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

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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 à 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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14  Source: 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 22  

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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