2023 Decorah Water Resources Investigation Report defines water protection area and reveals potential susceptibilities

University of Iowa Hydrogeologist Greg Brennan presented the findings of the 2023 Decorah Water Resources Investigation Report at a public meeting held at the Decorah Public Library Tuesday. The report, based on a geological survey conducted by Brennan this June, identified the most accurate Decorah drinking water protection area, called the Wellhead Protection Area (WHPA). The WHPA delineates where the source water originates that supplies Decorah's drinking water and uses city streets to demarcate the area. The report also revealed potential drinking water susceptibilities in case of a contamination event within the WHPA. According to Brennan, Decorah's drinking water comes from six wells located along the Upper Iowa River and can contain a mix of groundwater from a prolific alluvial aquifer and surface water. In times of high water need, like a dry June, the drinking water wells pump hard enough to draw an increasing amount of surface water, mainly from the river, into the wells to keep up with water demands. Drinking water susceptibilities In order to test the WHPA for susceptibilities, Brennan conducted two worst-case scenarios that modeled what could happen if contaminants entered the water supply inside the WHPA. In the scenarios, one side of the well field or the other was deemed temporarily unusable. Brennan and his team then ran field tests on the side of the well field that was supplying all of the water for a set period of time. In the first field test scenario (testing Wells 1, 2, 3 on the west side of the well field) he found that 100% of test particles used to represent possible contaminants made it to the wellheads within 10 days and 20% of the total amount of water pumped in the wells came from re-charge (water flowing into the aquifer) from the river, due to increased pumpage. Within 30 days, 50% of the water in the aquifer came from the river and test particles were present in the well. In the second field test scenario (testing Wells 5, 6, 7 on the north side of the well field) he found similar results, but the time of travel was 30 days and the amount of water pulled from the river was about 13%. "The main thing is that 10 days, even 30 days, isn't very long to react," Brennan said. Next steps to protect Decorah's drinking water Decorah City Engineer Jeremy Bril said the most significant takeaway from Brennan's report is that while the capture zone, or WHPA, was found to be smaller than previous models, the travel time for water to move within the WHPA was found to be extremely short. "For many communities, the travel time within a well's capture zone is typically on the order of years, decades, or even centuries. Greg's model showed that for our wells' capture zone we could have a travel time as short as 10 days under certain conditions," Bril said. "This has substantial implications for how we report and respond to hazardous spills, monitor contaminants in the wells, inventory potential contaminant sources, etc." According to Bril, the City of Decorah Drinking Water Protection Team will use the findings from Brennan's report to help create a full Drinking Water Protection Plan Report. The Team, led by Iowa Water Quality Consultant Becky Ohrtman, will use the data, along with other information, to develop a list of recommendations for ensuring that the City is doing everything possible to produce a high quality drinking water. This includes recommendations like increasing frequency of testing the wells for hazardous substances, updating the City's Drinking Water Emergency Response Plan, considering installation of monitoring wells, and modifying City Code to best protect the WHPA. Protection and prevention Ohrtman explained that it is critical for the City of Decorah to have a prevention plan in place because "of the 527 susceptible water supplies in Iowa, Decorah is one of them." Ohrtman encourages Decorah residents to do what they can to help protect drinking water, by: Installing backflow prevention devices in private homes to decrease contamination risks. Plugging abandoned wells. Knowing the Wellhead Protection Area (WHPA), and saying something if you see something, like a potential contaminant, in the area. Reporting contaminant spills. She emphasized that protection and prevention is crucial not just for health and environmental reasons, but for economics, too. "It's cheaper to prevent contamination than it is to treat it," Ohrtman said. The full 2023 Decorah Water Resources Investigation Report should be available on the Iowa Geological Survey website within the next week or so.