Regional and local hydrogeology of calcareous fens in the Minnesota river basin, USA

Six calcareous fens in the Minnesota River Basin, USA are in regional hydrogeologic settings with large discharges of calcareous ground water. These settings juxtapose topographically high areas of ground-water recharge with fens in lower areas of discharge, thus creating steep upward hydraulic gradients at the fens. Coarse glacial deposits with high permeability connect recharge areas to discharge areas and transmit large amounts of ground water to the fens. Calcareous fens in the Minnesota River Basin are associated with two regional landforms, river terraces and glacial moraines. The calcareous drift is the likely source of carbonate for the fens; carbonate bedrock is not required. Five of the calcareous fens form peat aprons over broad areas of diffuse ground-water discharge on river terraces. One of the calcareous fens is a peat dome over an aquifer window, a relatively small area (about 15-m radius) of localized ground-water discharge through a breach in the clayey confining layer of the underlying aquifer. Carbonate content of calcareous fen peat averaged about 27% (calcium carbonate equivalent, dry weight basis) in the surface layer, which commonly overlies a carbonate-depleted zone with a carbonate content of 10% or less. Hydraulic conductivity (K) of calcareous fen peat determined from slug tests ranged from 2.7×10⁻⁷ to 9.8×10⁻⁵ m s⁻¹ and had a geometric mean of 3.8×10⁻⁶ m s⁻¹. These values likely underestimate the true horizontal hydraulic conductivity (K_h) and overestimate the true vertical hydraulic conductivity (K_v) because of errors in assumptions commonly used in slug-test analyses. Median (over time) hydraulic heads in wells screened below the base of the peat ranged from about 25 to 69 cm above the peat surface. Upward vertical gradients (dimensionless) through the peat ranged from 0.040 to 0.209. Vertical ground-water discharge was calculated by Darey’s Law and ranged from 2 to 172 L m⁻² d⁻¹. Because of bias in estimating K_v, these values likely overestimate the true vertical ground-water discharge and indicate the importance of better field methods to estimate K, especially K_v. Calcareous fens may need water tables sustained near the peat surface by large vertical ground-water discharges to allow carbonate precipitation, which is associated with the rare fen vegetation.