HR: 1340h
AN: H13B-0402    [Abstracts]
TI: Measuring a Small Hydraulic Gradient in the Presence of Noise
AU: * McElwee, C D
EM: cmcelwee@ku.edu
AF: Department of Geology, University of Kansas , 1475 Jayhawk Blvd., KS 66045 United States
AU: Devlin, J F
EM: jfdevlin@mail.ku.edu
AF: Department of Geology, University of Kansas , 1475 Jayhawk Blvd., KS 66045 United States
AB: In naturally occurring flow systems the hydraulic gradient may be small, often less than 0.002- 0.001. These small gradients are hard to measure accurately at all scales. On regional flow maps, the accuracy of the head contour lines and the gradient is usually determined by the accuracy of the elevation of the top of the casing and of the well location on the regional map. These limitations on the regional scale accuracy will be improved in the future by the use of Global Positioning System (GPS) technology. However, for the present, some regional gradients and many local gradients - even those based on measurements made at local scales - are problematic in low gradient areas. This paper uses field data to demonstrate some of the problems associated with determining a small natural gradient in the vicinity of a research site, the Geohydrologic Experiment and Monitoring Site (GEMS) at the University of Kansas. The site is contained in an area of about 50 meters by 50 meters near the valley wall in the Kansas River valley north of Lawrence Kansas. The difficulty of determining the natural gradient was discovered about 10 years ago when attempting to design and run a bromide tracer test; the distances between wells at GEMS are too small (about 20m max.) to accurately determine the hydraulic gradient. The task is further complicated by the presence of rural water district wells some distance to the west of the site. Monitoring the water levels at the site reveals a noisy environment, primarily caused by the periodic pumping of the rural water district wells. In 2003 two additional wells were installed near the site, one to the east (96m) and one to the south (147m) of GEMS. With these larger distances between wells, monitored water level differences were more pronounced, permitting reliable gradient estimates to be calculated by accurately surveying the elevations and locations of the wells. Water levels were measured using accurately calibrated pressure transducers recording data at a high frequency over an extended period of time. The amplitude of the noise in the area was found to be larger than the head differences measured between wells; subsequently, techniques for extracting the usable signal were employed and found to be successful. This paper presents the results of our measurement of the natural gradient in the presence of noise, points out some of the pitfalls to avoid when measuring small gradients, and quantifies the uncertainties in flow velocity (magnitude and direction) that can result from gradient measurements in the presence of noise.
DE: 1829 Groundwater hydrology
DE: 1832 Groundwater transport
DE: 1884 Water supply
DE: 1894 Instruments and techniques
SC: Hydrology [H]
MN: 2004 AGU Fall Meeting