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Our probe points consist of a four foot slotted rod driven approximately 5 feet below the water table. This places the top of the screen approximately 1 foot below the water table, and the center of the screen approximately 3 feet below the water table. Our plume diving calculations indicate that the top of the plume is 7.75 to 8.73 feet below the water table; 4.75 to 5.73 feet below the center of our sampling point. Had we used a typical 10 foot screened monitoring well we would not have faired much better since the concentration from the well would be the average of the entire 10 foot vertical section which would include 7.75 to 8.73 feet of relatively clean water.


Suggestion icon Don't be fooled by a low annual precipitation value, as it can be greatly enhanced from surface water (ponds, creeks), irrigation, recharge basins, septic systems and leaking water or drainage lines just to name the obvious. Remember, low recharge alone may not reduce the degree of plume diving. When low recharge is combined with a low hydraulic conductivity the potentil for plume diving can be just as great as an area characterized by our case site (high recharge/hydraulic conductivity). Try reducing the hydraulic conductivity by an order of magnitude (21.6) and reducing the recharge to 10 inches per year. Make sure the vertical migration route is adequately investigated before determining the boundaries of a dissolved plume.


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