Shallow Groundwater and Stream Water Interactions during High Intensity Rain Events

Abstract:

The study was conducted from early May through late July of 2013 in a small suburban watershed, called the Dry Run Creek (DRC), in northeast Iowa. Flooding is considered one of the largest hazards in the United States, causing economic damage in billions of dollars. Investigations across the country have documented dramatic changes in river discharge over the past decades due to an increase in rainfall events. Consequently, many areas in the U.S. have been experiencing flash flood occurrences. The objective of this study was to assess the impact of flash flooding on shallow aquifers and the associated changes in water quality. On May 29, 2.81 inches of rain fell on the watershed from 4:30 to 8 p.m. causing the water level at the selected site to rise from 1.99 ft at 4:15 p.m. to 9.38 ft at 8:45 p.m. The event produced a severe flash flood condition in the creek by releasing discharge that is 562% higher than the average flow rate at the site for that time of the season. The water level eventually exceeded the bankfull stage. A considerable impact of the flood on the shallow aquifer was observed. The hydraulic gradient in the hyporheic zone reversed, resulting in the adjacent shallow groundwater table to rise by 4 ft. Several water quality parameters rapidly changed in values. The water temperature dropped from 11.26 degrees to 10.95 degrees C, and the total dissolved solids went down from 432 ppm to 426 ppm. On the other hand, turbidity went up from 72.4 NTU to 80.4 NTU and the dissolved oxygen (DO) increased from 7.54 ppm to 8.43 ppm. The changes took place within a 4-hour time period. The dramatic rise in water level as well as the increase in DO indicate rapid influx of stream water into the aquifers. The groundwater measurements were taken in a well that was 45 feet away from the creek. The aquifer response of this nature indicates that the hyporheic zone alongside the creek is much wider than expected for this small watershed. Wide and active hyporheic zones in a small watershed can be very effective in channeling high stream discharge at times of flash flooding.