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 MertiAquifer

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

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200

300

me

ter

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 Habasweinuntil 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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