Hydrogeological investigation in the Omaruru Delta (OMDEL) Aquifer north of Swakopmund, Namib Desert, Namibia

Abstract

Summary provided by author:

The study is also based on the following data: (1) Geology and hydrogeology, described in detail by NAWROWSKI (1990, 1994); (2) Geophysical data (sections) determined by SENGPIEL et al. (1995) in1992/93; (3) Isotope hydrological parameters (3H, d13C, 14C) determined by VOGEL between 1969 and 1977; (4) Isotope hydrological parameters (d2H, 3H, d13C, 14C, d18O) determined by the Joint Geoscientific Institute of the German State Survey (GGA) in 1993/94 as part of the German-Namibian Groundwater Exploration Project (GNGEP); (5) Water chemistry data determined by the DWA between 1993 and 1996

The aim is to provide a summarising geological/hydrogeological overview of the investigation areas and to study the OMDEL aquifers by hydrochemical, isotope hydrological, aerogeophysical, and hydrogeological methods : (1) Summary of all up to now and new ascertained data about the geological and hydrogeological conditions of the OMDEL and OMAP areas (2) Construction of four sections based on geological profiles and aerogeophysical measurements (sections) (3) Evaluation of up to now and new hydrochemical and isotope hydrological data of the OMDEL aquifers for the investigation of the hydrochemical and isotope hydrological status (i. e. trends, variations, groundwater flow, groundwater chemistry and chemical reactions, hydraulical contacts between the different channels) (4) Isotope hydrological, hydrochemical, and hydrogeological investigation and proof of groundwater recharge as a result of the infiltrated water of the OMDEL Dam System. (5) To give recommendations for the future water supply management

Results: (1) The constructed sections demonstrated and specified the geological and hydrogeological settings of the OMDEL and OMAP area. Altogether, it was shown, that the aerogeophysical method is a helpful supplement with little restriction for section constructions or groundwater explorations

(2) The evaluation of the isotope hydrological parameters 3H, d13C and 14C between 1969 and 1993 predominantly refers to the results procured by GEYH (1994), GEYH & PLOETHNER (1995), PLOETHNER (1995), and PLOETHNER (1998) : The 3H values decreased from 1969 - 1977 to 1993 as a result of the radioactive decay, lower tritium concentrations in the rain precipitations in the last two decades, or lesser recharge of the OMDEL aquifer between 1985 and 1993. The wide scatter of the d13C values (i. e. - 16. 8 (1973 - 1977) and + 1. 7 (1993); Tab. 5) implies that these values are generated by different conditions (i. e. geology of the Omaruru River catchment, chemical reactions in the aquifer). Only the 14C water ages have been discussed in reference to the reservoir effect. It was shown, that the reservoir effect can not be indicated as a single value (i. e. - 800 a; PLOETHNER, 1998), but must be indicated between -1100 a and - 4200 a. This range of the reservoir effect value is caused by the differential geology of the catchment area. This result makes a detailed interpretation and discrimination of the 14C water ages impossible

(3) The evaluation of the isotope hydrological parameters d2H and d18O could not prove the infiltrated water of the OMDEL Dam System, but it has been demonstrated that the Main Channel (MC) of the OMDEL reaches two types of isotopical (lighter and heavier) rain precipitation

(4) The hydrochemical investigations proved the partial hydraulical contact between the MC and the other channels at three wells (WW 21501, WW 22188, WW 22195). Infiltrated water of the OMDEL Dam System was proved at selected wells by comparisons between EC/chloride values and infiltration rates, but calculations about the vertical and horizontal hydraulical conductivity were not possible, because of to few sampling campaigns restricted to five

(5) The hydrochemical developments between 1993 - 1998 demonstrate no significant changes. Therefore, the current hydrochemical status can be marked as stable

(6) Risks for the drinking water supply were not determined, except for WW 22191, WW 22193, WW 22194, which yield water of poor quality

(7) Fresh (recharged) groundwater replaces the more mineralised groundwaters to the lateral parts of the MC

(8) The infiltrated water of the OMDEL Dam System has been detected by a relationship between water levels of well WW 16501, and the infiltration rates since 1994. The calculated horizontal hydraulical conductivity (13. 8 m/d) is higher than NAWROWSKI (1994) and PLOETHNER (1998) had calculated. Moreover, the assumed vertical hydraulical conductivity is also very high (10 m/d)

Recommendations: (1) The aerogeophysical method should be used in future groundwater exploration or in investigations to improve the geological/hydrogeological knowledge in areas with already known geological settings (i. e. geological profiles). However, the costs and the benefits of this method must be considered carefully

(2) A d18O profiling of the OMDEL Dam water body should be carried out in order to find out whether or not a vertical isotope layering takes place

(3) WW 22191, WW 22193, and WW 22194 should be monitored carefully by frequent water chemistry analysis or should be retired, because of their poor water quality

(4) For a more detailed detection of infiltrated water from the OMDEL DamSystem it is recommend that: A. More water level monitoring should be accomplished at observation wells; B. Water samples should be taken more frequently (perhaps once a month), for hydrochemical analysis from production wells, and even from some observation wells

(5) WW 16501 should be sampled for hydrochemical analysis frequently, because changes in hydrochemical groundwater composition by artificial infiltration will affect this well first