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

Abstract

Six calcareous fens in the Minnesota River Basin, USA arc in regional hydrogeologic settings with large discharges of calcareous ground water. These settings juxtapose topographically high m'eas 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 t~iver 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 grotmd-water disch~ge on river terraces. One of the calcareoas 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 underIying aquifer. Carbonate content of calcareous fen peat averaged about 27% (calcium carbonate equivalent, dry weight basis) krt the st~rface layer, which commonly overlies a carbonate-depleted zone with a carbonate content of 10% or less. Hydraulic conductivity (K) of calcareous fen peat determhted from slug tests ranged from 2.7 × 10 .7 to 9.8 × I0 -~ m s ' and had a geometric mean of 3.8 × 10 ~" m s ~. These values likely underestimate the true horizontM hydraulic conductivity (Kh) and overestimate the true vertical hydraulic conductivity (142,) because of errors in assumptions commonly used in slug-test analyses. Median (over time) hydraulic heads in wells screened below ~he base of the peat rangeaJ 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 Darcy's Law and ranged from 2 to 172 L m -2 d-L Because of bias in estimating K~, these values likely overestim:~te the true vertical ground-water discharge and indicate the importance of better field methods to estimate K, especially K,,. Calcareous fens may need water tables sustained near the peat surface by htrge vertical ground-water discharges to allow carbonate precipitation, which is associated with the rare fen vegetation.

Groundwater-induced redox-gradients control soil properties and phosphorus availability across four headwater wetlands, New York, USA

“…The close correlation between soil composition, P-availability, and pore water chemistry along hydrologic gradients indicated that significant, non-random variation in soil properties occurred in response to the groundwater influence and suggested the importance of microbially-mediated biogeochemical reactions to controlling soil nutrient status. Previous hydrologic studies showed that stable water table elevations are sustained by discharge from multiple groundwater systems (Almendinger and Leete 1998b;Boomer and Bedford 2008). Equally important, we found that groundwater delivery of terminal electron acceptors, including NO 3 -and especially SO 4 2-, can induce distinct hydrochemical gradients extending across a wetland complex, on a scale of tens of meters (Boomer and Bedford 2008;Christensen et al 2000).…”

“…Water table elevations consistently occur within 10 cm of the land surface, and therefore soils remain saturated but not flooded (Almendinger and Leete 1998b;Amon et al 2002). These conditions result in formation of carbon-rich soils with more than 50% organic matter content (Verhoeven et al 1996), except where the soils consist predominantly of calcium carbonate precipitates (Bedford and Godwin 2003).…”

“…In alkaline, reduced environments, co-precipitation with calcium minerals reduces P availability (Moore and Reddy 1994). Under circumneutral conditions (pH range 5.5-7.4; Sharpley 1995) typical of fens in glaciated regions of the US (Almendinger and Leete 1998b;Amon et al 2002;Komor 1994), both mechanisms could influence P availability, but the relative importance of each mechanism remains uncertain. Elevated alkalinity (Drever 1988) and carbon dioxide (CO 2 ) degassing at the land surface in discharge areas (Boyer and Wheeler 1989;Komor 1994) has been observed to enhance calcium carbonate precipitation in fens (Almendinger and Leete 1998a;Futyma and Miller 2001).…”

Discrete wetland groundwater discharges revealed with a three-dimensional temperature model and botanical indicators (Boxford, UK)

“…However, links between phreatophytes (plants which draw water from the water table) and groundwater discharge are more established in arid and semi-arid regions than more humid regions (Batelaan, et al, 2003). Plants have been regarded as indicators of groundwater discharge in the Netherlands Klijn and Witte, 1999;Lucassen, et al, 2006;van Diggelen, et al, 1988;Wierda, et al, 1997) and Minnesota, USA (Almendinger and Leete, 1998;Batelaan, et al, 2003;Glaser, et al, 1990;Goslee, et al, 1997;Rosenberry, et al, 2000). Research in the UK has focussed on general water requirements of wetland plant communities and species (Gowing, et al, 2002;Newbold and Mountford, 1997;Wheeler, et al, 2009).…”

Field-trip guide to mafic volcanism of the Cascade Range in Central Oregon—A volcanic, tectonic, hydrologic, and geomorphic journey

“…Moderate discharge velocity and diffusive discharge are identified as potentially important conditions for peat development (Almendinger and Leete, 1998;Amon and others, 2002).…”