Spatial and seasonal variations of surface and groundwater quality in a fast-growing city: Lubango, Angola

Abstract

Lubango is a fast-growing city of Angola, with a very deficient water distribution public system. The majority of the population uses groundwater from shallow, rudimentary dug wells and also surface water from the Mapunda River, which crosses the city. The city has very poor systems for collecting domestic sewage and household wastes. The majority of domestic sewage is dumped directly into the Mapunda River or in pit latrines. In this study, the surface water quality and groundwater quality were assessed. In surface water, the low oxygen contents are related to the high temperature (T) in the warm rainy season and with high contents of oxygen-consuming organic pollution and nutrients, as shown by PCA, in both seasons. Anoxic conditions, with negative ORP values (− 55.7 mV), are caused by oxygen-consuming organic pollution, added with low flow regime during the dry, cold season. The very low ecological quality of the surface water is also indicated by the high contents of PO4 and Clt (up to 2.00 and 0.44 mg L−1, respectively). The phosphate contamination is higher in the warm rainy season, due to higher run-off and higher T, as expressed by PCA, both in river water and groundwater. Most groundwater sampling points are contaminated with nitrate (contents up to 132 mg L−1). The pH, total dissolved solids (TDS), alkalinity, Clt and K, determined in the surface water reflect the anthropic contribution to the river water. The variations in water bulk parameters (T, TDS, alkalinity, ORP, DO) and chemical species are indicative of pollution by diffuse sources (N-species, phosphates). The PCA shows that non-point sources such as the free discharge of household wastewater, intense laundering in the river, agriculture and animal grazing are the main contributors to low water quality, associated with seasonality and low contents of dissolved oxygen due to its consumption by organic matter. The multivariate analysis allows the evaluation of the changes in water quality, in fast-growing cities where the climate has a clear seasonality.