Hydrogeology of the Merti Aquifer

Impact of abstractions on drawdown of water level and salinity

Arjen Oord Jan de Leeuw (presenter)

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

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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

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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

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Geology of the Merti Aquifer

100

200

300

met

er

Archers’ Post(150 km)

Habaswein

Wajir (100 km)

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Ewaso Ng’iro

fresh

salt

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

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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

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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

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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)

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Increased salinity

• Two processes responsible for increased salinity risk

• Upconing of groundwater from below

• Lateral flow of water from peripheral areas

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fresh

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…

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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)

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14Source: Deltares.nl

Upconing

• Wells located over saltwater can draw the saltwater upward, creating a saltwater cone that might reach and contaminate the well

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• 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

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.

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MODFLOW model

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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)

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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:

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• Model was run several times with various depths of the fresh to saline water layer

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

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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

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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

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Examples of mitigation

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Freshwater Injection

Interception well

(River) water infiltration

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)

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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

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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

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•Thanks for your attention

•Questions?

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