Geostatistical Characterization Of Hydrogeologic Heterogeneity In A Glacial Lacustrine Aquifer System

Author : Sevin Ilhan Bilir
Publisher :
ISBN 13 :
Total Pages : 0 pages
Book Rating : 4.:/5 (269 download)

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Download or read book Hydrogeologic Characterization of a Glacial Aquifer Contaminated by Crude Oil Near Bemidji, Minnesota written by Sevin Ilhan Bilir and published by . This book was released on 1992 with total page 0 pages. Available in PDF, EPUB and Kindle. Book excerpt: This study investigates the hydrogeology of a glacial sand and gravel aquifer contaminated by crude oil near Bemidji, Minnesota. The goal of this study is to obtain an understanding of the effect of local flow phenomena, glacial stratigraphy, and climatic and surficial factors, on the groundwater flow behavior. The study determines the geohydrologic parameters of the study area that was affected by the oil spill, and identifies the dominant controls on fluid flow and the impact of seasonal variations. The study area lies on a flat to gently rolling outwash plain having a regional hydraulic gradient of 0.0028 towards N70°E. Locally, the shallow unconfined aquifer has a lower boundary consisting of a low permeability basal till unit located at approximately 23-31 m depth and is overlain by approximately 10 m of stratified morainal drift, containing discontinuous lenses of till, sediment-flow deposits, and lacustrine silt and clay. An unconformity separates the drift from approximately 7 m of outwash sands and gravels consisting of layers of fine grained sand and silt. The site is situated on a small recharge zone of a local flow system, which discharges into a small lake 350 m downgradient from the initial oil pool. Vegetation and geomorphological surveys, infiltration rate measurements, and soil organic carbon analyses were used to investigate the complexities of focused recharge. Surface water flow is directed to points of focused recharge. In the spray zone, where water flows over oily sediment, contaminated waters enter the unsaturated zone and possibly reach the water table. Monitoring wells were emplaced to gather stratigraphy and seasonal water level data, which indicated local flow variations particularly during times of extreme recharge and in areas with a shallow water table. Water levels are usually high from June to the early part of autumn and then drop off steadily throughout the year. In the wetland a water level decrease of 0.5 m occurred over one month during a dry summer. However, an average decrease of less than 10 cm was observed in the rest of the site. Mounding of the water table typically occurs in the summer and at points of focused recharge, probably in response to the uneven distribution of recharge and hydraulic conductivity. The wetland is a flow-thru lake or a discharge mound for most of the year, yet following large events of precipitation it is a recharge mound. Hydraulic conductivity and sediment anisotropy were quantified by measuring grain-size distribution and bulk and individual hydraulic conductivities of intact core sediments. Sediment is mostly medium sands with lenses of silt and gravel. Adjacent layers may differ in hydraulic conductivity by more than three orders of magnitude. Measured hydraulic conductivities ranged from 1x10−8 to 1x10−4 m/s with an average of 2.02x10−5 m/s, indicating mostly sand size sediments. Hydraulic conductivities of homogeneous sediments did not vary outside an order of magnitude, however, hydraulic conductivities of heterogeneous sediments were found to range over 4 orders of magnitude. Anisotropy ratios averaged 1.4 and 15 for calculated and measured hydraulic conductivities, respectively, indicating a complex flow field dominated by horizontal flow. Thin fine-grained layers affect the vertical flow rates to a large degree. MODFLOW, a three-dimensional finite-difference groundwater flow model, was used to simulate the effects of a changing hydrologic budget on the local flow system, incorporating the detailed information on hydraulic parameters and recharge rates determined during this study. Steady-state modeling confirmed that the complexities at the site could not be modeled under simple homogeneous conditions, but required a variable distribution of hydraulic conductivities. The influences of climatic factors were seen in consecutive steady-state models to support observations that evapotranspiration and recharge play an important role, particularly where the water table is shallow.