Relative impacts of shallow vs. deep groundwater inputs on eutrophication of Roosevelt Lake: a model for central Minnesota Lakes

Abstract

Roosevelt Lake located in Cass and Crow Wing Counties, Minnesota, has suffered from hypolimnion hypoxiaduring the late summer. The hypoxia has led to "summer kill" of cold-water fish species such as lake trout.The cause of depleted oxygen levels in the hypolimnion is excessive loading of particulate organic mattercaused by eutrophication. Generally, lake eutrophication is linked to excessive nutrient loading from activitiesin the lake watershed such as shoreline modification by roads and residential activities, shoreline disturbance byboat traffic, and direct input of nutrients from septic systems. However, there are natural processes that canlead to increased nutrient cycling in lakes. Because of its depth, Roosevelt Lake receives groundwater inputsfrom deep confined aquifers. The water in these deep aquifers is anoxic and highly reducing and therefore highin dissolved iron and sulfate that is derived from Cretaceous and Early Tertiary bedrock. Both dissolved iron and sulfate are critical in the cycling of phosphorus that is the primary nutrient controlling eutrophication. Inaddition, preliminary assessment of groundwater inputs to the lake suggests that the flux of anoxic groundwatermay be very large; a factor that contributes to hypoxia. To investigate the role that groundwater inputs may have on hypolimnetic hypoxia, we undertook a study of groundwater and lake water chemistry. Beginning in November 2008 and continuing through Nov of 2009, rates of groundwater seepage were measured at numerous locations and chemical composition of lake water and groundwater were determined. Repeated measurements of vertical profiles of the water column were conducted at four locations; two in North Roosevelt Lake and two in the south basin of Roosevelt Lake. Analyses included temperature, pH, conductivity, and dissolved oxygen (DO). Groundwater samples were collected from residential and commercial wells. Samples were submitted for analysis of major cations and anions. Results of this investigation suggest that Roosevelt Lake stratifies very early and that the hypolimnion of the north basin is fully anoxic by late June. Hypolimnetic oxygen levels in the larger south basin of Roosevelt Lake decline slowly throughout the summer and do not drop to zero until October. Both basins of the lake turned over in late October and appeared to be fully mixed by November. Groundwater seepage to the lake is large, with seepage to the north basin being 2-3 times that of the south basin. Contrary to our initial hypothesis, groundwater seeps to the lake everywhere around the perimeter with thepossible exception of a small reach of shore near the stream outlet. Groundwater seeping to the lake is anoxic and high in dissolved cations, including iron. We believe that the large seepage volumes of anoxic groundwater to the north basin of Roosevelt Lake are the cause of the early (June) hypoxia in this basin. The larger south suffers the same eventual fate but more slowly because of the lower seepage rates. Although it appears that natural processes ultimately control the oxygen deficiency in the hypolimnion, eutrophication of the lake likely plays an important role. Excessive algal blooms result in high biochemical oxygen demand and hasten the onset of hypoxia.