Groundwater Flow Modeling (Groundwater Modeling System)

Groundwater Flow Modeling (Groundwater Modeling System)
Puri (1984) used a one-dimensional finite difference model of Rushton and Redshaw (1978) for the resource evaluation studies of lower Greensand aquifer.

Groundwater Flow Modeling (Groundwater Modeling System)

Groundwater Flow Modeling (Groundwater Modeling System)

Simulation studies were carried out to refine annual recharge estimate. The groundwater flow model suggested that the regional abstraction affected water levels in the outcrop area of the aquifer.

The model showed that the pumping at distances as far as 30 kilometers from the outcrop area caused small but measurable drawdown. Eyre (1985) simulated groundwater flow in southeastern Oahu, Hawaii using a two dimensional finite element flow model duly modified for aquifers with a sea water interface. The model accurately simulated observed heads averaged over several years. Li et al. (1987) modeled the groundwater basin of San Juan valley, central California using a finite difference model. The calibrated model generated water levels that were in good agreement with the historic levels.

The study illustrated the impact of several choices on solution to the problem of optional distribution of artificial recharge in a real basin. Stoertz and Bradbury (1989) calculated groundwater recharge rates using a modified version of USGS modular groundwater flow model (McDonald and Harbaugh, 1988) for a basin in central Wisconsin. The model was calibrated against stream flow data and the recharge map prepared through model very well matched with the actual recharge map of the basin.

Rivera et al. (1990) applied a numerical model to study the coastal aquifer of EI Viejon, which comprised granular materials and scattered lenses of clay resting on impermeable andesitic bedrock with a sharp fresh/saline water interface. Simulations were made under both steady state and transient conditions for predicting the position and movement of the fresh/saline water interface amid different schemes of pumping.
Palma and Bentley (2007) used Visual MODFLOW to simulate groundwater flow in Leon Chinandega aquifer. It was demonstrated that pumping induced a decrease in base flow especially during dry periods. Transient modeling indicated that the response time of the aquifer was about one hydrogeologic year. Pisinaras et al. (2007) studied groundwater flow in a stream aquifer system using MODFLOW. The observed water levels were used for model calibration. Match of the calculated heads with the observed heads was excellent. The model results indicated that there must be approximately 33% decrease in withdrawals to stop a dramatic decline of groundwater levels. The results showed that the aquifer discharge to the nearby river would be very low after a period of 20 years.

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Mylopoulos et al. (2007) applied MODFLOW for the simulation of groundwater flow in a complex system of aquifers in northern Greece. Due to overexploitation of water the study area underwent negative water balance and severe water shortage. Gedeon et al. (2007) used MODFLOW 2000 to improve the understanding of the regional groundwater system including Boom Clay for disposal of high level radioactive waste in the northeast Belgium.

El Yaouti et al. (2008) integrated MODFLOW with Groundwater Modeling System (GMS) to model the unconfined aquifer of Bou-Areg in northeastern Morocco. Hydraulic conductivity distribution was estimated through modeling. The model was calibrated for the period 1990-2002 successfully. Predictions by the model were made period from 2007 to 2025 under different scenarios. Martinez-Santos et al. (2008) modeled the Mancha Occidental aquifer, Spain using MODFLOW of USGS. The model was calibrated in steady state and transient conditions. Predictions by the model showed that the system was more vulnerable to resource exhaustion along its boundaries. Under the worst scenario model predicted that some of the places might face significant water shortage by the year 2030. De Hammer et al. (2008) used MODFLOW for the evaluation of a small alluvial aquifer in Mnyabezi catchment area of Zimbabwe. They concluded that the aquifer could not store large volume of water.

Benner et al. (2008) performed numerical modeling of an arsenic contaminated aquifer in Mekong Delta, Cambodia using MODFLOW and MODPATH under both steady state and transient conditions. Bonomi (2009) applied MODFLOW for 3 D textural modeling of the aquifer system of Milan province in Po plain, northern Italy using computer based hydrogeologic database.

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The principal instrument used during the survey was ABEM terrameter. The instrument has an automatic signal averaging system (SAS 300). Other materials required were four steel electrodes, two plastic reels of current cables, and two potential cables, crocodile heads and two reels of already marked twines and pegs.
The figure below shows how this instrument is connected

Figure 1: Electro-spacing sampling
Field instrumentations and
P1    =    Potential electrode 1
P2    =    Potential electrode 2
C1    =    Current electrode 1
C2    =    Current electrode 2
O    =    Zero work point
In the arrangement, the current electrode and potential electrode are to the ABEM tetrameter which possesses and inbuilt current to the earth and at surefire increases the potential difference.
Other instrumentals includes
(1)    Hammer
(2)     Increasing tape
(3)    Connecting wire
(4)    Two plastic reds of current cables
(5)    Wooden clips for good electrical contacts with electrode
(6)     Battery
(7)    Four electrodes
(8)     Compass
(9)     Two resistivity potential cables
(10)    Computational sheets and field note books
(11)    Umbrella or tent canopy
(12)    Tool box (soldering iron, solder, screwdrivers, pliers, cutters plugs, connectors and crocodile clips).

The current and potential electrodes were planted according to the schlumberger array, and the electrodes were connected directly to the terrameter. While the current electrodes   were spaced out, the potential electrodes remained fairly constant. The values of the earth’s resistance at each spread of electrodes were read off through the digital display by the terrameter. A geometric factor K was calculated for each spread. The resistance, R and geometric factor K were used to calculate the bulk apparent resistivity of the measured sequence for every spread.

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Acquisition of Data and Precautions
In this field work, the vertical electrical sounding (VES) was employed and as such entails the lithologic contents using the schlumbeger electrode configuration. The schlumbeger was chosen over other methods because it has a continuous record with corresponding electrode separations at the end of the spacing.
A point is reached when the current electrode separation is so large that the potential difference between them becomes very small for accurate measurement.

1.    The configuration must be done in a straight line
2.    Battery should be charged before the fieldwork.
3.    A tight connection between the wire and the electrode must be made
4.    The electrode should be primed very well in the ground to ensure good contacts
5.    As sounding progresses terameter recharging should be taken regularly.
6.    Do not touch the electrode while taking the reading
Location and Geology of Study Area
Ohaozara is situated in the South Eastern part of Nigeria within latitude 60 17′ – 60 20′ N and longitude 80 5′ – 80 10′ E. It is located on the sedimentary coastal basin of Nigeria. The oldest sedimentary rocks in the basin are Cretaceous sandstone, shales and limestone which are generally believed to be of Albian Age. The sedimentary basin is characterized by crystal block faults trending in a NW-SE direction. Its origin is ultimately associated with the development of  the Benue trough both event being related to the opening of the South Atlantic which was active in early Cretaceous time under the Niger- Delta Miocene (Kogbe, 1989).

Sedimentation began with the initial deposition of fluvio-deltaic cross-bedded sand of the Awi formation. In early cretaceous time, the first marine invasion in middle Albian resulted in limestone deposition particularly on the horst and relatively stable plate from the area and their flanks.

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