VARIABLE HYDROGEOLOGIC RESPONSES IN A GROUNDWATER DEPENDENT ECOSYSTEM OF THE HURON-MANISTEE NATIONAL FOREST, MICHIGAN: DATA IN SUPPORT OF NATURAL RESOURCE MANAGEMENT

Expanding commercial development of groundwater resources in Michigan has generated strong public and regulatory responses. The objectives of my work as a Guest Scientist for the US Forest Service on the Huron-Manistee NF are to describe the nature of groundwater-surface water interactions that are potentially vital to groundwater dependent ecosystems. It is not explicitly understood how site–specific responses to hydrologic change, both short or long term, and human induced or “natural,” will manifest themselves. Long-term groundwater data are prerequisite for determining the role of groundwater in ecosystem maintenance, and any evidence of change. To those ends, a monitoring network was installed within the upland-riparian transition along a headwater reach of the White River to ultimately help resource managers understand and protect groundwater dependent ecosystems.

The White River is a Michigan State Natural River and a candidate Wild and Scenic River. Healthy populations of trout and salmon depend on streamflow that is dominated by groundwater discharge. Local groundwater flow systems are established in permeable glacial sands; point discharges of groundwater are easily observed in the channel bottom of the White River and other cold-water streams in the Manistee NF.

Six groundwater wells installed in two transects normal to the upland-riparian transition, and a staff gage in the adjacent White River, permit the evaluation of hydraulic gradients across 1) the transition to the river, 2) along valley, and 3) vertically across riparian peat. Three of the wells were instrumented in November 2008 to record hourly ground water levels. Data indicate that precipitation generates variable groundwater recharge responses and transient gradients. A sandy upland site displays a “typical” unconfined response to rainfall. Following a 0.73 inch (0.06 feet) rain event, GW level increased 0.14 feet. In contrast, the artesian sand beneath the peat showed a rapid, pressure-induced response with subsequent decay, and a net GW level response of only 0.015 feet, an order of magnitude less. A deep-shallow nest across peat deposits in the riparian corridor indicates artesian conditions in sand underlying the stream-marginal peat deposits, corroborating field observations of discharge through the stream channel.