My whole tank (3+ acre tank, 11' deep) sits on top of a thick shelf of rock that we couldn't break through with a PC300 excavator. I've owned the place for 9 years (dam is 30+ years old?) and the seepage below the L-shaped dam gets worse every year. The dam itself is dry but the ground 2-10 feet behind it seeps up enough to keep my neighbor's small tank full most of the summer. The seepage extends for probably 50 yards around the curve of the dam. This summer it averaged dropping 5" per week between rains(although here in central Texas they all seem to go up and down like yo-yo's due to the sun/wind/100+ days.) After watching all these years I think the water is seeping between the dam and the rock shelf it sits on. Has anyone faced and cured a similar problem?