Assessment of seawater intrusion potential from sea ...

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Managed aquifer recharge (MAR) has been identified as a potential major water management practice to support the groundwater storage in arid and semiarid ...
The Role of Managed Aquifer Recharge in Management of a Coastal Aquifer for Urban Purposes: Case Study of Samail Lower Catchment Aquifer, Oman 1

1

1,2

Ali Al-Maktoumi , Mustafa El-Rawy 1Sultan

and Slim Zekri

Qaboos University, P.O.BOX 34, Alkhoud 123 Muscat, Oman

2 Faculty

of Engineering, Minia University, , Minia 61111, Egypt

Introduction

Study Area

Results

In arid countries like The Sultanate of Oman where surface water resources are mostly absent and groundwater resources are often non renewable, the issue of water shortage is more likely (Al-Rashed and Sherif 2000; Mays 2013).

The study area located around 50 km west of Muscat. It covers an area of about 59 km2 (Fig. 2). The catchment is narrow in the upstream and spreads to form a delta fan closer to the coast. The mean annual rainfall is about 90 mm. The aquifer is an important source for potable water supply along with irrigation practices. Al Khawd recharge dam is located within the study area and about 7 km from Oman sea (red curve in Fig. 2). The aquifer is formed in predominantly alluvial sediments of different age and composition (mostly sand and gravel). The study area is mainly characterized by quaternary surface deposits. Tertiary sedimentary cover exists at the narrow upstream part of the study area. The ophiolites are exposed on both sides of the wadi Samail in the upstream boundary of the study area.

Six groundwater management scenarios were developed to determine the sustainable yield and the limits of utilization for the aquifer system. The developed groundwater flow model was used to predict the changes in the aquifer system under a set of different abstraction and injecting conditions. The transient simulation of the base model (Fig. 4) and all injection scenarios were done for 12 years. Details of Scenarios are presented in Table 1and Fog. 5 a-c. Fig. 5d presents the head distribution for the base case with Scenario 6. Fig. 6 presents the water balance of the base case and the six different scenarios.

Managed aquifer recharge (MAR) has been identified as a potential major water management practice to support the groundwater storage in arid and semiarid areas. MAR is practiced widely to store water during periods of surpluses and withdraw during deficits from an aquifer. MAR also known as artificial recharge, is the infiltration or injection of water into an aquifer. MAR implies aquifer recharge is accelerated by altering natural soil surface conditions to increase infiltration or water is injected directly via recharge wells.

Scenario

1 2 3 4 5 6

The importance of MAR is identified as: o Increase the groundwater storage volume and to recover the over exploited groundwater. o Improve the quality of groundwater. o Limit the salt-water intrusion in local area and to prevent the subsidence that may be caused by over pumping (Rapp 2008). o Reduce fresh water evaporation. o Reduce soil erosion and storm runoff (Gale 2000).

Objectives To assess the impact of managed aquifer recharge using treated wastewater on better management of the unconfined coastal aquifer of Samail Lower Catchment (SLC), Oman. Specifically, aiming to maximize withdrawals from the domestic wells with minimize salt water intrusion.

Domestic water pumping

51,488 51,488 51,488 45,604 45,604 34,180

Agric. Water pumping

Average change in groundwater level (m) 3.1 3.2 3.2 2.6 2.4 1.3

Injection rate

(m3/d ) 3,536 0 3,536 3,536 3,536 3,536

4,596 0 4,596 4,596 4,596 4,596

a

Fig. 5a

Fig. 5b

Fig. 5c

Fig. 5d

b

Projected coordinate System: WGS 1984 UMT Zone 40N Projection: Transverse Mercator Centeral Meridian: 57.00 Latitude of Orgin = 0.0 Source: Landsat

Fig. 2: Location of the study area SLC. It is bordered by wadi Taww to the west, wadi Rusayl to the east, Gulf of Oman to the North, and wadi Samail catchment to the south. Figure 4: Simulated groundwater head at time step of 144 (after 12 years simulation) under the base conditions : (a) top layer and (b) bottom layer.

Sensitivity analysis and calibration of the model are performed using time averaged measured heads (from January 2007 till October 2012) in 32 observation wells. The model found to be sensitive to the hydraulic conductivity, GHB conductance, and recharge parameters. The Mean Absolute Error between the observed and simulated groundwater head values is 0.63 m and the Root Mean Absolute Error is 0.70 m, which shows that the model performs well (Fig. 3) 50

Inflow from the sea Base case 10000 0 -10000

Outflow to the sea

Sc.1 5,930

413 Flow (m3/d)

 the aquifer modeled as four layers in the vertical direction. The Model area gridded by 692 rows and 687 columns of 30 x 30 m cell size. This makes a total of 475,404 active cells in each layer.  The coastline is modeled as a constant head boundary whereas the upstream boundary assigned a general head boundary. Noflow boundaries were assigned to both east and west sides of the model area.  Different recharge zones were assigned as presented by Fig. 1).  Outflows from both public (17000 m3/day 47 wells and irrigation wells (3636 m3/day through 68 wells) were assigned. The aquifer thickness gradually increases from the upstream (100 m) to the sea (350 m).

Sensitivity analysis and Calibration

Simulated head (m)

Model Setup

Injection water

Sc.2

4,596

5,884 0

-3,536

Sc.3

6,636

0 -647

Agricultural Water pumping

Domestic Water pumping

Sc.4

4,596

3,627

4,596

4,174

Sc.5

Sc.6

4,596

4,596

1,582

0

-555

-317

-3536

-3,536

-863

-3536

-2,494

-3,536

-12,837 -20000

-17,808

-30000

R² = 0.9969

40

-34,180

-40000

30

-45,604

-50000 -51,488

-51,488

-45,604

-51,488

-60000

20

Scenarios

10

Figure 6: Water balance of the base case and six different scenarios: inflow from the sea (red), outflow to the sea (black), injection water (grey), agricultural water pumping (green), and urban water pumping (yellow).

0 0

10

20 30 Observed head (m)

40

50

Conclusions

Fig. 3: Simulated versus observed heads for the steady state model calibration

• MAR acts as a hydraulic barrier to decelerate the seawater intrusion which allows higher abstraction of water from the aquifer.

Acknowledgement: Fig. 1: Recharge zones

-1,750

-3,536

This work was funded by SQU grant EG/DVC/WRC/14/02, and USAID-FABRI grant AID-OAA-TO-11-00049 (Project code: 1001626-104. Authors would like to acknowledge the support Ministry of Regional Municipalities and Water Resources, Oman.

• MAR along with redistribution of public wells allows abstraction of 2 times the base case amount • MAR using treated wastewater would increase the capacity to respond to emergency events.