Water is spontaneously seeping from my 80yr old mother's leg. what could be the cause & is it dangerous? she had heart valve surgery 3 times.

Venous hypertension: A history of multiple valve surgeries and longstanding valvular hert disease implies a risk of pulmonary hypertension. The high right sided pressures cause backup into the venous system with excessive venous hypertension in the legs (dependent legs makes worse). Chronic venous hypertension can lead to the weeping you describe. Although other things cause edema, repeat valve surgery makes likely.

Dramatic edema: Congestive heart failure or venous stasis are potentials. Infection can become a issue with skin leakage and breakdown. Get attention.

Heart failure: This is a common sign of heart failure and is quite worrisome. She should see her doctor or go to the er.