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.
No abstract
Quarrying in limestone aquifers can interfere with groundwater flow paths. Quarries can pirate karst conduit flow by physically breaking into the conduits and changing the groundwater discharge points. Another mechanism of groundwater flow interference occurs as quarry dewatering lowers the water table changing groundwater flow directions. Dye tracing is an effective tool to evaluate and quantify these impacts. In Minnesota, tracing investigations have been conducted at two quarries. The Big Spring quarry near Harmony, Minnesota is in the Ordovician Galena Formation. The quarry is 500 meters from Big Spring, the headwater spring of Camp Creek, a Minnesota designated trout stream. Although the quarry is nominally above the water table, beginning about forty years ago, the quarry intercepted conduits carrying groundwater to the spring. Groundwater that formerly discharged from Big Spring now rises in the quarry then flows overland joining Camp Creek 100 meters downstream of Big Spring. About 90 percent of the mapped groundwater basin of Big Spring is now routed through the quarry. The Osmundson quarry is in the Devonian Lithograph City Formation at LeRoy, Minnesota. This sub-water table quarry requires seasonal dewatering at 1,000-3,000 liters/minute. When the quarry is being dewatered, Sweets Spring, approximately 300 meters to the southeast, stops flowing. Dye tracing has verified that the quarry pirates the flow to the spring. Both of these cases demonstrate the utility of using dye traces to determine the impact of limestone quarrying on groundwater flow paths. This information can be used to evaluate
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