INVESTIGATION OF WATER-TABLE FLUCTUATIONS IN FRACTURED TILL USING MONTHLY, LONG-TERM (1996-2017) AND CONTINUOUS, SHORT-TERM MEASUREMENTS

Locating and monitoring the water-table surface are important tasks for environmental site assessment and regulatory enforcement. The water table in fractured till may behave differently than in a typical porous medium. For this study, we hypothesized that the lower porosity (and lower specific yield) in till affects the timing of water-table response to precipitation events and magnitude of its fluctuation. To test this hypothesis, we examined monthly, long-term water-table measurements from 1996 to 2017 and continuous, short-term measurements since October 2017 in a monitoring well in the Walnut Creek watershed, 7 km south of Ames, Iowa. The well was installed to a depth of 15 ft into oxidized and the upper part of unoxidized till of the Dows Formation of late Wisconsinan age. Fracture density decreases with depth in the till as well as hydraulic conductivity (K). The location is 30 ft north of an actively cropped field. Monthly measurements were made using a Solinst electric water-level tape. For continuous (15-minute) measurements, a pressure transducer/datalogger was installed into the well to measure water-table elevation. Precipitation data were obtained from the Iowa Environmental Mesonet. Data from the long-term measurements indicate the mean water-table elevation during the period was 994.2 ft, or about 6.4 ft below ground surface, with a standard deviation of 2.8 ft. The median elevation was 994.8 ft, or 5.8 ft below the ground surface. The range in amplitude was 15.3 ft during the study period. The largest change in the water table occurred from May 2012 to January 2013 when it declined 11.7 ft into the unoxidized zone due to a drought. Typically, there is a lag between precipitation events and water-table rise, but water-table decline can occur quickly. Tile drains, set four feet below ground surface, are near the site and our data indicate that the water table was below the tile drain elevation during most of the study period. Data from the pressure transducer show many smaller-scale, non-diurnal fluctuations that may reflect evapotranspiration. When compared to data from a sand aquifer, the till showed larger magnitude water-table fluctuations in response to similar precipitation. In summary, that data support our hypothesis about the nature of water-table fluctuations in fractured till.