Hydrogeochemical processes and groundwater evolution in complex volcanic highlands and alluvio-lacustrine deposits (Upper Blue Nile), Ethiopia.

Lake Tana basin encompasses a wide regional volcanic aquifer system, in the northwestern highlands of Ethiopia. It has significant water resource potential for water supply and irrigation purposes. The objective of this study was to assess the physiochemical characteristics of the surface water-groundwater and investigate the processes controlling groundwater chemistry. A total of 273 water samples were collected from different water points for major ions and stable isotope analysis, representing different physiographic and hydrogeologic regions. Multivariate statistical, graphical methods, saturation and speciation modelling were used jointly to characterize water chemistry and to define hydrogeochemical processes. The Piper plots in combination with cluster analysis and isotope hydrological data indicate that, generally, the groundwater chemistry of the basin can be classified into (1) low TDS Ca-HCO3, Ca-Mg-HCO3, and Ca-Na-Mg-HCO3 type water which is relatively enriched in δ18O-δ2H and depleted in δ13C. These are recharge waters, which are characterized by low rock-water interaction; (2) brackish Mg-Na-Ca-HCO3 type water which is relatively depleted in δ18O-δ2H and enriched in δ13C; and (3) low TDS Na-(Ca)-HCO3 and Na-HCO3 type water which varies from less to more depleted in δ18O-δ2H and is characterized by relatively enriched δ13C. The cross-plots of the major cations vs. HCO3- and stability diagrams show that the primary processes controlling the groundwater evolution in the Lake Tana basin are alumino-silicate weathering and dissolution. Studying the hydrogeochemical characteristics of a complex geologic system with integrated approach helps to understand the complex groundwater flow system and flow dynamics, which in turn helps for proper groundwater utilization and future management.

The Water Budget Myth and Its Recharge Controversy: Linear vs. Nonlinear Models.

The water budget myth, which is the idea that safe pumping must not exceed the initial recharge, gave rise to a controversy about the role of recharge in assessing the sustainability of groundwater development. To refute the concept of safe yield, a simplified water budget equation is used, which equals the total pumping rate to the sum of capture and storage change. Since initial recharge and discharge are canceled out from this equation, it is concluded that sustainable pumping has nothing to do with recharge. Investigating the assumptions underlying this equation, it is seen that it expresses the superposition principle, which implicitly assumes the groundwater reservoir can be depleted indefinitely and boundary conditions are an infinite source of water. To evaluate sustainability, however, the limits of the aquifer system must be examined accurately. Theoretically, this can only be accomplished applying nonlinear models, in which case setting up the simplified water budget equation is impossible without knowing the initial conditions. Hence, excluding recharge when assessing sustainable pumping may not be done inconsiderately, which is illustrated by two examples. An analytical solution, developed by Ernst in 1971 to simulate flow to a well in a polder area with a nonlinear function for drainage, even shows that it is not necessarily a misconception to assume the cone of depression stops expanding when the pumping rate is balanced by the infiltration rate.

Hydrogeochemical processes and groundwater quality of over-exploited Dupi Tila aquifer in Dhaka city, Bangladesh.

Dhaka is one of the highly populated cities in the world. Increased urbanization and population growth in Dhaka are mostly dependent on groundwater, with 78% of the supply water coming from the Plio-Pleistocene Dupi Tila aquifer. This research was conducted with the objectives of identifying ion chemistry, hydrochemical processes and their relationship to groundwater quality and finding out the impact of over-exploitation on Dupi Tila aquifer. Three consecutive semi-confined aquifers have been delineated up to the explored depth: the Upper Dupi Tila aquifer (UDA), Middle Dupi Tila aquifer (MDA) and Lower Dupi Tila aquifer (LDA). Hydrogeochemical processes and water quality have been inferred from bivariate plots, correlation of major ions, piper plot and stable isotopes analysis. The total dissolved solids show an increasing downwards trend from an average of 267 mg/l in UDA to an average of 284 mg/l in LDA. Waters in all three aquifers as well as surface water (SW) are mainly of Ca/Mg-HCO3 type. The weathering of aluminosilicates controls the concentration of the major ions. Even though there is a significant decline in piezometric level both in UDA (> -85 m Public Works Department reference datum (PWD)) and MDA (> -65 m PWD), there is no evidence of groundwater recharge through direct infiltration from the river. The stable isotopes indicate most of the UDA, MDA and LDA waters are mainly from local precipitation. Increasingly enriched mean values for stable isotopes for the subsequent aquifers UDA, MDA and LDA, as well as increasing PCO2 for UDA < MDA < LDA, indicate increasingly warmer recharge conditions. The overall groundwater quality in the aquifers is good. At present, we have no indication that groundwater exploitation has altered groundwater quality of the Dupi Tila aquifer. Therefore, it is of vital importance to regularly monitor water quality for the purposes in order to timely detect any potential water quality alteration that could be aggravated by the vast decline in piezometric level, in view of the long-term sustainable development of the groundwater resources of this city.

Groundwater inflow in rivers as a controlling factor to surface water nitrate concentrations and impact of groundwater age distribution on response times for remediation strategies.

Diffuse groundwater contamination by intense fertilizer use is a widespread problem in most of the agricultural regions in West-Europe and in many other countries worldwide. As a result elevated nitrate concentrations in groundwater are found, and as groundwater is the source of baseflow into rivers and streams, also surface waters show increased nitrate levels. Flanders in the north of Belgium is no exception and despite restrictions on the fertilizer use and remediation practices initiated in the mid-1990s, and still ongoing, in many stream monitoring points, peak concentrations are measured above the 50 mg/l criterium. In many monitoring stations there is apparently a long delay between the start of the restrictions and effect on water quality. Often this delay is related to high concentrations in the groundwater inflow and long residence and travel times of groundwater feeding the streams. To better understand this behavior and estimate response times a tool was developed that incorporates the main water flows in a typical catchment and considers the effects of varying groundwater ages. This tool is conceived as a parsimonious lumped parameter model, simple and easy enough to be used by many people but still capturing the main mechanisms end processes. In this paper the tool is explained and it is applied on two testcases in Flanders to show its performance.

Understanding the mechanisms of groundwater recharge and flow in periglacial environments: New insights from the Ledo-Paniselian aquifer in Belgium.

The Ledo-Paniselian aquifer in Belgium has been proposed to offer unique opportunities to study groundwater recharge and flow in periglacial conditions during the Last Glacial Maximum (LGM), due to its location in the permanent permafrost area, south of the ice sheet at that time. A palaeoclimatic record had been set up previously for this aquifer, consisting of major ion chemistry, stable isotopes, radiocarbon and noble gases. In this paper, methane data have been used to further refine the paleoclimatic model, along with revisiting in detail the set of chemical data, focusing on the area where groundwaters, recharged around the LGM, are known to occur. It was found that the high methane concentrations corroborate the hypothesis of groundwater recharge taking place during permafrost melting, from methane-bubbling lakes that had developed to the south of an eolian sand ridge. A relict flow path, existing in the aquifer during some period as permafrost was thawing, has been established, starting from these temporary recharge areas, based on various chemical parameters, radiocarbon model ages and noble gas recharge temperatures.

Groundwater salinization and freshening processes in coastal aquifers from southwest Bangladesh.

The hydrogeochemical processes in the multilayer aquifers of southwest (SW) coastal area of Bangladesh provide important information for quantifying hydrochemical differences between different aquifers. Hydrogeochemical processes affecting groundwater chemistry in this coastal area have been evaluated by interpreting conventional plots, ionic delta, HFE-diagram, stable isotopes and geochemical modelling. The median TDS distribution of the aquifers has an increasing trend from below 1000 mg/l in the deep aquifer (DA) to 2622 mg/l in lower shallow aquifer (LSA) and 7012 mg/l in upper shallow aquifer (USA). Na+ is the dominant cation in all the aquifers. HCO3- is the dominant anion in DA with high median concentration (495 mg/l), which is more than double than that in LSA (214 mg/l) and USA (159 mg/l). The groundwater in the DA is freshened NaHCO3+ type due to cation exchange process that enhances second stage of calcite dissolution. Few NaCl+ type DA waters found in the SW corner of the study area indicate the presence of connate water confined in the inter-basin during the Holocene transgression. The stable isotopes indicate that the DA waters have been deposited during warmer periods. In contrast, both the LSA and USA are dominated by NaCl- type water; some CaCl- type waters have also been found in these aquifers, indicating salinization. The salinization processes consist of infiltration of redissolved evaporite salts, reverse cation exchange, and mixing with marine influenced flood water. Thus, the knowledge of different processes controlling freshening and salinization will help properly manage and preserve the environmental characteristics in the aquifer systems of SW Bangladesh as well as similar complex coastal geological settings.

Groundwater recharge and water table response to changing conditions for aquifers at different physiography: The case of a semi-humid river catchment, northwestern highlands of Ethiopia.

Groundwater recharge estimation, aquifer response to meteorological variables, and evapotranspiration calculations have been performed on a semi-humid catchment, in northwestern Ethiopian plateau. The Soil Moisture Balance (SMB), WetSpass water balance model, Water Table Fluctuation (WTF), and Chloride Mass Balance (CMB) methods are applied to estimate the groundwater recharge. Accordingly, 431 mm, 462 mm, and 477 mm recharge amounts are estimated as mean annual value, respectively, using SMB, WetSpass, and CMB methods. Based on the WTF method, the annual recharge rates of the volcanic aquifers range from 157 mm to 760 mm. The SMB and WetSpass methods are less effective for the flat physiographic area, where the recharge rate is storage controlled rather than precipitation amount. The calculated high recharge for maintain-front aquifers using WTF is attributed to extra rising due to lateral groundwater flow, which restricts the reliability of the method for such aquifer geometries. High groundwater level rising rate (121 mm/day) has been observed for the steeply sloping, low rates (11 mm/day) for the flat floodplain, and intermediate rate (52 mm/day) for the gently sloping volcanic aquifers. Similarly, receding rates of 3.18 mm/day were found for the steeply sloping, 0.40 mm/day for the floodplain, and 1.14 mm/day for the gentle sloping aquifers. The recession, in all of the topographies, is happening with second-order polynomial decay function. A strong connection between the shallow and deep groundwater aquifers is noted. Storage change in the relatively deeper volcanic aquifers is due to vertical groundwater flow from the overlying alluvial aquifer. This indicates that the recharge mechanism is local, and may be the reason for the low aquifer productivity of the Dangila wellfield. Diurnal water table fluctuation is detrended from the receding trend of the dry period, and evapotranspiration from the groundwater is estimated at 28% of total ET.