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 wate r. These settings juxtapose topographically high areas of ground-water recharge with fens in lower areas of discharge, thus creating steep u pward hydraulic gradients at the fens. Coarse glacial deposits with hi gh permeability connect recharge areas to discharge areas and transmit large amounts of ground water to the fens. Calcareous fens in the Min nesota River Basin are associated with two regional landforms, river t erraces and glacial moraines. The calcareous drift is the likely sourc e of carbonate for the fens; carbonate bedrock is not required. Five o f the calcareous fens form peat aprons over broad areas of diffuse gro und-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 c layey 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 carb onate-depleted zone with a carbonate content of 10% or less. Hydraulic conductivity (K) of calcareous fen peat determined from slug tests ra nged from 2.7 x 10(-7) to 9.8 x 10(-5) m s(-1) and had a geometric mea n of 3.8 x 10(-6) m s(-1). These values likely underestimate the true horizontal hydraulic conductivity (K-h) and overestimate the true vert ical hydraulic conductivity (K-v) because of errors in assumptions com monly used in slug-test analyses. Median (over time) hydraulic heads i n 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 dis charge was calculated by Darcy's Law and ranged from 2 to 172 L m(-2) d(-1). Because of bias in estimating K-v, these values likely overesti mate the true vertical ground-water discharge and indicate the importa nce of better field methods to estimate K, especially K-v. Calcareous fens may need water tables sustained near the peat surface by large ve rtical ground-water discharges to allow carbonate precipitation, which is associated with the rare fen vegetation.