Groundwater Flow Assessment Using Modflow 6 and Model Muse: Application to Pointe-Noire Coastal Aquifers, Congo-Brazzaville

As numerical modeling is an effective tool for managing groundwater resources and predicting future responses, in this study, the author has in-tended to assess groundwater flow through Modflow 6 and Model Muse into Pointe-Noire coastal aquifers. The results showed that the fourth scenario has the biggest effect on the drawdown and seawater intrusion extent. Different parameters including evapotranspiration, recharge, model boundary, etc. were adjusted to run the model. The fourth scenario with the highest pumping rate value caused a slight increase of head values over the values simulated.


Introduction
The coastal aquifers of Pointe-Noire is a highly strategic area for daily needs of households and industrial; and the population depends on it and due to the proliferation of boreholes is a threat to the groundwater in Pointe-Noire, excessive pumping increases the risk of intrusion and the degradation of water quality, with decreases in piezometric levels since the pumping rate in coastal aquifers is higher than the intake and storage contribution, seawater is introduced into the aquifer and interacts with existing wells [1] [2] [3] [4].
In this paper, we are interested to model the risk of seawater intrusion in   [10]. The total water for Pointe-Noire demand was estimated to be 50,000 m 3 /day of potable water.

Aquifer
Aquifers (AQ) in Pointe-Noire comes in two types which are confined and unconfined aquifers, the wildly aquifer used is AQ-2 for water requirement. It consists of a hydrogeological complex including an aquifer system of several layers with hydraulic continuity according to geological and its direction of flow. A hydrogeological information system for the AQ-2 aquifer has been developed from a historical reconstruction of piezometric boreholes and piezometers from different measurement companions carried out using geographic information systems [1] [8] [11]. These aquifers are geologically characterized by their lithology or rock type, which can be successively determined as follows [1] [6]: • The unconfined aquifer AQ-1 contains a free water  [7] and the discharge takes place via streams and sources, by flowing towards the ocean [12]; • the confined aquifer AQ-2 contains a deep confined artesian or flowing well, in certain places, corresponding to the most permeable layers of the series of circuses (quaternary). This AQ-2 aquifer has an average thickness of 20 to 30 m and a depth between 80 and 180 m, consists of heterogeneous sands, silt sometimes alternating with levels argillit; • the aquifer AQ-3 (confined), contains a confined aquifer corresponding to the most permeable layers of iron sands, sands sometimes composed of heterogeneous conglomerates alternating with ferruginous concretions; • the aquifer AQ-4 (confined) also contains a confined aquifer, based on a low permeability substrate and bounded by a low permeability super-stratum, corresponding to the less permeable strata of the gresco-dolomitic series (secondary), consists of clay sands and dolomitic aggregates; • the potential aquifer AQ-5, is a so-called potential aquifer corresponding to the unit of dolomite and calcite of the calcaro-dolomitic series in which circulation losses have been observed in certain mineral exploration wells.
The configuration of the aquifers of the coastal sedimentary basin is presented in Figure 3.
The previous geological studies made in this region [1] [7], made it possible to define in detail the structure of the different aquifers horizons, Figure 4 shows the aquifer surfaces in Pointe-Noire region, the first layer contains shallower aquifers AQ-1, and its upper surface represents the topography of the study area, the second in pink color is essentially the impermeable layer, it does not containt whater, and the third layer represents the main aquifers AQ-2 which are saturated with water, where the wells are located in green. Table 1 shows the lithostratigraphy and hydrostratigraphy of the Pointe-Noire Coastal aquifers

Hydrodynamic Characteristics
The hydrogeological characteristics of the aquifer system in the Pointe-Noire   Hydraulic parameters (transmissivity and storage coefficient) data are rare; they come from long-term pumping tests carried out as part of study [2]. The

Groundwater Piezometric
Thanks to the work carried out by the Ministry of Hydraulics, we consider the piezometry of 2012 [3] that we georeferenced it using software ArcGIS 10.5.
A monthly monitoring over a hydrological year was carried out at the level of the piezometric network and of some cartesian wells located in certain districts of Pointe-Noire [3], shows the groundwater flow towards the Southwest.

Modeling of Fluid Flow
In this section, we present a state of current knowledge of the data for the nu-

Aquifer Geometry
The aquifer is bounded on the west by the ocean, to the east by Hinda made up of Quaternary sedimentary deposits based incoherently on impermeable facies, northwest and southeast boundaries are tight due to the flow direction which is perpendicular to the axes of hydraulic Kouilou and Loémé.  [2]. Wells considered to study groundwater into the AQ-2 of Pointe-Noire are shown in Table 2 below; • River simulation of river interaction with the aquifer, the conductance per unit length or area is 3E−6 and the elevation is the model top;

Boundary Conditions
• Evapotranspiration in zones with water table close to surface, several calculation methods have been developed to estimate evapotranspiration and for

Results and Discussions
The data processing steps undertaken in this GIS and modeling study are described in some detail above, and a critical assessment is given of the data availability and the requirements for successful monitoring and now we are modeling groundwater to assess the risks in heavily exploited AQ-2 aquifers. Wells are located from layer 3, which is confined. We are looking for the impact of high     a continuous decline and a negative effect in the groundwater level in these profiles due to fixed pumping and the selected results are shown in Figure 12 and Figure 13. However, Figure 14 and Figure 15 show that the hydraulic head is a negative constant in stress period 2.
Scenario 3 focuses on the impact of sustainable abstraction rate on groundwater level without considering climate change. Simulation results as shown in Figure 16 and Figure 17 there is a continuous decline and a negative effect in the groundwater level. These are due the fact that there is no impact of climate change. However, Figure 18 and Figure 19 show that the hydraulic head are constant in stress period 2.