Iowa Great Lakes Hydrology and Diagnostic Study

Thomas Doyle; Keith Schilling; Phillip Kerr; Matthew Streeter

doi:10.17077/rep.006705

This report is a synthesis of work conducted by the Iowa Geological Survey (IGS) in the Iowa Great Lakes Region (IGLR) from 2020–2024 for the Iowa Department of Natural Resources (DNR). The IGLR, located in Dickinson County, IA, is a major natural and economic resource for the state; however, several water bodies in the IGLR have water quality concerns that ultimately stem from the presence of excess nutrients. The purpose of the study was to complete a water budget for the Iowa Great Lakes (IGL) and to monitor water and nutrient concentrations entering the lakes from groundwater and surface water pathways to better understand how nutrient pollution is reaching the lake system. The three primary data collection elements of this project were groundwater monitoring, stream gauging, and water quality monitoring. These data were used to assemble water and nutrient balances for the IGLR. Groundwater data were collected from over 30 monitoring wells throughout the IGLR. Wells were installed to depths between two and ten meters to capture water table fluctuations and shallow groundwater quality. Manual water levels were collected monthly at all wells and transducers recorded hourly measurements at 9 wells. Groundwater monitoring revealed differences in recharge among the geological units present in the IGLR, with greater recharge occurring in coarser-textured deposits. Shallow groundwater movement appeared to be highly dependent on local topography as opposed to regional gradients. While an in-depth investigation of deep groundwater contributions to the IGLR went beyond the scope of this study, an assessment of glacial landforms and well records in the IGLR support the potential for West Okoboji Lake to be connected to a regional buried valley aquifer. Monitoring wells were sampled for nitrate and orthophosphorus (OP) nine times over the study period. Nitrate concentrations were higher in agricultural areas than in developed areas or areas with perennial vegetation but there was no clear association between OP concentrations and land use. Surface water was monitored at 20 locations over the four-year study period, with samples tested for nitrate, OP, particulate phosphorus, and several other field-measured parameters. Manual flow measurements were collected at all surface water sites, but the flashiness of the small watersheds that were monitored limited how these measurements could be used. Instead, an existing hydrologic model previously developed for the IGLR was used to estimate daily surface water flows at most sites. Sensors measuring continuous water levels and rating curves were used when these data were available. Surface water quality showed higher nitrate and OP concentrations in surface water draining agriculturally dominated watersheds. However the association between land use and particulate phosphorus was less clear. Weekly particulate phosphorus monitoring may not have been frequent enough to capture particulate phosphorus trends related explicitly to discharge. In watersheds where the monitoring point was downstream of wetland complexes or lakes, nitrate and OP concentrations were lower than in water sheds with comparable land uses that lacked these features. Using groundwater recharge estimates and surface water volumetric fluxes, an annual water balance was created for the major lakes of the IGLR. There were major differences in annual water balances during the study between dry (2021–2023) and wet (2024) periods. During the dry years, direct precipitation (39–59%) was the largest input and evaporation (71–95%) was the largest output. Ground water was an important input to the lakes during dry years (16–33%). In 2024, the region experienced increased precipitation and flooding and contribution of surface water fluxes in and out of the lakes increased substantially. Surface water fluxes comprised 60–65% of the inputs and 60 83% of the outputs to the lakes. Precipitation, groundwater inputs, and evaporation contributed to only 19–25%, 15 21%, and 16–33% to volumetric fluxes during the wet year. The nutrient balance was consistent with the water budget. Groundwater inputs contributed approximately 40% of the total nitrate load the lakes between 2021 and 2023 and decreased to 16–28% during 2024 (wet year). Groundwater was less significant in nitrate and OP inputs to the lakes. Groundwater OP contributions ranged from to 17% to 76% from 2021–2023 and 22% to 34% in 2024. Surface water contributions were much higher during the flood year. During the dry years, nutrient inputs were several times higher than outputs, likely a result of the lakes’ nutrient attenuation and storage capacity. This was also true for nitrate in 2024 but the amount of OP and particulate phosphorus outputs relative to the inputs was much higher in 2024, indicating that internal nutrient cycling is likely an important component of the lakes’ phosphorus cycle. The data collected and analyzed for this project can and should be used to help inform future work in the IGLR. Water quality data can be used to better understand how different factors affect water quality, and the updated understanding of the groundwater system can help with the creation of more accurate hydrologic models of the system. Monitoring results show the benefits of the best management practices that have been implemented and provide further support for their implementation to improve water quality. Based on our findings, recommendations for future work in the IGLR include additional focus on particulate phosphorus, mapping the extent of tile drainage in the watershed and assessing its impact on water quality, and conducting an investigation into deep groundwater sources in the region.

Iowa Great Lakes Hydrology and Diagnostic Study

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