MODELING SOLUTE DYNAMICS IN AQUIFERS: CASE STUDY OF IRON DISPERSION IN CHYULU AQUIFER, KENYA

Abstract

Chyulu in Makueni County falls in the Arid and Semi-Arid Lands (ASAL) parts of southeastern Kenya. The main source of water is boreholes whose water is mostly saline, harmful to human health and environment and sometimes fail to produce enough water to justify the high cost of drilling. Availability of both the groundwater potential and water quality index (WQI) maps would be paramount. Water quality parameters such as pH, turbidity, iron, fluoride, electrical conductivity, total dissolved solids, were used in assessment of the impact of stratum geochemistry on the groundwater quality. The results showed water whose parameters far exceeded the upper limits as per World Health Organization (WHO) standards. The negative impact of iron to storage and plumbing facilities was notable with tanks leaking by the third year of use due to corrosion and therefore reduction of iron using appropriate technology in the water would be a notable improvement of water quality. These parameters included iron, fluoride and nitrates among others and to understand their behavior in the aquifer/stratum, groundwater models are used. These models require modification to suite particular parameter because different ions react differently in the stratum. This study sought to develop tools for solute characterization that address strata like the Chyulu aquifer one that have excess iron among other salts. The study mapped groundwater in terms of quantity using drainage, rainfall, aspect, slope, topography, impact of the vadoze zone, and conductivity of the aquifer as inputs in (DRASTIC) modeling which utilizes both GIS and remote sensing techniques. The groundwater quality index (WQI) and quality map was also developed using the corresponding parameters’ concentrations. The study also tested a non-motorized aeration/filtration process using a fabricated unit that was used for aeration of feed water and its filtration. A user-defined Reactive Three–Dimensional Multi-Species Transport (RT3D) sub-routine that was linked to the Groundwater Modeling System (GMS) software that depicted the oxygenation of ferrous iron (II) at neutral pH was developed and tested using the non-motorized aeration filtration unit that now acted as batch reactor to predict chemical reactions in the aquifer. To validate the user-defined RT3D subroutine ferrous and ferric ions were measured every six months from June 2016 to December 2019. These test results were compared with those that were obtained from GMS/RT3D model after the subroutine had been linked to GMS software. This study concludes that the main consideration in deciding the site of a borehole in the study area be water quality because only 10% of the whole study area had portable water while 91 % of the area has either high or moderate groundwater potential. The non-motorized aeration/filtration system reduced the iron in groundwater by 58.3% and therefore this oxygenation process improves water quality. The other conclusion is that the User-defined RT3D subroutine predicted the iron flow through the stratum very accurately because the coefficient of correlation for ferrous ions was 0.8 while that for ferric ions was 0.82 which meant that the proposed subroutine as linked to the GMS model can be used to predict the solute (ferrous and ferric ions) in the Chyulu stratum/aquifer at Makindu. This study recommends that this user-defined model be tested and calibrated for use in other aquifers and that the model be tailor made/adopted to investigate the flow of calcium and its derivatives in aquifers.