Effect of sinkholes on soil moisture and ground water recharge at an abandoned underground coal mine near Wilton, North Dakota
Subsidence at abandoned underground mines in south-central and southwest North Dakota has produced numerous sinkholes. Previous studies in the Northern Great Plains have suggested depression-focused recharge may be a significant ground water recharge mechanism in this region. Thus, sinkholes may increase soil moisture and ground water recharge. Current reclamation practices include filling sinkholes with non-topsoil fill material. However, the benefits of increases in soil moisture and ground water supplies may outweigh the benefits of expensive reclamation. The purpose of this study was to determine whether soil moisture and, hence, the potential for ground water recharge, was greater in sinkholes than non-collapsed settings at an abandoned underground coal mine near Wilton, North Dakota. Site stratigraphy consists of till overlying interbedded sands, silts, clays, and coal. The water table lies within or slightly above the coal seam mined at the site, at a depth of 40 to 110 feet, with the exception of two wells set in structural highs in the coal seam. The topography is gently rolling hills with integrated drainage. Annual precipitation is 17 inches, 80% of which falls from April 1 through September, which is also the period of greatest evapotranspiration. Relative changes in soil moisture were measured in six sinkholes and five non-collapsed settings with a neutron probe. Tensiometers were installed to establish the direction of hydraulic gradients in the shallow unsaturated zone. Climatic data were obtained from the National Weather Service station in Wilton. Soil moisture conditions were monitored from the middle of February 1990 through early December 1990. These data show that soil moisture was greater in sinkholes than non-collapsed settings during this entire monitoring period, producing conditions more conducive to ground water recharge in sinkholes. Deep infiltration occurred only in sinkholes. Correlation of climate and soil moisture data suggest deep infiltration, possibly leading to storage or recharge, will occur only in sinkholes under normal climatic conditions. These data also indicate the greatest potential for deep infiltration and ground water recharge are in the spring and fall. Reasons for greater soil moisture in sinkholes include snow and run-off capture. Sediments in sinkholes also have greater porosity and permeability due to collapse. Although deep infiltration was measured in some sinkholes and not in non-collapsed settings, estimates of recharge indicate that the recharge measured was several orders of magnitude lower than the flux of water from the local water table to a lower aquifer. Reclamation would eliminate these sinkholes as recharge mechanisms but their loss as a recharge mechanism may not be that important. Because of the benefits of increases in soil moisture (creating more productive pasture) and ground water recharge potential, sinkholes at abandoned underground mines in this region could be left unreclaimed if they do not endanger surface structures and are not at risk of contamination. If they pose a threat to the general public or are at risk of contamination, they should be reclaimed.
Sinkholes--North Dakota--Wilton; Sinkholes--North Dakota--Burleigh County; Soil moisture; Abandoned coal mines--North Dakota--Wilton; Abandoned coal mines--North Dakota--Burleigh County, Geology
Networked Digital Library of Theses and Dissertations