Strontium isotopes as tracers for quantifying mixing of groundwater in the alluvial plain of a coastal watershed, south-eastern Tanzania

Abstract

Surface water and groundwater chemistry of the coastal aquifer systems in south-eastern Tanzania have been studied with a focus on isotopes and major ions, in order to assess the controlling hydrological and geochemical processes. 87Sr/86Sr isotope ratios have provided information about the hydrology that could not have been obtained by studying the physical and chemical characteristics of water quality alone. The data reveal a rather complex mixing pattern of various water source end-members that are subject to different hydro-geochemical processes, such as evaporation, weathering and other water–rock interactions, and seawater intrusion, which control the water quality. Data on 87Sr/86Sr ratio distribution and strontium (Sr2+) concentration delineate three types of groundwater in the coastal aquifer system:(1)water recharged by fresh precipitation/stream water infiltration and influenced by soil processes like weathering and evaporation,(2)groundwater subjected to rock–water interactions with different bedrock types, and(3)groundwater that is brackish due to seawater intrusion.Most of the groundwater data can be modeled as strontium-poor recharge water of meteoric origin. These waters are influenced to varying degrees by soil–water interactions in the surficial aquifer material. Elevated NO3− concentrations in a few shallow aquifer samples imply sewage infiltration from domestic wastewater. Data on the 87Sr/86Sr ratios did not appear to be affected by this anthropogenic influence. High Sr ratios (>0.711899) in the samples suspected of being contaminated by sewage as well as a couple of other shallow groundwater samples is likely to have been caused by weathering of surficial aquifer material. A lack of variation in Sr2+ content between the river water and groundwater samples reflects an efficient mixing of water during well discharge. The data also clearly demonstrate the influence of aerosol sources in rain water on the strontium isotope signatures.The results illustrate the strength of combining isotopic measurements with more conventional water chemistry data in order to obtain a better understanding of groundwater flow systems and the geochemical processes governing their chemistry.