Effects of Aquaculture on Mine Pit Lakes near Chisolm, MN: Restoration of Twin City-South pit lake by fallowing and status of Fraser pit lake

Abstract

Net-pen salmonid aquaculture was carried out from 1988 to 1993 in the Twin City-South mine pit lake on the Mesabi Iron Range in northeastern Minnesota. A water quality controversy enveloped the aquaculture operation from its inception in 1988. In 1992 the Minnesota Pollution Control Agency mandated that all intensive aquaculture operations in the Twin City - South mine pit lake be terminated by July 1993 and that restoration to baseline (i.e. preaquaculture) conditions be demonstrated within three years. This "fallowing" has led to a rapid recovery to near baseline water quality conditions and an oligomesotrophic, i.e. unproductive, status. Water column improvement in regard to phosphorus and hypolimnetic oxygen concentrations has been particularly rapid. Although baseline conditions were not well defined for TC-S, the P budget for the lake in September and November 1994 was typical of reference pit lakes in the area. Oxygen concentrations in near-bottom water remained above 5 mg02/L in November 1994 even without artificial mixing or aeration during the 1994 growing season. Algal growth was low in 1993, as expected due to artificial mixing, and remained low in 1994 without any artificial mixing. Ammonium has been naturally converted to nitrate which is decreasing faster than expected and at a rate similar to its increase during intensive aquaculture. More rapid reductions in water column phosphorus and nitrogen might have been accomplished during the first summer by allowing the lower hypolimnion to become anoxic in order to promote denitrification and minimize sediment resuspension. The natural burial of sedimented aquaculture wastes due to high ambient rates of erosion of inorganic sediment from the basin walls has effectively minimized sediment nutrient transport to the overlying water column. Fallowing for several years appears to be an effective method for lake restoration of these pit lakes. Our data, and our analysis of the NPDES monitoring data, has shown no change in the water quality of Chisholm's drinking water source, the Fraser pit lake, attributable to aquaculture impacts. This, and no apparent change in the water quality of two nearby pit lakes, Grant and Ironworld in recent years, suggests little or no significant off-site migration of aquaculturally impacted water.