Environmental impact of groundwater abstractions in the Salar del Huasco (northern Chile)

Abstract

In the Andes Range of the arid north of Chile there are elongated depressions containing salt flats (“salares”) and wetlands that are characteristic of the Subdesertic High–Andes Estepe (“bofedales”). One of them is the Salar del Huasco, declared RAMSAR site due to its high value as habitat and landscape. The aquifer system of the basin is in natural state, such as in the long–term recharge by rainfall infiltration, the only inflow, equals discharge by evaporation, the only proved outflow of the system. These aquifers are attractive to the mining industry, which needs large water flows for mineral processing. One of the copper mining companies has developed wells for about 900 L/s, and consequently has showed his interest in this water. Such a high flow needs a careful study of the global aquifer behavior and determining the potential environmental impacts on local humid ecosystems. Consequently a first conceptual basin model has been defined, in order to construct a numerical flow model to support decisions. In the area can be distinguished: a) an upper aquifer in the Recent Deposits and Upper Unit of Collacagua Fm., b) an intermediate aquifer in the Middle and Lower Units of Collacagua Fm., and c) a lower aquifer in the Huasco Ignimbrite and stratrovolcanoes of Sillillica Fm. About 80% of studied soils have coarse texture, with low water retention and high infiltration capacity, which favors recharge. This is a differential characteristic respect to many of the (semi–)arid areas elsewhere. Total average recharge, checked against the numerical model results, is close to 1100 L/s, and 80% of this quantity goes to the salt flat. Thus, there is a wetland that can be qualified as mixed hypogenic–epigenic one, in which groundwater abstractions of hundreds of L/s may produce a significant impact. Seeking a compromise between ecological conservation and supplying miner’s needs, an abstraction flow of about 200 L/s during 25 years seems to produce admissible ecological impacts, without significative level changes and chemical disturbances to the lagoons, since the springs will be almost no affected and groundwater contribution to the wetland is almost unchanged. The abstraction flow will come essentially from river water and storage in the mean watershed.