I'm trying to solve a water loss problem at an 8 acre "duck marsh" that is seasonally flooded. I took over the task of managing the shallow water impoundment and would appreciate any and all ideas that might be helpful in fixing the water leakage problem.
The impoundment is a dry agriculture field during most of the year and is flooded in October to create a flooded marsh for waterfowl hunting. It is located near Wichita, Kansas, and is within 5 miles of the Arkansas River. I don't have any information regarding the original dike construction. A small pond was built next to the marsh and doesn't hold water either.
The soil is sandy, but becomes very "gooey", sticky mud when wet.
I turned the well pump on October 20 and let it run 24/7. The water level increased for the first week, but began receding the second week. The water level is now very low and it appears to still be losing water despite the pump running non-stop. At best, only 1/3 of the marsh had surface water.
I've outlined the marsh in blue, the dry creek in orange, and the field drainage waterway in purple.

Shot at 2007-10-31
There is no obvious leak on the backside of the dike.
What should I do to identify the cause of water loss?