RECOVERY OF A MINE PIT LAKE FROM AQUACULTURAL PHOSPHORUS ENRICHMENT -MODEL PREDICTIONS AND MECHANISMS

A mine pit lake in northern Minnesota, USA, has received substantial n utrient loading from almost five years of intensive salmonid net-pen a quaculture. The lake, Twin City South (TCS), is typical of the approxi mately 200 mine pit lakes in the region that have formed from runoff, precipitation and groundwater intrusion into exhausted open-pit iron-o n mines. Most of the lakes are deep (similar to 20 to >200 m), with sm all surface area to volume ratios and have small watersheds with no su rface outflow. Basin walls are steep and comprised of unconsolidated g lacial sediments that are sparsely vegetated and highly erodible. The lakes are typically oligotrophic from phosphorus limitation but have a relatively high dissolved inorganic nitrogen content (similar to 100- 1000 mu g N/l). Regulatory concerns that included drinking water degra dation and long term eutrophication resulted in a stipulation agreemen t between the aquaculture company and the state. A part of this agreem ent required the company to terminate aquaculture and demonstrate the ability of TCS to recover from the effects of aquaculture to four targ et restoration goals within 2.5 years. With the availability of an ext ensive data base from several years of field work during aquaculture, this provided an opportunity to utilize two commercially available wat er quality models for predicting changes in surface and whole-lake pho sphorus during a restoration in which the entire phosphorus waste load was essentially eliminated. BATHTUB, an empirical eutrophication mode l. was used to predict the mean growing season surface total phosphoru s, a target restoration parameter. PHOSMOD, a mechanistic phosphorus b udget model, was used to predict total phosphorus in the water column. Both models encountered application problems in TCS. The most importa nt factors affecting the application of these models involved the wast e load characteristics, DO depletion, sediment P release and a high ba sin sedimentation rate. Future mine pit lake modeling efforts should c onsider these factors before applying any water quality model for simi lar purposes. (C) 1997 Elsevier Science B.V.