Water Resource Engineering

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Browsing Water Resource Engineering by Subject "Aquifers"

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    Integrated Three-Layer Groundwater Flow Modeling for Resource Assessment and Management for Part of the Upper Awash Basin, Central Ethiopia
    (Addis Ababa University, 2026-03-01) Ephrem Tadesse Tagegne; Tilahun Azagegn; Taye Alemayehu
    Effective management of future groundwater resources relies on mathematical models. This study presents a three-layer groundwater flow model for a segment of the Upper Awash Basin, aimed at identifying aquifers suitable for long-term use without significant depletion. Additionally, it offers a framework applicable to similar wellfields or areas. The model incorporates a variety of datasets, including measurements of 222Rn and Electrical Conductivity, resistivity soundings, hydrochemical and isotopic data, as well as streamflow records from 11 gauging stations collected over periods spanning 8 to 20 years. Four primary aquifer units have been identified through geological and hydrogeological assessments: the superficial deposit aquifer, the fractured Rift basalt aquifer, the acidic rock units (aquiclude), and the confined scoriaceous basalt aquifer. The superficial deposits and Rift basalt aquifers (layer 1) receive recharge from precipitation and nearby streams, characterized by mixed CaHCO3 or CaMgHCO3 water types with low TDS (< 200 mg/l), a high Ca2+/Na+ ratio (> 5), a low residual alkalinity (RA) (< 2.0), enriched isotopic signatures, and a high deuterium excess of 13.3. Conversely, the confined scoriaceous basalt aquifer (layer 3) is replenished by groundwater flow from the adjacent Muger Watershed, part of the Blue Nile Basin, as well as from surrounding highlands or escarpments. This is demonstrated by a high lineament density ranging from 0.96 to 3.0 km/km², high TDS (> 468 mg/l), a NaHCO3 water type, a low Ca2+/Na+ratio(< 1.0), a high residual alkalinity (RA > 2.0), significantly depleted isotopic signals, and a low deuterium excess of less than 11. The headwaters of both basins, parts of the Upper Awash and Blue Nile Basins, are closely linked to aquifers, as evidenced by high 222Rn and BFI values. The southern and southeastern areas act as the primary groundwater outlet zones, exhibiting very high 222Rn levels (ranging from 400 to 1467 Bq/m³) and BFI values (between 0.35 and 0.5). In contrast, the central plain area shows minimal connection with values (222Rn < 100 Bq/m³) and correspondingly low BFI values. Recharge rates calculated using the WTF and Darcy methods are 47.4 mm/y for the unconfined aquifer and 59.2 mm/y for the confined aquifer. The model area spans 3,440 km² and features a three-layer aquifer system composed of 113,825 active grid cells of 500 m x 500 m each. Utilizing Visual MODFLOW Flex, the model uses hydrogeological conditions of the year 2025 and projects stress-based scenario simulations extending to 2055 to assess system dynamics for management purposes. Calibration, achieved through a trial-and-error approach, exhibited a high degree of accuracy (RMS error: 9.16, mean residual: 0.81 m, R²: 0.99). A water balance analysis confirmed a state of equilibrium between inflows and outflows. Sensitivity analysis indicated that the unconfined aquifer is particularly sensitive to changes in hydraulic conductivity and recharge, while the confined aquifer responds to fluctuations in hydraulic conductivity and flux. Scenario based simulations after a steady-state calibration were conducted to assess aquifer responses to various scenarios, including drought, pumping, and injection. During drought conditions, recharge and flux were reduced by 50%, resulting in groundwater level declines of 25.8 meters in the unconfined aquifer and 4.3 meters in the confined aquifer. While river leakage inflow increased by 10.5%, outflows decreased by 43%. In the pumping scenarios, which involved the use of 100 and 150 boreholes at a rate of 50 l/s, the unconfined aquifer experienced drawdowns of 50 and 85 meters, respectively. Meanwhile, the confined aquifer experienced more modest declines of 2 and 4.2 meters. River discharge was reduced by 23.3%, while inflows saw an increase of 16.13%. In the fourth scenario, simulations involving the injection of water through 50 hypothetical deep wells, with varied daily injection volumes, resulted in an increase in groundwater levels by 1.05 to 4.45 meters. The water balance results indicated that higher injection rates enhanced groundwater outflow to rivers while concurrently reducing river leakage into the aquifer. The model simulations indicate that the regional confined aquifer (layer 3) can sustain long-term water withdrawals while ensuring groundwater stability and preserving its hydraulic properties. This study's model provides a framework for evaluating other wellfields, promoting sustainable resource management, and assisting policymakers in making informed decisions to safeguard the aquifer from long-term depletion.