Hydrochemistry, 18O/16O and 2H/1H Ratios of Groundwater and Surface water in Raya Valley, Northern Ethiopia.

English Abstract:

The uses of groundwater for irrigation, domestic supply and livestock water is increasing in the Raya Valley. Raya valley has been affected by recurrent draught and famine in few years back. Recent, study shows that the groundwater potential of the study area is high. The volcanic aquifer and alluvial aquifers are the host geological formation to hold and transmit groundwater.

In this study, we used the application of isotope hydrology and hydrochemistry to understand the possible interconnections between groundwater with surface water. A total of 128 water samples were collected from groundwater, river, swamp, lake and rainfall. Most of the groundwater samples from volcanic aquifers are characterized by fresh, low salt content and depleted isotopic composition, however, the low-lying alluvial aquifer is characterized by high dissolved chemical constituents, high salt content and enriched isotopic composition. A moderate salt content and slightly enriched groundwater samples are found from the foot of the escarpment, showing possible groundwater mixing. Almost all of the groundwater samples are plotted just below Local Meteoric Water Line, which indicates the groundwater origin is meteoric origin and undergoes significant evaporative fractionation prior to recharge.The surface water shows distinct physiochemical, hydrochemical and isotopic values. The average stable isotopes values for Lake Ashenge samples were found to be 4.27 ‰ for δ18O and from 29.23 ‰ for δ2H. Lake water sample plotted below the LMWL and shifting toward the right this shows the lake undergoes high level of evaporation.

The local evaporation line for Lake Ashenge (LEL) is constructed based on isotopic data and given as δ2H = 5.16 δ 18O + 8.5 with regression coefficient of 0.89. The LEL crosses the LMWL at the isotopic values of 1‰ in δ2H and -1.5 ‰ in δ18O. This input signal indicates the dominant groundwater inflow from the lake is from the groundwater and local rainfall. The slight enrichment of the input signal relative to the mean precipitation in the study area could be evaporation of river flowing toward lake during rainy season. The Isotopic signature for the Gerjale wampy water, which supports a lots of ecological fauna and flora, indicates the principal source from the local perched aquifer.