Conference Agenda

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Session Overview
9.4-2 Hydrogeology of arid environments
Thursday, 23/Sept/2021:
4:15pm - 5:45pm

Session Chair: Stephan Schulz, TU Darmstadt
Session Chair: Nils Michelsen, Technische Universität Darmstadt

Session Abstract

Most (semi-)arid regions are characterized by limited surface water and hence rely on other sources. While unconventional resources such as desalinated seawater can be a valuable option for domestic supply in coastal areas, groundwater withdrawal is still the only relevant option to produce volumes that can satisfy the demand of water-intensive sectors, above all irrigated agriculture.In many areas, however, groundwater resources are over-exploited, i.e., more water is abstracted from the aquifers than is naturally replenished. Considering global changes such as population and economic growth, and the predicted expansion of dry areas due to climate changes (e.g. in the Circum-Mediterranean region), the pressure on groundwater resources will be even amplified.This situation calls for a better water management, which is in turn dependent on an adequate hydrogeological understanding comprising both, available quantities and water quality. We hence envisage a session that covers a broad spectrum of hydrogeological aspects of (semi-)arid environments. Sub-topics could include, but are not limited to, groundwater recharge estimations, flow modelling, hydrochemical studies, and isotope applications. Interdisciplinary contributions, linking hydrogeology to neighboring disciplines, are encouraged. 

4:15pm - 4:30pm

Stormwater harvesting in ephemeral streams: how to effectively bypass clogging layers and thick vadose zones

Jose David Henao Casas1,2, Fritz Kalwa3, Marc Walther3,4, Randolf Rausch5

1Department of Integrated Water Resources Management, Tragsa, Calle Maldonado 58, 28006 Madrid, Spain; 2Upper Technical School of Agricultural Engineers, Universidad Politécnica de Madrid (UPM), Av. Puerta de Hierro 2–4, 28040 Madrid, Spain; 3Department of Hydro Sciences, Technische Universität Dresden, Chemie-Neubau, Bergstr. 66, 01069 Dresden, Germany; 4Helmholtz-Centre for Environmental Research – UFZ Leipzig, Permoserstrasse 15, 04318 Leipzig, Germany; 5Department of Geosciences, Technische Universität Darmstadt, Schnittspahnstraße, 64287 Darmstadt, Germany

In drylands, stormwater is often collected in surface basins and subsequently stored in shallow aquifers via infiltration to cope with water scarcity. These groundwater recharge schemes are often accompanied by high evaporation rates and hygiene problems due to low infiltration rates, which are a consequence of clogging layers on the topsoil and the presence of a thick vadose zone. The present study aims to develop a conceptual solution to increase groundwater recharge rates in stormwater harvesting systems. The efficiency of vadose-zone wells and infiltration trenches is tested using numerical models and sensitivity analyses. The constructed models are conceptualised in the dams built in the channel of ephemeral streams (wadis) and validated utilising analytical equations. The modelling demonstrated that the employment of vadose-zone wells and infiltration trenches contribute to starting the recharge 2250–8100% faster than via infiltration from the wadi dam bed surface. Furthermore, recharge rates are predominantly affected by well length and trench depth as per the sensitivity analyses. In terms of recharge quantity, the well is the most efficient solution contributing to infiltrating up to 1642% more water than an equivalent area of the wadi dam bed surface and between 336 and 825% more than a trench. Moreover, the well can provide the highest cumulative recharge per unit cost and high recharge rates when there are space limitations. The use of analytical equations proved the adequacy of the developed numerical models. The techniques explored can significantly improve groundwater recharge, providing practical solutions to enhance water availability in drylands.

4:30pm - 4:45pm

The Significance of Groundwater-in-Storage in Arid Environments

Heiko Dirks1, Randolf Rausch2

1Ingenieurgesellschaft Prof. Kobus und Partner, Germany; 2Institut f. Angewandte Geowissenschaften, TU Darmstadt, Germany

Sustainable groundwater use is commonly linked to groundwater recharge: If the long-term average of withdrawals is not higher than the inflows, the groundwater use is considered sustainable. In arid environments, natural inflows are small, but the amount of groundwater-in-storage might be large. Potential groundwater withdrawals cannot be referred to the small inflows only. Hence, the determination of groundwater-in-storage volumes and their exploitable parts, respectively, is of high importance.

In this study regional aquifer systems, like they exist on the Arabian Peninsula or in North Africa, are considered. Options and limits in the determination of groundwater-in-storage are investigated.

For the assessment of groundwater-in-storage the determination of aquifer geometry and storativity (under confined and unconfined conditions) are essential. Exploration, data interpretation, and knowledge of aquifer genesis allow for accurate determination of aquifer geometry. In contrast, determination of storage coefficients is difficult. Firstly, the logarithmic relation between storativity and drawdown makes determination from pumping tests indifferent. In regional aquifer systems, exploitable groundwater volumes relate to confined conditions, where the uncertainty of the storage coefficient may range over an order of magnitude. Secondly, there is also a lack in data interpretation: known heterogeneities in lithology, or assumable changes with increasing aquifer depth are rarely translated into corresponding distributions of storage coefficients.

Herewith, we want to emphasize the importance of the assessment of groundwater-in-storage and the related storage coefficients. Increasing occurrence of dry seasons lead to use of groundwater-in-storage in humid environments, too. Consideration of groundwater-in-storage is hence important for both arid and humid environments.

4:45pm - 5:00pm

Inverse geochemical modelling demonstrates how weathering and ion surface exchange control groundwater chemistry in the Pra Basin (Ghana)

Evans Manu1,2, Michael Kühn1,2, Thomas Kempka1,2, Marco De Lucia1

1GFZ German Research Centre for Geosciences, Germany; 2University of Potsdam, Institute of Geosciences, Germany

Effective implementation of sustainable water resources management is one of the daunting tasks in most parts of the world. The Pra Basin has a high economic importance, hosting most of Ghana’s mineral resources, including gold, bauxite, iron, manganese, and diamonds. Currently, the basin is faced with several water resources management issues, especially pollution arising from the discharge of untreated waste into water bodies and illegal artisanal mining. Considering this background, the present study aims to determine the geochemical processes controlling the Pra Basin’s groundwater chemistry and provide the baseline information for groundwater management. A total of 65 groundwater samples sourced from boreholes (depths >30 m) were analysed for their physico-chemical parameters. Hierarchical cluster analysis and inverse geochemical modelling were applied to the hydrochemical data to investigate the sources of variation in groundwater hydrochemistry in the area. Three major geochemical processes were determined as drivers for groundwater chemical evolution: dissolution of carbonates, weathering of silicates and ion exchange. Inverse modelling underlines the dissolution of primary biotite, dolomite, halite, plagioclase, and precipitation of secondary calcite and gypsum as the apparent dominating reactions, reflecting the general groundwater chemistry in the basin. Groundwater evolves, namely from CaHCO3 to NaHCO3, and finally into NaCl water along its flow path. The presented results improve our understanding of the hydrochemical controls of the groundwater resources and support the design and implementation of sustainable water resources management strategies for the Pra Basin.

5:00pm - 5:15pm

Groundwater exploration and production in arid Jordan

Falk Lindenmaier, Gerhard Kapinos, Mark Gropius, Klaus Holzner, Florian Brückner, Daniel van Rooijen, Rebecca Bahls, Mathias Toll

Bundesanstalt für Geowissenschaften und Rohstoffe, Germany

The water providers of Jordan are constantly seeking to find new water resources to supply the public. Water table depths of up to 250 m, dry aquifers and saline waters make it increasingly difficult to successfully strike exploitable groundwater. We provided geoelectrical measurements for the exploration of a new well field area and set up a preliminary a-priori model for the interpretation of the geoelectrical results. Borehole geophysical logs, data on salinity content and subsurface interpretations greatly helped to get a detailed mapping of subsurface structures in the pilot area. To improve drilling success, geoelectrical investigation should be regarded as a mandatory step in the exploration process for new boreholes in Jordan.
The sustainability of groundwater use started to be challenged in the 80es when the first deep water wells were drilled. In addition, pump capacities increased and groundwater-based irrigated agriculture expanded since then. However, there are still big challenges that need to be addressed at national level. This includes improving operation and maintenance of well fields, basic well monitoring in order to increase energy efficiency to bring rising costs of water production under control. Through support of the Jordanian Ministry of Water and Irrigation, the BGR aims at improving the sustainable management of groundwater resources.