The hydrogeological risk asesment on the merti aquifer
•Download as PPTX, PDF•
2 likes•1,396 views
World Agroforestry Center Production
Recommended
Recommended
More Related Content
What's hot
What's hot (20)
Viewers also liked
Viewers also liked (20)
Similar to The hydrogeological risk asesment on the merti aquifer
Similar to The hydrogeological risk asesment on the merti aquifer (20)
More from World Agroforestry (ICRAF)
More from World Agroforestry (ICRAF) (20)
Recently uploaded
Recently uploaded (20)
The hydrogeological risk asesment on the merti aquifer
- 1. Hydrogeology of the Merti Aquifer Impact of abstractions on drawdown of water level and salinity Arjen Oord Jan de Leeuw (presenter)
- 2. Impacts of abstractions? • Abstractions have two major geo-hydrological risks • Boreholes running dry – What is expected drawdown of groundwater as a result of the proposed abstractions • Boreholes turning saline – What is the risk of water from boreholes turning saline? • Additional risks of reduced recharge (Climate change, dams and abstractions upstream) • Acacia Water did research to assess these two risks 2
- 3. Overview of the presentation • Current knowledge about the Merti Aquifer • The Wajir Habaswein project • Impacts on water level – drawdown • Impacts on salinity level of the groundwater • Impacts of oil mining • Impacts of dams upstream • Mitigation options • Conclusion 3
- 4. What do we know about the Merti Aquifer Area Water Paper (2011) in Dutch • 139,000 km2 Merti Beds and related units • 61,000 km2 fresh-brackish groundwater • 10,000 km2 fresh water Habaswein + downstream Vertical • Water depth: ~100 m • Known aquifer thickness: 20 – 80 m • Possible thickness up to 300-400 m Water volume: 200 - 300 billion m3 (fresh-brackish) 50 billion m3 fresh water Large uncertainty around estimates
- 5. 5
- 6. Geology of the Merti Aquifer 100 meter 200 300 Archers’ Post (150 km) Habaswein Wajir (100 km) * * * * * * * * * * * * * * * Ewaso Ng’iro fresh salt
- 7. The Wajir Habaswein Water Supply Project • Total proposed yield Habaswein well field: 6000 m3/day • Well field: multiple wells (12) at safe distance (> 700m apart) • Fresh water is currently abstracted in Habaswein, this much is certain • The proposed water abstraction is far greater than current abstraction rates around Habaswein 7
- 8. Drawdown and the drying Boreholes? • Pumping leads to lowering of groundwater levels (drawdown) • If groundwater levels drop below pump level / well screen failure • Drawdown depends on aquifer properties (thickness, conductivity, storativity) and recharge. • Most of these parameters are uncertain • Uncertainty Modelling will give better estimates and insight 8
- 9. Modeling Drawdown • MODFLOW (USGS) is a model that allows to estimate drawdown for a given set of parameters • Model was run multiple times (stochastic model) using a range of parameters • Variables included (95% confidence intervals): – Volume abstracted: 6000 m3/day (from project report) – Recharge: 0.6 to 40 Million m3/year – Aquifer thickness: 40 – 200 m – Conductivity: 2 – 30 m/day (fine to coarse sand) • Uncertainty of the variables that affect the range of calculated drawdowns • Model was run 5000 times, using randomly selected parameter values from the estimated parameter ranges 9
- 10. Results – Drying Boreholes • Maximum drawdown: 10 m in 2050 • Not a problem: modern wells have screens of 20 m or more • Sphere of influence 10 km (in 2050) 10
- 11. Increased salinity • Two processes responsible for increased salinity risk • Upconing of groundwater from below • Lateral flow of water from peripheral areas 11 fresh
- 12. Salinity • Large volumes of fresh groundwater known to exist in central Merti Aquifer (Habaswein) • Quality (salinity) underneath fresh water is unknown • Quality is decreasing in some boreholes, so saline water is expected to exist underneath fresh water • Depth to saline layer? – At least 40 m underneath current boreholes (or we would have seen more saline boreholes). Could be more than 200 m • So, uncertainty approach… 12
- 13. Risk of upconing • Upconing is upward movement of saline groundwater caused by the abstractions • Significant cause of salinity increase in similar situations elsewhere (e.g. coastal water supply the Netherlands) 13
- 15. Upconing • Wells located over saltwater can draw the saltwater upward, creating a saltwater cone that might reach and contaminate the well 15 • Severity is influenced by: – Depth to salt water (40 – 250 m below top aquifer) – Density of salt water – Degree of mixing (or sharp interface) – Aquifer properties (e.g. porosity) • National Limit Kenya salinity: 1500 mg/L
- 16. Modeling approach upconing • To predict the upward movement of the saline water, the 3D MODFLOW groundwater model was used, combined with a transport routine (MT3DMS/SEAWAT) • Transport modelling is slow (one complex model run can take hours or even a full day) • Therefore, a limited number of (simplified) model runs was done, using different combinations of aquifer parameters and , more importantly, depth to the saline water. • Sensitivity analysis of the parameter shows that depth to the saline layer is the most sensitive parameter. Unfortunately, this is very uncertain. 16
- 17. MODFLOW model 17
- 18. Salinity – Model parameters • Aquifer properties: – Conductivity ( 2 – 30 m/day) – Aquifer thickness (40 – 200 m) – Depth to saline layer (40 m – 200 m) • Assumptions: – Sharp interface between fresh and saline layer – Salinity is lower than seawater – If the salinity in the well is higher than the national limit, the borehole cannot be used for water supply (failure) 18
- 19. Results upconing studies • Result: there is a 50% risk of boreholes turning saline in 2050 when depth is 40 - 200 m below top aquifer • Risk is much reduced at depth of >120 m below top aquifer • Lateral movement of salt water: very small chance One run of the salinity risk assessment: 19 • Model was run several times with various depths of the fresh to saline water layer
- 20. Salinity in 2050 • This illustrates the differences in salinity in 2050, depending on at what depth the saline water is found currently: if more than 140 m below top aquifer, chances of drawing saline water decrease 20
- 21. Conclusion salinity risk assessment • Risk of salinity depends on depth of boundary between fresh and saline water : • It is very high when the boundary is at 40 m below aquifer top • It is very low when boundary is at 200 - 250m below top aquifer • Depth of boundary layer is unknown 21
- 22. Mitigation options salinity • Establish depth of fresh to saline water boundary before implementation of phase 2 of the project • Design – create separate boreholes for Habaswein 1 to 10 km away from the main borehole field • Provide artificial groundwater recharge • Intercept saline water below the boreholes • Mitigation is costly: should be incorporated in feasibility studies 22
- 23. Examples of mitigation 23 Freshwater Injection Interception well (River) water infiltration
- 24. Scenarios – Oil drilling • Scenario: Oil Drillings – Comes with significant groundwater abstraction – Distance to nearest exploration site: more than 20 km – Assumption: abstraction will be less than – Estimated additional abstraction – 400 m3/day nearest site • In 2050, the Habaswein well field is not (yet) influenced by oil exploitation: oil drilling outside sphere of influence (based on current exploitation sites) • If oil drilling takes place at a distance of less than 20 km, (chances of project failure will increase) 24
- 25. Scenarios - Upstream Dams • Upstream Storage dams – Plans for large scale storage dams in Ewaso Ng’iro – Dams decrease flooding, but increase baseflow – Recharge Merti is believed to (partly) depend on flooding of Ewaso Ng’iro – Scenario: dams decrease Ewaso Ng’iro recharge by 50% (rough estimate) • Decrease in recharge as a result of dams is a slow process. Groundwater levels will not (yet) be influenced by dams in 2050, due to current distance to the flood area (over 50 km) • These scenarios do not change the situation in Habaswein until 2050 25
- 26. Conclusions hydrological risks • Drawdown - Maximum drawdown 250 is 10 m; planned abstractions unlikely to lead to drying of boreholes. • Existing (shallower) boreholes in Habaswein might be effected, depending on their depth. • Salinity – this could be a serious (irreversible) problem. Two ways to manage this – Establish depth to the fresh to saline water boundary allowing more accurate predictions of upconing. – Design the boreholes such as to mitigate negative effects on water supply to Habaswein (costly) • Abstractions upstream – not considered a threat in 2050, might be significant in the longer run 26
- 27. 27
Editor's Notes
- Good morning. This is a presentation on the hydrology of the Merti aquifer, and particularly how the proposed abstractions will affect the water level and its salinity. Arjen Oord of Acacia Water has generated these results, I am presenting on their behalf
- One way to look at the aquifer is to summarize what is known from the literature. It is basically a two dimensional perspective bringing together what can be observed from the surface. Arjen Oord wrote a paper in Dutch summarizing current knowledge. Central part freshwater (Blue), lateral parts brackish to saline water. There are estimates of the volume of fresh to brackish water, but these highly uncertain.
- Another two dimensional perspective. The blue areas are known to contain fresh water. The Red dots show where the saline water is known to exist. The isolines show the gradient of the groundwater: it flows from Mt. Marsabit towards the Somalian border.
- Another way to look at the aquifer is to take a three dimensional perspective. Habaswein on top of the Merti Aquifer. Old rift valley that has been filled up with deep layers of sediment. The sediments in the central rift have freshwater, and the Habaswein area is at the core of this. So ideal place to abstract groundwater. Bottom of rift valley normally has salt water, and the freshwater is likely to sit on top of heavier saline water. One of the unknowns of the proposed project is the depth of the boundary between the salt and the freshwater. Abstraction of water has a risk to draw up the saline water with the boreholes turning saline. Lateral parts of the rift valley system have brackish water. Abstraction of groundwater indices risk of lateral salt water intrusion.
- It is good to realize that where we speak of aquifer thickness or depth below the top of the aquifer, that the aquifer starts at 100-120 m below surface level. So: 40 m aquifer thickness means that the bottom of the aquifer is at 140-160 m. 200 m thickness means 300 – 320 m below surface level. Same goes for the slides about salinity.
- The well field is proposed in the middle of the fresh aquifer. The distance to the saline ‘flanks’ of the aquifer is greater than the estimated distance to the saline layer under Habaswein. Travel time is such that lateral intrusion is not likely to pose a problem in 2050. Focus of our study on upconing of saline water.
- Several boreholes are known to have decreasing quality over time. This suggests that saline or brackish water is present underneath the fresh layer. The question is: where and at what depth, specifically near Habaswein.
- Example of what happened in the Netherlands coastal dunes. Upconing is well known in the home country of the modeller: the Netherlands. Most water supply in the coastal area is from the dunes. There is a large fresh water body present, which is a result of high active recharge. Water supply from groundwater started in 1870. By the 1950s, many monitoring boreholes were showing serious upconing and several boreholes started to yield brackish / saline water. This problem was stopped by artificial recharge, decreased groundwater abstraction and relocation of several pump stations further inland (expansive). The ‘cones’ or ‘spikes’ of saline layer are still present in the groundwater system.
- Showing the MODFLOW 3D finite difference grid, showing the salinity ‘spike’ right underneath the well.
- Most of the model parameters are the same as for the drawdown/dry boreholes scenarios. New is the depth to saline layer parameter.
- If saline water is located at 40 m below the top of the aquifer (140 m blow ground level), there will be a serious problem within possibly 10 years. If it is 200 m below top aquifer (300 m below ground level) chances are slim that there is a problem. The interpretation should be like this: 40 m, almost certain, 200 m highly unlikely.
- Injection (from harvested water in rainy season) can mitigate the intrusion/upconing of salt water An interception well can be used to draw the interface downward, creating an opposing force to the fresh water abstraction
- Most exploitation sites are quite far from habaswein. The outcome of the model runs will only be different if the oil exploitation will take place < 20 km from water supply well field
- It will take longer than 2050 for effects of decreased recharge in Lorian Swamps to reach the well fields. This is because currently (!) the flood areas of the Ewaso ngiro are quite far from Habaswein. This used to be different. It is likely that the groundwatersystem is still reacting to a decrease in recharge because of the retreating Ewaso Ngiro.