Integrated management of coastal freshwater resources

Resumo

Freshwater is a scarce resource - only 2.5% of the total water volume on earth is freshwater, with the largest portion of it lying underground (Ranjan et al., 2006a). Groundwater resources, which are very important for human activities such as domestic consumption, agricultural uses and industrial processes, are being extensively used to supplement the available surface water in order to meet this ever-increasing water demand in the world. This is especially true in African countries such as Kenya where groundwater is a major source of domestic and agricultural water (MacDonald et al., 2012). These water resources are indisputably declining at an alarming rate all around the world mainly because demand for freshwater is rising with factors such as population growth, water pollution and economic as well as technological progress, together with land use change and climate variations (Schleich and Hillenbrand, 2009, Carter and Parker, 2009, Ding et al., 2014, Sun et al., 2013). This is expected to render availability of freshwater in the future uncertain (Davies and Simonovic, 2011, Gain and Wada, 2014, Khawaji et al., 2008). With growing water scarcity, mismanagement of the available water resources exacerbates the situation (Ding et al., 2014). Proper management of groundwater resources in the face of a changing climate and land use requires a reliable knowledge of their current status, availability, recharge rate of aquifers and demand by populations making use of the aquifers (Durham et al., 2003). The key to determining changes in groundwater systems lies in the interactions between physical and human-induced processes, which then need to be incorporated in the broader environmental change scenarios (Davies and Simonovic, 2011), particularly by policy makers who are concerned with the future availability of water supplies and the sustainability of water use (Gain and Wada, 2014). Though it is widely accepted that the effects of population growth and climate change will challenge future freshwater availability, water demand driven by an amalgamation of these factors has barely been explored in coastal aquifers (Sun et al., 2013). The main aim of this study is to assess the expected water stress on the freshwater resources on the Kenyan coast with particular emphasis on the Shela aquifer in Lamu by determining how land use change (specifically the construction of the new port in Lamu) and climate change will impact the recharge of the aquifers, the water storage capacity of the soils, as well as availability and quality of freshwater resources of the Shela aquifer in Lamu, Kenya. 1.1 Scope and Justification Lamu, located in the northern part of the Kenya coast is the site of Kenya¿s second harbour which is part of the Lamu-South Sudan-Ethiopia Transport Corridor (LAPPSET) project. The project will radically change the landscape of the entire Lamu area moving it from a once predominantly rural setting to a completely urban centre which is expected to trigger an unprecedented population growth rate in Lamu (RoK, 2011). As Lamu Island¿s principle source of freshwater is the Shela aquifer, the assessment of the impact on groundwater resources is of utmost importance for policy makers and water resource managers as this development is expected to increase the demand on this already overexploited freshwater source (Kuria, 2008). The study area lies in Kenya between longitudes 1º61' N and 4º68' S and between latitudes 39º00'E and 41º59' E, covering the coastal plains and the adjacent hinterlands that extend to a height of 100 m.a.s.l. The Lamu Island, which is the focal point of this study, has a total land surface of about 50 km2; about 19 km2 is covered by a double row of longitudinal sand dunes located along the entire length of the southern coastline (Kuria, 2008). The Shela aquifer¿s unconfined section lies underneath these sand dunes and has a well field that comprises 30 wells located on the eastern side of the aquifer. The dunes have a height ranging from 20 to 65 m and are almost entirely covered with fine¿medium grained Pleistocene carbonate sands as well as loamy sands and pink coral limestone sediments. The area is underlain by sand, silt, and silty clays deposits as well as by weathered to fresh corals which are in turn overlaid by coral breccia. 2. Materials and Methods The study was carried out in three phases: first, the impact of RCP 2.6 and 8.5 climate change scenarios (IPCC 2013) on infiltration rate and storage capacity of three main soil types along the Kenyan coast, extending inwards to areas below 100 m.a.s.l, as well as the sandy soils of the Lamu sand dunes for a soil column of 100cm was computed using a numerical model ¿ HYDRUS-1D. The size and shape of the Shela aquifer (Fig. 2) was then defined and characterised by using analytical solutions to quantify (1) the size and total volume of the freshwater lens (2) the elevation of the water table, (3) the depth of the freshwater/salt water interface of the Shela aquifer, and (4) the expected change in volume under the influence of climate change considering recharge and sea level rise variations for the Intergovernmental Panel on Climate Change (IPCC) A1b and A2 SRES for up to 2100 (IPCC 2007). Finally, the contributions of population growth (human development) in the context of these climate change scenarios to the future state of freshwater resources available in the Shela aquifer were identified and computed using models based on analytical solutions. 3. Results and discussion The results of this study demonstrated that climate change would have a positive impact on the recharge of aquifers and soil water storage overall. The soil properties were found to play a significant role on the infiltration rate with clay soils having the lowest rate of infiltration and highest surface run-off. Clay loam had the second lowest infiltration rate of 12.5% and a 1.1% surface run-off followed by sandy clay loams with ¿ 16% of the total precipitation infiltration rate. Sandy soils showed the highest infiltration rate reaching ¿ 26% of the precipitation and, together with sandy clay loam, had the lowest surface run-off of less than 1%. The impacts of climate change were also investigated and results show that the infiltration and run-off rates are expected to be higher in 2100 for both RCP 2.6 and 8.6 scenarios than they were for the reference period. The average deep percolation is expected to increase by 14% for RCP 2.6 scenario and 10% for the RCP 8.5, while the average run-off is expected to increase by 8% for RCP 2.6 scenario and 16% for the RCP 8.5. The average water content is expected to increase by 1% the RCP 2.6 scenario and decrease by 2% for the RCP 8.5 scenario, further lending evidence to the impact climate change will have on groundwater resources. With limited data about Lamu Island in general and the Shela aquifer in particular, this study demonstrated that analytical solutions can be used to calculate the size and shape of the freshwater lens, the location of the freshwater/saltwater interface as well as the hydraulic conductivity in an aquifer where very limited hydrogeological information is available. Quantification and characterization of the aquifer and the effect of climate change under SRES scenarios A1b and A2 at the end of this century on freshwater resources have been established. The water table elevation at various points of the aquifer (ranging 1.2¿4 m.a.s.l.) has been established with different methodologies such as Fetter¿s Infinite Island Strip and using DEMS and ArcGIS. From these elevations, the corresponding depth of the freshwater/saltwater interface at these points was also calculated. The total saturated volume of the freshwater lens before abstraction (128 x 106 m3) and the effective volume after discharge and saltwater intrusion (124 x 106 m3) were approximated, representing 3.4% loss in volume to human abstraction. Future climate change scenarios have also been modelled and show that under the A1b SRES, a 136 % increase of recharge is expected and an increase in the effective freshwater volume to 199 x 106 m3. The aquifer¿s vulnerability is expected to reduce by half (50 %) from the current state under these climatic conditions. This is a sharp contrast to the A2 scenario where an opposite impact is expected. The recharge in A2 is expected to decrease by 95 % and the effective volume is expected to reduce to 27 x 106 m3. The results further show that the ¿no industrial development¿ population is expected to reach ¿50,000 by 2065. The population is projected to reach 1.25 million by 2050 when the port reaches its full cargo-handling capacity ("LAPSSET development projected"). The groundwater abstraction as of 2009 was 0.06 m3 daily per capita, while the results of the exponential growth equation shows an annual growth rate of 0.003%, resulting in a daily per capita demand rise to 0.1 m3, as recorded in Mombasa and Nairobi. The RCP climate change scenarios will have a lesser impact on the effective volume than the SRES. The overall results suggest that population growth exacerbated by land use change will be a more significant driving force that will affect the availability of freshwater than climate change. The aquifer is not expected to experience any stress by 2065 for the ¿no industrial development¿ population, while for the "LAPSSET development" projected population, it will occur much sooner (between 2020 and 2028). New water management policies that take into consideration these expected changes should be formulated and implemented in order to conserve Shela aquifer¿s status as a viable freshwater reservoir. Barring this, the aquifer is in danger of being exploited to its full capacity. Import of water from a secondary location and desalination of seawater are feasible options to offset the increasing demand, which should be considered as viable alternative water supplies. The choice of method should be informed by the cost of the necessary infrastructure, the availability of funds, as well as the efficiency of the technology. 4. Conclusions and recommendations Climate change is expected to impact the recharge rate of aquifers and water storage capacity of soils in the Kenyan coastal area. It will also affect the volume of water in the Shela aquifer and increase its vulnerability to salt water intrusion. However, the anthropogenic impacts are expected to be greater than climate change. Population growth due to land use change after the completion of the LAPSSET project will increase demand for water in the area exponentially. Therefore, it is important to consider the impact of human activities and land use change that complement the role soil types as well as climate change play in recharging aquifers. Water management policies of the Kenyan coast generally and the Lamu area specifically should take into account the effect the new harbour will have on population growth and land use changes. Considering that industrialized agriculture contributes to climate change and biodiversity decline, this policy should ensure that coastal groundwater resources with their associated ecosystems are safeguarded. This can be done by ensuring that the freshwater/saltwater balance is maintained. A vulnerability indicator that combines the effects of climate change, sea level rise and the anthropogenic contribution would give a better risk assessment to saltwater intrusion, providing policymakers a clearer analysis of the situation. This study provides sound methods of data collection, analysis, and interpretation in areas with limited data that are needed by policy makers in regard to groundwater assessment, research and development, monitoring, and controlling of groundwater exploitation. However, for the results to be fortified, a test pumping program should be carried out on all wells to establish their safe yields. This will contribute in the alleviation of saltwater intrusion issue by managing the amount of water drawn from each well. Therefore, for a more wholesome approach in tackling water management issues, more robust, multi-disciplinary studies must be carried out to include anthropogenic influences such as agriculture and urbanization. A more encompassing water stress analysis using an indicator that combines the effects of climate change, sea level rise and the anthropogenic contribution (population growth and urbanization) would give a better risk assessment to freshwater degradation through saltwater intrusion, providing policymakers a clearer analysis of the situation needs to be carried out to further augment the findings of this study. Management policies that encourage infiltration especially in areas covered by clay soil such as damming of surface run-off and better agricultural practises should be implemented. The mitigation measures highlighted by this study should also be investigated further to assess their viability, both ecologically and economically. Furthermore, a monitoring program of the water quantity and quality in the well field should be established as soon as possible to serve as an early warning against the deterioration of the freshwater lens. References Carter, R. C. & Parker, A. 2009. 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