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 nutrient loading from almost five years of intensive salmonid net-pen aquaculture. The lake, Twin City South (TCS), is typical of the approximately 200 mine pit lakes in the region that have formed from runoff, precipitation and groundwater intrusion into exhausted open-pit iron-ore mines. Most of the lakes are deep (~ 20 to > 200 m), with small surface area to volume ratios and have small watersheds with no surface outflow. Basin walls are steep and comprised of unconsolidated glacial 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 (~ 100-100 μg N/l). Regulatory concerns that included drinking water degradation and long term eutrophication resulted in a stipulation agreement between the aquaculture company and the state. A part of this agreement required the company to terminate aquaculture and demonstrate the ability of TCS to recover from the effects of aquaculture to four target restoration goals within 2.5 years. With the availability of an extensive data base from several years of held work during aquaculture, this provided an opportunity to utilize two commercially available water quality models for predicting changes in surface and whole-lake phosphorus during a restoration in which the entire phosphorus waste load was essentially eliminated. BATHTUB, an empirical eutrophication model, was used to predict the mean growing season surface total phosphorus, a target restoration parameter. PHOSMOD, a mechanistic phosphorus budget model, was used to predict total phosphorus in the water column. Both models encountered application problems in TCS. The most important factors affecting the application of these models involved the waste load characteristics, DO depletion, sediment P release and a high basin sedimentation rate. Future mine pit lake modeling efforts should consider these factors before applying any water quality model for similar purposes.