Ground Water and Surface Water Influence on the Water Quality in the Antequera River basin, Bolivia : A Minor Field Study

Sammanfattning: The Antequera Basin of 150 km2 is a sub-basin to Lake Poopó, located at an altitude of 3800 meters above sea level on the Bolivian high plateau. Lake Poopó is a terminal lake with high salinity and heavy pollution from centuries of extensive mining activity. The climate is semiarid-cold with an average precipitation of 450 mm/year and potential evaporation of 1700 mm/year. Almost all rain falls during December to March and the temperature varies from -2 to 18°C during the summer and from -10 to 14°C during the winter. Antequera River is seasonal, only carrying water during the rainy season. The social situation is marked by poverty and scarcity of water. The sub-basin Antequera has a population of about 4300 inhabitants. The inhabitants of lower part of the basin mainly earn their living from agriculture and livestock and in the upper part mining activity is important. The water quality is fairly good in the uppermost part of the basin while there are problems with pollution due to mining activity further down. This study aims to investigate the ground water and surface water system in River Antequera and how the water quality changes along the river. Also to find out if anthropogenic and natural influences affect the surface water quality. A field study was done at the end of the rainy season, in the beginning of March 2008, to collect water samples and measure river flow, pH and TDS. The samples were analyzed regarding concentrations of Ca2+, Mg2+, Na+, K+, SO42-, NO3-, Cl- and alkalinity. The water quality and flow was compared along the river to find out if anthropogenic influence and ground water affect the surface water quality. In the uppermost part of Antequera basin the river water quality is of fairly good quality except in regards of nitrate. A bit further downstream the area is full of old mine residue and active and deserted mines surround the river. pH in the river drops to around 2-3 here and remains low throughout its course. All concentrations of analyzed ions increase significantly in the reach through the mine district due to extensive weathering. Further south a tributary is connecting and this result in dilution. As the river approaches the low land all ion concentrations increase which indicates intrusion of shallow ground water. Sodium and chloride keeps increasing toward the saline Lake Poopó. Measurements of total dissolved solids (TDS) in wells show ground water becoming more saline close to the lake.