Hypolimnetic Withdrawal as Lake Restoration Technique

Hypolimnetic withdrawal is a lake restoration technique based on the damming of surface outflow and the discharge of hypolimnetic water. In 17 lakes, hypolimnetic withdrawal has been used for 1–10 yrs. In general, anoxia and total phosphorus (TP) concentrations of epiand hypolimnetic water decreased after the commencement of withdrawal. The decrease of epilimnetic TP concentration correlates to TP export via hypolimnetic withdrawal and becomes stronger with years of operation. While it is advisable to maximize TP export to improve lake conditions, two constraints have to be observed: (1) the lake level should remain constant; and (2) the thermostructure must not be disturbed. Maximum TP export can be obtained by positioning the withdrawal pipe deep in the hypolimnion and operating the system when hypolimnetic TP concentrations are high. Under these circumstances treatment of the withdrawn water might become necessary to avoid fertilization of downstream waters.