A homogeneous silty sand aquifer has the following properties: effective porosity = 25%, hydraulic conductivity = 100 gpd/ft2, saturated thickness = 200 feet, width = 2 miles, length = 15 miles. Six wells penetrate the aquifer, and three have water table elevations as shown.         a. What are the flow direction and gradient?         b. A tracer is introduced at well A. How long will it take to be detected at wells B and C?         c. Calculate the flow.         d. What assumptions did you make?         e. What other uses can be made of tracers in ground water?