Riverbank filtration (RBF) is a low‐cost water treatment/pretreatment technology that is used in many countries around the world for water supply. When wells are situated close to rivers or lakes and pumped, the surface water is induced to flow to these wells. During soil and aquifer passage, chemical, biological, and particulate contaminants are removed.

The possibility of developing a well field taking advantage of bank filtration for large scale water supply is being studied for the unconfined aquifer shown in the figures below. The aquifer is developed in the alluvial sand deposits of perennial river. The first phase of investigation consists of drilling monitoring wells in and around the aquifer, collecting data on aquifer geometry, water table elevations, and elevations of surface water level along the river. This information is shown in table 1. The average hydraulic conductivity, K, of the aquifer is 2.4x10^-3 m/s as determined from slug tests.

TASKS:

Using all available data, develop a preliminary groundwater model for assessing the aquifer’s potential and planning the second phase of investigation. In particular, you are asked to address the following questions, assuming the total demand for water is 4000 gpm.

1. Is this feasible?
2. What is the maximum sustainable pumping rate? 
3. What are the optimal pumping locations?

NOTE: Your proposed pumping network should avoid inducing migration of contaminants into the wetland areas or to the water supply wells themselves. 

Figure 1: Proposed site for large scale water supply. 

Table 1: Field data collected on site.

Figure 2: Hydrogeologic cross-sections of the alluvial aquifer (vertical scale is exaggerated).

MAGNET/Modeling Hints:

• Use ‘Synthetic mode’ in MAGNET to create a model domain with the following dimensions: 8km (north-to-south) by 5.5 km (west-to-east)
• Overlay the provided SiteMap image file included in the problem description. Choose ‘Use Domain Extent’ to fit the image to the established domain size.
• Use the provided data to delineate the aquifer top elevation surface (the land surface) and bottom elevation surface (aquifer base). You can use scatter point data to interpolate to a spatially continuous surface using by creating a zone across the aquifer layer and assigning elevations using the scatter point import tool. (See the real-time Help button in the interface menus for proper implementation and formatting.)
• Use a zone feature and the Red River altitude data to conceptualize the river as a head-dependent boundary feature. Assume some reasonable value for leakance of the riverbed (hydraulic conductivity per unit thickness of the bed sediments).
• The recharge to the alluvial aquifer  and sandstone unit is unknown. But you can use the observed water table information as calibration targets to determine recharge (recall the K is given) for 'natural' (pre-pumping) groundwater conditions. 
• Use a relatively large grid size (e.g., NX = 150).