentration increases again. This is due to the base flow from very old (not dated, but chemically highly evolved) aquifers in that sector that have higher arsenic content, besides other toxic elements.