Aquifer Storage & Recovery (ASR) system improvement in northern Ghana

Abstract

The northern Ghana climatological conditions are favourable for agricultural production. The annual average precipitation of 800-1250 mm/y is theoretically sufficient for farmers to be year-round self-sufficient. However, the majority of precipitation falls in a 4-month wet season spanning from late May to October. As a consequence, the region is subjected to both seasonal flooding and long periods of drought. The dry season agriculture is of moderate intensity, takes place at small-scale and is groundwater dependent. In the near future it is not unthinkable that extraction exceeds natural recharge and groundwater withdrawal is no longer sustainable in the northern Ghana regions. The small-holder farmers' use of Aquifer Storage and Recovery (ASR) systems can potentially contribute to the continued sustainable use of groundwater in northern Ghana. An ASR system acts as a seasonal bridge. The system recharges flood water and extracts groundwater in periods of drought. A feasibility study on the sustainable use of an ASR system in northern Ghana has been performed by taking present conditions and multiple system improvements into account.

Aquifer tests are carried out at five study sites in northern Ghana to determine local geohydrological conditions. The TTim analytic element modelling environment is used to analyze the obtained groundwater drawdown data and derive parameters for subsurface characteristics. TTim allows for the inclusion of additional model parameters (e.g. borehole storage, well skin resistance and multiple model layers) and outperforms the analytic Theis method in this research. Although some uncertainties are present in the derived subsurface parameters, plausible values for transmissivity (T) and storativity (S) are suggested to be present in the ranges of respectively 1 to 100 (m2/d) and 1e-3 to 1e-2 (-).

The year-round performance of a northern Ghana single ASR system is studied with a MODFLOW model. The potential types of ASR system improvements that are examined are (a) the extension of daily pumping time, (b) the enlargement of the borehole diameter, and (c) the reduction of the well skin resistance. The ASR systems sensitivities to changing environmental conditions are explored by (a) the degradation of well depth by clogging, (b) the shortening of the wet season inundation time, and (c) the reduction of the wet season inundation levels. Research results show that well maintenance is key for the performance of existing (and new) ASR systems. The recharge and discharge volumes can be improved by cleaning of the borehole depth and well screen. In the case of a new ASR system, the performance can positively be influenced by an enlargement of the borehole diameter. Furthermore, the construction of a proper permeable well skin (screen and gravel-pack around the well) can also result in increased system capacities. Despite the imposed options of system modifications, the geographic position of an ASR system remains of utmost importance for system performance. The construction of an ASR system at a location sensitive to flooding (riverbank overtopping or rainfall based) can be beneficial from a sustainable perspective. Recharge volumes are normative for the sustainable use of an ASR system. The recharges are (approximately linear) dependent on the time-span and levels of inundation. Moreover, the research contains soil scenarios, and demonstrates that the ASR system performs significantly better in regions with higher transmissivity (T) values.

To give insight on some financial aspects of an operational ASR system, the obtained (improved) ASR system discharge capacities are transformed to agricultural and financial yields. A subdivision of the dry season into a tomato and a groundnut cropping season demonstrates that financial yields are crop type dependent. The ASR system revenues are dominantly affected by the choice in crop type(s) and crop-specific market prices. The yields are compared to the ASR system pumping costs. The importance of pump selection is demonstrated by the implementation of the Pedrollo 4" submersible pump efficiencies. The use of a pump that is tuned to local conditions can be beneficial for the operational costs of an ASR system. Although no distinctive conclusion on the financial feasibility can be drawn, the examined system improvements are substantially beneficial for the revenues of a northern Ghana ASR system.