Background

A sewer construction program has been proposed as a method of controlling eutrophication in a small lake which is surrounded by suburban community.  The lake receives practically all of its water from a phreatic groundwater system which surrounds the lake.  Evaporation from the lake surface is equal to precipitation.  Suburban development is distributed throughout the groundwater basin.  Domestic waste water is currently discharged into individual septic tanks which drain to the groundwater system.  Water supply is also obtained from the ground and consumptive use is small. 

The discharge of waste water into the aquifer has produced an increase in nitrate concentration in the aquifer and the lake.  Biologists have concluded that nitrogen is the limiting nutrient in this lake, and suggest that the concentration of nitrate-nitrogen in the lake will have to be reduced to less than 1 ppm in order to control the excessive eutrophication. 

Problem 1:

Using an analytical approach, estimate the time required after completion of the sewer system for the eutrophication control to become effective.  Assume that nitrate is conservative in the groundwater system and the lake, and that the lake is fully mixed.  Natural nitrate levels in this area are less than 0.1 ppm NO₃-N. Other useful information:

                        Lake                                                                Aquifer

                      Surface area 2 mi²                                                      Area 10 mi²

                                                 Average depth 25 ft.                                           Average saturated thickness 75 ft.

                                       Current NO₃-N concentration 3 ppm                      AverageNO₃-N concentration 4 ppm

                                                                                                                        Average effective porosity 0.33

                                                                                                             Net recharge 12 in/yr

 Note:  

Problem 2 

In Problem 1, you assumed that the aquifer and lake were completely mixed. Now develop a numerical model that explicitly represents the spatial location of the sources of sewage by placing sources of NO₃-N at the home locations depicted in the figure above. You do not have to predict solute concentrations within the aquifer system; rather, use groundwater flow tracing techniques to visualize the movement of NO₃-N in the aquifer system. Make reasonable assumptions about the difference in elevation between the average lake water level and the average land surface elevation.

Present the results in a short memo that includes a visualization of the groundwater flow paths (for different times) from the sources to the lake.

MAGNET/Modeling Hints:

• Use ‘Synthetic mode’ in MAGNET to create a model domain.
• Overlay the provided SiteMap image file included in the problem description. Choose ‘Use Domain Extent’ to fit the image to the established domain size.