Characterizing groundwater recharge sources using water stable isotopes in the North Basin of Lake Kivu, East Africa

The delta O-18 and delta H-2 of rivers and springs were investigated in order to characterize the groundwater recharge sources around Nyiragongo and Nyamulagira volcanoes, in the Democratic Republic of the Congo and Rwanda. Water samples were collected monthly between November 2013 and October 2014 from 5 major rivers, 3 major cold springs, 3 tepid springs and 1 hot spring. The temperatures of each spring were nearly constant over the sampling period attesting for their groundwater character, while the temperatures of the rivers were much more variable. The rivers monthly delta H-2 and delta O-18 range from -6.8 parts per thousand to 1.9 parts per thousand and -3.1 parts per thousand to 1.6 parts per thousand, respectively, while springs showed depleted values that span from -10.2 to -1.1 parts per thousand for delta H-2 and - 3.6 to -1.9 parts per thousand for delta O-18. Catchment morphology (formed of depression, upper footslope and medium to high gradient-mountains) and the local tectonic discontinuity (fissures and faults) regulate the surface runoff and subsurface flow, control the precipitation infiltration zones and hence the aquifers recharge areas. Chemical and isotopic (delta O-18 and delta H-2) compositions of springs and rivers reveal the presence of shallow and deep aquifers, with some waters having intermediate isotope composition. Three different recharge zones characterized by different altitudes were identified: the first is found at low altitude ranging from similar to 1800 m to similar to 2150 m, the second and intermediate recharge zone in the altitude range from similar to 2180 m to similar to 2500 m at the upper footslope area, while the third and highest recharge area is located in the altitudes range from similar to 2620 to similar to 3220 m. The two upper recharge areas are the most fractured and fissured zones allowing rapid infiltration of depleted precipitations which recharge deep aquifers found in the tepid and hot springs. Based on their chemical and isotopic composition, waters from the shallow and deep aquifers have been considered representative of mixing end members. During their ascent to the surface, water from the deep aquifer mixes with that of shallow aquifer yielding the tepid springs of intermediate chemical and isotopic composition, while the other keep their original fingerprint corresponding to the isotopically depleted hot spring.

Automated summary

The delta O-18 and delta H-2 values of rivers and springs were investigated to characterize groundwater recharge sources around Nyiragongo and Nyamulagira volcanoes, revealing the influence of catchment morphology and tectonic discontinuity on surface runoff and subsurface flow.