A conceptual model of the aquifer in Tiquipaya, Bolivia : A minor field study

Sammanfattning: SummaryThe area of study in this project is in Bolivia, South America. The specific area is theMunicipality of Tiquipaya, situated in Cochabamba valley. Cochabamba valley consists of a gently sloping plain bounded by steep slopes of the Cordillera. The vegetation in the Cordillera is mostly high steppe and high mountain prairie. The average annual precipitation is 600-1000 mm/ year. When it rains in the mountain range the rivers flow down to the valley with high velocity and cause great problems with erosion. In the valley the average annual precipitation is 500 mm/year and the land use is mostly agricultural. Because of the intense agriculture and the rapid population growth, Cochabamba Central Valley suffers a constant shortage of water. Different irrigation systems and groundwater from the aquifer supplies Tiquipaya with water.The aquifer in Tiquipaya is one out of three thick sedimentary bodies in the Central Valley. The aquifer is an alluvial fan delta and the thickness is approximately 500 m. It is recharged through infiltration from the mountain rivers, precipitation on the valley area and by irrigation. The discharge from the aquifer consists of wells and springs. There are 37 flowing springs in Tiquipaya and they have been in use for hundreds of years. During the last 10 years the habitants have noticed a decreasing amount of flow in the springs. This indicates that the groundwater level is sinking. In order to find out whether or not the aquifer reserve is decreasing a water balance is calculated. To estimate the recharge to the aquifer we had access to precipitation data, but we had to estimate the evapotranspiration and the percentage which infiltrates. Concerning the discharge we had access to specific spring data over two areas and a list of registered wells. We know that there are more wells then the ones registered, but we did not know how many and what flow rate they have. Based on the existing data and our assumptions we have estimated a possible discharge for the Tiquipaya area. To get a truthful picture of the reality, we have made three water balances based on different data and variations. The one we find most likely show that the aquifer is decreasing.The aquifer is under a lot of environmental threats. The town of Tiquipaya is situated on the aquifer and the town’s residual water is used as irrigation water without any treatment. Also the river beds, which are very permeable recharge zones, are being used as garbage heaps. Concerning the aquifer the most impending problems are the use of residual water and the garbage handling. To obtain a sustainable development the habitants' residual water and garbage have to be taken care of.As a conclusion we do not think that the actual situation in Tiquipaya is sustainable. The groundwater reserve is decreasing and can not supply enough water for the municipality. If the agriculture stays as intensive as it is today the municipality needs to supplement with water from watersheds outside of the Cochabamba Valley. This would also give the aquifer a chance to restore its reserve. The completion of the Misicuni project will hopefully provide Tiquipaya with the water supplement needed. In order to determine the exact extension and capacity of the aquifer more reliable data is needed. We recommend geophysical tests, drilling and field investigations to obtain this data.