By Hydrosimulatics INC  

You are interested in recharge, residence time and flow velocities in the regional confined aquifer shown below. You have the following information: (1) the hydraulic head distribution, as shown on the map; (2) the aquifer thickness in the area between Black Hills and wells A and B is 100 ft; (3) the aquifer is in a geological unit composed of consolidated, cemented sandstone; and (4) the groundwater ages are 4250 years at well A and 15900 years at well B.

Problem A. Using the available information ( and perhaps constructing a flow net), and making reasonable (but only necessary) assumptions regarding aquifer properties, perform appropriate calculations and discuss the following:

(a) Assuming the mountain front recharge (over shaded area) provides all water to the aquifer, determine how much recharge there is. Express your results as per unit area (in units L/T) and for the entire recharge area (units L3/T).

(b) What is a reasonable range of specific discharges (consider all locations within the map)?What is a reasonable range of flow velocities?

(c) Propose at least two explanations for the less-densely spaced equipotentials in the middle of the system (between 2000 and 1800 ft contours).

(d) Assuming that the flow system in the past was the same as it is today, estimate the age of groundwater in well C. Take into account part (c).

(e) Which area is the best for long-term, large-scale pumping? Why?

Problem B. Create a MAGNET model to help aid in the explanation of the head patterns

MAGNET/Modeling Hints 

  • Use ‘Synthetic mode’ in MAGNET to create a model domain. Resize the domain to be approximately the same size as the study area shown in the figure.
  • No-flow boundary conditions are applied to the lateral and bottom model boundaries. Consider using a prescribed head line feature near the right and/or left edges to generate the proper flow patterns. 
  • Add prescribed flux zone feature to represent the recharge along the front range of the Black Hills
  • Use the Nodal Value tool to inspect model results (flow velocities) at different locations in the model ( 'Analysis Tools' > 'Analysis' > 'Nodal Values' )
  • Use reverse particle tracking to determine travel times / groundwater age