Controlling Mine Drainage Problems in Minnesota - Are All the Wetland Treatment Systems Really Above Average?

Abstract

Five surface flow wetland treatment systems were built in the 1990's to remove copper, nickel, cobalt and zinc from mine drainage in northeastern Minnesota. All but one of the drainages were neutral, and metal concentrations ranged from around 0.02 to 10 mg/l. Nickel was the primary metal generally accounting for about 90 % of the load. The one acid seep was pretreated with a peat / limestone bed prior to discharge to a surface flow wetland. System design, maintenance and performance of the wetland systems varied. Some systems have required essentially no maintenance and have produced water that has always been in compliance, while others have required a variance to maintain compliance and have required a considerable amount of reconstruction. All the systems included a series of berms to control the hydraulic gradient and to provide access to the wetland. Some of these berms included elaborate under drains, which were generally ineffective since only a small amount of the total flow could be transmitted. The key factor in performance was the size of the wetland. An areal nickel removal rate of about 40 mg/m 2 day was determined in pilot cell tests prior to wetland construction. In general, the systems that met this requirement produced the most consistent compliance with water quality standards.